Working strokes of Diesel Engine
Suction Stroke
In this stroke, the piston moves down from the top dead center towards the bottom dead center. As a result, the inlet valve opens and the air is drawn into the cylinder.
After a sufficient quantity of air with pressure is drawn, a suction valve closes at the end of the stroke. The exhaust valve remains closed during this stroke.
Compression Stroke
In this stroke, the piston moves up from the bottom dead center to the top dead center. During this stroke, both inlet and exhaust valves are closed.
The air drawn into the cylinder during the suction stroke is entrapped inside the cylinder and compressed due to the upward movement of the piston.
In a 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 Kg/cm², at this pressure, the temperature of the air is reached 1000° centigrade which is enough to ignite the fuel.
Constant Pressure Stroke
In this stroke, the fuel is injected into the hot compressed air where it starts burning, at constant pressure. When the piston moves to its top dead center, the supply of fuel is cut off.
It is to be said that the fuel is injected at the end of the compression stroke and injection continues until the point of cut-off, but in actual practice, the ignition starts before the end of the compression stroke to take care of the ignition tag.
Working or Power Stroke
In this stroke, both the 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 a stroke, the piston finally reaches the bottom dead center.
Exhaust Stroke
In this stroke, the piston again moves upward. The exhaust valve opens, while the inlet and fuel valve is closed. A greater part of the burnt fuel gases escapes 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 an exhaust stroke, the exhaust valve closes and the cycle is thus completed.
As there is some resistance while operating in the inlet and exhaust valve and some portion of burnt gases remains inside the cylinder during the cycle, resulting in the pumping losses.
These pumping losses are treated as negative work and therefore subtracted from actual work done during the cycle. This will give us a network done from the cycle.
Actually, all these strokes are performed at such a fast speed; you cannot see it step by step but it happens in every four-stroke engine.