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The rapid expansion of the market for remotely piloted aircraft (RPA) includes a particular interest in 10-25 kg vehicles for monitoring, surveillance, and reconnaissance. Power-plant options for these aircraft are often 10-100 cm3 internal combustion engines. Both power and fuel conversion efficiency decrease with increasing rapidity in the aforementioned size range. Fuel conversion efficiency decreases from ∼30% for conventional-scale engines (>100 cm3 displacement) to <5% for micro glow-fuel engines (<10 cm3 displacement), while brake mean effective pressure decreases from >10 bar (>100 cm3) to <4 bar (<10 cm3). Based on research documented in the literature, the losses responsible for the increase in the rate of decreasing performance cannot be clearly defined. Energy balances consisting of five pathways were experimentally determined on two engines that are representative of Group-2 RPA propulsion systems and compared to those in the literature for larger and smaller engines.

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.

In this work, in-cylinder pressure was measured in a 55 cc single cylinder, 4.4 kW, two stroke, spark ignition engine. In cylinder pressure measurements were taken using two different pressure transducers to determine if the performance differences between the two transducers are discernible in a small, spark ignition engine. A Kistler brand measuring spark plug was compared to a Kistler brand flush mount high temperature pressure sensor. Both sensors employ piezo-electric pressure sensing elements and were designed to measure indicated mean effective pressure as well as to detect knock at high temperature engine conditions. The pressure sensors were installed and adjusted to ensure cylinder volume after sensor installation matched the engine's original configuration within reasonable manufacturing tolerances. A series of tests at four throttle settings ensued to determine if either device altered the combustion volume or the engine's performance.

As IC engines decrease in displacement, their cylinder surface area to swept volume ratio increases. Examining power output of IC engines with respect to cylinder surface area to swept volume ratio shows that there is a change in power scaling trends at approximately 1.5 cm−1. At this size, it is suggested that heat transfer from the cylinder becomes the dominant thermal loss mechanism and performance and efficiency characteristics suffer. Furthermore, small IC engines (>1 cm−1) have limited technical performance data compared to IC engines in larger size classes. Therefore, it is critical to establish accurate performance figures for a family of geometrically similar engines in the size class of approximately 1.5 cm−1 in order to better understand the thermal losses that contribute to lower efficiencies in small IC engines. The engines considered in this scaling study were manufactured by 3W Modellmotoren, GmbH.

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.