Automotive-Engineering

Internship Report by Khalid Tantawi, 2003

Introduction to Automotive Engineering 

by Khalid Tantawi

      Department of Mechatronics Engineering      

      University of Jordan 

      August 2003

SECTION 1:  Vehicle Mechanical Structure

Automobiles basically consist of six units from a mechanical point of view [1]:

1. The Basic Structure

2. The Prime Mover and Power System

3. The Transmission System

4. The Steering System

5. The Braking Systems

6. The auxiliaries

1. THE BASIC STRUCTURE:

It consists of the following:

- Frame: it is made of compressed steel to which all the mechanical units are attached and on which the body is superimposed.

- Suspension system: prevents transmitting shocks on the road to the vehicle. The two components of a suspension system include the suspension springs and the shock absorbers. There are many types of suspension springs that may be found in vehicles, ranging from leaf springs (such as the one shown in the figure below) to coil springs, rubber springs, and hydraulic springs. Shock absobers dampen the vibrations that result from the shock by converting the shock energy into heat. 

- Axles:  front and rear, supported beams. The following loads are applied on the axles:

        a) The vertical load due to the weight of the vehicle.

        b) The "for- and- aft" load resulting from driving and braking.

        c) The torque reactions.

        d) Centrifugal force resulting when turning about a curve.

- Wheels

2. The Prime Mover and Power System:

The prime mover in a vehicle is the internal combustion engine. Which may be of either spark-ignition or of compression- ignition type. Some driving power systems are hybrid engines, they consists of a combustion engine backed by a brushless dc motor of a power rating of about 50 KW to provide a four wheel drive. Other power systems use the hydrogen cell which was bilt by Ford automotives, but is still under research. Technological advances that lead to improvement in the storage capacity of batteries made it possible for complete electric vehicles (such as the Nissan Leaf) to take a big portion of the automotive market in recent years.

Figure 1.1: A suspension system that consists of a shock absorber and a leaf spring.

3. THE TRANSMISSION SYSTEM:

This system consists of a gear box and a clutch, the propeller shaft, and diiferential. this system is discussed in chapter 2.

4. THE STEERING  SYSTEM:

Every vehicle should have an accurate steering system, that is stable in terms that it should tend to get to the vehicle to the straight direction position after turning.  The steering system is usually accompanied by a power steering system to make the turning effort minimum. The main part of the steering system is the steering gear that transmits the rotary motion of the steering wheel to the driving axle.

Figure 1.2: The universal joint connecting the propeller shaft and the differential gear of the rear axle (left).  The front axle of a four-wheel drive Mitsubishi Pajero vehicle. 

5. THE BRAKING SYSTEM: 

Friction force is the principle of operation of the brakes. Therefore braking pads should be replaced when they wear.   The braking system is composed of a hydraulic system that applies an equal force on the brake pads to push them in to be in contact with the wheel disc. This primary braking system is for stopping a veicle in motion, to hold a standing vehicle in position the 'emergency braking' is used.

6. THE AUXILIARIES

There are many auxiliary systems in a typical vehicle. Some of these are:

6.1.  Electric Supply System: this includes the alternator and battery. The alternator, as the name suggests, is a synchronous generator that generates an alternating current,the current is then rectified to supply the vehicle with its needs of dc current,  but why not use a dc generator that supplies a dircet current rather than using an alternator that needs a rectifing stage afterwards? the answer comes from experience, cars in the 1960's were equipped with  dc generators,  they suffered from several difficulties associated with the output power vs speed characteristics of dc machines. Another major problem associated with dc generators is that their brushes need much more maintenance than brushes of a synchronous generator (alternator).  It worths mentioning here that the vegicle's body is connected to the negative terminal of the electric system.

6.2.  The starter: Internal combustion engines cannot start from rest by themselves, they need some kind of a prime mover that will supply them with an initial high torque for them to run. This high torque  that initiates the engine is supplied from a small and strong electric motor.  Due to the space limitation under the hood, this electric motor should be small and at the same time strong enough to rotate the heavy engine.  In other words, it has to have a high output power-to- weight ratio.  The best machine that can meet these requirements is called the dc serial motor.  As the name suggests, the rotor and the stator of this electric motor are serially connected together, resulting in a huge current passing through the motor coils.  In fact this motor draws a huge amount of current when it runs that it can even damage itself if run for a long duration, this is why cranking the engine should not last for more than a few seconds at a time.

