How It Works : LiquidPiston X Engine
Jun 14, 2016

How a 4-Pound Engine Can Replace a 40-Pound Engine
by Avery Thompson

Engine startup LiquidPiston has radically shrunk a go-kart engine, but it's the military applications that has everyone really excited.

Connecticut-based startup LiquidPiston announced today that they have built a small, compact engine that is powerful enough to drive a go-kart. Their X-mini engine weighs just 4 pounds and has three moving parts, and yet can produce 3 horsepower, enough to replace the default 40-pound piston engine that normally powers the go-kart.

LiquidPiston says that their X-mini is still in a testing phase, and they hope to get the weight down to 3 pounds and the power up to 5 hp. (The 40-pound piston engine produces about 6.5 hp.) In the meantime, their little engine already packs a punch, as you can see in the video below.

LiquidPiston announced last year that they received a $1 million DARPA grant to develop their X-mini engine, and it appears that they've succeeded. The 4-pound, 3-hp engine is small enough to fit in the palm of your hand, yet it can power everything from vehicles to generators to drones. The X-mini uses LiquidPiston's proprietary rotary engine design and thermodynamic cycle which offer vast improvements over both a traditional Wankel rotary engine and common piston engines.

LiquidPiston says the X-mini can run on Jet Propellant 8, the military's fuel of choice, making it an ideal candidate for all sorts of military applications. For instance, the X-mini is small and light enough to power a UAV, it can be part of a generator that can be carried in a backpack, or it can even be used to power military robotics. According to Alec Shkolnik, LiquidPiston's co-founder and president, "[DARPA] is kinda agnostic as to the actual application … they have so many different applications that need power."

Shkolnik said that the X-mini is still early in testing, and they have only just built their first working prototype, but he's hopeful that the engine could see a commercial release sometime in the next few years. When that happens, the X-mini could find its way into lawnmowers, emergency generators, and even small vehicles like mopeds.

Oct 17, 2012

LiquidPiston's Hyper-Efficient Engine: Turning the Rotary Inside Out

A new spin on the internal combustion engine by startup LiquidPiston aims for a leap in efficiency from 20 percent to 50 percent.


Ben Wojdyla

As automakers augment the reciprocating piston engine with hybrid systems and improved accessories, independent inventors are busily working to make huge improvements to the basic efficiency of the internal combustion engine. Novel designs are popping up at engineering expos everywhere, and the newest comes from Bloomfield, Conn.-based LiquidPiston. Its X1 engine is a simple machine with just three moving parts and thirteen major components, but it aims to raise thermal efficiency from the 20 percent of a normal gas engine to more than 50 percent, with drastic reductions in weight and size. How? By wasting much less energy during the course of an combustion cycle.

Up to 80 percent of the energy in fossil fuels is thrown away normal engines through the heat and pressure of exhaust, or dumped to the atmosphere through the radiator. LiquidPiston's design attempt to capture all of that waste within a tiny package. "We stretched the performance curves in every direction to get much higher efficiency," said Alec Shkolnik, President and CEO of LiquidPiston, "We took the best parts of many different thermal cycles and combined them." The design is theoretically capable of 75 percent thermal efficiency, but the group is targeting 57 percent in real world applications, still a huge jump.

The basic idea is similar to a Wankel rotary, but turned on its head. Where the rotor holds the seals in a normal Wankel, the housing does that job in the X1 engine. This allows significant reduction in oil consumption over a regular rotary motor. Other enhancements include direct injection, a high compression ratio at 18:1, and a dramatic change to the geometry of the combustion chamber, which maintains a constant volume during ignition. This change means the air-fuel mixture auto-ignites like a diesel, and can be burned much longer than normal. The result is a more complete combustion ending in low emissions and very high chamber pressures. This high pressure is allowed to act on the rotor until it reaches nearly atmospheric pressures, so almost all the available energy is extracted before the exhaust is physically pushed out. Again, this is different than a normal internal combustion engine, which releases very energetic, high-pressure exhaust gas.

Some other slick features: Since the engine is designed to convert so much more heat energy into mechanical force, less heat has to be removed from the block, so there's actually no water cooling system. In cases where the engine is under load and needs to cool down, it can skip an fuel injection event and just suck in cool air, which is then heated by the block and gets exhausted. Another option is to inject water into the combustion chamber. This has three effects: cooling the engine, reducing NOx emissions, and converting some of the water to steam, which increases power.

