Question about compression

[JoysiBoy]

I read that a Honda CB350 can have compression as high as 170 psi. The compression ratio is 9.5 to 1. If you multiply 9.5 times 14.7 psi standard atmospheric pressure you get just over 139 psi. How could you get a higher than that number reading a compression gauge? Thanks for replies.

[jeremyr62]

You should try asking ChatGPT for a good answer. It told me the the CR is raised to a power which n, which for a petrol engine is about 1.3.

The constant
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n in the formula represents an exponent that accounts for the efficiency and characteristics of the compression process in the engine. This exponent is often referred to as the adiabatic or polytropic exponent and varies depending on the specific heat ratio of the gas being compressed, as well as the efficiency of the compression process itself.

In the context of internal combustion engines, the value of
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n typically ranges between 1.2 and 1.4 for gasoline engines. The variation accounts for factors such as heat loss to the cylinder walls during compression and the speed of the compression process (whether it is closer to an isothermal or an adiabatic process).

If the compression process is very efficient and approaches an ideal adiabatic process (no heat transfer with the surroundings),

n would be closer to the adiabatic exponent for air, which is about 1.4.
If there is significant heat loss during compression or the process is slower, making it closer to an isothermal process (temperature remains constant),

n would be closer to 1.0. However, in practice, values below 1.2 are less common for engine compression calculations because the process is generally much faster than an isothermal process and involves some level of adiabatic compression.

[A100man]

[stinkwheel]

I can't be bothered thinking about it too much but are you sure it's a direct multiplication and doesn't have a square or even a cube in there because you are dealing with volumes and surface areas?

Also, it's a fuel/air mix charge, not just air so there will be a fair bit of atomised fuel in there along with the air which doesn't compress much due to being a liquid.

Also quoted compression ratios are often little more than a guess based on the crown height and swept volume. To calculate it properly takes a fair bit of fucking about with burettes and a reasonable amount of maths.
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