LiquidPiston, a developer of advanced combustion engine technology, received the award for the first phase of a program to realize the full capability of its patented high-efficiency hybrid cycle (HEHC) engine. This is the second award DARPA has made to LiquidPiston in the past 18 months, following an earlier test program for a different LiquidPiston rotary engine prototype, which achieved promising results. (Earlier post.)
LiquidPiston will be developing its X4 test rig: a prototype 30 kW diesel X-engine, with targeted brake specific fuel consumption (BSFC) of 186 g/kWh (better fuel efficiency than heavy duty truck engines, and twice the fuel economy of a gasoline engine over a typical drive cycle).
The X4 is designed for extremely high specific power, with an objective of 1.5 hp/lb, making it up to ten times (10x) more powerful than a standard diesel engine with the same weight. The entire 40 hp engine is targeted to fit into a 10" x 8" x 8" box and weigh just 30 lbs. In this first phase of the program, LiquidPiston will build a test rig that will support future optimization efforts towards these objectives.
Our goal is to create a reliable X4 test rig that will support optimization of the X-engine architecture and the HEHC cycle. Combined, the cycle and architecture offer a compression-ignition, diesel-fueled rotary-engine power solution that is a fraction the size and weight of a comparable piston engine, while offering the advantages of greater fuel efficiency, quieter operation and reduced vibration.—Alexander Shkolnik, CEO and Founder of LiquidPiston
LiquidPiston recently installed its 70cc X-mini rotary engine prototype into a go-kart demonstrator (earlier post), replacing a 40 lb, 6 hp gasoline piston engine with the 4 lb, 3-5 hp 70cc X-mini engine.
LiquidPiston’s X Engines are non-Wankel rotary embodiments of the company’s High Efficiency Hybrid Cycle (HEHC). In contrast to other rotary engines, the X engine has a higher CR, and a stationary conical/spherical combustion chamber suitable for direct injection (DI) and compression ignition (CI). As with the Atkinson or Miller cycles, the X engine takes advantage of over-expansion. 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.
Three combustion events per rotor revolution result in high power density. The company earlier built functional proof-of-principle 70 and 40 HP diesel engine prototypes (X1 and X2), which demonstrated the initial operating capabilities of the engine architecture.
The X Engine’s few moving parts consist of a rotor (the primary work-producing component) and an eccentric shaft. Except for ancillary parts such as injectors, fuel pumps, and oil pumps, there are no other moving parts. LiquidPiston’s X Engine architecture geometry allows for standard materials and 2-D manufacturing to be used, greatly decreasing the design, build and testing cycle.
The HEHC combines high compression ratio (CR), constant-volume (isochoric) combustion, and overexpansion, and 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%. The cycle elements include:
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.
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.
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.
Alexander Shkolnik, Daniele Littera, Mark Nickerson, and Nikolay Shkolnik et al. (2014) “Development of a Small Rotary SI/CI Combustion Engine”, SAE Technical Paper 2014-32-0104 doi: 10.4271/2014-32-0104