This paper describes the development of small rotary internal combustion engines developed to operate on the High Efficiency Hybrid Cycle (HEHC). The cycle, which combines high compression ratio (CR), constant-volume (isochoric) combustion, and overexpansion, has a theoretical efficiency of 75% using air-standard assumptions and first-law analysis. This innovative rotary engine architecture shows a potential indicated efficiency of 60% and brake efficiency of >50%. As this engine does not have poppet valves and the gas is fully expanded before the exhaust stroke starts, the engine has potential to be quiet. Similar to the Wankel rotary engine, the ‘X’ engine has only two primary moving parts - a shaft and rotor, resulting in compact size and offering low-vibration operation. Unlike the Wankel, however, the X engine is uniquely configured to adopt the HEHC cycle and its associated efficiency and low-noise benefits. The result is an engine which is compact, lightweight, low-vibration, quiet, and fuel-efficient.
Two prototype engines are discussed. The first engine is the larger X1 engine (70hp), which operates on the HEHC with compression-ignition (CI) of diesel fuel. A second engine, the XMv3, is a scaled down X engine (70cc / 3HP) which operates with spark-ignition (SI) of gasoline fuel. Scaling down the engine presented unique challenges, but many of the important features of the X engine and HEHC cycle were captured. Preliminary experimental results including firing analysis are presented for both engines. Further tuning and optimization is currently underway to fully exploit the advantages of HEHC with the X architecture engines.
The internal combustion engine enjoys widespread use as an inexpensive and reliable power conversion system. While piston engines date back 150 years, various alternative engine architectures and cycles have been considered. Today's small piston engines can be inexpensive, and have suitable reliability to serve a variety of applications including mobile propulsive power for scooters, motorcycles, all-terrain vehicles (ATVs), boats, and small aircraft including unmanned aircraft vehicles (UAVs). Small engines are also used for mobile power generation, including electric or auxiliary power, and to directly provide mechanical power for lawn and garden equipment. While piston engines enjoy prolific use, their efficiency is remarkably low. Literature indicates peak engine efficiencies of 15-18% for 30cc 1hp 4-stroke, and average part-load efficiency significantly lower .
The rotary engine has some advantages that make it a formidable contender for some of the markets currently served by reciprocating engines. The piston in a 4-stroke reciprocating engine momentarily comes to rest four times per cycle as its direction of motion changes. In contrast, the moving parts in a rotary engine are in continuous unidirectional rotational motion. High power density, smooth operation, simple design, low vibration, compact size, and lightweight architecture are a few of the benefits . However, the rotary type engine has some drawbacks. A major problem of the Wankel rotary engine is that it does not measure up to the fuel economy of reciprocating engines in part due to the long combustion chamber shape and low compression ratio (CR). The gas sealing of the rotary engine, such as apex seals and side seals, are less efficient and durable due to poor lubrication compared to piston rings. However, these drawbacks have been steadily improved for gasoline spark-ignited (SI) rotary engines [3, 4, 5, and 6]. Recently, the rotary engine has become more attractive to applications where the typical merits of rotary engines are becoming more important (such as size, efficiency, and power density).
The LiquidPiston X Engine architecture is a rotary engine embodiment similar in some aspects to the Wankel-type engine, however, several differences lead to advantages. Merits of the X engine are described, and a comparison to the Wankel engine is provided in this paper. A primary motivation for development of the new engine architecture is the ability to embody an optimized 4-stroke cycle, dubbed the High Efficiency Hybrid Cycle (HEHC), which can change the operation of internal combustion engines fundamentally [7, 8, and 9].
The combination of this cycle's features is expected to achieve higher thermodynamic efficiency - analytical modeling studies have previously indicated that the HEHC is able to reach 50%+ brake efficiency (75% ideal cycle efficiency) with CR of 18 [10, 11, and 12].
The company has been working on new engine architectures, with a focus on rotary engines, in order to realize the HEHC cycle. The X engine platform has been developed after several years of research and development efforts. The X engine is simple, and has no reciprocating parts - features common to the conventional rotary engines. However, in contrast, the X engine has a higher CR, and a stationary conical/spherical combustion chamber suitable for direct injection (DI) and CI. As with the Atkinson or Miller cycles, the X engine takes advantage of over-expansion. This is done simply by changing the locations of intake and exhaust ports asymmetrically which allows for the extraction of more energy during the expansion stroke. Further, a power stroke of the X engine occurs 3 times per rotor revolution resulting in a high power density . LiquidPiston has developed initial prototypes to demonstrate the principles of the engine, including the X1, 1370 cc (70HP) and the XMv3, 70cc (3HP).
The paper is organized as follows: 1) the HEHC cycle is reviewed, and efficiency of the cycle is discussed. While the real engine is still at relatively early stages of development, the cycle serves as a primary motivation for the development of the engine. 2) the ‘X’ engine architecture, including its structure and operation, is described; 3) a brief discussion of Cooling strategy, and then 4) Sealing strategy are described, as these are necessary for enabling successful operation as an engine; 5) differences between the X rotary engine and the traditional Wankel rotary engine are highlighted. 6) a summary of potential benefits of the cycle and engine is presented; 7) experimental methods and 8) results describing initial experiments with the X1 (70 HP 1.3L CI engine) and XMv3 (3 HP 70cc SI engine) are presented, and 9) we conclude discussion.