The performance and emissions characteristics of heavy fuels in a small, spark ignition engine

Date of Award


Degree Name

M.S. in Mechanical Engineering


Department of Mechanical and Aerospace Engineering


Advisor: Sukh S. Sidhu


This thesis research was conducted in pursuit of the DoD's plan for the universal use of a heavy, low volatility hydrocarbon fuel, and the increased interest in bio-derived fuels for small Unmanned Aircraft Systems (UAS's). Currently a majority of small UAS's use small spark ignition engines for their high power densities. Typically, these systems use commercial off-the-shelf power plants that are not optimized for fuel efficiency. Increased fuel efficiency is being pursued alongside the ability to utilize military heavy fuels. A test stand using a 33.5 cc four-stroke, spark ignition, air-cooled, single cylinder engine was constructed. Research was conducted to establish the feasibility of converting the existing system to utilize JP-8 with the stock mechanical carburetion. The stock carburetion had difficulty maintaining a consistent air/fuel ratio across the entire engine operating range. To resolve this, an electronic fuel injection system was developed to gain greater control over fuel mixture. An air-assisted electronic fuel injector was sourced from a scooter and adapted to work with the 33.5cc four-stroke engine. An aluminum injector mount was designed and machined and electronic controls were employed. Sensors on the valvetrain and crankshaft were developed as control signals for the injection system. The injector was characterized for flow rates and droplet size. The test stand consisted of a small dynamometer coupled to the engine. Servo throttle actuation was designed and throttle position was monitored with a throttle position sensor. The air-assisted injector was supplied with regulated shop air, and the fuel pressurized using regulated nitrogen. A fuel flowmeter and mass air flowmeter monitored equivalence ratio. Work was done to facilitate smooth measurement of unsteady air flow intrinsic to single-cylinder engines. Performance testing showed a decrease in brake specific fuel consumption (BSFC) while utilizing the injection system for the baseline fuel (Avgas 100LL), as greater mixture control (closer to stoichiometric) was realized. The engine was started using gasoline. Heavy fuel testing showed the ability to achieve required torque values at certain engine speeds. JP-8 was tested on the carbureted engine and fuel injected engine, showing a decrease in BSFC over baseline (carbureted avgas) with the carburetor and a further decrease in BSFC for the injected system. Biofuels that were tested were plant-based Camelina (carbureted and injected) and a UDRI grown and extracted algae-based fatty acid methyl ester (FAME) biofuel blended with D2 diesel in a 20% algae/80% diesel blend. Performance results for the Camelina showed a decrease in BSFC for the carbureted engine and the largest decrease of all the test fuels for the injected Camelina fuel. The algae blend showed less decrease in BSFC than the 100% diesel fuel. Emissions data were recorded as well. The injection system demonstrated less CO emissions for the injected fuels over the carbureted fuels due to closer to stoichiometric mixtures. Similarly, unburned hydrocarbon emissions decreased when injection was employed. NOx emissions were higher for the fuel injected engine, as peak NOx emissions will typically occur at slightly lean conditions and the injected fuels were closer to peak NOx emission conditions.


Spark ignition engines Alternative fuels Testing, Fuel pumps Technological innovations, Drone aircraft Motors Fuel injection systems, Drone aircraft Energy consumption, Biomass energy Testing, Spark ignition engines Exhaust gas Testing

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Copyright 2011, author