MAIN ENGINE ORAL QUESTIONS
Q.A diesel engine crankcase ventilation system is provided to
Ans. remove the combustible gases.
Inside the crankcase, the oil is getting spread all around due to the moving parts of the engine. At the same time, this oil is coming in contact with the hot surfaces thus causing evaporation of oil. If this evaporated oil is allowed to condense, it forms oil mist. This oil mist is a potential hazard for the crankcase explosion. The only thing that lacks is a spark. If there is any spark present due to the blowpast or running of any hot part, crankcase explosion will take place. In order to avoid such occurrence, crankcase is fitted with ventilation so as to remove the combustible gases which are formed.
Q. Most fuel injector nozzles are opened by
Ans. (a) fuel oil pressure.
Most of the fuel injector nozzles presently utilized on the internal combustion engines are hydraulically operated. This hydraulic pressure is produced by the fuel itself which is going to burn in the cylinder. Hence when the fuel pump plunger is moving upwards, oil pressure is developed which acts on the underside annular space of the fuel valve needle. This results in the lifting up of the needle and the oil, under high pressure, finds its way through the already unseated fuel injector needle and is forced through the fuel injector holes as a fine stream. As soon as the pressure drops, the spring pressure will be more than the hydraulic pressure and the valve snap closes on the seat.
Q. The bearing used to support the crankshaft are generally called
Ans. (c) main bearings.
Main bearings are the bearings which support the crankshaft. The full load or the full power developed by the engine is transmitted to the crankshaft through various moving parts such as pistons, connecting bearing, crosshead bearing, bottom end bearing and ultimately to the crankshaft. From crankshaft, it is transmitted to the propeller shaft. At the same time, the downward forces acting on the crankshaft are taken care of by the bearings which supports the crankshaft. Hence they are called the main bearings as they are the most important bearing of the system. If they are not of proper strength or are worn out, the crankshaft may get deflected and there is a possibility that the crankshaft may crack. Also, the complete alignment of the engine is fully dependent on these bearings; hence they are called main bearings.
Q. Injection pressure in a common rail fuel system is controlled
Ans. (d) by a bypass valve.
In common rail system of fuel injection, common piping supplies fuel to the individual injectors by a branch pipe which is the feature of the common rail system. This pipe and the full system is under high pressure i.e. the injection pressure. To control this, a bypass valve is utilized which is fitted at the end of the common pipe. By closing or opening this valve, more or less, the fuel is bypassed from the system back to the suction of the pump, thus reducing the pressure.
Q. In internal combustion engines, the crankcase vent outlets must be equipped with
Ans. (b) corrosion resistant flame screens.
Corrosion resistant flame screens are required on the vent outlets of the crankcase, in internal combustion engines, in order to prevent spark from outside to enter the crankcase. As the crankcase atmosphere is full of oil mist, it is only waiting for the spark to come in contact to have an explosion. Under any circumstances, the spark must be prevented from entering the crankcase. For this purpose, it is necessary to have the flame arrester fitted on the crankcase breather. At the same time, this flame arrester should be maintained in clean condition so that it should have better heat transfer property so that it can extinguish the flame before entering the crankcase. It is therefore necessary that there should be not any rust formation on the screen because of which the heat transfering capacity and the efficiency of the arrester would be reduced. Taking into consideration the above conditions, it is necessary to provide a rustproof flame arrester screen on the crankcase vent outlet.
Q. What is to be installed on an internal combustion engine if a cylinder bore
exceeds 200mm?
Ans. (d) Explosion relief valves.
The engines having cylinder bore that exceeds 200mm are generally highly rated engines with high peak pressures. At the same time, most of these engines are compressed air started engines. Taking into consideration these conditions, it is necessary to prevent the excessive build-up of pressure inside the cylinder as this excessive pressure may put the parts of the engine under heavy stresses, which may cause mechanical failure of these parts. Hence, in order to avoid this, it is necessary to provide a cylinder relief valve on these engines.
Q. Cylinder lubricating oil for a low-speed main propulsion diesel engine is admitted to each cylinder during
Ans. (b) the compression stroke.
Cylinder lubricating oil, for lubricating the piston rings and the liner, has to be admitted when the piston, piston rings and the liner are in cool condition and the piston is moving upward so that oil can be retained on the piston rings and sprayed by the piston rings on the liner walls. This is only possible during the compression stroke. Otherwise, the piston is hot and if the lubricating oil is sprayed on it, it will evaporate very fast and will not carry out any work of lubrication. At the same time, if lubricating oil is injected during the expansion stroke, i.e. when the piston is moving downwards, it will have a scrapping effect rather than lubrication.
Q. What is the purpose of the window installed in the housing of an individual jerk-type fuel pump?
Ans. (a) To allow the pump to be timed with the engine.
Through the window of the fuel pump housing, it is possible to see the movement of the sleeve up and down. The sleeve has one marking only. At the same time, the sides of the window also have one reference mark. When the reference mark on the sleeve starts moving upward, it indicates that the plunger is moving upward. When the mark on the upward moving sleeve coincides with the mark on the pump body, it indicates that the upward moving plunger has closed the suction port of the fuel pump. This is the beginning of the injection. This can be checked on the flywheel and, if required, can be adjusted. The end of the injection will be dependent on the amount of fuel to be injected.
Q. Which of the following listed construction details of the internal combustion engines is required?
Ans. (a) A warning notice of caution against the opening of a hot crankcase for a specified period of time after shutdown.
On most of the modern engines, the crankcase doors has been put up with a warning or notice as not to open before particular time has lapsed. The crankcase is full of oil spray. This oil spray will be spread over the hot surfaces of the crankcase and will get evaporated into oil mist. This oil mist, when comes in contact with the hotspots, causes the explosion. Most of the times these crankcase explosions do not occur as the atmosphere in there is too rich to cause an explosion. If the crankcase is hot due to some reason and oil mist has been already formed, under such circumstances, if the crankcase door is opened before allowing sufficient amount of cooling, there will be ingress of fresh air. This will try to dilute the crankcase atmosphere and bring it into explosive range and an explosion might occur. Thus it is necessary to wait for specified amount of time before opening a crankcase door.
Q. Which statement regarding the arrangement and location of explosion relief valve used on internal combustion engines is true?
Ans. (b) Minimising the danger from emission of flames is a key consideration.
When a crankcase explosion takes place, the crankcase relief door opens and allows the hot gases to get relieved through the relief door. Along with this, there is a possibility of a flame emerging from the crankcase relief door. An arrangement should be made on the crankcase relief door so that this flame and hot gases will not emerge directly and create danger for the personnel working in the engine room. Hence it has to be deflected to a safer place. This is carried out by a deflector plate on the outside of the crankcase relief doors. At the same time, inside of the crankcase relief door is provided with a flame arrester valve which also cools the flame before it emerges in the engine room atmosphere. Thus it prevents the spreading of fire in the engine room.
Q. The term “proper metering” as applied to a diesel engine injection system can be best defined as
Ans. (a) delivering the same quantity of fuel to each cylinder for each power stroke depending upon the engine load.
It is necessary to inject particular quantity of fuel oil, for a specified load, in order to have proper combustion of the fuel and the best thermal efficiency of the engine. Also it is necessary that this fuel oil should be injected at the right time. In addition, it is necessary that all the units get equal quantity of fuel so that the engine will be mechanically and thermally balanced. For this purpose metering is done. The process of injecting the right quantity of oil at the right time and to distribute equally for all the cylinders so as to have the full power balancing of the engine is called metering. Actually, metering is the word used for measuring the fuel equally for all units.
