Innovative Engines for Small Unmanned Aerial Systems (SUAS)

With the specific output of any engine being directly proportionate to the weight of air consumed, the current Otto, Diesel, and Brayton Cycles have common UAS propulsion constraints in terms of power density, specific weight, and fuel consumption while operating on logistically available kerosene-based fuels. The combustion system efficiency is the most dominate parameter that defines the overall UAS propulsion system efficiency, while military fuel characteristics widely used (JP-8/F-24) are regulated by flash point alone. Fuel variations in viscosity, lubricity and cetane dictate the need for a true multi-fuel combustion system capable of extracting the utmost thermal yield from the fuel. The dominate use of the gas turbine in military applications is beneficial in terms of being lightweight and fuel agnostic but suffers from low pressure ratio and overall poor brake thermal efficiency. (BTE: 15~20%)  The popular use of spark ignited (SI) engines are limited by low fuel volatility/octane of turbine fuels and suffer from difficult cold start, chronic spark plug fouling, and ultimate detonation setting an early and harsh limit on the operating BMEP and/or bore size.(BTE: 10~15%) Conversely, the modern compression ignited (CI) Diesel engine is equally sensitive to kerosene fuel properties meaning variations in fuel viscosity and lubricity disrupt injection timing, injection pump life and adverse ignition delay proportionate to altitude. High altitude descent can lead to ultimate combustion flame out which now dictates additional fuel burn during high altitude descent. Scaling the (CI) engine down suffers from internal leakage and heat loss that requires exceedingly high compression ratios to operate on lower cetane turbine fuels with poor efficiency(18~20% BTE) and specific weight (3.3 lbs/hp). By contrast, this proposal outlines an innovative breakthrough in high-speed multi-fuel injection and heterogenous charge combustion that is insensitive to engine scale and/or compression ratio ideally suited for combine cycle engine design. Design attributes include passive pneumatic fuel injection combined with pilot ignition and true multi-fuel combustion chamber geometry extracting the utmost thermal yield from logistically available fuels (JP-5 / JP-8 / DF-2 / F-24).