4 STROKE DIESEL ENGINE

 In a 4-stroke engine, the engine crankshaft makes two revolutions for each working cycle.

They are mostly used as auxiliary engines as the prime mover for the alternators onboard ship

The four corresponding piston strokes are as follows

 1. Suction Stroke

 2. Compression Stroke

 3. Power Stroke 

 4. Exhaust Stroke

The engine has an air inlet and exhaust valves.

 By opening and closing these valves in proper sequence, the piston can be made to perform its main function of transmitting power to the crank.

In addition, the piston also performs subsidiary functions of drawing air into the cylinder, compressing the air, and expulsing exhaust gases.

4-stroke engines carry out useful work once per two revolution of the crankshaft.

This means fuel and exhaust must function once in two crankshaft revolution. The camshaft must rotate at half the speed of the engine crankshaft. 

BELOW IS A SIMPLIFIED EXPLANATION OF EACH STROKE OF 4 STROKE MARINE DIESEL ENGINE

1. SUCTION STROKE

In this stroke fresh air is sucked into the cylinder while the piston is moving down and the suction valve is opened.

The crankshaft is rotating clockwise and the piston is moving down the cylinder. The inlet valve is open and a fresh charge of air is being drawn or pushed into the cylinder by the turbocharger.

(A): Piston at top of the stroke, inlet valve open and air intake begins

(B): Piston descending, the air being taken in.

(C): Piston at the bottom of the stroke, all valves closed, air intake complete

2. COMPRESSION STROKE

Here the fresh air is compressed until it reaches the autoignition temperature of the fuels with both the valves closed.

The inlet valve has closed and the charge of air is being compressed by the piston as it moves up the cylinder. Because energy is being transferred into the air, its pressure and temperature increase. By the time the piston is approaching the top of the cylinder (known as the Top Dead Centre or TDC), the pressure is over 100 bar and the temperature is over 500°.

COMPRESSION STROKE 

3. POWER STROKE

In this stroke expanding hot gases as a product of combustion due to injected fuel, forces the piston downwards.

Just before TDC fuel is injected into the cylinder by the fuel injector. The fuel is "atomised" into tiny droplets. Because they are tiny these droplets heat up very quickly and start to burn as the piston passes over TDC. The expanding gas from the fuel burning in the oxygen forces the piston down the cylinder, turning the crankshaft. It is during this stroke that works energy is put into the engine; during the other 3 strokes of the piston, the engine is having to do the work.

a                                                               b
POWER STROKE

A.  Piston at the top of the stroke. Inlet and exhaust valves closed. Injector spraying oil into hot air.

B.  Piston descending. All valves closed. Hot high-pressure gas forces the piston down.

4. EXHAUST STROKE

The upward movement of the engine piston along with the opened exhaust valve expelled the spent gases out of the engine cylinder.

As the piston approaches the bottom of the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting primarily of nitrogen, carbon dioxide, water vapour and unused oxygen) are expelled from the cylinder.

EXHAUST STROKE

A.Piston at the bottom of the stroke. The exhaust valve opens.

B. Piston rising. The exhaust valve opens. Exhaust gas being driven out of the cylinder.

As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.