1.5 Identify Components of diesel engines listed below

1.5.1 Generator Engine

Basic knowledge of main and auxiliary engine

The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into the hot, high – pressure air in the combustion chamber. In common with all internal combustion engines the diesel engines operate with a fix sequence of event, either in four strokes and two strokes. Stroke beings a travel of the piston between its extreme points TDC and BDC (TOP dead center and Bottom dead center. Each stroke is accomplished in half a revolution of the crank shaft.

Auxiliary Engine

Main Engine

Four stroke engine

The four stroke cycle is completed in four stroke of the piston, or two revolution of the crank shaft. In order to operate this circle the engine requires a mechanism to open and close the inlet and exhaust valves

Consider the piston at the top of the stroke, a piston known as top dead center (TDC). The inlet valve opens and fresh air is drawn in as a piston move down. (Suction Stroke)

At the bottom of the stroke i.e. bottom dead center (BDC), the inlet valve close and the air in the cylinder is compressed (and consequently raised in temperature) as the piston move up as raises. (Compression Stroke)

Fuel is injected as the piston reaches top dead center and combustion take place, producing very high pressure in the gases. The piston is forced down by these gases. (Power stroke)

At the bottom dead center the exhaust valve open. The final stroke is the exhausting of the burnt gases as the piston rises to top dead center to complete the cycle. (EXHAUST STROKE)

1.5.2 Main Engine

Two stroke Diesel Engine

The two stroke cycle is completed in two stroke of the piston or one revolution of the crank shaft. In order to operate this circle where each event is accomplished in a very short time, the engine requires a number of special arrangements.

First, the fresh air must be forced in under pressure. The incoming air is used to clean out or scavenge the exhaust gases and then fill or charge the space with fresh air

Scavenge Port

Instead of valve holes, known as “Scavenge ports” are used which are opened and closed by the side of the piston as its moves up and down.

Compression stroke

The scavenge ports are just covered, exhaust valve is close, the fuel injector is shut (see in fig 1 and 2). The cylinder is full of air (in modern engine, air is compressed to about 90 bar and above. The temperature at the end of compression will be about 550 deg C).The fuel is injected (approx. 10 deg before TDC) at the end of compression in the form of fine spray.(see in Fig 3).

Expansion Stroke

The pressure in the modern engine goes up as high as 130 -140 bar after combustion and temperature is about 1700 deg C (Note: This is not exhaust temperature). The hot gas, containing sufficient heat energy do the work on the piston and push toward the end of the stroke. The gas fall in pressure as they expend. (See in fig 4, 5)

Exhausting & Scavenging in two stroke engine

There are no separate strokes for suction and Exhaust. These function is completed between Expansion and Compression stroke. Towards the end of the stroke exhaust valve at the cylinder head or ports on the cylinder are open. Scavenge port are uncover by the piston immediately after this stage. Scavenge air has two functions in two stroke engine

(a) Pushing remaining gases out of the cylinder

(b) filled up the cylinder with fresh air.

The four stroke engine (usually rotating at medium speed between 250-750 revs/min) is used for auxiliaries such as alternators.

On board ship have resulted in the slow speed (80-100 rev/min) main propulsion diesel operation on the 2 stroke cycle.

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