Optical Engine Study of Variable Energy Assisted Compression Ignition using a Glow Plug for Unmanned Aircraft Propulsion Systems
Eri R. Amezcua Cuellar, David Rothamer, Kenneth Kim, Chol-Bum Kweon
Abstract
An experimental investigation of energy-assisted compression ignition using a glow plug was performed in an optically-accessible engine. Current glow plugs are designed to assist ignition at cold start engine cranking conditions (~200-300 rpm). This study investigated the glow-plug-assisted ignition capabilities and limitations of a metal glow plug at higher engine speeds. The glow plug was instrumented with a surface welded thermocouple to measure its surface temperature during motored engine operation. Glow-plug temperature measurements as a function of engine speed for motored operation showed a decrease in glow plug surface temperature as engine speed increased due to higher rates of heat transfer from the glow plug surface. The results indicated a need to overdrive the glow plug voltage at higher engine speeds to reach surface temperatures comparable to those achieved at engine cranking conditions. Fired engine experiments consisted of testing two jet fuels with different CNs (34 and 48) at different glow plug input voltages. Hydroxyl radical (OH) chemiluminescence high-speed imaging and in-cylinder pressure were recorded during the experiments. OH chemiluminescence imaging showed that the glow plug induces a multi-stage ignition process originating near the surface of the glow plug. Heat release analysis of the in-cylinder pressure measurements demonstrated the ability of the glow plug to advance and help stabilize ignition. The results demonstrate that glow plugs can effectively assist ignition at engine speeds higher than those associated with engine cranking, but also highlight the limitations of metal glow plugs to achieve the required surface temperatures to assist ignition at higher engine speeds.