6.3.  The ignition system:  this is the system that ignites the fuel. In gasoline engines, ignition is done by using spark plugs.

6.4.  Other devices:   Head and back lights, signal lights, internal lighting system, windscreen wipers, horn,...etc.

SECTION 2: THE TRANSMISSION SYSTEM

The transmission system may be either manual (regular) or automatic.  A regular transmission system consists of a gearbox, a clutch, a propeller shaft, and a differential gear. Each of these components is designed to allow the transmission system to disconnect the engine from the wheels and reconnect the engine to the wheels, with reducing the engine speed. 

The components that perform these functions are:

The Clutch: to engage and disengage the engine from the wheels.

The Gearbox (Transmission): provides leverage variation between the engine and wheels.

The Bevel pinion and crown wheel : to reduce the engine speed while maintaining torque at the wheels.

Universal joints : they transmit the rotary movement of the engine to the wheels, while allowing the drive shaft to be not aligned with the driven shaft. 

Differential: this is required to allow the inner and outer driving wheels to turn at different speeds.

Figure 3.1: The differential

SECTION 3: THE COMBUSTION ENGINE  

An automobile's engine belongs to the family of Internal Combustion (IC) engines, which besides the diesel and gasoline engines includes other ones such as turbo jets and turboprops, however the latter two are used on aircraft.

 

3.1 CONSTRUCTION

There is a number of basic terms that are used to describe and compare engines. I will discuss some of them here briefly:

Before I continue further in discussing the construction of the Internal Combustion (I.C) engine, I think it is important to discuss the two commonly used cycles:

1.      The Otto Cycle: it consists of two adiabatic processes and two constant volume processes. Hence combustion occurs in a constant volume.

2.      The Diesel Cycle:  two constant pressure and two adiabatic processes. hence combustion here occurs at a constant pressure.

The main parts of an automotive engine are:

1.  Cylinder block: this is the engine's frame,  It is here where the engine cylinders are located, there is also a network of paths for the circulating cooling water and the lubricating oil.

2.   Cylinder head: this is the combustion chamber over each of the cylinders in the block. It is attached to  top of the cylinder block,  leak is prevented by the use of gaskets. It also contains spark plugs for the case of gasoline engines or injector holes for diesel engines.

   

Figure 3.1: Cylinder head of a Mitsubishi Pajero gasoline-engine vehicle (left). Cylinder head of a Mitsubishi diesel engine (right).

3.   Pistons: they transmit the force generated from the explosions to the crankcase, and seal the high pressure gases in the combustion chamber, they also guide the connecting rod.

4.   Connecting rods: the function of the connecting rod is to convert the reciprocating motion of the piston into the rotary motion of the crankshaft. Two forms of stresses are applied on it: axial stresses due to cylinder gas pressure, and bending stresses due to centrifugal effects. Notice in the figurethat the cross section of the connecting rod is an I-section, this is to provide maximum stress resistance. The big end as shown in the figure, is connected to the crankshaft. The small end connects to the piston pin.

Figure 3.2:  Connecting rods of a Mitsubishi Motors engine

5. Crankshaft: this is the engine component from which the power is taken. It receives the mechanical power from the connecting rods in  a designed sequence.  The sequence is necessary for onward transmission to the clutch.

Construction of the crankshaft (refer to figure 3.3):

(1) Main journals: these form the axis of rotation of the crankshaft. Their number is always one more or one less than the number of cylinders.

(2) Crank pins: the journals for the connecting rod big end bearings and are supported by the crank webs. The distance between the axis of the main journal and the crankpin centerline is called the ' Crank-Throw'.

(3) Crank webs

(4) Counter weights: these are necessary to avoid bending of the crankshaft dueto  the centrifugal force.

Figure 3.3: The components of a Crankshaft of a Mitsubishi pickup truck.

6.    Camshaft: A shaft with cams on it, each with an accurately chosen angle,  which  operate the valves at specific timings in relation to the movement of the pistons, the sequence is governed by the firing order.  the camshaft also drives the ignition distributor.

Figure 3.4: The cam shaft

7.   Poppet valves: to admit the air-fuel mixture in the engine cylinder and to force the exhaust gases out at correct timings, all these necessitate the use of valves. Generally, inlet valves are larger than the exhaust valves, because speed of incoming air-fuel mixture is less than the speed of exhaust gases which leave under pressure. The main parts of a poppet valve are the head, margin, face, stem, and tip. Please refer to figure 5.1 for a view of the poppet valves.

