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U.S. Army to field test basketball-size 25-hp inverse-Wankel engines for power generation

The LiquidPiston 10-kW Genset, incorporating the XTS-210 X-Engine, is capable of replacing multiple existing generator power classes with a single genset platform, greatly simplifying Army genset procurement and logistics. [Credit: LiquidPiston]





In a bid to revolutionize and downsize battlefield power generation, the U.S. Army is investing over $8 million to try out LiquidPiston's basketball-size experimental inverse-Wankel rotary engine integrated into a compact 10-kW generator set that is 75% percent lighter and smaller than currently fielded Advanced Medium Mobile Power Sources.

LiquidPiston's award supports an existing $14.9 million Other Transaction Award (OTA) prime contract for U.S. Army Expeditionary Intelligent Tactical and Expeditionary Power that was awarded to Polaris Alpha Advanced Systems, a Parsons Company, by the Department of Defense Ordnance Technology Consortium (DOTC) in 2022.

LiquidPiston X-Engine prototypes are claimed to deliver up to 5x the power density of traditional piston diesel engines. [Credit: LiquidPiston]



LiquidPistons' recently launched, high-performance rotary X-Engine, called the XTS-210, will be the generator unit's power plant for this heavy-fuel application. It is the company's latest-generation X-Engine platform architecture. The 25-hp X-Engine is inherently simple in design, with just two primary moving parts: a rotor and shaft. It is also much quieter and exhibits very little vibration. The XTS-210 engine addresses the fuel efficiency, lubrication, and fuel-type limitations of the traditional Wankel rotary engine, an influential engine design developed in the 1950s.

According to LiquidPiston, "Current military mobile generators are too heavy and inefficient for modern use cases, which also makes them a potential hazard; during the height of the combat in Iraq and Afghanistan, approximately one-half of American casualties were sustained while moving fuel to the front [Forbes, 2009]. With the power density of LiquidPiston's 10-kW genset, nearly 4x the power could be deployed to the battlefront for the same transport volume as currently-fielded gensets; this would reduce the frequency of refueling and therefore the risk to the troops."

The LiquidPiston XTS-210 engine is the size of a basketball. [Credit: LiquidPiston]



The XTS-210 adds up to one bar of boost through supercharging and operates as a two-stroke, producing six combustion events per revolution of the rotor, to deliver smooth power from a lightweight package, roughly the size of a basketball.

The LiquidPiston generator set (genset) will be approximately one-quarter the size and weight of the currently fielded U.S. Army Advanced Medium Mobile Power Source (AMMPS) generator system. Given its smaller footprint and inverter-type operation, LiquidPiston's new genset could replace multiple existing generator power classes with a single genset platform, greatly simplifying military genset procurement and logistics.

The XTS-210 engine has just two primary moving parts: a rotor and shaft. [Credit: LiquidPiston]





The very beginnings of LiquidPiston and its innovative engine trace back all the way to 2003, when it was a "garage" project. To date, the company holds more than 70 patents for the technology.

Designfax has been following LiquidPiston since 2014 when it was a startup at MIT. Back then, MIT described the novel engine as running "a high-efficiency hybrid cycle (HEHC)" -- developed by Alexander (Alec) Shkolnik (who holds a PhD from MIT) and his physicist father, Nikolay -- "that achieves combustion at constant volume and overexpansion for greater energy extraction. With only two moving parts, a rotor and shaft, and no poppet valves ... the engine also has reduced noise, vibration, and harshness characteristics."

In those early days, a prototype 3.5-hp version -- about as tall as an iPhone 6 -- was considered a great alternative to the small piston-based internal combustion engines used in lawn and garden equipment, particularly because it was so much smaller, lighter, more efficient, quiet, and did not vibrate much.

LiquidPiston X Engine. from LiquidPiston on Vimeo.

In the traditional Wankel engine design, a rounded triangular rotor spins in an eccentric orbit within an oval chamber, with each rotation producing three power strokes -- where the engine generates force. In the LiquidPiston X engine designs, an oval rotor spins within a modified, rounded triangular housing.

LiquidPiston - Intro from LiquidPiston on Vimeo.

According to a LiquidPiston company brochure on the unique engine design, "The X-Engine [development platform] can be likened to an 'inverted' Wankel rotary design, having a trochoidal rotor and three-lobed housing. It leverages a stationary combustion chamber, which enables a higher compression ratio with direct injection of fuel, leading to better efficiency and optimized fuel air mixing. Additionally, stationary apex seals reduce blow-by and offer direct lubrication, improving efficiency, emissions, and durability. The X-Engine has few moving parts and either three or six combustion events per rotor revolution, resulting in very high power density. Except for ancillary parts, there are only two primary moving parts -- the shaft and the rotor -- making the X-Engine simple and elegant in its implementation."

Overall, LiquidPiston says its engines are scalable from one to over 1,000 hp and capable of utilizing efficient fossil or renewable fuel.

LiquidPiston says the XTS-210 by itself uniquely addresses four major market requirements to solve many applictions:

  1. 5x the power-to-weight (specific power) and power-to-volume (power density), and up to 3x the torque-to-weight (specific torque) compared with diesel piston engines of similar power.
  2. Ultra-portable and configurable for operation in both engine-only and hybrid-electric modes -- capable of running up to 7,000 RPM, which matches well with small and lightweight electrical machines, enabling increased mobility for power generation and hybrid applications.
  3. Liquid-cooled and capable of either spark ignition (SI) or compression-ignition (CI) -- designed for military-grade robustness from its inception.
  4. Multi-fuel capability: Initial testing focuses on JP-8/Jet-A fuels for defense and aerospace applications.

"There are essentially no diesel engines in the 25-horsepower power class today that are suitable for aerospace and mobile military applications, where size and weight parameters are especially critical," said Dr. Alec Shkolnik, co-founder and CEO of LiquidPiston. "The reduced weight, size, and heavy- or multi-fuel capability of the XTS-210 delivers significant end-system capability and utility benefits, especially for the military to reduce supply chain and logistical burdens in an era where 'power on the move' is increasingly important."

The 25-hp XTS-210 engine core (left) compared to a 25-hp Kohler KDW1003. Not shown on the XTS (as the engine is presented here in its preliminary "Alpha" development stage) -- are ancillary systems such as fuel pump, water pump, and oil system. [Credit: LiquidPiston]





The company has previously prototyped several variants of rotary X-Engines, demonstrating significant versatility in engine architecture, including naturally aspirated four-stroke versions ranging from 5 to 40 hp.

In addition to a recent $9 million award to contribute to the development of the XTS-210, LiquidPiston has also received a $1.7 million Army contract to power a hybrid-electric VTOL UAV demonstrator.

In October 2023, the company also announced a $35 million contract to support hybrid power system development by AFWERX, a Technology Directorate within the Air Force Research Laboratory (AFRL), in partnership with AFLCMC, the U.S. Air Force Life Cycle Management Center. LiquidPiston will design, develop, and qualify a 90-hp (or higher) heavy-fueled rotary engine, which will be incorporated into a modular Rotary Engine Hybrid Power System. This engine and platform can serve as the "power pack" for a variety of applications of interest to the Air Force, including unmanned aerial systems (UAS) propulsion, human- and cargo-carrying Organic Resupply Bus (ORB) capabilities, mobile operating bases, and vehicle auxiliary power units (APUs).

To date, the company's Department of Defense contracts total well over $70 million.

Source: LiquidPiston, Forbes (LiquidPiston referenced)

Published November 2023

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