Mumbai university > MECH > SEM 3 > THERMO

Marks: 4M

Year: May 2014

• The cutoff ratio is the ratio of the volume after combustion to the volume before combustion.

• The Air standard efficiency of a diesel cycle is given as:

  $η = 1 - \frac{1}{R_0^{γ-1}}[\frac{γ_c^γ - 1}{γ (γ_c - 1)}]$

• where,

  $R_o$ is the compression ratio

  $r_c$ 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, $R_o$, by decreasing the cut-off ratio, $r_c$, or by using a gas with large value of γ.

• Since the quantity $[\frac{γ_c^γ - 1}{γ (γ_c - 1)}]$ in above equation is always greater than unity, the efficiency of a Diesel cycle is always lower than that of an Otto cycle having the same compression ratio.

• However, practical Diesel engines uses higher compression ratios compared to petrol engines and are often quite efficient.