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Small remotely piloted aircraft (10-25 kg) powered by internal combustion engines typically operate on motor gasoline, which has an anti-knock index (AKI) of >80. To comply with the single-battlefield-fuel initiative in DoD Directive 4140.25, interest has been increasing in converting the 1-10 kW power plants in the aforementioned size class to run on lower AKI fuels such as diesel and JP-8, which have AKIs of ~20. It has been speculated that the higher losses (short-circuiting, incomplete combustion, heat transfer) that cause these engines to have lower efficiencies than their conventional-scale counterparts may also relax the fuel-AKI requirements of the engines. To investigate that idea, the fuel-AKI requirement of a 3W-55i engine was mapped and compared to that of the engine on the manufacturer-recommended 98 octane number (ON) fuel.

Loss mechanisms in 1-10 kW spark-ignition, two-stroke engines may be grouped into five categories: thermal losses, frictional losses, sensible enthalpy in the exhaust gases, incomplete combustion, and short-circuiting of fresh fuel and air mixture. These loss mechanisms cause small two-stroke engines to have fuel conversion efficiencies 50%-70% lower than similar larger engines. Previous studies of loss scaling in small engines have estimated the short-circuiting using heuristics derived for larger engines or grouped it with other combustion losses to complete the energy balance. This work describes and compares two methods for measuring short-circuiting on a commercially available, two-stroke, naturally aspirated, spark ignition engine with 55 cm3 displacement. One method used oxygen as an analyte (the Watson method), nitrogen as an internal standard, and gas chromatography with a thermal conductivity detector for quantification.

The engines used to power small unmanned aerial systems are often modified commercial products designed for use by hobbyists on small model aircraft at low altitude. For military applications, it is desirable to fly at high altitudes. Maintaining power from the engine at the reduced ambient air pressures associated with high altitudes requires some method of increasing air delivery to the intake manifold. Conventional turbochargers and superchargers are typically very inefficient for the low mass flows associated with small engines. Due to its unique characteristics, a pressure wave supercharger (PWS) can avoid many scaling-related losses. This project designed a small-scale PWS for turbo-normalization of a Brison 95 cc two-stroke engine for a small unmanned aerial vehicle. A larger PWS called the Comprex®, designed by Brown Boveri Company, was simulated using a quasi-one-dimensional Computational Fluid Dynamics (CFD) code developed at the NASA Glenn Research Center.