Q. One function of the fuel pump delivery chest valve is to
Ans. (c) help the injector needle reset without dribbling at the injection nozzle holes.
The fuel pump delivery valve is designed in such a manner that it has got one portion of the valve in the form of a piston. This piston form of the valve will help in preventing the smart closing of the valve. In other words, it will delay the closing of the valve because of the drag effect. This delayed closing of the valve will help in relieving the pressure wave caused due to sudden closing of the fuel valve needle on the seat. As this pressure wave is reduced, it will not have any effect on the fuel valve needle. If this pressure wave is not reduced, then as the delivery valve of the fuel pump is closed, it will cause the fuel injector to open for the second time thus causing secondary injection. This is not desirable as it would increase the exhaust temperature and reduce the power generated and increase the specific fuel oil consumption. Hence the fuel pump delivery valve does this function quite efficiently.
Q. The rate of fuel injection in a diesel engine cylinder depends primarily on
Ans. (a) the size of holes in the fuel nozzle.
Taking into consideration the size of the holes, the volume of the fuel injected through each hole will depend upon the diameter of the hole because the length of the hole is constant for a particular fuel injector nozzle. Also, as the hole size of the injector increases, the droplet size increases. As the droplet size increases, it has more penetration into the cylinder volume. Hence it can overcome the compression pressure of the cylinder. Taking into consideration both these aspects, the diameter of the hole of nozzle will be the main governing factor for the rate of fuel injection.
Q. The fuel injection system meters, atomize, and
Ans. (c) inject the fuel at the right time.
Fuel injection system has many functions to perform. One of the functions is to inject the proper amount of the fuel which is determined by load on the engine and through the governor. Secondly, it has to atomize the fuel by passing the high-pressure fuel through the fuel injector nozzle having tiny holes so that it would be spread in the form of oil mist. One more important function of the system is to inject the fuel at the proper time. Right time fuel injection is the most important aspect of the efficient burning of the fuel. This is carried out by the helix cut on the plunger or with the help of the
*spring valve. Hence, in most of the engines, the proper timing is the beginning of injection time because the end of the injection timing is governed by the quantity of the fuel injected. This is achieved by the time when the upward moving plunger closes the inlet port of the fuel pump or when the fuel pump plunger moves upward and the suction valve closes.
Q. The valve cam slope angle determines the
Ans. (b) acceleration rate of valve closing and opening.
As steeper the slope angle of the valve, more abrupt is the opening and closing of the valve. At the same time, this gives lot of stresses on the operating mechanism of the cams, the rollers and the pushrods. The best possible efficiency can be obtained from the cam when the flank of the cam is kept tangential to the base surface which gives snap opening and snap closing, without undue stresses on the operating mechanism of the cams. Especially in the case of the inlet and exhaust valves, the closing and opening should be fast enough to avoid the throttling effect which may be encountered during the opening and closing. Steeper the angle, the valve will open and close faster.
Q. The engine displacement is equal to the cylinder
Ans. (b) area x stroke x the number of the cylinder.
The displacement of the engine is the displacement volume caused by the engine or the swept volume of the engine piston, together for all units. Hence the displacement is calculated by multiplying the cross-sectional area of the cylinder and the stroke of the piston which gives the swept volume for one cylinder. When this is multiplied by the number of cylinders, it gives the total swept volume for the engine.
Q. Cylinder diameters greater than 230mm require additional safety devices
when the scavenging spaces are openly connected to the cylinders. Which of the following devices will be used to protect such spaces?
Ans. (b) Explosion relief valves.
When the under piston scavenge spaces are directly connected to the cylinder, in such conditions if there is blowpast due to the any reasons (such as broken piston rings, excessive liner wear, seized piston rings in the groove), it will cause scavenge fire. When scavenge fire takes place, there is a possibility of explosion occurring inside the scavenge space because of the availability of combustible material, hot gases or sparks blowing past and plenty air available in the scavenge space. In order to avoid such explosion, it is necessary to relieve the pressure if it gets generated. This is carried out by the explosion relief valves.
Q. A loop or cross scavenge engines utilizes the motion of its piston and turbocharger to provide scavenge air. Which of the listed mechanical design prevents the air under the piston from being pumped back through the scavenge ports during the piston travel?
Ans. (b) length of the piston skirt.
Q. The duration of fuel injection developed by the individual port and helix fuel injection pump is determined by
Ans. (b) effective pump stroke.
Effective pump stroke is the stroke when fuel is delivered by the pump. It will not take into consideration the idle stroke before the up-moving fuel pump plunger covers the suction port or the movement of the fuel pump plunger after the completion of the delivery stroke. Hence the position the fuel pump will be decided by the time through which the fuel is actually delivered. When the fuel pump plunger is moving upwards, as soon as it covers the ports, fuel injection is started. This is the beginning of the effective delivery stroke. As the plunger moves upwards, depending upon the load on the engine, the rack position of fuel pump will rotate the fuel pump plunger. Thus, for every load, there will be a definite
disruptive
position between the helix cut on the fuel pump plunger surface and the suction port. When the plunger is moving upwards, a point will come when the top of the plunger, i.e. the high pressure side, will get connected to the suction side thus relieving the high pressure and causing the end of injection. This is the end of effective delivery stroke. The quantity of the fuel injected will vary as per the effective delivery stroke which is controlled by the governor through fuel pump rack.
Q. The plunger in a jerk pump is rotated until the release port is uncovered. If the port remains uncovered all the time which of the listed operation will occur?
Ans. (a) no fuel.
When the plunger is moving upwards, the fuel pump as soon as the top of plunger covers the suction port, the oil on top of the plunger will get compressed and pressure will be developed and injection will commence. As soon as the top of the plunger will get connected through the helix to the suction port, the suction and the delivery side will get short-circuited and hence no pressure will be developed on top of the plunger which will cause the end of the delivery stroke. If the rack position is nearly zero, even though the plunger is moving upwards and is covering the suction port, the helix position is such that it will always connect the discharge side of the pump with the suction side of the pump thus causing the short-circuiting and no pressure will be developed throughout the stroke of the plunger. If no pressure is developed throughout the stroke of the plunger in that case there will be no delivery of the fuel throughout the stroke of the plunger. This is called as a zero delivery stroke.
Q. When the lower edge of the spiral begins to uncover the release port, in a jerk
pump,
Ans. (b) effective pumping stroke of the plunger ends.
As the plunger is moving upwards and the fuel is getting compressed, the pressure is developed and it will cause the fuel injector nozzle to get lifted and the fuel is injected. Depending on the position of the rack which governs the position of the helix with respect to the suction port, a point will come when the helix bottom portion will come in contact with the suction port. This will cause the short-circuiting of the fuel oil between the high pressure and the low-pressure side i.e. between the top of the plunger and the suction side. This will cause a sudden drop in the delivery pressure due to which the fuel valve will close under the spring pressure thus causing the end of fuel injection and hence the end of effective delivery stroke.
Q. The amount of fuel delivered by a helical plunger fuel injection pump is controlled by
Ans. (c) the rotation of the pump plunger.
The surface of the pump plunger has a helix cut on it. These helical grooves on the surface will be responsible for changing the effective delivery stroke of the plunger. The moment at which the helical groove connects the delivery side of the plunger with the suction side will be the end of injection. The position of the helical groove can be changed with respect to the fuel pump barrel suction hole by means of rotating the fuel
pump plunger while it is moving up and down in vertical position. This movement of the plunger with respect to the pump barrel is due the fuel pump rack which is connected to the governor. Hence depending upon the fuel load, the governor will change the position of the rack. Due to this, the fuel pump plunger will rotate and change the position of the helix with respect to the suction holes. Hence the quantity of fuel delivered will vary as this causes change in the end of injection timing.
