What is the effect of compression ratio (CR) on the air standard efficiency of otto and diesel cycles?

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RRB JE CBT 2 (Mechanical Engineering) ReExam Official Paper (Held On: 04 Jun, 2025)

Answer 1

Explanation:

The thermal efficiency of Otto cycle is given by:

ηotto = 1 - \(\frac{1}{r^{γ~-~1}}\)

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The thermal efficiency of the Diesel cycle is given by:

ηDiesel = 1 - \(\frac{1}{r^{γ~-~1}}~×~[\frac{{S}^{γ}~-~1}{γ~×~(S~-~1)}]\)

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Higher CR always leads to higher efficiency

This correctly states that increasing the compression ratio improves the air standard efficiency for both Otto and Diesel cycles. This is because a higher compression ratio allows for better utilization of the energy in the fuel by increasing the temperature and pressure of the working fluid (air or air-fuel mixture). This results in a higher thermal efficiency as indicated by the formulas for both cycles.