Combustion engines can be approximated as the isometric expansion of the fuel air mixture. In the gasoline engine the fuel-air mixture is compressed, which is then ignited by a spark. For the higher compression diesel engine, the fuel is injected into the engine at the end of the compression stroke, resulting in combustion. Omitting the opening & closing of valves, and the injection of fuel, as steps, then one can consider that an idealistic diesel engine has four steps, as follows:
1) A to B: Compression stroke: Compression of the cool fuel-air mixture.
2) B to C: Isobaric combustion: Explosion of the fuel-air mixture.
3) C to D: Power stroke: Expansion of the hot gases from combustion.
4) D to A: Isometric exhaust stroke.
Fig 1.8.13 illustrates the idealistic curves for both the compression stroke (A to B) and the power stroke (C to D), which are considered as being isothermal, e.g. the following drawing of isothermal lines on a P-V diagram. Note: Tcd >>Tab.
Reality: During the compression stroke, the cylinder’s gaseous molecule’s temperature increases with pressure. Conversely, during the power stroke, the exploded fuel-air mixture performs work hence its temperature decreases. Also, combustion is not isobaric. Nor is the exhaust stroke isobaric. Accordingly, Fig.1.8.14 shows more realistic curves for a diesel engine. Note: In Fig.1.8.14 point E was added, which represents a fifth step, that being the isobaric intake stroke (E to A).
It should be stated that often the idealistic curves for both the compression stroke and power stroke are discussed as being adiabatic curves. This is simply not the case. One must remember that adiabatic implies isolated and at no times are these systems truly isolated, e.g. during the power stroke work is done in displacing the atmosphere. In some ways isothermal maybe conceptually better, namely because the engine block and coolant regulate the temperature but even then that is simplification over a complete cycle at the operating temperature e.g. heat is continually created, then carried and radiated away.
Copyright Kent W. Mayhew