Between real and theoretical diesel cycles there are differences and similarities in the form and in the values of pressures and temperatures. Differences that also appear in the Otto cycle. Some of these similarities correspond to those of the Otto cycle; for example, the similarities due to the variation of the specific heats, to the loss of heat and to the opening time of the exhaust valve.
Others differ in part and are caused by dissociation and loss by pumping. Finally, one is peculiar to the diesel engine, namely; the one concerning that the combustion is not verified at constant pressure in the case of the real cycle.
Differrences in the 4-stroke engines
The differences between the actual and theoretical cycle of a diesel, compression ignition engine are slightly different from those of a two-stroke engine. We first analyze the differences in the four-cycle cycles.
Combustion at constant pressure
As seen in the diagram indicated, in practice the combustion takes place under such conditions, that the pressure varies during the process, whereas in the theoretical cycle we had assumed that it remained constant.
Actually, part of the combustion takes place at a constant volume, and another part at constant pressure, almost like in the real Otto cycle. Only in the case of very slow motors does the theoretical process develop slightly.
Dissociation of combustion products
In the compression ignition engine, dissociation does not have as much effect as in the spark ignition engine ( gasoline engine). So the excess air and mixture of the products of combustion are such, that reduce the maximum temperature. Consequently, this excess also reduces the dissociation of said products.
Loss by pumping
The losses due to pumping of a diesel engine are lower than those that are produced in an Otto cycle gasoline engine. This is because there is no strangulation in the suction air. In compression ignition diesel engines there is no throttle valve, characteristic of spark ignition engines, equipped with a carburetor. Therefore, the negative surface of the real diesel cycle is smaller than that of the Otto cycle.
Differrences in 2-stroke engines
The points analyzed above correspond to the four-stroke engine.
Two-stroke engines are fairly widespread among diesel engines. In the two-cycle cycle, the loss due to pumping and that caused by the interruption of the expansion before the bOttom dead center (P.M.I.) to produce the escape is important.
Included in the loss by pumping should also be considered the work necessary to perform the sweep of the cylinder, which is often done by a compressor.