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Technical Paper

Development of the X4: a 30kW heavy fueled rotary engine with target 45% brake thermal efficiency

This paper presents a progress towards the development of the LiquidPiston 'X4' 30kW heavy fueled rotary Compression Ignition engine prototype. The X4 Engine is the newest version of the LiquidPiston unique rotary 'X' engine architecture, which can be likened to a Wankel type rotary engine that has been inverted. The engine operates on an optimized thermodynamic cycle, called the High Efficiency Hybrid Cycle (HEHC), which combines: 1) a high compression ratio; 2) constant volume combustion, effected by a dwell in combustion volume for a significant period of time; and 3) over-expansion, e.g. a greater expansion ratio compared to compression ratio. The engine development is funded by DARPA, and the long term objectives of the program target: 45% brake thermal efficiency; > 1 hp / lb operating in CI of heavy fuel; and the 30kW (40hp) engine will fit in a 10" box and weight <40lbs.
Technical Paper

Performance of a low-blowby sealing system for a high efficiency rotary engine

The X engine is a non-Wankel rotary engine that allies high power density and high efficiency by running a high-pressure Atkinson cycle at high speeds. The X engine overcomes the leakage (blowby) issue of the Wankel engine by ”inverting” the Wankel engine, so that the apex seals are stationary, located within the housing, and interface directly against the face seals located in the rotor without any significant gaps at the interface. This is in contrast to the Wankel rotary engine which has moving apex seals located in the rotor that interface with the face seals using a corner seal that has gaps allowing significant blowby at this interface. This paper demonstrates the sealing performance that can be achieved by this new type of seal configuration for a rotary engine based on dynamics models and validated with experiments.
Journal Article

Measurement and Prediction of Heat Transfer Losses on the XMv3 Rotary Engine

Abstract This paper describes predictive models and validation experiments used to quantify the in-chamber heat transfer of LiquidPiston’s rotary 70cc SI “XMv3” engine. The XMv3 engine is air cooled, with separate cooling flow paths for the stationary parts and the rotor. The heat transfer rate to the stationary parts was measured by thermal energy balance of that circuit’s cooling air. However, because the rotor’s cooling air mixes internally with the engine’s exhaust gas, a similar procedure was not practical for the rotor circuit. Instead, a CONVERGE CFD model was developed, and used together with GT-POWER to derive boundary conditions to estimate a ratio between rotor and stationary parts heat transfer, thus allowing estimation of rotor and total heat losses. For both cases studied (5000 and 9000 rpm under full load), the rotor’s heat loss was found to be ∼60% that of the stationary parts, and overall heat losses were less than 35% of supplied fuel energy.
Technical Paper

Development of the XMv3 High Efficiency Cycloidal Engine

The demand for lighter, smaller, more efficient, and more powerful engines calls for a rethinking of the traditional internal combustion engine (ICE). This paper describes development progress of LiquidPiston's small rotary engine, the XMv3, which operates on a Spark-Ignited (SI) variant of its patented High Efficiency Hybrid Cycle (HEHC). This thermodynamic cycle, which combines high compression ratio (CR), constant-volume combustion, and overexpansion, has a theoretical efficiency of up to 75 percent using air-standard assumptions and first-law analysis. XMv3 displaces 70cc (23cc per each of three working chambers) and is gasoline fueled. The engine is simple, having only two primary moving parts, which are balanced to prevent vibration.
Technical Paper

Development of a Small Rotary SI/CI Combustion Engine

Abstract 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.
Technical Paper

High Efficiency Hybrid Cycle Engine

The High Efficiency Hybrid Cycle (HEHC) is a thermodynamic cycle which borrows elements of Diesel, Otto and Atkinson cycles, including: Air compression to a high ratio, followed by fuel injection and compression ignition (Diesel). Constant volume combustion (Otto) Over-expansion (Atkinson) Optionally, internal cooling heat recovery via steam generation (Rankine). Simple air standard analysis predicts this cycle to be 17% more efficient than diesel and 19% more efficient than Otto. The construction of a prototype rotary engine implementing this cycle is also described in detail. The main engine components consist of a rotor in pure rotation and two reciprocating gates directly driven by overhead cams. This combination separates the working mixture into three separate volumes. At a given rotor position each volume operates at a different part of the cycle. For instance, intake/compression, combustion, expansion/exhaust are occurring simultaneously in separate chambers.
Technical Paper

Rotary High Efficiency Hybrid Cycle Engine

In this paper we discuss a rotary implementation of the High Efficiency Hybrid Cycle (HEHC) engine. HEHC is a thermodynamic cycle which borrows elements of Diesel, Otto and Atkinson cycles, characterized by 1) compression of air only (e.g. Diesel), 2) constant volume heat addition (e.g. Otto), and 3) expansion to atmospheric pressure (e.g. Atkinson). The engine consists of a compressor, an isolated combustion chamber, and an expander. Both compressor and expander consist of a simple design with two main parts: a rotor and an oscillating rocker. Compared to conventional internal combustion engines, in which all processes happen within the same space but at different times, in this engine, all processes are occurring simultaneously but in different chambers, allowing for independent optimization of each process. The result is an engine which may offer up to 57% peak efficiency, and above 50% sustained efficiency across typical driving loads.