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

摘要

Comparisons are made for propulsion systems for unmanned flights with several hundred kilowatts of propulsivepower at moderate subsonic speeds up to 50 h in duration. Gas-turbine engines (turbofans and turboprops), two- andfour-stroke reciprocating (diesel and spark-ignition) engines, and electric motors (with electric generation by acombustion 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, and2) a mission with intermittent demand for high thrust and/or substantial auxiliary power. The weights of the propulsionsystem, required fuel, and total aircraft are considered. Nowadays, diesel engines for airplane applications are rarely achoice. 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 drivingpropellers. It is shown that hybrid-electric schemes and configurations with several propellers driven by one powersource are less efficient. At the 500 KW level, one gas-turbine engine driving a larger propeller is more efficient fordurations 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.