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LiquidPiston receives $3M Rapid Innovation Fund award from US Army for 2kW hybrid-electric genset

LiquidPiston, Inc. (LPI), a developer of advanced multi-fuel-capable rotary combustion engine technology, has been awarded a $3-million Rapid Innovation Fund (RIF) award from the US Army to develop an innovative ultra-portable 2kW diesel Compact Artillery Power System (CAPS). The RIF process is extremely selective, with only five percent of whitepapers ultimately being selected for the award.

CAPS is a compact, lightweight, quiet, low-vibration and efficient hybrid-electric diesel generator set capable of supplying up to 2kW of electric power while running on Jet Propellant 8 (JP8) or diesel fuel. The CAPS Genset prototype objectives include less than 30 pounds (13.6 kg) (dry weight), 1.5 ft3 (bounding volume), and less than 60db at 7 meters. This is a 75% reduction in generator weight compared to the MEP-501A, the current 2kW JP8 generator in use today, which weighs 124 pounds (56.3 kg).

This marks the first hybrid application of LPI’s engine technology.

The project team is led by LiquidPiston and also includes HDT Expeditionary Systems (HDT), a leading supplier of expeditionary military equipment including generators sets; and General Atomics (GA), a leading developer of military technology including battery and energy systems.

The CAPS hybrid electric 2kW diesel generator integrates a Lithium-ion Fault Tolerant (LiFT) battery technology developed by GA, which safely increases the CAPS’ power delivery quality, provides a “silent watch” capability, and optimizes the usage and life of the combustion engine. The 2kW CAPS generator, either as a standalone genset or as a hybrid electric power solution, is a capable solution for powering various military as well as non- military applications.

To be demonstrated initially to power the Howitzer M777 Digital Fire Control System, the new CAPS will address the Army’s need to extend mission duration, reduce fuel consumption, and reduce noise and heat signatures in powering digitized towed howitzers.

This marks a shift from research towards a product and a first real commercial application. We also are happy to be working with partners. Developing the technology is one thing, developing the business unfortantely is an almost equal aspect. You can’t develop the technology without developing the business. It’s really hard to start an engine company from scratch. Our model is to work with partners. We bring technology development and innovations. What we find in partners like HFT and General Atomics is the proven capabuility to deliver systems to end users, sales, manufacturing and distribution. We leverage leverage each others’ strengths.

—Dr. Alexander Shkolnik, Founder, CEO LiquidPiston

LiquidPiston develops compact, quiet, fuel-efficient, low-vibration, multi-fuel- capable rotary combustion engines that are scalable from 1hp to more than 1,000hp based on the company’s patented High Efficiency Hybrid thermodynamic Cycle (HEHC) and rotary X engine architecture. (Earlier post.) The company has been looking at a variety of engine architectures to embody HEHC, Shkolnik said, and has patented dozens of types of engines that look different.

HEHC is a patented thermodynamic cycle that combines the advantages of Diesel, Otto and Atkinson thermodynamic cycles. 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.


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.

LiquidPiston previously developed the “X Mini” prototype, a 2.2kW, 70cc SI engine operating LiquidPiston’s HEHC, which was demonstrated in a go-kart. (Earlier post.)

XMv3 engine gas path flows. Green is the intake charge, red is the exhaust, and blue is the cooling flow. The blue outlines represent the outer engine shroud, which separates the “stationary part” and “rotor cooling” flow paths at the orange dashed lines. Shown in red, three triangular openings on each side of the engine allow the passage of rotor cooling air and exhaust flows. Costa et al. (2016) Click to enlarge.

LiquidPiston was also recently awarded a contract by the US Defense Advanced Research Projects Agency (DARPA) to develop an ultra-efficient and compact 30kW diesel rotary X engine that fits in a ten inch box and weighs just 30 pounds, with fuel efficiency better than a heavy duty truck engine. (Earlier post.)


  • Costa, T., Nickerson, M., Littera, D., Martins, J. et al., (2016) “Measurement and Prediction of Heat Transfer Losses on the XMv3 Rotary Engine,” SAE Int. J. Engines 9(4):2368-2380 doi: 10.4271/2016-32-0033.



While a 2Kw generator is interesting for a digital fire control system, these are not very common, so a 20 - 30 Kw one would be of more use as a range extender for EVs.
It would be especially useful if it could use a low evaporation fuel (like diesel?)


They can run on several different fuels, the first one was over 30 kW.


On the forefront of power is the generation of Graphene a single carbon based element. Can be used for numerous applications and to power this 2KW power system. please look into graphene you will be very suprised


Too good to be true.

Brian P

This suffers from quite a few of the same bad design aspects that a Wankel does, plus a few new ones (even though it isn't a Wankel). Rotor sealing, and tip seal lubrication, will be issues just like they are with a Wankel. The main combustion chamber to rotor seals have combustion spaces on both sides (unlike in a piston engine, where the rings have combustion space on one side and lubrication on the other side). And, as with a Wankel, the combustion chamber is a horrible shape with a lousy surface-area-to-volume relationship (think: lots of heat transfer, i.e. poor efficiency). Having the induction and exhaust go through the rotor means even that space will have limited lubrication ... and a lot of thermal loading.

Too good to be true, indeed.

Josefina Madeira Neto

O pistão líquido realmente merece essa recompensa de três milhões de dólares pela fabricação de tecnologia de nível avançado de motor de combustão rotativa com capacidade para vários combustíveis. Visito a fonte para preparar meus documentos a tempo. Essa tecnologia pode trazer um grande avanço no campo respeitado.

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