written 7.6 years ago by | • modified 7.6 years ago |

Mumbai university > MECH > SEM 3 > THERMO

**Marks:** 4M

**Year:** May 2014

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

written 7.6 years ago by | • modified 7.6 years ago |

Mumbai university > MECH > SEM 3 > THERMO

**Marks:** 4M

**Year:** May 2014

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written 7.6 years ago by |

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.

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