Four stroke engine compression ratio
In between the many non-aviation related chores I need to tend to these days my thoughts wandered off and continued to mull over how the engine for my Ibis homebuilt aircraft is going to look like.
The thing I contemplated the last couple of weeks is ‘compression ratio‘.
After collecting a lot of information from various sources I was flabbergasted at the large spread of data points.
Ultra high compression high-revving engines that apparently get by with using low-octane fuel as well as engines with a very low compression ratio that still need Avgas 100LL to run smoothly.
Obviously there was something wrong with my data points….
Then it dawned on me that one could measure compression rato in multiple ways:
- The maximum possible compression ratio would be calculated by determining the ratio of head volume above the piston (when the piston is at TDC) with the volume displaced by the piston between TDC and BDC.
- The ‘net’ compression ratio would also take the position of the valves into consideration. Think about it: if the intake valve is still opened a bit when the piston is at bottom dead center (i.e. the intake valve closes with the piston after BDC) this would reduce the effective/net compression ratio, as the full piston swing between TDC and BDC should not be used for compression ratio calculations.
Likely old hat for you engine buffs, but something I never considered before. Clearly, I somehow need to normalise my data points if I ever am going to base any decision on these….