Diesel, Spark-Ignition, and Turboprop Engines for Long-Duration Unmanned Air Flights

Abstract

Comparisons are made for propulsion systems for unmanned flights with several hundred kilowatts of propulsive power at moderate subsonic speeds up to 50 h in duration. Gas-turbine engines (turbofans and turboprops), two- and four-stroke reciprocating (diesel and spark-ignition) engines, and electric motors (with electric generation by a combustion engine) are analyzed. Thermal analyses of these engines are performed in the power range of interest. Consideration is given to two types of generic missions: 1) a mission dominated by a constant-power requirement, and 2) a mission with intermittent demand for high thrust and/or substantial auxiliary power. The weights of the propulsion system, required fuel, and total aircraft are considered. Nowadays, diesel engines for airplane applications are rarely a choice. However, this technology is shown to be a very serious competitor for long-duration unmanned air vehicle flights. The two strongest competitors are gas-turbine engines and turbocharged four-stroke diesel engines, each type driving propellers. It is shown that hybrid-electric schemes and configurations with several propellers driven by one power source are less efficient. At the 500 KW level, one gas-turbine engine driving a larger propeller is more efficient for durations up to 25 h, whereas several diesel engines driving several propellers become more efficient at longer durations. The decreasing efficiency of the gas-turbine engine with decreasing size and increasing compression ratio is a key factor.