The traditional reciprocating I.C. engine has evolved to a point where significant improvements in thermal efficiency and specific power are not expected. Modifications to existing engines may prove to be difficult and expensive while resulting in only marginal gains. In addition, most modifications result in added components that often increase cost and decrease reliability of the system as a whole. For applications requiring major advances in performance, such as unmanned vehicles, meeting mission requirements will likely stem from a revolutionary rather than an evolutionary engine design. The slider crank mechanism is a major impediment to the traditional reciprocating I.C. engine. Although this mechanism has been used for the past 100 years, it is very wasteful of the available energy supplied by the combustion process, where piston-liner interactions from this arrangement accounts for 50–70% of the total friction losses in this engine design. Eliminating the slider crank could significantly reduce friction losses and provide additional benefits that can increase fuel conversion efficiency. The HiPerTEC engine is an opposed, free-piston engine arranged in a toroidal configuration with two counter reciprocating sets of pistons. The counter reciprocating masses eliminate the vibration found in linear free-piston engines. The HiPerTEC employs a unique shared volume configuration where the swept volume is twice the physical cylinder volume. This attribute offers a significant increase in specific power, while the free-piston characteristics provide for substantial gains in thermodynamic cycle efficiency. An eight cylinder/chamber arrangement offers balanced operation in both two and four-stroke cycle modes to allow for a wide operating envelope. The final HiPerTEC configuration will require advanced materials to address lubrication and cooling requirements. This paper discusses the HiPerTEC design, operating characteristics, development progress to date, and the challenges that lie ahead.

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