Cut off ratio: The cut off ratio is the ratio of the volume after combustion to the volume before combustion. The Air standard efficiency of diesel cycle is given as:

Where,

rp is the compression ratio.

rc is the cut off ratio

From the above equation it is observed that the thermal efficiency of the diesel engine can be increased by increasing the compression ratio, by decreasing cut off ratio or by using a gas with larger value of γ.

Cut off ratio plays an important part is the efficiency of the diesel engine. In diesel engines the fuel cut off ratio depends on the output and therefore unlike the otto cycle the efficiency of the diesel cycles depends on the output. When the cut-off ratio of diesel cycle increases, the air standard efficiency of the cycle is decreased when compression ratio is kept constant.