During an experiment on a 4-stroke SI engine, it was observed that the maximum and minimum pressure inside the cylinder during each working cycle was 5 bar and 200 bar respectively. If the compression ratio was 8, then the mean effective pressure is approximately:

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BHEL ET Mechanical Held on May 2019

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Answer 1

Concept:

The Mean Effective Pressure (MEP) of the otto cycle is given by,

Substituting the Q₁ and Q₂ in terms of temperatures considering the process, we will get,

where r_p = pressure ratio, γ = ratio of specific heats

Calculation:

Given:

P_max = P₃ = 200 bar, P_min = P₁ = 5 bar,

As we know, For isentropic process 1 – 2

Pressure ratio is:

Mean Effective Pressure (MEP) is:

For an Otto cycle, an increase in the compression ratio leads to an increase in the mean effective pressure as well as the thermal efficiency.