10.2514/6.2017-0389">
 

Power loss pathways and energy balance of a small four-stroke internal combustion engine

Document Type

Conference Proceeding

Publication Date

1-5-2017

Abstract

With the global demand for Unmanned Aerial Vehicles (UAVs) increasing from $4 billion to $14 billion annually over the next decade, a constant drive to lower operating costs and improve range and endurance will increase. The United States Air Force (USAF) has identified UAV propulsion methods and efficiency as key areas study for future improvement. Group 2 UAVs (10-25 kg) typically employ the use of electric motors or small (1-10 kW) internal combustion engines (ICEs) for propulsion. The high specific energy of hydrocarbon fuels (13000 Wh/kg for gasoline), but low efficiency for small ICEs (4-15%) means that small improvements in overall efficiency of small ICEs can pay big dividends on improvements in range and endurance. Prior studies have developed generalized power and efficiency scaling laws for ICEs of many types and displacements based on manufacturer advertised data. Little information is available however for the efficiency of ICEs in the 10-200 cm3displacement range, especially for four-stroke engines. Air Force Research Laboratory’s (AFRL’s) Small Engine Research Lab (SERL) has previously studied the power and efficiency of two-stroke engines in the 10-100 cm3displacement range. In this study energy balances were performed for a 118 cm3displacement single cylinder Honda GX120 four-stroke ICE. Energy pathways were characterized as a percentage of the total chemical energy available in the test fuel. Energy pathways were characterized into four categories: brake power, cooling load, exhaust sensible enthalpy and incomplete combustion. Results showed that the engine produced a maximum brake power of 3.05 kW at 3600 RPM. Fuel conversion efficiency ranged from 22.5% to 25.5% as engine speed was swept from 2000 - 3600 RPM, from 20.9% to 29.1% as equivalence ratio was swept from 0.8 to 1.2, from 15.1% to 24.6% as throttle was swept from 25% to 100%, from 23.3% to 24.5% as CA50 was swept from -2° aTDC to 18° aTDC, and from 24.2% to 24.8% as cylinder head temperature was swept from 117°C to 155°C. Peak efficiency was 29.1% under extremely lean conditions (φ = 0.8), and minimum efficiency was 15% with an intake air flow rate of 1.33 g/s (throttle 25% open).

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Source Publication

55th AIAA Aerospace Sciences Meeting

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