There are different types cycles in thermodynamics.Such as Otto cycle,Carnot vapour cycle,Diesel cycle etc.Out of that cycle diesel engine works on diesel cycle.This cycle is also known as constant pressure cycle.Diesel engine is mostly employed in Stationary Power plants,Ships,Heavy Motor Vehicles.
In Petrol Engine,the air-fuel mixture after being compressed in the engine cylinder to a high pressure,is ignited by an electric spark from a spark plug.In diesel engine,diesel oil and light and heavy oil used as fuel.This fuel is ignited by being injected into the engine cylinder containing air compressed to a very high pressure,the temperature of this air is sufficiently high to ignite the fuel.That is why there is no spark plug used in diesel engine.This high temperature compressed air used in the form of very fine spray is injected at a controlled rate so that the combustion of fuel proceeds at constant pressure.
Diesel Engine is mainly worked on below strokes.
In this stroke,the piston moves down from the top dead centre.As a result,inlet valve opens and air is drawn into the cylinder.After sufficient quantity of air with pressure is drawn,suction valve closes at the end of the stroke.The exhaust valve remains closed during this stroke.
In this stroke,piston moves up from the bottom dead centre.During this stroke both inlet and exhaust valve are closed.The air drawn into the cylinder during suction stroke,is entrapped inside the cylinder and compressed due to upward movement of the piston.In diesel engine,the compression ratio used is very high as a result,the air is finally compressed to a very high pressure up to 40 kilogram per centimeter square,at this pressure,the temperature of the air is reached to 1000 degree centigrade which is enough to ignite the fuel.
03)Constant Pressure Stroke:-
In this stroke,the fuel is injected into the hot compressed air where it starts burning,maintaining the pressure constant.When the piston moves to its top dead centre,the supply of fuel is cut-off.It is to be said that the fuel is injected at the end of compression stroke and injection continues till the point of cut-off,but in actual practise,the ignition starts before the end of compression stroke to take care of ignition tag.
04)Working Or Power Stroke:-
In this stroke,both inlet and exhaust valve remain closed.The hot gases (which are produced due to ignition of fuel during compression stroke) and compressed air,now expand adiabatically,in the cylinder pushing the piston down and hence work is done.At the end of stroke,the piston finally reaches the bottom dead centre.
In this stroke,the piston again moves upward.The exhaust valve opens,while inlet and fuel valve are closed.A greater part of the burnt fuel gases escape due to their own expansion.The upward movement of the piston pushes the remaining gases out through the open exhaust valve.Only a small quantity of exhaust gases stay in the combustion chamber.At the end of exhaust stroke,the exhaust valve closes and the cycle is thus completed.
As there is some resistance while operating in inlet and exhaust valve and the some portion of burnt gases remains inside the cylinder during the cycle, resulting the pumping losses.This pumping losses are treated as negative work and therefore subtracted from actual work done during the cycle.This will give us net work done from the cycle.
I hope,this information will help you to increase knowledge about diesel engine.
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