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Small internal combustion engines (ICEs), (<7.5 kW), possess low thermal efficiencies due to high thermal losses. As the surface area to volume ratio increases beyond 1.5 cm2/cc, the increase in thermal losses leads to a drop off of engine efficiency and power. This effort describes the development and validation of a test stand to characterize thermal losses of small ICEs, optimize combustion phasing, and eventually enable heavy fuel operation. The test stand measures torque, rotational speed, brake power, intake air mass flow, up to 48 temperatures (including ambient, intake, cylinder head, fuel, and exhaust), 8 pressures (including ambient, intake, and exhaust), throttle position, and fuel and air mass flows. Intake air temperature and cylinder head temperature are controlled and adjustable. Three geometrically similar engines with surface area to volume ratios near 1.5 cm2/cc were selected from 3W Modellmotoren.

An experimental study is performed to investigate the possibility of relaxing the octane requirement of a Rotax 914 engine equipped with a pre-chamber jet ignition system. A pre-chamber jet igniter with no auxiliary fuel addition is designed to replace the spark plug in cylinder two of the test engine and is evaluated across engine speeds ranging from 2500 to 5500 RPM. Experiments are performed across both normally aspirated and boosted configurations using regular 87 AKI gasoline fuel. Normally aspirated results at 98 kPa manifold absolute pressure show a 7-10° burn rate improvement with the jet ignition combustion system. Tests to determine the maximum load at optimal combustion phasing (no spark retard) are then conducted by increasing boost pressure up to maximum knock limits.

A two-port fuel-injection (PFI) system is added to a Rotax 914 four-cylinder spark-ignition engine to allow two fuels of different reactivity to be injected simultaneously in order to vary the fuel octane number during engine operation. Engine performance using the dual-fuel PFI system is compared to that using injection of primary-reference-fuel (PRF) blends via a single-PFI system for fuel octane ratings of 50, 70, and 87 octane. The on-the-fly octane control of dual-PFI system is found to control fuel-octane well enough to produce maximum indicated mean effective pressure (IMEPn) results within ± 2% of single-PFI PRF IMEPn results. IMEPn is compared among dual-PFI blends from 20 to 87 octane, neat n-heptane, neat JP-8, and JP-8/isooctane blends. Maximum IMEPn for these fuels is established for the Rotax 914 engine operating from 2500 to 5800 rev/min.