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I have a doubt, I always thought that if
you increase the compression ratio, exhaust gas temperature will increase.
Same case as if you increase boost pressure.
It depends on ignition timing a lot. But what you are definitely increasing is the burn rate and exhaust gases velocity since they have to fit in smaller volume than before so the pressure and temperature raise in combustion chamber but by the time the exhaust valves start opening they drop down quicker than with lower compression ratio.
Here is a picture of theoretical comparison of hi and low CR engines on how they loose pressure in combustion chamber
Attached Files
Screenshot_20250718_091812_cn.wps.moffice_eng.jpg
- Screenshot_20250718_093616_cn.wps.moffice_eng.jpg
I think Shota may have that the wrong way round, as in his images and text less energy is passed out the exhaust with the higher compression.
Remember, with higher compression the fuel-air mass is denser and burns faster, that increases the cylinder pressure more early in the combustion process and as it expands there is more energy used for power, and so less is available at the exhaust to drive a turbine.
I need to clarify- when I said exhaust gases velocity is increasing I was talking about burn rate and pressure building up (correced original text). That additional pressure helps to rotate crankshaft quicker over time comparing to lower CR. After that, as I also said, the pressure drops down much quicker than lower CR engine - it is obvious from picture attached.
