Otto cycle is more efficient than the diesel cycle for the same compression ratio while the diesel cycle is more efficient than the Otto cycle for the same maximum pressure and temperature.
The efficiencies of the otto and diesel cycle can’t be compared directly because the otto and diesel cycle works at different maximum pressure, temperature, and compression ratio.
Therefore the efficiencies of these cycles can be compared by keeping the same compression ratio and by keeping the same maximum pressure. Efficiency comparison for both cases is discussed below.
Which is more efficient otto or diesel?
1] Efficiency comparison for the same compression ratio:
We are here comparing the efficiency of the otto and diesel cycles for the same compression ratio and for the same amount of heat supplied.
As shown in the above figure, cycle 1234 represents the otto cycle and cycle 123′4 represents the diesel cycle.
The area covered by the cycle in the PV diagram gives the net work done by the cycle.
As Area (1234) > Area (123′4)
∴ `W_{\text{otto}}` > `W_{\text{diesel}}`
As the same amount of heat is supplied to diesel and otto cycle,
∴ `\frac{W_{\text{otto}}}{Q}` > `\frac{W_{\text{Diesel}}}{Q}`
`\mathbf{\eta_{\text{otto}}}` > `\mathbf{\eta_{\text{diesel}}}`
Therefore for the same compression ratio, the otto cycle is more efficient than the diesel cycle.
2] Efficiency comparison for same maximum pressure & temperature:
Here we are comparing the efficiency of the otto and diesel cycle for the same maximum pressure and temperature.
In the above figure, cycle 1234 represents the otto cycle while cycle 12′34 represents the diesel cycle.
The area covered by the cycle on the PV diagram gives the net work done by the cycle.
Here the area covered by the diesel cycle is more than the otto cycle.
As Area (12′34) > Area (1234)
Therefore `W_{\text{Diesel}}` > `W_{\text{Otto}}`
As the same amount of heat is supplied for diesel and the otto cycle,
∴ `\frac{W_{\text{Diesel}}}{Q}` > `\frac{W_{\text{Otto}}}{Q}`
`\mathbf{\eta_{\text{diesel}}}` > `\mathbf{\eta_{\text{otto}}}`
Therefore for the same maximum pressure and temperature, the diesel cycle is more efficient than the otto cycle.
FAQ’s

Is diesel more efficient than Otto?
For the same maximum pressure and temperature, the diesel cycle gives more work output. Thus it is more efficient than the otto cycle.

Which is more efficient Otto cycle or diesel cycle?
For the same compression ratio, the otto cycle is more efficient than the diesel cycle and for the same maximum pressure and temperature, the diesel cycle is more efficient than the otto cycle.

Why is Otto cycle more efficient than diesel cycle for the same compression ratio and heat input?
For the same compression ratio and the heat input, the area covered by the Otto cycle on the PV diagram is more than the diesel cycle. It means that the W𝗼𝘁𝘁𝗼 > W𝗱𝗶𝗲𝘀𝗲𝗹. And thus for the same amount of heat input, the otto cycle is more efficient than the diesel cycle.