Lightweight and quiet power to enable armored military exoskeletons

General Atomics is looking at a hybrid-electric power unit with a liquid piston engine that fits in the palm of the hand that will run at 10,000 RPMs. A lightweight and quiet power source is the key to the new powered military exoskeletons.

LiquidPiston’s X Engine is a non-Wankel rotary embodiment of the company’s innovative High Efficiency Hybrid Cycle (HEHC). The X Engine has few parts and three combustion events per rotor revolution, resulting in tremendous power density.

LiquidPiston is currently developing and testing the X Mini, a power-dense, low-vibration, quiet, 70 cubic centimeter gasoline powered rotary four-stroke engine prototype. The compact engine (4-pound core) has only two primary moving parts.

The X Mini is based on LiquidPiston’s patented thermodynamic cycle and engine architecture. To date, the X Mini prototype has demonstrated 3.5 horsepower (net indicated) at 10,000 RPM and the ability to run steady state with air-cooling.

When mature, the engine is expected to weigh 3 pounds, produce over 5 horsepower (over 3728 watts) at up to 15,000 RPM, and be over 30 percent smaller and lighter than comparable four-stroke piston engines. The engine can fit in a 6.6” x 6.2” x 5.4” box. The compact, quiet, high-efficiency, low-vibration, multi-fuel capable combustion engines that are scalable from 1 horsepower to over 1000 horsepower. The power density will be about 2 horsepower per pound.

The X Mini prototype demonstrates that LiquidPiston’s innovative engine technology can scale down in size – prior prototypes focused on 40 and 70 HP designs – and is multi-fuel capable – able to run on gasoline (spark ignition), in addition to previously demonstrated diesel and JP-8 (compression ignition).

The X Mini will enable many small engine applications to be smaller, lighter, and quieter, including hand-held power equipment, lawn and garden equipment, portable generators, mopeds, unmanned aerial vehicles, robotics, marine power, range extenders, and auxiliary power units for boats, aviation and other vehicles. The engine’s improved noise, vibration and harshness (NVH) characteristics will also increase product performance, enhance operator comfort and prolong application life.

* engine runs the novel high-efficiency hybrid cycle (HEHC)
* achieves combustion at constant volume and overexpansion for greater energy extraction.
* Only two moving parts, a rotor and shaft, and no poppet valves—commonly used in other four-stroke ICEs to control fuel intake
* engine also has reduced noise, vibration, and harshness characteristics
* theoretical efficiency of 75 percent for HEHC using air-standard assumptions and first-law analysis
* up to 2 HP per pound (3.3 KW / kg)
* innovative rotary engine architecture shows a potential indicated efficiency of 60% and brake efficiency of over 50%.

In an exoskeleton the engines would only be run to recharge batteries.

Revision Military unveiled an armored exoskeleton for the US Special Operations TALOS (Tactical Assault Light Operator Suit) project. Revision is based in Essex Junction, Vermont, Revision develops innovative capabilities for integrated, performance-enhancing soldier systems.

* Armor resists rifle rounds

Technical Background on HEHC engine

MIT News discussed the engine

A standard 35 HP diesel engine (left) next to LiquidPiston’s 40HP diesel engine (right)

(2014) Development of a Small Rotary SI / CI Combustion Engine

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 over 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.

Wikipedia has a discussion of high power to weight ratio electric and non-electric engines. Electric engines would still need high energy density batteries and a regular engines needs a fuel tank.

Drones and commercial markets

SOURCES – Liquid Piston, MIT News, Wikipedia, Vimeo, SAE, Revision Military