Cylinder pressure fundamentals

I have a basic question about the pressure achieved in a conventional cylinder compression test. I have a fair understanding of gas laws and pvt relationships but I must not understand how our engines work.

To start, I believe the theoretical pressure achieved in a SINGLE compression stroke in a 9/1 compression ratio engine with no leaks at sea level would be about 118 psi (if ignoring temperature effect). But I know that real tests achieve much higher pressure readings: 150 psi or more.

Why is that? One possibility could be that the pressure in the cylinder is higher than atmospheric when the piston starts the compression stroke. But I guess the opposite is the case. There must be a slight vacuum in the cylinder {ie. less than atmospheric pressure) in the cylinder at the bottom of the intake stroke when the valves close. And we know that there is a vacuum in the intake manifold. If so, compressing at 9/1 ratio should lead to a pressure lower that 118 psig.

Another guess is that the energy of compression is transferred to the gas as heat and the temperature rises.
That would result in pressures higher than 118. Is that the correct answer or am I missing something else?

Incidentally I have searched about a hundred posts on the subject of cylinder pressure going back over 10 years and have not found to answer to my question.

Hello Phillip, a good starting place for learning about internal combustion gasoline engines is to look up explanations of the Otto Cycle. The compression step in your question first can be approximated simply as an adiabatic compression, it is not a step where the ratio of starting and final volumes equals the ratio of starting and final pressures. The simple adiabatic approximation is a good starting place for understanding the compression step , after which reality requires some modest further adjustment of the predictive equations.

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The difference between the calculated pressure rise vs the actual pressure rise(as measured by gauge)is explained by the increase in temperature, as Roger expresses. This heat increase is assumed to be adiabatic because the pressure rise happens so rapidly there is little time for that heat to be dissipated.

However, the static compression ratio is NOT achieved at cranking speeds because the intake valve is still open as the piston begins it’s compression stroke. IIRC, the valve remains open for about 57 crank degrees AFTER the piston starts to rise. This will yield somewhere in the neighborhood of a 7.5/1 “dynamic compression ratio”. The mechanical static CR will not be achieved until the engine’s torque peak…somewhere in the 2500 to 3000 RPM range for streetable engines.

thanks for great replies

You mean we’re right!?!?

If we say so…:nerd_face:

Ha!..fair enough…