Small Internal Combustion Engine Dependence on Inlet Pressure and Temperature at Altitude

Abstract

The effects of atmospheric pressure and temperature variations on the performance of small internal combustion (IC) engines operating at altitudes significantly above sea level are not widely documented. Most small IC engines are designed and manufactured for ground based applications such as chain saws and lawn mowers. However these engines are finding application as power sources for Remotely Piloted Aircraft (RPA) for both hobbiest and military applications. A test stand with the capability to control atmospheric conditions was developed for representative small IC 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 gasolineoil 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 BMEP) of nominally 4% per 300 m of elevation due to pressure while improvements of 1% per 300 m due to temperature effects. This resulted in a nominal 3% loss per 300 m of altitude. These trends were validated against a mathematical model. These measurements provide a better understanding of small IC 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.