Compression Ignition engines are mostly used in marine applications, power generation and heavier transportation vehicles. Here, in a typical four-stroke cycle, air is drawn into the cylinder in the intake stroke and then compressed during the Compression Stroke. At near maximum compression, finely atomized diesel fuel is sprayed into the hot air, initiating auto-ignition of the mixture. During the subsequent power stroke, the expanding hot mixture does work on the piston, then the burnt gases are purged during the exhaust stroke.
The Diesel Cycle is an air-standard model of the actual cycle described above. The Diesel Cycle differs from the Otto Cycle only in the modeling of the combustion process: In a Diesel Cycle, it is assumed to occur as a reversible constant pressure heat addition process, while in an Otto Cycle, the volume is assumed constant. The four steps of the air-standard Diesel Cycle are outlined below:
- (1-2) Isentropic Compression (Compression Stroke)
- (2-3) Reversible, constant pressure heat addition (Ignition)
- (3-4) Isentropic expansion to initial volume (Power Stroke)
- (4-1) Reversible constant-volume heat rejection (Exhaust)
Typical pv and Ts diagrams for Diesel Cycle are shown below where steps (1-2) and (3-4) are Isentropic and step (2-3) is Isobaric while (4-1) is Isochoric
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