We aren’t attempting to make mechanics out of pilots by writing about compression ratio in aircraft engines, but we desire to help both groups by providing a simplified description as it relates to our engines.
In order to gain a reasonable amount of work from an internal combustion engine, we must compress the fuel/air mixture during each power stroke. The fuel/air charge in the cylinder can be compared to a coil spring in that the more it is compressed, (within limits), the more work it is potentially capable of doing.
Engineering tells us that the compression ratio of an engine is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke. For example, if there are 140 cubic inches of space in the cylinder when the piston is at the bottom and 20 cubic inches of space when the piston is at the top of the stroke, the compression ratio would be 140 to 20 or usually represented at 7:1.
Although we can create a more efficient engine by increasing the compression ratio, there are limits and a compromise is needed. If the pressure is too high, premature ignition will occur and produce overheating. Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present day fuel grades and the high engine speeds and manifold pressures required for takeoff.
Our normally aspirated engines are generally categorized as either low-compression or high-compression powerplants. In surveying the complete range of all Lycoming engine models, we note that compression ratios vary all the way from a low 6.5:1 to a high of 10:1. Engineering has generally established the low-compression group as those with a compression ratio of 6.5:1 to 7.9:1; and the high- compression group from 8:1 and higher.
All Lycoming engines in the high-compression category require a minimum of Grade 100LL (blue) or 100/130 (green) octane, FAA approved aviation fuel, and nothing less. With high-compression engines we must stress the importance of the manufacturer’s recommendations as outlined in the Engine Operator’s Manual or in the Pilot’s Operating Handbook. These engines require not only the correct fuel, but the proper oil, precise timing and a good air filter. All are very important in order to protect this high performance powerplant.