The engines of heavy motor vehicles(like trucks, buses), stationary power plants, big industrial units sed ships mostly operate on diesel cycle or constant pressure cycle. It was introduced by Dr. Rudol Ph. Diesel in 1897. Diesel cycle differs from otto cycle in one respect. The beat is added at constant pressure instead of at constant volume. Thus, it comprises two adiabatic processes, one constant pressur heat addition process and one constant volume beat rejection process. Diesel cycle engines mostly use heavy oils diesel oil is the most common. In diesel cycle engines only air is compressed in the cylinder to a high pressure, the temperature of this compressed air becomes sufficiently high to ignite fuel Diesel is injected in the cylinder at the end of compressed air. There is no spark plug in diesel engine In a four-stroke, diesel-cycle compression, ignition engine, the four strokes are as shown in Figure


During the suction stroke, the piston moves down from top dead centre position Air is sucked into the cylinder through the open inlet valve which closes at the end of this stroke. The exhaust valve remains closed during this stroke.


The piston moves upward from bottom dead centre position. Inlet and exhaust valves remain

closed. The air is compressed in the cylinder with the upward movement of the piston. As the

compressed to a pressure as high as 40 kg/cm at which its temperature is as high as 100°C, enough

to ignite the fuel.


At the end of compression stroke the fuel (diesel) is injected into the hot compressed air where

it starts burning maintaining the pressure constant. The piston finally reaches to the bottom dead centre. Both the valves remain closed during this stroke.


The piston moves upward. The inlet valve remains closed and the exhaust valve opens. The greater part of the burnt gases escape because of their own the end of the compression stroke and injection contigammaed till the point of cut off. In actual practice, the ignition starts before the end of the compression stroke.

The upward movement of the piston pushes the remaining gases out through the open exhaust valve. The cycle is thus completed.

Law of Thermodynamics

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