Q. Which of the following will occur when lower edge of the spiral on the plunger of a jerk pump uncovers the spill ports?
Ans. The pressure drops and the fuel delivery stops.
When the lower spiral edge cut of the helix cut on the fuel pump plunger surface uncovers the suction port or the spill port, it will have a short-circuiting effect between the top of the plunger and the suction side. As the top of the plunger is high-pressure side, when it is connected to suction low-pressure side the pressure will drop suddenly on the delivery side. As a result, the fuel pump will have the end of effective delivery stroke and the fuel valve will close smartly.
Q. In a diesel engine, the pistons are attached to the crankshaft by
Ans. (c) connecting rods.
The transfer of the power from a piston to the crankshaft is achieved by the use of connecting rod and gudgeon pin assembly or connecting rod and crosshead assembly. This connecting rod also helps in converting the reciprocating motion of the piston into rotary movements of the crankshaft. This part of engine connects the piston to the crankshaft hence it is called the connecting rod.
Q. Which of the component listed is not found in a single acting engine?
Ans. (b) Double crankshaft.
In single acting engine, the action of the fuel burning and load transmission is carried out only on the top of the piston and during the downward movement of the piston Hence it requires only one crankshaft to transfer the power.
Q. In diesel engines designed with a crosshead the motion of the engine can be described as
Ans. (c) straight reciprocating.
In an engine with crosshead, the piston has a piston rod which is rigidly connected to the bottom of the piston and the other end is connected to the crosshead. Hence when the piston moves up and down, the piston rod also moves in a vertical direction thus causing the reciprocating movement which follows the movement of the piston. At the same time it is passes through a stuffing box which separates the scavenge space from the crankcase space. This vertical movement of the piston rod is converted into the rotary movement by the use of crosshead and connecting rod combination.
Q.Cast iron piston used in large propulsion diesel engines are constructed with
Ans. (d) the crown being tapered and smaller than the skirt.
The crown of a large engine is always subjected to more heat rather than the skirt and hence the expansion of the crown is higher than the skirt. Clearance is provided for the expansion. Otherwise, the crown will expand to such an extent that it may start to touch the liner and it may cause a damage. In order to avoid such things, clearance is kept at higher valve at the top of the piston. This is achieved by using a tapered crown so that the clearance is maximized at the top of the crown where the temperature is maximum and reduced at the bottom where the temperature is minimum.
Q. Which of the manufacturing method is recommended diesel engine fuel injection line piping?
Ans. (c) Seamless drawn.
The diesel engine fuel injection piping is subjected to very high pressure, in the range of about 1200bar. Also, there is sudden increase and decrease of pressure which causes the fatigue in the material. Taking into consideration these two things it is necessary that the material and the manufacturing process of the piping should be such that it should be able to withstand the high pressure and fatigue. It is also of prime importance, from the point of view of safety of the ship and fire hazard, that the fuel pipe should not get cracked under these high pressures. In order to achieve these requirements, it is necessary to use fuel pump pipes made up of seamless, solid drawn tubes.
Q. The surface of combustion in the cylinder of a diesel engine depends on
(i) fine atomizing.
(ii) high temperature.
(iii) good mixing of fuel and air.
(iv) All of the above.
Ans. (iv) All of the above.
The efficient combustion of fuel will depend on the proper mixing of the fuel and air in the combustion space and evaporation of the fuel as fast as possible so as to form a combustible mixture. This is achieved by breaking the fuel into fine particles and to spread the fuel in the entire combustion space available. This is carried out by the fuel injector which will atomize the fuel into fine spray due to which the fuel droplets will be broken down into small particles and cause the evaporation of the fuel. Higher the temperature of the fuel or the compressed air, the time required for the fuel to evaporate and form a combustible mixture reduces and hence greater combustion effect is achieved. As good mixing of fuel and air takes place, entire volume of air is available for combustion. Hence full quantity of fuel can be burnt effectively as there are no pocket formations which will cause local combustion.
Q. Which of the combustion parameters listed is use in diesel engine but not related to the injection system?
Ans. (c) Effective stroke.
Effective stroke is not directly related with the injection system. It will control only the quantity of fuel which is injected. It will not be related to any of the parameters required for efficient combustion such as atomization, metering or penetration.
Q. Fuel oil penetration in the cylinder of a diesel engine is
Ans. (b) reduced by the finer atomization.
Atomization is carried out by injecting the fuel with high pressure through the fuel injector nozzle holes and it breaks the fuel into very minute particles. If the atomization is too much, the particle size will be very small which will have less weight and hence it will not have sufficient amount of kinetic energy to overcome the pressure of compression which is existing inside the cylinder at the time of injection. Hence it will not be possible for the fuel to penetrate deep into the combustion space, which is filled with high-pressure compressed air, at the end of compression stroke. This may cause the improper burning of the fuel as pockets of high concentration of fuel will be created and complete amount of air available will not be use for combustion.
Q. Proper atomization of fuel in the diesel engine combustion chamber will
Ans. (b) improve the combustion.
Proper atomization means breaking the fuel oil droplets into fine particles or to inject the fuel under the influence of high pressure which will convert the fuel oil into fine spray. this will cause more surface area of the fuel available for the contact with air. As the surface area increases, the fuel and air mixing will be properly done. At the same time, because of availability of more area, the fuel will evaporate fast and will form the combustible mixture which is the main important point from the point of view of greater combustion of the fuel and increasing the thermal efficiency of the engine.
Q.What is required for crosshead type engines that have a scavenging space in open connection to the cylinder?
Ans. (c) The scavenge space to be permanently connected through an approved fire extinguishing system, entirely separated from the extinguishing system of the engine room.
When the scavenge space is connected to the under side of the piston, there is every possibility of scavenge fire if the blowpast of the hot exhaust gas takes place due to worn Out cylinder liner or broken piston rings or piston ring cut off in the groove. In such circumstances, the hot gases or the spark, entering the scavenge space, will start burning the sludge and accumulated oil inside the scavenge space. As plenty amount of air is available this scavenge fire may continue for a longer period and it may also go beyond control. Hence it is necessary that a permanent arrangement must be provided so that if all other means, such as stopping of the engine and covering the exhaust and the turbocharger suction filter with a cloth, fail. In such circumstances, it should be possible to extinguish the fire by the use of a permanent extinguishing medium which is connected to the underside of the piston scavenge space mostly CO2 or steam is utilized. The requirement for this is very small and it should be independent of the engine room system because if it is connected to the engine room fire fighting system, it may possible that you may discharge the complete safe battery of the cylinder which are actually kept for the engine room fire. In such condition, engine room fire fighting equipment capacity will be reduced. Hence the scavenge space fire fighting installation should be entirely independent of the engine room fire fighting installation.
Q. Which of the following statement is an accurate description of the fuel injection piping used in a diesel engine with a cylinder bore of 250mm and above?
Ans. (b) The piping is to be effectively shielded and secured to prevent fuel or the
fuel mist from reaching the source of ignition on the engine or the surrounding. In case of engines of high capacity or those utilized in UMS ships, it is necessary that if the high-pressure pipe do burst, the oil or the oil mist should not come in contact with hot surfaces such as exhaust manifold so as to prevent the fire. For this purpose, it is necessary to double-walled the high pressure injection piping between the fuel pump and the fuel valve. Hence in case of any rupture in the pipe, the out coming high-pressure oil will not get injected directly over the hot surfaces and will not cause fire in the engine room. Also in the double-walled pipes, the outer portion of the pipes is connected to a leak-off tank whereby if any high pressure pipe does crack, the leak-off will led to the leak-off tank where it will give an alarm which indicates that there is a crack in the fuel piping
Q. A large slow speed main propulsion diesel engine may become overloaded by
(a) heavily fouled hull.
