LiquidPiston unveils 70cc rotary gasoline engine prototype embodying HEHC; power dense, low-vibration

19 November 2014

LiquidPiston X1 engine features three separate combustion chambers creating 3 power strokes per revolution for high power density. Source: LPI.
Click to enlarge.

LiquidPiston, Inc. (LPI) the developer of engines based on its High Efficiency Hybrid Cycle (HEHC) (earlier post), has unveiled the alpha prototype of the X Mini—a power-dense, low-vibration, quiet, 70cc spark-ignited, non-Wankel rotary embodiment of the HEHC. Dr. Alexander Shkolnik, President and Co-Founder of LiquidPiston, is presenting the engine in a paper at the SAE International/JSAE 2014 Small Engine TechnologyConference today in Pisa, Italy.

The compact engine with a 4-lb (1.8 kg) core has only two primary moving parts—a rotor (the primary work-producing component) and an eccentric shaft—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 more than 5 hp at up to 15,000 RPM, and be more than 30% smaller and lighter than comparable four-stroke piston engines.

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The X Mini 70cc rotary gasoline engine. Click to enlarge.

HEHC is an improved thermodynamic cycle optimized for fuel efficiency that combines features of four existing cycles: high compression ratio (Diesel); constant volume (isochoric) combustion (Otto); over-expansion to atmospheric pressure (Atkinson); and internal cooling with air or water (Rankine).

The cycle has a theoretical efficiency of 75% using air-standard assumptions and first-law analysis. The rotary engine architecture shows a potential indicated efficiency of 60% and brake efficiency of 50%. As the engine does not have poppet valves and the gas is fully expanded before the exhaust stroke starts, the engine also has potential to be quiet. The cycle elements include:

• 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—and hence somewhat lower efficiency than the CI implementations, Shkolnik said.

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

• Combustion products are over-expanded using a larger expansion volume than compression volume, as in the Atkinson Cycle. (This is done by changing the locations of intake and exhaust ports asymmetrically which allows for the extraction of more energy during the expansion stroke.)

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

• 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.

By combining HEHC with a rotary engine architecture, LiquidPiston is creating engines up to ten times lighter, quiet, and two to three times more efficient at part-load than conventional engines. LPI selected a rotary architecture because it offers more flexibility in optimizing each part of the cycle.

LiquidPiston is emphatic that its rotary engines are not Wankels; the X engine has a fundamentally different thermodynamic cycle, architecture and operation. The Wankel is characterized by a low compression ratio, no constant-volume combustion and no over-expansion. By contrast, the LPI X engine is characterized by high compression ratios, constant-volume combustion and over-expansion, the company says.

LiquidPiston earlier introduced the larger X1 (rotary) compression-ignition prototype (1370 cc, 70 hp). With the new X Mini, said Dr. Shkolnik:

What we’ve done is taken everything we’ve learned from the larger engine and, especially due to customer interest, focused on the very small engines. This one is spark-ignited, not compression ignition. It has a lower compression ratio, but it still has the constant-volume combustion. We don’t get the same efficiency as with the true compression ignition version, but we still get a significant efficiency improvement over gasoline engines.

In the paper being presented in Pisa, Shkolnik and his team explain that while the reduction in compression ratio (from 18:1 for the X1 to 9:1 for the X Mini) causes a reduction in efficiency compared to CI, the dwell in combustion volume near TDC results in higher peak pressure and efficiency than piston-engines operating with SI. This, the LPI team says, is related to the slower variation of displacement in proximity to TDC than piston engines.

Overexpansion further increases efficiency, similar to Atkinson cycle. The dwell in volume at TDC allows the engine to more closely achieve true constant-volume combustion (isochoric head addition), compared to a piston implementation of the Otto cycle.

Also in the Pisa paper, LPI notes that while the X1 has demonstrated 33% indicated efficiency at medium load at 1800rpm, diesel fueled, the early X Mini prototype is capable, at this stage of its development, of providing 10% indicated efficiency.

Those initial results indicate that the target HEHC efficiency of 60% is not yet achieved, but they support the feasibility of development of this engine architecture and the potential for rapid improvement. Future work and publications will focus on demonstrating efficiency and power density benefits, including running the engines at full load and in continuous (steady-state) operation over a wide range of engine speeds.

The X Mini is meant to be a low-cost engine, really targeted especially towards the outdoor power equipment market, and especially in the hand held aren, Shkolnik said.

49cc Honda Metropolitan moped engine (left) and 70cc LPI X Mini (right). Click to enlarge.

We’ve studied 60 rotary engine embodiments and patented dozens of rotary and pistons engines. This [X engine rotary] is by far the simplest strategy that there is. We really converged on this design and demonstrated that it’s working…so it looks like this is the one for us.

What we would like to do is to get it into production as quickly as possible. That’s why we’re speaking with so many customers behind the scenes. We really designed based on what we’ve heard, and we’re doing it in markets that don’t take a decade to get into production. After we get the X Mini into the market, we can go back to higher efficiency diesel engines.

The X Mini will enable many small engine applications to be smaller, lighter, and quieter, including handheld power equipment, lawn and garden equipment, portable generators, mopeds, unmanned aerial vehicles, robotics, range extenders for electric vehicles, and auxiliary power units for boats, aviation and other vehicles.

In addition to improving existing engine applications, the X Mini may enable entirely new applications not possible with current engine technology, LPI suggests. In early 2015, LiquidPiston will host an open call for ideas regarding these new applications. The company will award a cash prize for the most innovative submission.

Resources

• Shkolnik, A., Littera, D., Nickerson, M., Shkolnik, N. et al. (2014) “Development of a Small Rotary SI/CI Combustion Engine,” SAE Technical Paper 2014-32-0104 doi: 10.4271/2014-32-0104