In a paper to be presented at next week’s 2010 SAE World Congress in Detroit, LiquidPiston, Inc. will publicly reveal the architecture of its High Efficiency Hybrid Cycle (HEHC) engine. (Earlier post.) The paper describes LiquidPiston’s third-generation engine prototype, which was first test-fired in January 2010. It provides an air-standard analysis of efficiency, including a comparison to conventional engine technologies.
The thermodynamic efficiency of the HEHC cycle offers a 20% to 50% improvement over the best diesel engines, according to the company. In addition, the engine is ultra-compact, delivering one horsepower per pound, and operating quietly without a muffler.
|“Imagine what you could do with a light weight, quiet, 20 HP diesel engine the size of a shoe box.”|
|—Edward Richards, CEO of LiquidPiston|
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); and
- Optionally, internal cooling heat recovery via steam generation (Rankine).
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.
As the rotor moves, the cavity formed by the side of the rotor, the retracting compressor gate, and the stationary housing is decreasing in volume, producing compression. The gate fully retracts, as the rotor passes beneath. The air is fully compressed into a combustion chamber within the housing and held at constant volume. Fuel is injected, and combustion occurs at relatively constant volume.
As the rotor continues its motion, the volume defined by the housing, expander-gate, and the rotor is increasing through the completion of the expansion stroke. Due to the geometry, a higher expansion ratio is achieved relative to the compression ratio. Predicted output is 143 hp/L and 30% thermal efficiency.
Combining the high peak pressures produced during constant volume combustion with Atkinson expansion, which continuously extracts energy as exhaust gases expand to atmospheric pressure, delivers unprecedented fuel efficiency. Also, using a ported design rather than conventional poppet valves makes for extraordinarily quiet operation.
—LiquidPiston co-founder and Chief Technology Officer Nikolay Shkolnik
Although the company is not pursuing the automotive primary propulsion market due to the long design-in times and prohibitive development costs, it believes HEHC engines for conventional passenger cars could deliver 100+ miles per gallon within a decade. In the near term, the company plans to produce and license flex-fuel engines in the 5 hp (3.7 kW) to 100 hp (75 kW) range for a variety of commercial, industrial, and military applications.
The prototype engine was brought from art to part in 8 months by a team of six full time employees. Since its founding in 2004, the company has used only a small amount of venture capital to achieve multiple design iterations and a working prototype.
Stephen Nabours et al. (2010) High Efficiency Hybrid Cycle Engine (SAE 2010-01-1110)