Diesel Cycle

The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated during the compression of air in the combustion chamber, into which fuel is then injected.

Diesel cycle is similar to Otto cycle except in the fact that it has one constant pressure process instead of a constant volume process (in Otto cycle).

This is an important cycle on which all the diesel engines work. It is also known as constant pressure cycle as heat is received at a constant pressure.

The ideal diesel cycle consists of two reversible adiabatic or isentropic, a constant pressure and a constant volume processes. These processes are represented on a P-V and T-S diagram as shown below:

 Process:

• Process (1-2): Reversible adiabatic compression.
• Process (2-3): Constant pressure heat addition.
• Process (3-4): Reversible adiabatic expansion.
• Process (4-1): Constant volume heat rejection.

Heat Supplied = mC_p(T₃-T₂) 

Heat Rejected = mC_v(T₄-T₁)

Efficiency:

The air standard efficiency of Diesel cycle is given by:

Where, 

r = V₁/V₂ = Compression ratio

ρ = V₃/V₂ = Cut-off ratio

Note:

• The efficiency of the diesel cycle is lower than that of Otto cycle, for the same compression ratio.
• The diesel cycle efficiency increases with the decrease in cut off and approaches maximum (equal to Otto cycle efficiency) when cutoff is zero, i.e. ρ=1.