(b) strong head wind and heavy seas.
Ans. (c) both of the above.
When the hull is fouled, the resistance offered by the ship for the movement increases. Hence for the same rpm, the engine will be overloaded as it will have to produce more power to overcome the resistance. Similarly, when there is strong head wind, the ship will have to work against the head wind and overcome the force created by the headwind. In this condition also, the engine will be overloaded to maintain the same revolution on the engine shaft.
Q. Immediately after any diesel engine is started, the engineer should check the
Ans. (b) lubricating oil pressure.
Lubricating oil is the most important component of the engine. If the lubricating oil pressure is lost, even for a fraction of a second, there will be metal-to-metal contact between the bearing and the shaft, and they will get damaged. Hence the lubricating oil pressure should be checked as soon as the engine is started.
Q. In a diesel engine lubricating oil system, which of the following part should be lubricated first?
Ans. (b) Main bearings.
In case of the four stroke engines, the oil is first supplied to the main bearings. From the main bearings, through the holes of the crankshaft, it comes to the bottom end bearings. From the bottom end bearings, through an oil passage, it passes to the top end bearing. This passage shows that the main bearing is lubricated first. In case of large two-stroke engines, where separate lubrication is given to the crosshead bearing, both the main bearing and the crosshead bearing get the lubrication together.
Q. Poor combustion in a diesel engine can be caused by
Ans. (b) low intake air temperature.
If the intake air temperature is low, then the temperature of the air at the end of compression stroke will also be low. In such condition, if the fuel is injected it will have improper combustion because it will take more time for the oil to evaporate and form a combustible mixture. Due to this, there will be more amount of fuel combustion and loss of thermal efficiency.
Q. Diesel engine exhaust gas temperature can be used to determine individual cylinder
Ans. (a) performance.
Q. Any increase in exhaust gas back pressure in a four-stroke cycle diesel engine will
Ans. (a) reduce diesel engine power output.
High back pressure will cause the piston to push the exhaust gas out of the cylinder with the back pressure acting over it from the exhaust manifold. This will cause loss of power because the power required to drive out the exhaust gas from the cylinder will be taken from other units which are firing. Thus the total power output of the engine will be reduced because of the pumping effect of the trapped exhaust gas by the use of the piston.
Q. In diesel engine, blowby
Ans. (b) causes excessive crankcase pressure.
Especially, in case of four-stroke engine, if there is blowpast, high-pressure gases will enter the crankcase by blowing through across the piston rings. This will cause increase in the crankcase pressure.
Q. A crack in the cylinder liner can be caused by
Ans. (b) restricted cooling water passage.
Due to the restricted cooling water passages, the cooling of the liner flange on top will get affected. The liner will run at the higher temperature and because of that the material of the liner will lose its strength. Hence the liner will crack under the load.
Q. A cracked cylinder head may be indicated by
Ans. (c) combustion gases vented in expansion tank.
When the cylinder head is cracked and, under the compression stroke, when the fuel is injected at high pressure, the combustion gases produced will find its way into the cooling space through the cracks (in the cylinder head). Those gases which are lighter will come out of the cooling water from the expansion tank vent.
Q. A direct cause of crankcase explosion can be attributed to
Ans. (c) an overheated bearing.
Crankcase explosion will take place only if there is hotspot present inside the crankcase. If the bearing gets heated, due to various reasons, it will act as a hotspot and crankcase explosion may take place.
Q. Crankcase explosion in propulsion diesel engine results from
Ans. (d) the ignition of unburnt fuel and air in the crankcase.
If there is unburnt fuel oil in the crankcase due to contamination of lubricating oil with fuel oil, the crankcase will consist of lubricating oil as well as fuel oil vapors. This fuel oil will ignite in presence of large amount of air present in the crankcase and thus cause crankcase explosion.
Q. Combustion knock occurring in a diesel engine can be caused by
Ans. (a) low coolant temperature
.If the coolant temperature is low, it will take large amount of heat from the compressed air at the end of the compression stroke. As a result, the temperature of the air will drop and ignition will be delayed. When the ignition gets delayed, fuel will get accumulated and get ignited resulting in knocking in the unit.
Q. Persistent knocking of one cylinder of a diesel engine ceases when the fuel supply of that cylinder is secured. This problem may be a result of
Ans. (b) faulty combustion in that cylinder.
When the engine cylinder is knocking, and after shutting of the fuel if the knocking stops, it is a clear indication that the knocking was a result of problem related to fuel injection or quality. If the knocking persisted even after the fuel injection was cut-off, it indicates that the knocking is a mechanical knocking which is a serious problem.
Q. Which of the condition will occur when in an operating diesel engine if air pockets are formed within a cylinder head circulating water passage?
Ans. Hotspots will be developed.
If the air pocket is formed, as air is a low conductor of heat, the heat will not be conducted form the hot surface and it will not be carried away by water. Hence that particular spot will get overheated and hotspot will be developed.
Q. If a diesel engine exhaust temperature is abnormally higher, the cause could be
Ans. overload of the engine or the compression pressure would be too low.
If the engine is overloaded, the exhaust temperature of all units will be higher. If the temperature of only one unit goes up, it is an indication that there is some thing wrong with that particular unit i.e. either incomplete fuel combustion or improper timing adjustment of that unit or too low compression pressure of that unit due to worn out cylinder liner or leaky piston ring.
Q. Heat damage to the fuel injection nozzle can be prevented by avoiding
(a) excessive fuel oil temperature.
(b) long period of engine overload.
(c) hard carbon deposits and varnish on the nozzles.
(d) All of the above
Ans. (d) All of the above.
If excessive fuel oil temperature is permitted and if there is no separate cooling is given to the injector and the circulating fuel itself is part of cooling system, in such conditions the hot fuel oil will keep the nozzle at high temperature. This will cause damage to the fuel injector because at higher temperature the fuel injector nozzle and the valve seats will lose its strength and it will fail. Secondly, if the engine is working at high load for long time, the fuel injector nozzle will be subjected to high temperature from the combustion space due to which the fuel injector nozzle may get damaged. Thirdly, if the quality of the fuel is bad, there is a possibility that hard carbon deposits may be formed. Also the needle guide may get coated with hard varnish and because of this the fuel oil nozzle operation may get affected and it would ultimately cause to seep in place or have excessive wear-down and it will get damaged.
Q. The main propulsion diesel engine is normally shutdown by
Ans. (c) securing the fuel supply.
Under normal circumstances, the fuel oil supply supplied to the engine is stopped.
Q. Excessive diesel engine back-pressure may be indication of
Ans. (a) carbon built up in exhaust manifold.
When the carbon is built up in exhaust manifold the exhaust gas will have to overcome resistance of the built up carbon to pass through the exhaust manifold. Due to this there will be excessive back-pressure. In order to overcome this back-pressure, the piston has to work against the exhaust gas and hence there will be loss of power.
Q. White smoke from the exhaust of diesel engine can result from
Ans. (b) low cooling water temperature.
Due to low cooling water temperature, the entire atmosphere inside the cylinder liner will be at lower temperature because of which the exhaust gas temperature will drop down. When the exhaust gas temperature will drop down, the water vapor in the exhaust gas will get condensed to form white smoke.
Q. What is the best way of stopping an over-speeding diesel engine?
Ans. (d) secure the fuel supply and block the air intake.