The compact design of LiquidPiston's lab engine currently tips the scales at 80 lbs for the 40-hp model. It would weigh less than 50 lbs in production, the company claims, far less than a comparable 40-hp diesel that would tip the scale at around 400 lbs. LiquidPiston's current aim is to continue developing the engine with an eye on the sub-100 hp market — compressors, hybrid range-extenders, military applications, boat engines — and license the intellectual property to manufacturing customers. We love seeing plucky inventors like these to completely rethinking the gasoline engine.

LiquidPiston develops advanced rotary engines based on the company’s patented thermodynamic cycle and engine architecture.

LiquidPiston, Inc.
1292a Blue Hills Avenue
Bloomfield, CT 06002
Phone: (860) 838-2677
Paper No. ICEF2005-1221, pp. 835-845; 11 pages
ASME 2006 Internal Combustion Engine Division Spring Technical Conference (ICES2006); Aachen, Germany, May 7–10, 2006

High Efficiency Hybrid Cycle Engine

Nikolay Shkolnik and Alexander C. Shkolnik


A “High Efficiency Hybrid Cycle” (HEHC) thermodynamic cycle is explored. This four-stroke cycle borrows elements from Otto, Diesel, Atkinson, and Rankine cycles. Air is compressed into an isolated combustion chamber, allowing for true isochoric combustion, and extended duration for combustion to proceed until completion. Combustion products expand into a chamber with greater volume than intake. We provide details of a compact HEHC design implementation using rotary pistons and isolated rotating combustion chambers. Two Pistons simultaneously rotate and reciprocate and are held in position by two roller bearings. One Piston performs intake and compression, while the other performs exhaust and expansion. We predict a reduction of energy losses, moving part counts, weight and size over conventional engines.
Paper #: 2008-01-2448

Rotary High Efficiency Hybrid Cycle Engine


In this paper we discuss a rotary implementation of the High Efficiency Hybrid Cycle (HEHC) engine. HEHC is a thermodynamic cycle which borrows elements of Diesel, Otto and Atkinson cycles, characterized by 1) compression of air only (e.g. Diesel), 2) constant volume heat addition (e.g. Otto), and 3) expansion to atmospheric pressure (e.g. Atkinson). The engine consists of a compressor, an isolated combustion chamber, and an expander. Both compressor and expander consist of a simple design with two main parts: a rotor and an oscillating rocker. Compared to conventional internal combustion engines, in which all processes happen within the same space but at different times, in this engine, all processes are occurring simultaneously but in different chambers, allowing for independent optimization of each process. The result is an engine which may offer up to 57% peak efficiency, and above 50% sustained efficiency across typical driving loads.

Development of a Small Rotary SI/CI Combustion Engine

Alexander Shkolnik, Daniele Littera, Mark Nickerson, and Nikolay Shkolnik et al., SAE Technical Paper 2014-32-0104, 2014, doi:10.4271/2014-32-0104.

This paper describes the development of small rotary internal combustion engines developed to operate on the High Efficiency Hybrid Cycle (HEHC). The cycle, which combines high compression ratio (CR), constant-volume (isochoric) combustion, and overexpansion, has a theoretical efficiency of 75% using air-standard assumptions and first-law analysis. This innovative rotary engine architecture shows a potential indicated efficiency of 60% and brake efficiency of >50%. As this engine does not have poppet valves and the gas is fully expanded before the exhaust stroke starts, the engine has potential to be quiet. Similar to the Wankel rotary engine, the ‘X’ engine has only two primary moving parts – a shaft and rotor, resulting in compact size and offering low-vibration operation. Unlike the Wankel, however, the X engine is uniquely configured to adopt the HEHC cycle and its associated efficiency and low-noise benefits. The result is an engine which is compact, lightweight, low-vibration, quiet, and fuel-efficient.

Two prototype engines are discussed. The first engine is the larger X1 engine (70hp), which operates on the HEHC with compression-ignition (CI) of diesel fuel. A second engine, the XMv3, is a scaled down X engine (70cc / 3HP) which operates with spark-ignition (SI) of gasoline fuel. Scaling down the engine presented unique challenges, but many of the important features of the X engine and HEHC cycle were captured. Preliminary experimental results including firing analysis are presented for both engines. Further tuning and optimization is currently underway to fully exploit the advantages of HEHC with the X architecture engines.



