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What is cut off ratio? How does it affect the air standard efficiency of a diesel cycle?

Mumbai university > MECH > SEM 3 > THERMO

Marks: 4M

Year: May 2014

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• 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.