The best way to stop an over-speeding engine is to cut-off the fuel to all the units simultaneously and immediately. If the fuel cannot be cut-off, then even if the air intake suction filter or the turbocharger suction filter is blanketed, it will prevent the air entry into the engine. Hence due to lack of availability of air fuel, combustion will not take place and the speed of the engine will drop and ultimately it will stop.
Q. By comparing the exhaust gas outlet of each cylinder in a diesel engine, one method of determining if the engine load is balanced is by the use of
Ans. (d) pyrometer.
Pyrometer will show the exhaust gas temperature of each unit. Generally and roughly, it is accepted that the exhaust gas temperature is the measure of power developed by individual unit. Hence higher the exhaust temperature, higher will be the power developed and lower the exhaust temperature lower is the power developed. This is a rough estimation and cannot be utilized unless it is backed up by other parameters such as * compression and * maximum.
Q. One cylinder of a diesel engine is persistently knocking and does not ceases when the fuel oil supplied to the cylinder is secured. What is the following problem which can cause this?
Ans. (d) A mechanical defect in working part.
If the diesel engine one cylinder is knocking the knocking may fuel related or mechanical In order to know the exact cause of knocking first the fuel is cut-off. If after cutting of the fuel the knocking stops, it indicates that the knocking is due to the fuel timing or the fuel quality. But if the knocking continues even after the fuel is cut-off, it is a clear indication that somewhere metal-to-metal contact is taking place and there is mechanical defect in the working parts.
Q. A diesel engine is warmed up and white vapor is noted in the exhaust. What does it indicate?
Ans. leaky cylinder liner.
If engine is properly warmed up and is working at proper load and under such circumstances if there white vapor is indicated in the exhaust, it is an indication that at some point water is finding its way into the combustion space. This may be due to a liner or cylinder head crack.
Q. A smoky exhaust from an operating diesel engine could be caused by
Ans. (c) late fuel injection.
In case the fuel injection is too late, there will be after-burning. When afterburning takes place the efficiency of combustion reduces and excessive black smoke may occur.
Q. Oil accumulating in exhaust gas piping or manifold of a diesel engine can be caused by
Ans. (b) worn valve guide.
If the valve guide is worn out, the clearance between the valve guide and valve stem will increase and the valve guide will be lubricated by the use of rocker lubrication oil. If the clearance will be more, then the finer particles of oil will, along the stem of the valve, enter into the exhaust manifold.
Q. Diesel engine operating conditions are indicated by color of exhaust smoke. Blue smoke can indicate
Ans. (c) clogged drain holes in the oil control ring.
If the drain holes in the oil control ring get clogged, the oil which is trapped in the oil control link will not get drained into the crankcase but it will remain on the cylinder liner and it will get burnt with the exhaust gas. This burnt oil vapor will find its way along with the exhaust gas and hence the exhaust gas will have a blue tinge.
Q. If the diesel engine is smoking excessively under load, the cause could be
Ans. (d) plugged injection holes or low compression pressure.
If the compression pressure in one of the cylinder is low, the fuel will not burn efficiently and black smoke will be emitted by the engine. Also if the injector holes are plugged or are choked, there will not be proper penetration and the engine will have late ignition and because of this, there will be excessive black smoke.
Q. Sticking of diesel engine compression rings may be caused by
Ans. (c) excessive cylinder lubrication.
If the cylinder lubrication is excessive and the excessive oil is not been scrapped by the oil control rings, the excessive lubricating oil will get burnt and form carbon or calcium carbonate due to which the piston ring will get stuck up in the grooves. Also the improper rotation of the rings may cause them to get stuck up in the grooves because rotation of the piston rings give the self-cleaning effect for the piton rings. If they are not allowed to rotate, the carbon may get deposited and get choked up or stick the piston ring in the groove
Q. Which of the following condition can cause excessive lubricating oil consumption in a diesel engine?
Ans. (d) High lubricating oil temperature.
If the lubricating oil temperature is high, excessive vaporization of the lubricating oil will take place and ultimately it will cause excessive lubricating oil consumption.
Q. Late fuel oil injection in a diesel engine may result in
Ans. (d) high exhaust temperature.
When the fuel oil injection is late, there is little time for fuel and air to mix properly. Hence there is improper combustion will take place. Also there will be after-burning and hence the exhaust temperature will be on the higher side.
Q. Which of following condition is likely to develop if the thermocouple element of a pyrometer becomes coated with excessive amount of combustion byproducts?
Ans. (b) Pyrometer response will be retarded.
The carbon deposits around the pyrometer sensor will retard the heat transfer because that coating will form an insulating material and hence heat transfer will be affected. Also the heat transfer will take more time. Due to this, the response in change in temperature will be retarded and the change in temperature will be registered not immediately but comparatively late.
Q. When the opening pressure injector is greater than specified by the engine manufacturer which of the following problem can be expected?
Ans. (a) quantity of fuel to be injected tends to decrease.
The effective delivery stroke of the pump plunger will reduce because it will take more time for the plunger to travel before the sufficient pressure is developed. Hence as the effective delivery stroke is reduced, it will automatically reduce the quantity of the fuel injected.
Q. Late fuel injection occurring, at or after TDC, is indicated by excessive exhaust smoke and
Ans. low firing pressure.
When the injection is late, the fuel will get ignited late when the piston is already on its downward travel and because of this the firing pressure will reduce and afterburning will take place.
Q. In a diesel engine, late fuel injection is indicated by black or gray exhaust smoke with
Ans. (a) low firing pressure.
As the firing is late, the combustion of the fuel is late when the piston is moving downwards and because of that the volume will increase. Hence the firing pressure will reduce.
Q If a crankcase explosion occurs in a diesel engine you should stop the engine and
Ans. (b) allow the engine to cool naturally .
The crankcase doors should not be opened immediately as if they are opened there is a possibility that outside air may enter the crankcase and result in secondary explosion. In order to avoid the secondary explosion, which is of higher intensity and it will cause large scale destruction, it is necessary to allow the engine to cool down naturally and not by forced ventilation.
Q.A diesel engine will lose power if the injection occurs too late in a cycle because of
Ans. (c) maximum expansion of burnt fuel cannot take place in the cylinder.
If the fuel injection occurs too late then in such conditions the maximum pressure will be reduced. At the same time, when the actual burning of the fuel takes place, the piston will be on its downward movement under which the volume will increase and the pressure cannot be increased. Ultimately, the power developed by the engine will reduce because the power developed by the engine will be given by
Power developed = PLAN
In the above formula as P reduces the power developed is reduced
Q. Which of the following condition will be developed when unburnt fuel contaminates the crankcase of a diesel engine?
Ans.
When the unburnt fuel oil finds its way into the crankcase, it will mix with the lubricating oil from the crankcase and bring down the viscosity of the lubricating oil. Also it will bring down the flash point and fire point of the lubricating oil
Q. The ignition quality of diesel oil becomes less critical as
Ans. (d) the engine speed decreases.
As the engine speed decreases, there is ample amount of time available for the fuel and air to mix and form a good combustion mixture. Hence the quality of the fuel does not matter much under the low speed engine as compared to medium or high-speed engines.
Q. Higher than normal temperature air passing through the intake of a diesel engine will result in
Ans. (c) low horse power .
If the air entering the cylinder is at high temperature, the volumetric efficiency of the cylinder will reduce because the quantity of the air which enters the cylinder reduces. When this happens, automatically the fuel injected will not burn properly. Hence the thermal efficiency will drop down and will cause black smoke and higher exhaust temperature and low power is developed.
Q. When the fuel injector nozzle overheats which of the following problems may developed?
Ans. (a) The fuel metering will vary.
When the nozzle gets overheated, the clearance between nozzle and needle guide will change due to which there will excessive leakage pass the needle. Also the needle will not seat properly on the seat and there will leakage pass the seat. Due to all this the total quantity of fuel which is injected will vary.
