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While many of LiquidPiston’s prototypes are compression ignition, the 5-hp X-Mini is a spark ignition engine fit for most Group 2 (21 lbs. to 55 lbs) UAVs. At 6.6 in by 6.2 in by 5.4 in, the X-Mini is the company’s smallest design (Image source: LiquidPiston).

X marks the spot

Smaller, powerful X engine promises increased UAV flight endurance

LiquidPiston Inc., an advanced internal combustion engine (ICE) technology company, has reimagined combustion engines with a new platform that can run on multiple fuels, including diesel, jet fuel, and gasoline. This platform uses an optimized thermodynamic cycle and a new rotary engine architecture. This architecture involves only two primary moving parts, a shaft and a rotor, and corrects the major performance, sealing, and lubrication issues of previous rotary engine designs such as the Wankel.

The lightweight rotary X engine can achieve compression ignition in a single stage at a weight of just 1/10th of typical diesel-fueled, piston engines. In terms of unmanned aerial vehicles (UAVs), LiquidPiston’s fuel-efficient “X engine” could increases flight endurance over conventional UAV engines by greater than 50%.

LiquidPiston’s X engine development began 13 years ago with an idea for a new thermodynamic cycle. This cycle was designated the high-efficiency hybrid cycle (HEHC). The HEHC combines the advantages of the Diesel, Otto, and Atkinson thermodynamic cycles. To achieve these advantages, the engine architecture utilizes a high compression ratio, constant volume combustion, and an expansion volume that substantially exceeds the compression volume. The resulting power-to-weight and fuel efficiency advantages enable improved UAV payload capabilities and increased mission flight duration.

As a result, the X engine design significantly reduces vibration, noise, and heat output. The reduced vibration and noise minimizes engine impact on the airframe and intelligence, surveillance, and reconnaissance (ISR) equipment.

The design also reduces UAV heat and acoustic signatures, because of its lack of poppet valves, piston slap, and has a lower pressure exhaust. The thermal signature is further reduced by the over-expansion process.

LiquidPiston has developed a range of advanced ICE prototypes, initially focusing on small engines from 5 hp to 40 hp. While some of the larger prototypes have been compression ignition, the 5-hp X-Mini is a spark ignition engine. The X-Mini is a fit for most Group 2 UAVs, while the larger 40-hp X4 engine is a fit for most Group 3 UAVs. (A Group 2 UAV has a maximum weight in the range of 21 lbs. to 55 lbs. whereas a Group 3 UAV may weigh up to 1320 lbs.)

“As the UAV space grows into a $50 billion-plus market by the mid-2020s, we believe LiquidPiston’s technology will help UAV manufacturers and operators reimagine unmanned flight possibilities” said Alexander Shkolnik, CEO and Founder of LiquidPiston.

The company is working with the Defense Advanced Research Projects Agency (DARPA) and is in discussions with a number of potential partners, toward the development of a 30kW propulsion system that could be used for a UAV. This 30kW platform is projected to demonstrate up to 45% brake thermal efficiency.

In addition to propelling UAVs, the X engine design holds promise for many other applications where a high power-to-weight ratio and excellent fuel efficiency are particularly advantageous. The company is also interested in the prospects involved with vehicle electrification.

“The 30kW platform we are developing for DARPA could be perfect as a range extending platform for electric cars,” said Shkolnik. In regard to manufacturing costs, Wankel engines historically have been comparably priced to piston engines. With the use of conventional high-volume production processes, LiquidPiston anticipates that their engines will be significantly less expensive than traditional piston engines.

The company, based in Bloomfield, CT, presented for Starburst Accelerator’s Selection Committee in Israel on Wednesday, March 7th. The company’s appearance at Starburst’s event comes at a moment of growing interest in LiquidPiston’s propulsion solution by UAV manufacturers and operators. In November 2018, the company took home the top prize at the Association for Unmanned Vehicle Systems International’s (AUVSI’s) NOVUS Unmanned competition in San Francisco.

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