The Otto Cycle: Compression Ratio vs. Efficiency

Increased compression, by a change in displacement, chamber volume, or both, increases torque throughout the RPM range, and is especially helpful at part-throttle. However, the increase is most effective in cases where the original compression ratio is relatively low. Adding 1 point of compression (e.g., 5.0-1 becomes 6.0-1, a change of +20% in ratio, increases efficiency by 7.79%), is more effective than the same percentage increase beginning with a higher ratio (10.0-1 becomes 12.0-1, also +20%, but only increases efficiency by 4.65%).     “Efficiency” refers to the relative use made of the energy contained in the fuel, not the amount or percentage of fuel or air consumed. The efficiency assumes that no other changes are made when the compression ratio is altered, such as quench, valve shrouding, piston dome masking, etc. and that fuel of sufficient octane is used to permit the best ignition advance.     To use this table: locate the original compression ratio (“C-Ratio”) in the 1st, 3rd or 5th row across. The table cell directly underneath it will show the efficiency calculated from the formula below, where E = efficiency, R = static (nominal) compression ratio, and K = 1.4 (the ratio of variable heats for air).     To compare ratios for efficiency, divide the Efficiency value of the new ratio by the Efficiency value of the original.