Q..An increase in fuel injection pump discharge pressure can be caused by
Ans. (c) plugged fuel injector spray holes.
When the fuel injector spray holes are choked, it will develop excessive backpressure on the fuel pump. This will cause excessive fuel injection discharge pressure.
Q. If fuel injection in a diesel engine begins earlier than designed start of injection, ignition may be delayed because of
Ans.
If there is excessive early fuel injection, as the compression pressure in the cylinder has not sufficiently reached to cause the ignition of the fuel and because of this the fuel will get accumulated and burn with a heavy knock. This is the reason for not advising early injection of fuel.
Q. Prior to starting, the purpose of turning over the main propulsion diesel engine with a cylinder test cock open is to
Ans. (b) remove the condensation from the cylinder.
When condensation takes place of the moisture in the air on the top of the piston or if there is any water or fuel accumulation and an attempt is made to start the engine, as the liquid is in-compressible, there will be hydraulic pressure developed which will act over the piston, cylinder head, connecting rod and crankshaft thus causing them to bend, break or fracture. Hence it is advisable every time to blow through the engine before starting after a long stoppage.
Q.A fuel injection valve opening at pressure below than normal will result in
Ans. (b) early fuel injection.
If the fuel injection valve opening pressure is low, the pressure required to lift open the needle valve will be less. This low pressure can be reached with a very little upward movement of the plunger and because of this fuel may enter the chamber too early and this may also cause knocking.
Q. Faulty operation of diesel engine fuel injection nozzle can be directly caused by
Ans. water in the fuel oil supply or excessive fuel nozzle holder cooling or distorted fuel pattern or leakage past the plunger into the oil drain.
(All the above noted things are possible for the faulty operation of diesel engine fuel nozzle.) Water will cause sticking of the needle in the guide and excessive wear-down of between nozzle and guide and missing of the firing in between and so it will cause the improper combustion and surging of the engine. Excessive cooling of the nozzle will cause the formation of sulfuric acid and it will get corroded. Due to the corrosion of nozzle, the spray pattern of fuel which is injected will change. If the plunger and barrel clearance is more then also there will be excessive leakage between the plunger and barrel and this will also cause the late opening of the fuel injector. At the same time, because of the drop in pressure, the spray will not be proper and atomization and penetration will not be good.
Q. Water in the fuel can prevent the engine from starting, preventing it to develop full power or run at full speed.
Ans.
If there is water in the fuel the power developed by the engine will be reduced and it will miss firing. When missing of firing takes place, the rpm will go on fluctuating continuously and smooth running of the engine will get hampered.
Q. Air in fuel lines of a diesel engine can cause
Ans. ignition failure.
As air is compressible fluid, it will get compressed and will not transfer the pressure to the fuel injector valve. Hence it will not get lifted up and there will be loss of power and ignition failure.
Q. Before starting a diesel engine you should always check
Ans. (b) crankcase oil level.
As oil is the most important part of the engine, it is necessary to check the oil level. If the oil level is less and the engine is started, the oil level will go down and lubricating oil pump may lose suction. If the lubricating oil pump loses suction, even for a fraction of a second, the bearing will get damaged.
Q. Dirt in a fuel oil system of a diesel engine may cause
Ans. (d) to the fuel oil injector damage.
Due to the dirt, there will excessive wear-down between the needle and guide. At the same time, there will be scratching on the needle and the guide. The fuel injector may get worn out faster or the dirt may also cause the needle to stuck up. All this will lead to damage to the fuel injector.
Q. If cooling water flow through after cooler is interrupted the power output of a turbocharged diesel engine will drop because the
Ans. (b) density of the air will decrease.
When the water flow through the after cooler is interrupted, the air temperature increases. Automatically, the volume of the air increases and the density decreases. Hence for the same said volume, the quantity of the air from the point of view of weight, as concerned, will be always less than what it would be if the temperature was lower. Higher the temperature, the weight of the air reduces. When the weight of the air reduces, the fuel injected will not burn efficiently. Hence, there is automatically a drop in the power output of the engine.
Q.Operating a propulsion diesel engine at less than 30% of designed normal load for prolonged periods will result in
Ans. (d) carbon formation on the combustion chamber surfaces.
When an engine is operated at such low loads, the turbocharger efficiency drops. When the turbocharger efficiency drops, less quantity of air is supplied to the cylinder. Hence the quantity of air available in the cylinder for combustion is reduced. This will automatically reduce the quantity of fuel which is burnt. At the same time, the fuel will not burn efficiently. Thus unburnt fuel will lead to carbon formation and the combustion chamber surfaces will get coated with the carbon deposits.
Q. Diesel engine fuel oil contamination often results in
Ans. (b) fuel injection system malfunction.
The fuel oil of diesel engine will get contaminated with water or with solid particles such as silica, rust, or alumina. All these three components will result in improper functioning of the fuel combustion injection equipment. Because of the water, there will be rusting and lots of misfiring in the combustion stroke. Also if there is a hard particle, silica or allumina, present then it will give abrasion on the needle and guide and cause the leakage of fuel. Any catalysts and fines entering the clearance spaces of part of fuel injection equipment's can lead to accelerated wear of these parts. The wear may be extensive with few days of operation with contaminated fuel. Also it will block the injection holes on the nozzle and cause complete malfunctioning of the fuel injection system.
Q. Operating a diesel engine under light load and at lower temperatures for extended period can result in
Ans. (a) formation of carbon on the inlet and exhaust valves.
When the engine is operated at low load and low temperature for a longer time, the turbocharger efficiency drops. When the turbocharger efficiency drops, the quantity of air supplied to the engine reduces. This results in improper combustion and hence large amount of carbon formation takes place in the combustion chamber. When running at low load, the exhaust gas pressure and exhaust gas energy are not sufficient. Hence there will be backpressure exerted on the exhaust. If the same overlap is maintained between exhaust and inlet valve is maintained during that time, then when the inlet valve opens the exhaust pressure is not sufficiently dropped. Under such circumstances, momentarily there will be back-flow of exhaust gas from the cylinder into the air inlet valve before the actual flow of the inlet air to the unit starts. During this time, the formation of carbon takes place in the air inlet valve.
Q. Which of the conditions listed occurring in the diesel engine would cause carbon deposits to develop in the piston ring belt.
Ans. (d) all above i.e. faulty combustion, excessive ring temperature, over-lubrication.
The carbon deposits develops in the piston ring belt because of
1. Improper combustion: The improper combustion of fuel results in carbon being deposited.
2. Excessive ring temperature: If ring temperatures are excessive, then lubricating oil will get burnt which also leads to the formation of carbon.
3. Over lubrication: The excessive lubricating oil that is supplied to the piston ring will get burnt and convert into ash or carbon thus forming the excessive coating of the carbon on the ring.
Q. If a crankcase explosion occurs in a diesel engine which of the listed action should be taken.
Ans. (c) Allow the engine to cool before opening the crankcase.
There will be two kinds of crankcase explosion, one a primary explosion and other the secondary explosion. When the primary explosion takes place because of the oil mist and the appearance of hot spot in the crankcase, there is a high-pressure rise in the crankcase which is released into the engine room atmosphere with the help of crankcase relief valves. At the same time, the non-return valve will shut and not allow the new air to enter into the crankcase. The crankcase atmosphere will be highly rich and hence there is a less possibility of exposure. Under such circumstances if by mistake the crankcase door is opened, fresh air will enter the crankcase and bring the crankcase atmosphere in the range of explosive limit. When this happens, there will be heavy explosion and a secondary explosion may take place. So the best thing to do is to allow the engine to cool down before opening the crankcase doors.
Q. The burning of fuel oil in a diesel engine having a high sodium content will cause
Ans. corrosion and grooving of exhaust valves.
In general, fuels leaving the refinery have sodium content well below 50mg/kg. If the sodium content increases, which is normally caused due to seawater contamination. A 1% seawater contamination represents potentially a 100mg/kg increase. Vanadium is also present in the fuel oil, which combines with oxygen to form V2O5 (vanadium pentoxide), which combines with sodium to form sodium/vanadium complexes. It is well known that there are low melting temperatures of sodium/vanadium complexes of certain critical ratios. The most critical sodium/vanadium ratio is about 1:3. This will form a sodium/vanadium complex with a low melting point which will flow with the exhaust gases. It will get deposited as a hard and brittle layer on the cold surfaces such as exhaust valve spindles, turbocharger nozzles and turbine blades. This layer is highly corrosive and corrodes the metal. It is also brittle and breaks away exposing the metal for fresh attack especially when they get deposited on exhaust valve seats. The hard layer breaks and gives a cutting effect on the seat. Preventive measures can be taken such as keeping the temperature of the exhaust below the melting point of V2O5 and removal of sodium by proper purification and proper draining of the settling tanks. Therefore high sodium content in the fuel oil will result in corrosion and grooving of exhaust valves.
Q.The longer the ignition delay period resulting from improper use of low cetane fuel, the
Ans. more rapid the rise in the combustion pressure.
Longer the period of ignition delay, more quantity of fuel will accumulate before the actual firing will takes place. As a result, there will be sudden increase in the combustion pressure. When the large quantity of accumulated fuel ignites, there is a sudden development of the high-pressure wave. At the same time if the ignition delay is too much, there will after-burning. When after-burning takes place, there will be high enforced temperature.
Q. If the relief valve on the diesel engine cylinder lifts, the cause could be
Ans. (a) liquid in the cylinder.
When there is a liquid in the cylinder, the piston is moves upward. Hydraulic pressure will develop because of the presence of liquid. As liquid is in-compressible, large amount of hydraulic pressure is developed. In order to save the engine components from damage, the relief valve will lift and release the pressure.
Q. The substantial increase in the crankcase pressure could be an indication of
Ans. (c) a worn cylinder liner.
When the pressure in the crankcase of a diesel engine increases, it could indicate blow-past by the piston. This could be due to worn out cylinder liner or the piston rings are leaking or piston rings are broken. In that case large amount of high-pressure gas will enter into the crankcase and produce high pressure in a crankcase. Hence the substantial increase in the crankcase pressure could be an indication of a worn out cylinder liner.
Q. An indication of an overloaded main propulsion engine.
Ans. (b) High exhaust pyrometer readings.
As a general rule the exhaust temperature is related with the load of the engine. Higher the load, higher the exhaust temperature of the engine. Hence it is the direct indication of the load on the engine. Hence if the exhaust temperature is higher, then the engine is overloaded.
Q. The dry-type exhaust silencer clogged with soot will cause
Ans. (b) loss of engine power.
When the dry-type exhaust gas silencer gets clogged with soot, there will be large amount of exhaust gas to flow and because of that the exhaust gas pressure will be higher. Hence the complete removal of exhaust gas will not be possible which will cause the mixing of the exhaust gas with the incoming air. Thus it dilutes the incoming air with the exhaust gas. This will cause improper combustion because of the starvation of the oxygen for the combustion of the fuel. The exhaust temperatures will increase and consequently there will be loss of power in the engine.
Q. The color of the exhaust from the diesel engine should be
Ans. (a) clear.
The clear color of the exhaust gas from the engine exhaust will indicate the best possible combustion without any carry over of the fuel, lubricating oil, water in the fuel or improper combustion. So it should be noted that the color of the exhaust indicates the health of the engine or any defects in the combustion. If the color of the exhaust is black, it indicates improper combustion that may be due to some fuel system malfunctions or an overloaded engine. If the color of the exhaust is white, it is consists of water. If the color of the exhaust is bluish, then it indicates the burning of lubricating oil in the engine. So clear exhaust is an indication that the combustion is perfect.
Q. Engine operating conditions maybe indicated by the color of the exhaust smoke. Black smoke could indicate.
Ans. (b) an overloaded engine.
If the engine is overloaded beyond particular limit then, the quantity of the fuel will be more but comparatively the quantity of the air will be less. Hence larger quantity of fuel, which is injected into the unit, will be difficult to burn in allotted time and so
after-burning will occur. This will increase the exhaust temperature and black smoke will be formed.
Q. Restricted air intake to a diesel engine may result in
Ans. (a) engine failing to reach the rated speed.
When the air intake to the engine is restricted, automatically the fuel will not burn properly. Hence power generated by the engine will be less. When power generated by the engine is less, it may be possible that it will fail to reach the rated speed at a lower condition.
Q. Color of engine exhaust form diesel engine should be
Ans. (a) clear.
Q. With regards to diesel engine crankcase explosion, the most violent is
(i) secondary explosion.
(ii) primary explosion.
Ans. (ii) only.
Q. Some diesel engines are fitted with thermometer in cooling water outlet of each cylinder. The cooling water temperature from all the cylinder being risen above the normal, you should suspect
Ans. overloading of all the cylinders i.e. overloading of the engine.
If overloading of engine takes place, then the quantity of heat taken out by the cooling water will be very high and in that case jacket cooling water temperature will increase. Also, it may increase due to the malfunctioning of the control valve fitted on the water cooling system.
Q. Some diesel engines are fitted with thermometers in the cylinder. If the cooling water temperature from an individual cylinder begins to rise you should suspect
Ans. (b) overloading of that cylinder.
If the heat generated in that cylinder is excessively high, heat carried out by water from that unit will be very high and hence the jacket cooling temperature of that unit will increase. It is an indication that there is a overloading of that particular unit. This may be because of leaky fuel valve, or fuel pump rack stuck up in the high fuel condition.
Q. A diesel engine using lubricating oil having too high viscosity will exhibit
Ans. increase in starting difficulty in cold weather.
If we are using high viscosity oil and having low viscosity index, because of the excessively high viscosity, there will be too much of drag and excessive friction between the bearing. The loss of power to overcome this drag will be so high that it will be difficult to start the engine from the cold and most of the power, which is used for starting and turning the engine, will be wasted to overcome this friction caused by the high viscosity.
Q. One result of operating a diesel engine at light load with excessively low cooling water temperature is
Ans. (b) increase in the cylinder misfiring.
If the engine is operating at low load and with low cooling water temperature, the compression temperature and compression pressure will be very low. If the fuel is injected, then it may stop igniting and may cause misfiring of the engine.
Q.Which of the following condition may tend to increase the ignition delay period of combustion in a compression ignition engine?
Ans. (b) Decreasing the charge air temperature.
If the charge air temperature is decreased, automatically the temperature at the end of compression stroke will also decrease. When the temperature at the end of compression decreases beyond certain limit, it will be difficult for the fuel which is injected to convert into combustible mixture and large amount of time will be required to achieve this process. In this situation, the time delay will increase, especially in compression ignition engines
Q. In a diesel engine excessive cylinder liner wear will cause
(i) increase blow-by.
(ii) wear between the piston ring and the groove.
Ans. Both answers are correct.
If there is excessive cylinder liner wear, there will be blowpast because piston ring will not seal the liner properly. When the piston rings will be sliding over the worn surface of the liner, which are not evenly worn-out, in order to keep the contact with the liner, the piston ring will move in radial direction in and out continuously. Due to this, piston rings will slide over bottom surface of the ring groove thus causing the excessive wear of the bottom of the ring groove.
