Why is the heat addition process in otto cycle assumed constant volume while in diesel cycle it is considered constant pressure?
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In an Otto cycle, fuel is introduced prior to compression and ignited quickly when the piston is at or near top dead center. This produces a sharp rise in pressure because combustion happens quickly compared to how fast the piston is moving. In the Diesel cycle, fuel begins to enter near the end of the compression stroke, and continues past top dead center, into the expansion stroke. For this reason, the pressure is kept more-or-less constant because fuel is being added as the volume is increasing.
This paper is a bit dated, but well written and contains a brief explanation for the constant-pressure assumption on page 16.