Methods, devices and systems for power generation through liquid piston internal combustion engine. The liquid piston internal combustion engine of the invention, utilizes a novel, synergetic combination of internal combustion and steam piston engines within the framework of one and the same system. The engine may comprise or a plurality of cylinders, each having a liquid piston. The ICE (Internal Combustion Engine) system comprises six modules viz PGM (Power Generating Module), ERS (Energy Recovery System), PCM (Power Conversion Module), HAS (Hydraulic Shock Absorbers Module), DAC (Data Acquisition & Control Module) and AEM (Auxiliary Equipment Module).

Seal Assembly for a Heat Engine

A seal assembly includes first and second seal elements configured to lie adjacent to one another with a lower portion of each one disposed in a groove and an upper portion of each one projecting above the groove. The groove has a length disposed transverse to the direction of relative motion of a housing and a moving member and is located in the housing. The seal elements are further configured so that a contact surface of the upper portion of each seal element abuts the moving member and configured to allow independent movement of each seal element relative to each other in a direction transverse to the groove length. The seal elements are shaped to define a lubrication channel therebetween that is configured to allow the passage of a lubricant therein so as to lubricate motion of the seal elements relative to each other and relative to the moving member.

Isochoric Heat Addition Engines and Methods

Engines and methods execute a high efficiency hybrid cycle, which is implemented in a volume within an engine. The cycle includes isochoric heat addition and over-expansion of the volume within the engine, wherein the volume is reduced in a compression portion of the cycle from a first quantity to a second quantity, the volume is held substantially constant at the second quantity during a heat addition portion of the cycle, and the volume is increased in an expansion portion of the cycle to a third quantity, the third quantity being larger than the first quantity.

Internal Combustion Engine and Components Therefor

A rotary internal combustion engine includes crank-driven gates to synchronously form chambers for the intake, compression, combustion, expansion and exhaust of a working medium during a high-efficiency hybrid engine cycle. A variety of rotor geometries and sealing apparatuses may work with a rotary engines in the execution of various engine cycles including, but not limited to, a high-efficiency hybrid engine cycle.

Cycloid Rotor Engine

A rotary engine has a cycloid rotor and a sealing grid including a face seal that rotates with the rotor, and including other seals that do not rotate with the rotor. As the rotor rotates within a housing, the rotor, housing and seal grid form at least one working chamber between them, the chamber undergoing a change from initial volume V1 to V2, which is less than V1, thus compressing a working medium, and subsequently expanding to volume V3, which may be larger than V1, such that the chamber volume is a smooth and continuous function of rotor's rotational angle.

Air-cooled rotary engine

An internal combustion rotary engine includes an air passage configured to allow cool air to flow through the rotor as the rotor moves relative to the housing within the engine. Some embodiments include a removable fuel cartridge.

Hybrid Cycle Rotary Engine

An internal combustion engine includes in one aspect a source of a pressurized working medium and an expander. The expander has a housing and a piston, movably mounted within and with respect to the housing, to perform one of rotation and reciprocation, each complete rotation or reciprocation defining at least a part of a cycle of the engine. The expander also includes a septum, mounted within the housing and movable with respect to the housing and the piston so as to define in conjunction therewith, over first and second angular ranges of the cycle, a working chamber that is isolated from an intake port and an exhaust port. Combustion occurs at least over the first angular range of the cycle to provide heat to the working medium and so as to increase its pressure. The working chamber over a second angular range of the cycle expands in volume while the piston receives, from the working medium as a result of its increased pressure, a force relative to the housing that causes motion of the piston relative to the housing.

Isochoric Heat Addition Engines and Methods

Engines and methods execute a high efficiency hybrid cycle, which is implemented in a volume within an engine. The cycle includes isochoric heat addition and over-expansion of the volume within the engine, wherein the volume is reduced in a compression portion of the cycle from a first quantity to a second quantity, the volume is held substantially constant at the second quantity during a heat addition portion of the cycle, and the volume is increased in an expansion portion of the cycle to a third quantity, the third quantity being larger than the first quantity.