nkaminar
Well-known member
I have a set of Audels Engineers and Mechanics Guide, published in 1921. In volume 4 there is a table that shows the pressure and temperature at the end of the compression stroke given the compression ratio (page 1,343). The table goes from a compression ratio of 3 to 6. For the compression ratio of 6:1, the pressure ratio is 11.233 and the temperature ratio is 187.22. The text says that “The accompanying table has been prepared for determining the compression pressure and temperatures which usually occur in gas engines.” I think this table was prepared by pressure cards (indicator diagrams) taken off of running engines.
Here is how the final pressures and temperatures are calculated: Final compression pressure is determined by the initial pressure. Assuming 14.7 psi the final pressure for 6:1 is 14.7 X 11.233 = 165 psi. The final temperature depends on the initial temperature and absolute temperature is used in the calculation. Assuming 70 F the final temperature is (70+459.2) X 187.22/100 = 991 R or 532 F.
Because of the intake valve timing the actual final pressure and temperature in a compression test will be less. Plus, because in a compression test the time of the compression stroke is longer compared to actual engine running, the pressure and temperature will be less because there is more time for the temperature to be reduced by transfer to the room temperature cylinder walls and other parts. In a running engine the actual pressure and temperature will depend on the valve timing, carburetor and manifold flow rate, engine design, engine rpm, and throttle setting.
If anyone needs the results for different compression ratios, please let me know and I will calculate them for you. I will try to publish the table later.
Here is how the final pressures and temperatures are calculated: Final compression pressure is determined by the initial pressure. Assuming 14.7 psi the final pressure for 6:1 is 14.7 X 11.233 = 165 psi. The final temperature depends on the initial temperature and absolute temperature is used in the calculation. Assuming 70 F the final temperature is (70+459.2) X 187.22/100 = 991 R or 532 F.
Because of the intake valve timing the actual final pressure and temperature in a compression test will be less. Plus, because in a compression test the time of the compression stroke is longer compared to actual engine running, the pressure and temperature will be less because there is more time for the temperature to be reduced by transfer to the room temperature cylinder walls and other parts. In a running engine the actual pressure and temperature will depend on the valve timing, carburetor and manifold flow rate, engine design, engine rpm, and throttle setting.
If anyone needs the results for different compression ratios, please let me know and I will calculate them for you. I will try to publish the table later.
Last edited: