The term cycle refers to one complete sequence of operation required to produce power in an engine.This cycle operation is continuously repeated while the engine is running.For a design engine iy consists of four operations within the cylinder
1.Compression of charge air.
2. injection of fuel which then ignites
3. expansion of the hot gases formed during combustion.
4.. Expansion of the used gas to exhaust.
cycle of operation in a four stroke diesel engine are shown below :-
In a 4 stroke diesel engine it takes four strokes of the Piston to complete one working cycle of operations. The four strokes in one cylinder are :
1. Induction stroke
During an induction stroke Piston moves down from the top of a stroke Piston, a position known as top dead Centre (TDC ), to bottom of a stroke, a position called bottom dead Centre (BDC ),drawing in fresh air through open inlet valve,At BDC the inlet valve closes.
During the following stroke called compression stroke the Piston moves up from BDC to TDC, compressing the air in the cylinder and consequently raising its temperature.
3. power stroke
fuel injected as the Piston reached top dead centre and combustion takes place, producing very high pressure in the gases. The piston is now forced down by the these gases to BDC. This stroke where the piston is forced down from TDC to BDC by the high pressure of combustion gases is called the power stroke of piston.
4. exhaust stroke
The exhaust valve opens as the Piston recharge BDC. Exhausting of burnt gases now take place as the Piston travels second time from BDC to TDC. This stroke is called the exhaust stroke. This stroke is the last Stroke and it completes one full cycle.
Cycle of operation net two stroke diesel engine
In a two stroke diesel engine it takes just two strokes of the Piston to complete one working cycle operations. The various operations that happen during these two strokes in one cylinder are:-
compression and firing:-
Piston moving up, the exhaust and scavenge ports are covered by the piston and the fuel valve is shut.Air previously taken into the cylinder is being compressed to above 40 bar and 550 degree Celsius at the end of compression.
This is called compression stroke. Next sketch shows fuel being injected into the cylinder. This fuel mixes with air,burns and give out heat.This is called combustion and high pressure gases is produced thus push the piston downward towards the end of the stroke. This is called power stroke.
exhausting and scavenging
As the piston is still moving downwards and has just begun to uncover the exhaust ports,the exhaust gases (having a little pressure to do any further useful work on piston ) are blown out under own pressure.The Piston continues to move downwards to uncover scavenge ports.Fresh pressurized air is blown into,first to scavenge out the leftover exhaust gases which are still hanging around in the space and secondly to provide charges air for next compression.