Diesel or compression ignition (CI) engines operate with a much higher compression ratio than
spark ignition (SI) engines and, thus, have higher efficiencies. Modern CI engines start to inject the
fuel much earlier in the cycle, somewhere around 20 before top dead center (bTDC), instead of
injecting the fuel late in the compression stroke near TDC, as was done in early engines. The first
fuel then ignites late in the compression stroke, and some of the combustion occurs almost at
constant volume at TDC, much like the Otto cycle [1]. The air standard cycle used to analyze this
modern CI engine cycle is called a Dual cycle in which the heat input process of combustion can
best be approximated by a dual process of constant volume followed by constant pressure.
Therefore, the Dual cycle is a better approximation to the modern high speed CI engine than
either the Diesel cycle or the Otto cycle