An unmanned air vehicle (UAV) capable of sustained flight in the upper limits of the tropopause is a relatively new technology which has seen increasing interest during the past decade. Mission lengths for High Altitude Long Endurance (HALE) applications are typically measured in days rather than hours with operating altitudes ranging from 50,000 to 100,000 feet. An Otto cycle propulsion system offers significant performance advantages over other cycles. This paper provides a technical assessment of a liquid cooled turbocharged, reciprocating engine concept capable of meeting the requirements for a HALE vehicle. A properly designed spark ignition engine with a two or three stage series turbocharger system utilizing state-of-the-art aerodynamic design can meet the challenges presented at these altitudes. Several records for long endurance and high altitude flight have already been set with this type of propulsion system. A comparison with other candidate engines will also be made. The ability to operate with low brake specific fuel consumption (BSFC) across a broad operating range will be identified. With sufficiently high exhaust gas temperatures, the addition of a power turbine for turbocompounding can further reduce the BSFC and brake specific air consumption (BSAC). A version of the turbocharged spark ignition engine is capable of providing high thermal efficiency with the least BSAC and minimum turbomachinery weight.

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