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.

Resources

• 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