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Equation 57 shows that under the cold-air-standard assumptions, the thermal
efficiency of an ideal Otto cycle depends on the compression ratio of the engine and the
specific heat ratio of the working fluid. The thermal efficiency of the ideal Otto cycle
increases with both the compression ratio and the specific heat ratio. This is also true for
actual spark-ignition internal combustion engines.

4.3 Diesel Cycle
The Diesel cycle is the idealized cycle for compression ignition engines (ones that
don't use a spark plug).
The CI engine, first proposed by Rudolph Diesel in the 1890s, is very similar to the
SI engine discussed in the last section, differing mainly in the method of initiating
combustion. In spark-ignition engines (also known as gasoline engines), the air–fuel
mixture is compressed to a temperature that is below the autoignition temperature of the
fuel, and the combustion process is initiated by firing a spark plug. In CI engines (also
known as diesel engines), the air is compressed to a temperature that is above the
autoignition temperature of the fuel, and combustion starts on contact as the fuel is
injected into this hot air. Therefore, the spark plug and carburetor are replaced by a fuel
injector in diesel engines (Fig. 4.4).
In gasoline engines, a mixture of air and fuel is
compressed during the compression stroke, and
the compression ratios are limited by the onset of
autoignition or engine knock. In diesel engines,
only air is compressed during the compression
stroke, eliminating the possibility of autoignition.
Therefore, diesel engines can be designed to
operate at much higher compression ratios,
typically between 12 and 24. Not having to deal
Fig. 4.4 - In diesel engines, the spark plug with the problem of autoignition has another
is replaced by a fuel injector, and only air
is compressed during the compression benefit: many of the stringent requirements
process. placed on the gasoline can now be removed, and
fuels that are less refined (thus less expensive)
can be used in diesel engines.
The fuel injection process in diesel engines starts when the piston approaches TDC
and continues during the first part of the power stroke.
Therefore, the combustion process in these engines takes place over a longer interval.
Because of this longer duration, the combustion process in the ideal Diesel cycle is
approximated as a constant-pressure heat-addition process. In fact, this is the only
process where the Otto and the Diesel cycles differ. The remaining three processes are
the same for both ideal cycles.
The similarity between the two cycles is also apparent from the P-v and T-s diagrams
of the Diesel cycle, shown in Fig. 4.5.
The difference between the Diesel cycle and the Otto cycle is that heat is supplied at
constant pressure.

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