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LiquidPiston introduces latest X-series version of HEHC rotary engine; targeting 58% peak BTE, 2 hp/lb power density
18 October 2012
|Isuzu D201 35 hp diesel engine (left) and the LiquidPiston 40 hp X2 Prototype (right). Click to enlarge.|
LiquidPiston, Inc. (LPI), the developer of engines based on its High Efficiency Hybrid Cycle (HEHC)(earlier post), introduced the latest version of its X-series of rotary engines embodying HEHC at the US Department of Energy’s (DOE) 2012 Directions in Engine-Efficiency and Emissions Research (DEER) Conference in Dearborn, Michigan.
The X2—which can be considered a beta version of the X1 alpha (referenced by Dr. Alexander Shkolnik, President and CEO, at the SAE High Efficiency IC Engine Symposium earlier this year, earlier post)—is a 40 hp (29 kW) multi-fuel capable rotary engine that requires no valves, cooling systems, radiators, mufflers, or other components. Expected realized brake efficiency for the X2 is 58% (peak) and 50% at partial load; power density is targeted to be around 2 hp/lb.
Other features of the X2 include:
- Quiet operation: engine lacks poppet valves and over-expansion minimizes exhaust noise.
- Power modulation by cycle skipping increases efficiency and recaptures heat.
- Low heat signature from over-expansion and cycle skipping eliminates water cooling.
By combining HEHC with a rotary engine architecture, LiquidPiston has created an engine that is 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, Shkolnik had noted during his presentation at the SAE symposium.)
HEHC is an improved thermodynamic cycle optimized for fuel efficiency that combines features of four existing cycles:
- High air compression ratio (Diesel).
- Constant volume combustion (Otto)
- Over-expansion to atmospheric pressure (Atkinson)
- Internal cooling with air or water (Rankine)
The combination of high compression ratio, true constant volume combustion, expansion into a larger volume than intake, and (optionally) water turning to high pressure steam cumulatively add to the efficiency of the engine. An air-standard analysis predicts an ideal thermodynamic efficiency of 74% at an 18:1 compression ratio.
Liquid Piston is married to the High Efficiency Hybrid Cycle—e.g. to the thermodynamics, not to a particular engine. Our engine architecture has evolved substantially over time, since 2007. We started with a “liquid piston”, moved to a spit-cycle rotary (compressor rotor with gate + combustion chamber + expander rotor with gate), collapsed the split-cycle rotary into a single deck with two gates (the M line), and finally we got rid of the gates and ancillary systems, leaving us with just a rotor and eccentric shaft (X line)—yet all of these engine designs executed our HEHC cycle.
We have explored and patented dozens of engine architectures capable of implementing the HEHC cycle. It took a lot of learning and refinement to figure out how to make such a simple engine as the X engine, which is 1) sealable, and 2) implements the HEHC cycle.—Alexander Shkolnik
The X2, which is still a research-grade prototype, is very similar in architecture to X1, Shkolnik said, but is generally a simpler engine to manufacture and to assemble. It has a different eccentricity ratio for the rotor; the X2 rotor has a convex profile (the X1 rotor looks more like a figure-8), which helps with sealing; and the X2 engine is smaller (40 HP, instead of 70 hp for the X1). LPI has moved towards a wider rotor (axially), with shorter rotor radius, for sealing purposes.
|Click to enlarge.|
LPI has placed the “M” engine line on hold, for now, to focus on developing the X architecture.
The X1 engine—brought from a concept on paper to an engine on the dyno in less than one year—is firing reliably and consistently in the lab on synthetic diesel fuel with compression ignition, and producing positive power, Shkolnik said. Using a model calibrated to the data from the engine, LPI has shown that if it achieve sealing, heat transfer coefficients, and friction similar to today’s Mazda Wankel engine, it can achieve its efficiency goals.
LPI will continue to use the X1 for development work, while evaluating the performance differences after the X2 arrives. The company expects to transition to the X2 engine for development work probably around the end of the year. The X2 engine is less sensitive to prototyping/machining errors, and is very fast to assemble and disassemble, so ultimately it will improve our development time, Shkolnik said. It will also serve as a better platform for strategic partners in testing and development.
LPI has also designed in the ancillaries—including the fuel pump, oil pump, oil filter, starter, etc,—thereby bringing the X2 closer to a standalone engine capable of running outside the lab.
The X2 engine is planned to be available as a beta prototype for outside testing by the first quarter of 2013.
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