Dependence of Small Internal Combustion Engine’s Performance on Altitude

Abstract

The effects of atmospheric pressure and temperature variations on the performance of small internal combustion engines operating at altitudes significantly above sea level are not widely documented. Most small internal combustion engines are designed and manufactured for ground-based applications such as chainsaws and lawn mowers. However, these engines are finding application as power sources for remotely piloted aircraft for both hobbyist and military applications. A test stand with the capability to control atmospheric conditions was developed for representative small internal combustion engines. During previous research, data were recorded to identify the impact of varying engine intake pressure on engine performance both with a carburetor and with a throttle body fuel-injection system delivering a gasoline–oil mixture. Using the stand and fuel-injection system, data were collected while varying air temperature along with pressure. Results revealed a decrease in performance (power and brake mean effective pressure) of nominally 4% per 300 m of elevation due to pressure, while improvements of 1% per 300 m were due to temperature effects. This resulted in a nominal 3% loss per 300 m of altitude. These data were then used to establish an updated correction factor for power due to altitude changes. A speed-dependent model was validated for this small engine. These measurements provide a better understanding of small internal combustion engines operation at altitude. Quantifying these effects is vital for establishing the capability of operating spark-ignition engines at altitudes well above those for which they were designed.