Q. The change in the degree of fuel atomization in a fuel diesel engine will mostly affect
Ans. (b) fuel penetration.
Penetration and atomization are inversely proportional to each other. Hence if there is excessive penetration there will be less atomization and if there is excessive atomization there will be less penetration. When atomization takes place, the fuel particles are divided into very small minute microscopic particles which will have less amount of energy to overcome the dense atmosphere in the combustion chamber and hence it will not penetrate deep into the combustion space. On the other hand, if the atomization is less the particle size will be bigger and their mass will be higher and hence they will have high amount of kinetic energy to penetrate into the dense atmosphere air deep into the combustion chamber.
Q.The longer the ignition period occur in a diesel engine the more rapid rise in the combustion pressure.
More delay will give more accumulation of the fuel in the combustion space before the actual firing of the fuel takes place. As a result, the accumulated fuel will get ignited. When the accumulated fuel will get ignited suddenly or instantaneously, it will cause excessive amount of pressure rise in the cylinder.
Q. Diesel engine exhaust temperature may be used to indicate
(a) leaky exhaust valve.
(b) overloaded cylinder.
(c) clogged injector nozzle.
(d) All of the above.
Ans. All of the above.
The leaky exhaust valve will indicate increase in temperature because of the high-pressure gases leaking past the exhaust valve. An overloaded cylinder will be indicated by excessive exhaust temperature as higher the load, more will be the quantity of fuel which is injected. When the quantity of fuel injected increases, automatically the temperature of the exhaust gas increases. Thirdly, as the injector nozzle is clogged automatically the quantity of the fuel injected will be decreased. In that case, the temperature may drop down for that particular unit but it may increase for the other units. Also, if one or two holes of the injector are clogged, there is a possibility that more fuel will be injected through the other holes resulting in unbalanced distribution of the fuel in the combustion chamber which may cause the afterburning effect and increase the temperature.
Q. Maintaining proper fuel oil temperature will result in
Ans. (b) improved atomization.
Maintaining proper fuel oil temperature will maintain proper viscosity and viscosity is important from point of view of atomization. If viscosity is properly maintained, there will be proper atomization and proper penetration. Hence there will be proper balance between penetration and atomization, and there will be better combustion.
Q. Combustion knock can occur in the cylinder of a diesel engine under any condition permitting
Ans. (c) excess fuel in the combustion chamber.
Excessive fuel in the combustion chamber may be due to ignition delay and cause sudden increase in the pressure. This excessive pressure will cause excessive knocking effect in the engine.
Q.587 Persistent knocking in one cylinder of a 8-cylinder diesel engine is most likely to be caused by
Whether the knocking in the cylinder is by fuel oil or not, it can be checked by cutting of the fuel oil and checking whether the knock persists or not. Even if the knock persist after cutting the fuel oil it indicates that there is some excessive clearance in some of the running parts. Most possible place is badly worn piston pin and bush. Hence every time when the piston is changing its direction from upwards to downwards it will give the knocking effect.
Q.A crack cylinder head in an operating engine may be indicated by
Ans. (b) combustion gases venting from expansion tank.
Due to the crack, high-pressure gases will enter through the cracks into the water space during the exhaust stroke and these gases may come out of from the expansion tank or the expansion tank may overflow. At the same time, this will be indicated by high jacket cooling temperature of that particular unit.
Q. Coolant can be lost from jacket cooling water system by leakage from
(a) cracked cylinder head.
(b) piping joints.
(c) pump seal leaking .
(d) All of the above.
Ans. (d) All of the above.
Water can be lost if there is crack in the cylinder liner or head during the suction stroke when the pressure inside the cylinder is low. When the pressure in the cylinder is low, water will leak into the combustion space. When during the compression and ignition stroke there is high pressure gas in the cylinder it will enter the cooling space. Thus there will be loss of water. The water loss can also be due to leaking piping joints that can be seen externally. The pump sealing leakage will also result in loss of water and it may enter the crankcase or it may come out of the * hole fitted between the crankcase and the pump.
Q. When oil vapors, oxygen and hotspots are present at the same time with in a crankcase which of the following hazards could develop?
Ans. (a) Explosion.
When oil vapor, hotspot and oxygen is present the triangle is completed for fire resulting in explosion. This is called primary explosion which will be followed by secondary explosion.
Q.A possible cause for an individual piston to knock when at TDC on a slow speed two-stroke diesel engine could be due to
(a) early fuel injection.
(b) excessive bearing play between the gears.
(c) overloading of the cylinder.
(d) All of the above.
Ans. (d) all of the above.
The main reason is that if there is excessive early fuel injection, fuel will get accumulated and there will be sudden firing of the cylinder and cause the knock. Excessive bearing clearance will cause the mechanical knocking. Also excessive bearing clearance between the running gears will cause low compression pressure. Due to low compression pressure automatically the ignition will get delayed and knocking will take place. Also because of overloading of engine, excessive fuel will be loaded in the engine and the time available to burn will be less. Hence more fuel will accumulate and that accumulated fuel will ignite violently to give knocking. In other words, knocking will generally be because of increase in the ignition lag.
Q. An automated diesel engine should normally shut down due to
Ans. (c) low lubricating oil pressure.
Lubricating oil is the most important element of the engine. If there is low lubrication oil pressure, the engine will not run even for short time and may cause excessive damage. Hence the first and foremost important thing for the engine is low lubrication oil pressure cut-off.
Q.Routine monitoring of a diesel engine should include
(i) checking for leak.
(ii) checking for temperature and pressure.
(iii) listening for abnormal noise.
(iv) All of the above.
Ans. (d) All of the above.
All of the above aspects will indicate proper running of the engine and to ensure proper running of the engine, all the parameters should be continuously monitored and checked, and noted in the log book.
Q. One remedy for a high firing pressure, in addition to high exhaust temperature, in one cylinder of a diesel engine is to
Ans. (c) adjust the fuel rack.
If both the exhaust temperature and peak pressure is high, it indicates that the quantity of fuel injected is more. Hence it is necessary to reduce the quantity of fuel in the cylinder. This is done by adjusting the position of fuel rack. Due to this the quantity of fuel injected will automatically get reduced and knocking will be stopped.
Q. If a diesel engine is difficult to start and operates at engine temperature lower than normal you should suspect that
Ans. (a) compression pressures are below normal.
If the compression pressure is below normal, it will be difficult to start the engine immediately as the fuel does not burn immediately. As the engine is cold and compression pressure is low, it will be difficult to start the engine.
Q. A distorted spray pattern of a fuel injector can cause
Ans. (d) loss of power.
When there is distorted pattern of fuel injector, there will not be a proper distribution in the combustion space. Hence fuel and air will not mix properly and complete air will not be available for combustion. This will cause the improper combustion of fuel and because of this there will be loss of power and high exhaust temperature.
Q. White smoke issuing from exhaust of an auxiliary diesel engine would mean
Ans. (b) engine is cold.
If the engine is cold, the exhaust will have condensation of the moisture. At the same time, the exhaust gas generated will cool down and the moisture in the exhaust gas will also get condensed. This will result in white smoke from the exhaust gas outlet.
Q. White smoke in the exhaust of a diesel engine can be caused by
Ans. low combustion temperature.
Q.Bluish smoke in the exhaust of an operating diesel engine can be caused by
Ans. (b) cold cylinder liner
When the cylinder liner is cold, lubricating oil will not get scrapped out of the liner into the crankcase but it will get burnt along with the fuel into exhaust gas. This will result in blue smoke emitting from the exhaust manifold