No-load vs loaded engine 1

[stephthecot]

Greetings,

How does the P-V curve of an engine differ between a loaded and no-load state?

Thanks in advance

 

[BrianPetersen]

You mean the P-V diagram?

On the unloaded example (e.g. idling, in neutral, etc), the positive-work areas will be smaller and be such that the areas of positive work offset the areas of negative work by only just enough to overcome the FMEP and other extraneous loads that may still be present at idle (e.g. air-con compressor, power steering pump, etc).

If the engine is a conventional throttled Otto-cycle spark ignition engine, the intake stroke will have significant negative work when running unloaded due to having the piston pull a vacuum against the almost-closed throttle. The reduced absolute pressure at the end of the intake stroke is also going to mean less pressure through the compression and power strokes - that's how it achieves making the positive-work area smaller.

If the engine is turbocharged, that's not going to be doing much of anything at idle, and it's going to make all relevant pressures bigger when the engine is running loaded under boost.

You asked a vague, qualitative question, so only vague, qualitative answers are possible.

 

[stephthecot]

Thank you very much for the help