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DARPA awards additional $2.5M to LiquidPiston for development of 30kW X4 rotary diesel engine prototype

DARPA awards additional $2.5M to LiquidPiston for development of 30kW X4 rotary diesel engine prototype

The US Defense Advanced Research Projects Agency (DARPA) has awarded LiquidPiston Inc., an advanced internal combustion engine technology company, an additional $2.5 million to continue development of its 30kW X4 rotary diesel engine prototype, bringing DARPA’s total funding of the engine technology to $6M. (Earlier post.)

LiquidPiston received this award after meeting the objectives for Phase I of the program, which had focused on the clean sheet design of the X4, and demonstrating the structural integrity of the new engine platform while operating under compression ignition of diesel fuel. LiquidPiston engineers presented a development update on the engine at the SAE International WCX World Congress Experience in Detroit in April this year.

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.

LiquidPiston’s X engine (right) essentially inverts the Wankel engine (left). While a Wankel has a 3-sided rotor and a 2-lobed housing, the X engine has a 2-lobed rotor in a 3-sided housing. The X engine captures the main advantages of the Wankel (high power-to-weight ratio; simplicity; and inherent balance), but also addresses the design deficiencies of the older engine.

Because the combustion chamber is located in the stationary housing with most of the gas displaced during compression into this chamber, the X is uniquely suitable for high compression ratio operation with direct injection and compression ignition. The combustion chamber can take any geometry and be optimized for surface-to-volume ratio.

The apex seals of the X are located within the stationary housing. Because they do not move with the rotor, the seals do not experience centrifugal forces. Lubrication is simpler, with oil consumption lower.

The unique sealing geometry also has 3-5 times less blowby than the Wankel. Nickerson et al. (2018) Click to enlarge.

The objectives for the $2.5-million phase II of the program are demonstrating 30kW of power and reaching 45% net indicated fuel efficiency from the .75L X4 prototype. Development will be executed at LiquidPiston’s dynamometer & engineering test facility in Connecticut.

Phase II also lays a foundation for future work. When development of the fully packaged engine is complete, the 30kW X4 engine is expected to weigh just 30 lbs (13.6 kg) and fit into a 10"x10"x10" box, while achieving 45% brake thermal efficiency—approximately an order of magnitude smaller and lighter than traditional piston diesel engines, and also 30% more efficient.

The efficient, lightweight, and powerful rotary Diesel/JP-8 X4 engine offers a disruptive power solution for direct as well as hybrid electric propulsion and power generation.

Exceeding DARPA’s objectives for the first phase of the program validates the potential for an entirely new category of military-grade, rotary diesel engines. In combustion testing, the X4 prototype handled peak cylinder pressures reaching 150 bar at a compression ratio of 26:1. This is the first time that a rotary engine has shown the capability to achieve true compression ignition at this ratio in a single stage of compression and without any supercharging.

We’re excited to continue working with DARPA in demonstrating the power and efficiency capability of the X4, while identifying transition pathways for the technology within specific branches of the armed forces.

Our novel cycle and rotary engine architecture promise dramatic performance improvements while also reducing engine heat signature and minimizing vibration impact on intelligence, surveillance, and reconnaissance equipment.

—Alexander Shkolnik, CEO and Founder of LiquidPiston

LiquidPiston is in discussions with development partners considering potential applications in range-extended electric vehicles and drones in both the military and commercial sectors. Bringing the efficiency of a large diesel engine to markets typically served by small gasoline engines could nearly double the fuel economy or range of these types of vehicles.


  • Nickerson, M., Kopache, A., Shkolnik, A., Becker, K. et al. (2018) “Preliminary Development of a 30 kW Heavy Fueled Compression Ignition Rotary ‘X’ Engine with Target 45% Brake Thermal Efficiency,” SAE Technical Paper 2018-01-0885 doi: 10.4271/2018-01-0885

  • Leboeuf, M., Dufault, J., Nickerson, M., Becker, K. et al. (2018) “Performance of a Low-Blowby Sealing System for a High Efficiency Rotary Engine,” SAE Technical Paper 2018-01-0372 doi: 10.4271/2018-01-0372



Seems to me if you need a transportable fuel type engine for military use, you would look at hydrogen fuel cells with electric drive, not at obsolete ICEs.

Nick Lyons

@Lad: Seems to me that diesel/JP8 is a lot more practical: energy dense and transportable--no high pressure or cryogenic liquefaction required. It also does not require any new infrastructure.


At least better than a Wankel engine, which is not well-suited to the high-efficiency diesel cycle. RR once worked on a diesel version of a 2-stage Wankel engine to get high compression ratio but it never went into production and I presume it was plagued with various technical issues.

It should be noted that they refer to "net indicated efficiency" once and "brake thermal efficiency" the second time. Assuming that they would get 45% net indicated efficiency, this roughly equals 40% brake thermal efficiency (depending on what you assume about pumping losses and friction, of course...). This would be lower efficiency than a passenger car diesel engine (~43%) but, on the other hand, this engine is smaller (e.g. ~100 kW for a passenger car engine), which always comes with a penalty in efficiency. Nevertheless, it would be much more efficient than a corresponding gasoline engine of this size. And, as Nick indicated, diesel and/or jet fuel is always available when the military is present.

Account Deleted

JP8 or drop in equivalent will probably remain the military fuel for some time. It used in all applications, i.e. air, land, and sea.
The Liquid Piston X4 rotary diesel engine would be an excellent UAV engine since it is light and more efficient than gas turbines.
For ground applications the Achates Power two-stroke opposed piston diesel looks like a winner. Already developed as the Advanced Combat Engine (ACE) by Cummins with up tp 1500hp can power all military vehicles even replacing the gas turbine in the M1 tank which is very inefficient. A similar diesel engine is in the Ukrainian T-84 tank (the KMDB-6) replaced the gas turbine in the T-80 tank (both the Achates Power and KMDB engine are based on the Junkers Jumo aircraft diesel).


Reform JP8 for HTPEMs.


Reform JP8 for HTPEMs.

Account Deleted

The military tried reformed JP8 over 10 years ago. They found that high sulfur content quickly chokes fuel cells. While JP8 refined in the United States may have a sulfur content of 15 parts per million, the military buys its fuel regionally. In the first Gulf War, the military bought fuel from Saudi Arabia with a sulfur content of 3,000 parts per million ratio.


They can remove sulfur, it has been done for a long time.
Excuses are just that.

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