Figure 3.5: Poppet Valves of a Mitsubishi Motors diesel engine

8.   Manifolds: The intake manifold is a set of pipes attached to the cylinder head, in which the air-fuel mixture is carried. There is also an exhaust manifold. There are two methods for metering and injecting fuel into the engine: by using a carburetor (for older vehicles, no more used), or an Electronic Fuel Injection (EFI).

Figure 3.6: The intake manifold

9.  Other engine components: there are other components that are not integral parts of an engine but the engine cannot operate with out their presence such as the water pump, the fuel pump, air cleaner and intake fan, ignition system, starting motor,  radiator, and the alternator.  

Figure 3.7:  The air cleaner and intake fan for a Mitsubishi Pajero (left).  The author troubleshooting a radiator (right). An alternator on a Toyota Camry 91 vehicle (right). The alternator generates the electric power enough to supply the vehicle's demands and to charge the battery.  About few decades ago, the alternator (which is a synchronous ac machine) replaced the older dc generator.

3.2 THE FUEL SUPPLY SYSTEM

The basic fuel supply system in an automobile with gasoline engine consists of a fuel tank, fuel pump, fuel filter, air cleaner, and the fuel injection system.

3.3 CYLINDER FIRING ORDER

The firing order of the cylinders of an IC engine is such that a maximum distribution of stress and heating is made among the cylinders, and a minimum back pressure is developed [3], common firing orders are as below:

Four-cylinder in-line engine : 1-3-4-2, other possible order is 1-2-4-3

6-cylinder V-shape engine: 1-5-3-6-2-4, Other possible orders are 1-5-4-6-2-3 , 1-2-4-6-5-3, and 1-2-3-6-5-4

3.4 THE TURBO ENGINE

Turbo engines operate by supplying the air-fuel mixture to the engine under an elevated pressure. This process is referred to as supercharging.  In ordinary engines, a vacuum is created when the piston moves to its bottom dead center, this vacuum force draws in the the air-fuel charge.  On the other hand, the turbocharger supplies the air-fuel mixture at a higher density resulting in  a higher power and torque output of the engine.  Since the mixture's temperature increases as it is compressed, an inlet cooler is used to cool the mixture before it enters the engine.

Figure 3.8: The inlet cooler for the turbocharger of a Mitsubishi 2500 CC engine

SECTION 4: AUTOTRONICS

Autotronics may refer to the automobile control systems, such as the Electronic Fuel Injection (EFI) system, and the ignition system. Some of the control systems that are found in most vehicles include:

1 Electronic Fuel Injection:  meters and injects fuel into the engine. It replaced the carburetor in older vehicles.

2 The ACTIVE STABILITY CONTROL  A safety control system that provides a quick responce in emergency, such as when the drive tries to avoid an obstacle on the highway, this system takes a momentary control of the steering wheel to prevent spinning of the vehicle.losing

3 The ACTIVE TRACTION CONTROL: To avoid losing traction

4 ENGINE BRAKE ASSIST CONTROL (EBAC) substitutes for engine brake when lost

5 ANTILOCK BRAKE SYSTEM (ABS) If one side of the vehicle is on a slippery surface, this system allows the vehicle to stop without spinning. It also allows steering when the brake pedal is fully on. 

6 ELECTRONIC BRAKE DISTRIBUTION (EBD) distributes the braking power between the front and rear of the vehicle.  A safety system to prevent spinning of the vehicle.

7 ELECTRONIC POWER STEERING (EPS) This system works to boost te steering when parking the car, and at the same time still give the driver a good feel for the road when speeding.

8 ACTIVE YAW CONTROL (AYC) A safety-related control system, that  aims at improving  performance of cornering and acceleration.

9 ACTIVE CENTER DIFFERENTIAL (ACD) Improves traction by distributing torque between the front and rear.  It works in conjunction with Active Yaw Control.

10 ELECTRONIC STABILITY PROGRAM (ESP)  A spin-avoidance control system, that applies a brake at an individual wheel when it senses that the vehicle is losing control.

11 CRUISE CONTROL SYSTEM controls vehicle speed, disengages automatically when braking.

References

1. Kerpal Singh, Automoblie Engineering Vol. 1: Automobile Chassis & Body, Standard Publishers Distributors,  1993

2. Kerpal Singh, Automoblie Engineering Vol. 2: Engine and Electrical Equipment, Standard Publishers Distributors, 1993.

3. V. Ganesan, Internal Combustion Engines 2nd ed. Tata McGraw Hill, 2003