Increasing Reliability of a Two-Stroke Internal Combustion Engine for Dynamically Changing Altitudes

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Increasing Reliability of a Two-Stroke Internal Combustion Engine for Dynamically Changing Altitudes

Crosbie, Steven C. / Polanka, Marc D. / Litke, Paul J. / Hoke, John L.

Remotely piloted aircraft typically use commercial internal combustion engines as their power sources. These engines are designed to run at sea level, but these aircraft are often pressed into service at higher altitudes where the performance characteristics deteriorate. The performance characteristics of a Brison $5.8 {in.}^{3}$ two-stroke engine at altitude were investigated using a test facility that can measure these characteristics over a range of pressures and temperatures. With its stock carburetor tuned to sea-level standard conditions, the engine created 5.5 peak horsepower, and brake-specific fuel consumption ranged from $1.2 to 4.0 lb / (hp \cdot h)$ . At an altitude of 10,000 ft, the peak horsepower decreased by 40%, whereas the off-peak horsepower decreased by more than 90%. Furthermore, the carburetor caused operation at high altitudes to be unreliable. Specifically, if the engine was tuned at sea-level standard, it would not operate above 10,000 ft. To increase reliability, a throttle body fuel injection system was installed on the engine. The fuel injection system matched the carburetor peak power at sea-level standard conditions while increasing the power at high-altitude operating conditions by as much as 90% at low rpm and 12% at high rpm. Brake-specific fuel consumption was decreased to a consistent 1.0 to $1.3 lb / (hp \cdot h)$ across operating speeds of 3000 to 7000 rpm, thus enhancing the range for the aircraft. Finally, both reliability at high altitude and startup reliability were increased with the throttle body fuel injection system, whereas the need for tuning by the end user was eliminated.