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# compression ratio vs. peak cylinder pressure and thermal efficienty

## compression ratio vs. peak cylinder pressure and thermal efficienty

(OP)
Hello, Merry Christmas to everyone! I wanna ask: if I increase compression ratio, cylinder peak pressure also increase? And how compression ratio increases thermal efficienty?

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

In short, sure. Figure in all the other variables, valve and ignition timing, fuel octane, piston and combustion chamber shape and you could approximate the deltas.

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

See this entry in Wikipedia for analysis of how ideal thermal efficiency is affected by compression ratio, in Diesel and Otto cycle engines.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

(OP)
And why in diesel engine we have less temperature in end of combustion than in petrol, but pressures are nearly same? It's because diesel have higher "expansion ratio"? And what is "expansion ratio"? Temperature and pressure difference in the beginning and in the end of combustion?

P.S. sorry for my english :)

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

In the ideal case, expansion ratio is the ratio of the cylinder volume at the bottom of the exhaust stroke to the volume at the end of combustion.
In reality, neither volume is easy to determine directly. Yes, the volume at BDC is a known value, but because the exhaust valve is opened before BDC, the real cycle has departed from the ideal cycle in the P-V diagram.
With cylinder pressure high speed data acquisition combined with heat release analysis, it is possible to analytically determine the "effective expansion ratio" (EER), which directly correlates to combustion efficiency, as you would expect.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

(OP)
another question, which i don't understand about CR, is how you can lower compression ratio by fitting shorter pistons or shorter rods? i understand, thaht in TDC, with shorter pistons or rods, i have more space (volume), but in BDC i also have more space, so how it can lower the CR? let's say i have 100cm3 at BDC and 10cm3 at TDC, and CR will be 10:1, if i fit shorter piston or rod, i have 120cm3 at BDC and 12 cm3 at TDC, so CR also will be 10:1?

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

(OP)
or maybe i have not 120cm3, but 102cm3 at BDC with shorter piston or rod?

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

Your math is wrong. Shorten the piston/rod and you add the same NUMBER of cc to both ends, not the same percentage!

110 / 10 = 11
120 / 20 = 6

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

Also, your comparison of diesel versus gasoline peak cylinder pressure is off. Firstly, turbodiesels normally DO have much higher peak cylinder pressure because they are not detonation-limited. But also, diesels manage the combustion process differently. Spark ignition engines always have ignition well before top-dead-center when running at rated load so that the bulk of the combustion happens near TDC or shortly after. Diesels have the fuel injected at a certain rate starting before top-dead-center but extending well after when running at rated load. Fuel that isn't in the cylinder yet (at TDC) can't burn and contribute to peak cylinder pressure.

### RE: compression ratio vs. peak cylinder pressure and thermal efficienty

(OP)
Thank You very much :)

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