Ignition Assistance Device for Diesel Aviation Engines

Diesel piston engines are used in military unmanned aerial vehicles (UAVs) because they are compatible with jet fuel, they are generally reliable, and they are efficient. An example would be the MQ-1C Gray Eagle which uses a Thielert Centurion diesel engine. Relative to typical automotive diesel applications, military UAV diesel engines are confounded by operation at altitude, and cetane number variability in military jet fuels. This can lead to combustion stability issues for UAVs operating low cetane number fuel or at high altitude. We developed an Ignition Assistance (IA) device to enhance combustion stability and robustness in diesel UAV engines. The IA Device is similar to a diesel glow plug, except that it is designed to operate continuously (not just during cold start), and it has a high continuous operating temperature of 1300°C. This paper describes the design, development, fabrication, and testing of IA Devices. Two embodiments are presented. The first involves a commercial heating element repurposed for this use with a suboptimal geometry. The second involves a custom heating element developed specifically for this application. The devices are made of silicon nitride ceramic with internal electrical heater traces. Testing involves thermal testing in air, combustion testing in a high-pressure combustion rig, and testing in a diesel automotive engine. The IA Device has promise both for the UAV application, and for automotive applications where increased combustion performance is desired. An example would be engines running alternative fuels such as low cetane number diesel, or fuels with reduced reactivity such as methanol or ammonia.