Working Of A Four Stroke Diesel Cycle Engine With Suitable PV Diagram

 In diesel or CI engines, the ignition takes place due to the heat produced in the engine cylinder at the end of the compression stroke.

The four-stroke cycle of a CI engine are discussed below -

(i) Suction Stroke - In this stroke, the suction valve opens and pure air is sucked into the cylinder as the piston moves downwards. This will continue till the piston reaches BDC (Bottom Dead Center).

(ii) Compression Stroke - In compression stroke, both the inlet valve and exhaust valve are closed and the air is compressed as the piston moves upwards toward TDC (Top Dead Center).  As a result of compression, pressure, and temperature of the air increase considerably. This completes one revolution of the crankshaft.

(iii) Expansion or Working Stroke - Before the piston reaches the TDC during the compression stroke, fuel oil is injected in the form of fine spray into the engine cylinder through the fuel injection valve. At this moment, the temperature of the compressed air is sufficiently high to ignite the fuel. It suddenly increases the pressure and temperature of the products of combustion. Due to increased pressure, the piston is pushed down with great force. The hot burnt gases expand due to the high speed of the piston. During this expansion, some of the heat energy is transformed into mechanical work. It may be noted that during this working stroke, both the valves are closed and the piston moves from TDC to BDC.

(iv) Exhaust Stroke- In exhaust stroke, the exhaust valve is open as the piston moves from BDC to TDC. This movement of the piston pushes out the products of combustion from the engine cylinder through the exhaust valve into the atmosphere. This completes the cycle and the engine cylinder is ready to suck the fresh air again.

 The real and ideal p-V diagrams for the 4-stroke diesel cycle are shown in the upper figure. 

The difference between them is due to the following reasons - 

(i)  The suction of air inside the cylinder is possible only if the pressure inside the cylinder is below atmospheric.

(ii) The exhaust of gases is possible only if the pressure of the exhaust gases is above atmospheric pressure.

(iii) Compression and expansion are not isentropic, as there are some heat and pressure losses. 

(iv) The combustion is not possible at constant pressure as the fuel will not burn as it is introduced into the cylinder. 

(v) The sudden pressure release after the opening of the expansion valve is not possible and it takes through some degrees of crank rotation.