Hot on the heels of our coverage of Mazda’s all-new Wankel engine, comes a different bit of rotary engine news.

Apparently, the U.S. Pentagon-backed engineering firm, LiquidPiston, plans to commercialize its XTS-210 unit and then install it into a ton of applications. This 25-horsepower, two-stroke, supercharged, liquid-cooled, 210cc rotary engine is pretty remarkable too, as it basically turns the traditional Wankel design inside-out.

This news follows the signing of a $9 million contract with the U.S. Army in late 2022, which landed the compact power systems manufacturer the opportunity to develop a prototype around the XTS-210 architecture for military applications.

By utilizing an engine that is nearly 80-percent smaller and lighter than the diesel piston engines currently powering its military vehicles, the U.S. Army aims to make one of the most “revolutionary” moves in engine implementation since the shift to fuel injection.

But in order to test (and implement) the XTS-210 on a broad scale, all while retaining the same power output as before, some seriously tricky engineering and ingenuity had to be brought forth. Which come to think of it, is really what LiquidPiston is all about in the first place.

A Closer Look at LiquidPiston

After inventing the High Efficiency Hybrid Cycle (HEHC) thermodynamic cycle, along with an engine design to encapsulate this form of cycling in 2003, Dr. Nikolay Shkolnik and Dr. Alexander Shkolnik went on to form LiquidPiston, Inc.

Two decades later, the MIT-award-winning technology startup now has a 12,000 square-foot facility in Bloomfield, CT, where it develops and tests its engines. The firm produces pretty much everything in-house, with CNC machining and assembly capabilities being utilized once all of the software engineering and modeling engine system configurations are complete.

Following nearly two decades of breakthrough thermodynamics research with various types of rotary diesel engines, we are excited to converge on this latest generation as our first X-Engine model that we plan to bring to market. — Dr. Alec Shkolnik, co-founder and CEO of LiquidPiston

And while the company’s patented HEHC design is still a core part of the company’s architecture, fuel-energy conversion solutions for hybrid power systems, including for UAS, mobile power generation, and APU applications are all on the table nowadays.

Furthermore, this brand has become hell-bent on becoming the global leader in scalable combustion engine development, which leads us back to the U.S. military, and its acute interest in the XTS-210 prototype.

XTS-210 engine core compared to 25-horsepower Kohler KDW1003. Not shown on the XTS (as the engine is still in its preliminary “Alpha” development stage) are ancillary systems such as fuel pumps, water pumps, and oil systems.

Whatever You Do, Don’t Call it a Wankel

In a press statement back in April of this year, LiquidPiston claimed that the XTS-210 engine “…addresses the fuel efficiency, lubrication, and fuel type limitations of the traditional Wankel rotary engine…”

Do you hear that sound? That’s the dirge of a million Mazda fanboys simultaneously screeching in outrage, as they angrily hammer hastily written hate mail to yours truly on their keyboards.

But here us out all ye “apex predators,” because unlike old Felix Wankel’s design, the X-Engine promises a far simpler solution to producing power. As opposed to the Wankel design, there are only two primary moving parts within LiquidPiston’s design, with a rotor and a shaft playing the parts of the dynamic duo. It’s also a high-compression rock star, and excels in the efficiency department, something that the Wankel rotary has long struggled with in stock form.

The XTS-210 is 100% heavy-fuel compatible (diesel, Jet A, JP-8, kerosene) and multi-fuel capable (gasoline, propane, hydrogen) and is further optimized for hybrid-electric applications.

As for performance, the 25-horsepower XTS-210 relies upon one bar (14.5 psi) of boost via supercharging, which can be adjusted and scaled all the way up to 1,000 horsepower, if so desired. This design follows a simple two-stroke pattern, allowing it to produce six combustion events per each revolution of the rotor. This results in what is marketed as “…smooth power from a lightweight package, roughly the size of a basketball.”

To date, LiquidPiston has received over $30 million in Department of Defense contracts, with the most recent being a $9 million development contract for the U.S. Army to develop a prototype “core engine” based on the XTS-210 design for military applications.

Why the U.S. Military is Backing LiquidPiston

Returning to the U.S. military’s invested interest in this “sports-ball” sized, yet fully scalable platform, we encounter the four major market requirements that the XTS-210 had to address in order to be labeled a viable power solution.

• Portable & Configurable: Can operate in both engine-only and hybrid-electric modes, and is capable of running up to 7,000 rpm. (Ideal for small/lightweight electrical machines, as it enables increased mobility for power generation and hybrid applications.)Produces five times the power-to-weight (specific power) and power-to-volume (power density), and up to three times the torque-to-weight (specific torque) compared to traditional diesel piston engines of similar power.
• Spark-Ignition (SI) or Compression-Ignition (CI) Capable: Designed with military-grade robustness in mind from inception, this design can be configured to rely on either compression or spark for ignition.
• Liquid-Cooled: Because air-cooled Humvees are a recipe for disaster.
• Multi-Fuel Ready: Focuses on JP-8/Jet-A fuels to simultaneously provide coverage for both ground defense and aerospace applications.

The XTS-210 rotary engine is the size of a basketball, and it reduces size and weight by nearly 80 percent, compared to current diesel piston engines with comparable power output.

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. 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. — Dr. Alec Shkolnik

LiquidPiston Eyes Providing Power to the Military Masses

LiquidPiston’s plan is to not just turn its XTS-210 model into an entire line of auxiliary power units for land, air, and marine vehicles. It wants to make this rotary engine the most reliable, viable source of power on the planet. Something so robust and easy to use, that commercial and military heavy-fuel applications can utilize the engine without issue.

From there, its goal is to formulate mobile electric power generation via the use of “primary or hybrid-electric propulsive power for vertical take-off and landing aircraft (VTOL) and small unmanned aircraft systems (sUAS).”

On a more short-term narrative, LiquidPiston has targeted the delivery of an XTS-210 prototype to the U.S. Army sometime in 2024. The company has previously prototyped several variants of rotary X-Engines for the military, but out of every option the engineering firm has produced, this supercharged, basketball-sized variant appears to be the most promising.