LiquidPiston Rotary Engine

The Wankel rotary engine looked to be a promising technology for motorcycle application back in the seventies. So promising, in fact, that Suzuki devoted huge development dollars to bring the liquid-cooled RE5M to the market in 1975 and a revived Norton Motors brought out air-cooled Wankel type rotary bikes in 1979.

The RE5M lasted only two years in the showroom, and the Norton rotary was gone by the early 1990s despite some impressive racing success including winning the British F1 and Supercup championships in 1989 and Steve Hislop riding one to victory in the Senior TT at the Isle of Man TT in 1992.

Despite some theoretical advantages of the Wankel type engine, practical problems beset the concept in its early days; apex seal wear and emissions problems, to name a couple. Despite technological advances in those areas, Wankel-type rotary engines have not found their way into the motorcycle market since those early efforts.

Now, an emerging rotary engine design may be able to make inroads in motorcycle markets where the Wankel-type could not. That design is the LiquidPiston X Mini prototype. While it is a rotary engine, it is not of the Wankel design.

The Bloomfield, Conn., company says its patented High Efficiency Hybrid Cycle (HEHC), is an improved thermodynamic cycle, and the engine architecture “enable LiquidPiston to create engines optimized for fuel efficiency.

LiquidPiston’s engines have only two primary moving parts – a shaft and rotor, resulting in compact size and low-vibration operation.” Needless to say, the absence of a mechanical valve train reduces weight, complexity and potential sources of noise.

The company details the engine architecture and HEHC cycle this way: “The cycle, which combines high compression ratio (CR), constant-volume (isochoric) combustion, and over expansion, 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 X Mini prototype displaces only 70cc and utilizes a four-cycle design, but the simplicity of its design allows it to easily be scaled up to larger displacement. It is described by LiquidPiston as a “power-dense, low vibration, quiet, 70-cubic centimeter gasoline powered rotary four-stroke engine prototype.

The compact engine has only two primary moving parts, a 4-pound core, and fits in a 6.6” x 6.2” x 5.4” box. In prototype testing, the spark-ignited X Mini engine has shown high power density, producing 3.5 horsepower (indicated at 10,000 RPM).

When mature, the engine is expected to weigh 3 pounds, produce over 5 horsepower at up to 15,000 RPM, and be over 30 percent smaller and lighter than comparable four-stroke piston engines.”

At present, the developers have small implement, marine and aviation applications in mind for the engine; lawn and garden equipment, generators, auxiliary power units for marine and other applications, robotics, range-extenders for electric vehicles and UAVs.

But mopeds are in the mix and with the mix of characteristics that the engine has, can motorcycle applications be far off? After all, what motorcycle designer would not want a small, light, power-dense, low vibration, quiet, efficient power plant to wrap a motorcycle around? That would seem a real possibility as the company says the engine design can be scaled up from 1 hp output to over 1,000 hp.

The X Mini has demonstrated the ability to run steady state with air-cooling. The engine design is based on LiquidPiston’s patented thermodynamic cycle and engine architecture. The company has already demonstrated compact, low-vibration, high-efficiency 70 horsepower and 40 horsepower compression ignition diesel engines.

The X Mini on the other hand, is gasoline-powered, demonstrating that the engine technology can scale down in size and can be configured for spark-ignited operation to support both gasoline and diesel applications. Indeed, the company says the engine design will allow multi-fuel operation.

The compression ignition operation cycle is explained by LiquidPiston this way:

“LiquidPiston’s High Efficiency Hybrid Cycle (HEHC) is a patented thermodynamic cycle that combines the advantages of Diesel, Otto and Atkinson thermodynamic cycles. The cycle elements include:

1. Compression: For maximum efficiency, air is compressed to a high compression ratio, fuel is injected and compression ignited (CI-HEHC). The X Mini utilizes a spark-ignition (SI-HEHC) version of the cycle with a lower compression ratio standard for gasoline engines.

2. A dwell near top-dead-center forces combustion to occur at nearly constant-volume conditions.

3. Combustion products are over-expanded using a larger expansion volume than compression volume, as in the Atkinson Cycle.

4. Cycle-skipping power modulation allows high efficiencies at low power settings while simultaneously cooling the engine’s walls internally and providing partial heat recovery.

5. Water may be injected to internally cool the engine. Some of this cooling energy is recuperated, as the water turns to steam, increasing the chamber pressure.”

Item number five would suggest that version of the engine would be used in fixed installation settings such as facility emergency power generators.

With the unveiling of the X Mini engine, Dr. Alexander Shkolnik, President and Co-Founder of LiquidPiston, said: “We are currently in discussions with interested industry partners looking to enhance their existing product lines. The X Mini is an incredible breakthrough, and we expect to further optimize the engine for increased power (greater than 5 horsepower) and efficiency, and even lower operational noise.”

SM Shahed, a LiquidPiston advisor, past President of SAE International, and former VP, Advanced Technology at Honeywell, added: “The high power density and smooth vibration signature of the X Mini together with its simplicity (fewer parts) make it an ideal candidate for its intended markets. Further developments are likely to greatly expand
its range of applications.”

LiquidPiston is currently speaking with companies regarding licensing the engine and developing it for specific market applications that require compact, lightweight, quiet, low-vibration, powerful, fuel-efficient engines.

Ultimate MotorCycling readers can put their creative thinking caps on, too, at the invitation of LiquidPiston.

The company is seeking input on the ways their engine can be used to improve existing engine applications, as well as ways the X Mini may enable entirely new applications not possible with current engine technology.

In early 2015, LiquidPiston will issue an open call for ideas for new applications of its rotary engine technology.

The company will award a cash prize for the most innovative submission. Click here for more information.

To learn more, visit LiquidPiston.