A free-piston engine offers a number of benefits, including optimization of the combustion process through variable compression ratios, making multifuel operation possible; reducing frictional losses because of fewer moving parts; and allowing homogeneous charge compression ignition (HCCI) to be more easily achieved. A linear alternator can use the linear piston force without requiring additional mechanical components necessary in a rotary configuration.

A free piston engine eliminates the entire mechanical drivetrain of a conventional engine, allowing ultra-efficient combustion cycles to be developed and reducing the parts count and cost. It completely removes the crankshaft constraint placed on piston movement that actually hinders efficient combustion, and allows piston motion to be optimized to deliver a cleaner, more efficient combustion process.

—Libertine CEO, Sam Cockerill

A free piston engine is a heat engine with pistons that move back and forth to extract work from a fluid, or to act as a pump. Unlike a conventional internal combustion engine, a free piston engine does not have connecting rods and a crank shaft to govern each piston’s movement and convert the reciprocating action into rotary motion. Each piston is free to move along its axis under the action of linear forces including those due to combustion pressure. An internal combustion engine generator based on this format has a combustion chamber at one or both ends of the free piston, and a linear electrical generator to capture power from the piston during its movement cycle. Source: Libertine LPE. Click to enlarge.

However, motion control is a significant design challenge for free piston engines, Libertine notes. Multiple pistons must be accurately positioned and synchronized in the absence of a crankshaft which, in a conventional engine, serves this purpose. If each piston’s motion is not controlled adequately and arrested in the right place, the compression rate and ratio will vary leading to combustion variations and potential misfires. Both could reduce the engine’s efficiency and increase tailpipe emissions. If pistons are not synchronized, the engine will not be electrically and mechanically balanced, leading to harsh vibrations and power spikes.

Libertine’s technology leverages a combination of piston geometry, electrical machine design and cylinder construction. All three are relatively long and smaller in diameter than those currently used by other free-piston developers. This novel architecture reduces the moving mass of the piston relative to the electrical machine’s force, giving more effective and accurate control of the piston motion.

The Libertine free piston power generator features a piston, electrical machine and cylinder that are relatively long and smaller in diameter than those currently used by other free-piston developers. Source: Libertine LPE.Click to enlarge.

This geometry also reduces heat losses from the combustion chamber which, at top-dead-center, has a much improved surface-to-volume ratio. By integrating the electrical machine into the cylinder wall, rather than attempting to isolate it with seals, the high friction losses often encountered by free piston engine developers are significantly reduced, the company says.

Comparing a 10 kW portable generator based on Libertine’s technology architecture to the most efficient currently available systems for static power generation shows a saving of 90% in package size, 80% in weight, significantly reduced cost and up to 30% higher efficiency, the company says. This technology architecture can be applied to many different combustion cycles, and its modular design can be readily scaled for applications from 1kWe to over 100kWe.

One of the first institutions to invest in the new technology is the Universiti Teknologi PETRONAS (UTP) in Malaysia, which is supported by the PETRONAS oil and gas company. The university has entered a partnership with Libertine, funding a feasibility study that is expected to lead to the supply of free piston research engine hardware upon successful completion.

UTP, which has an established program of free piston engine research and development, is primarily interested in the combustion process and requires a reliable, efficient linear power system as a platform for its work, according to the university’s deputy vice chancellor for Research and Innovation, Professor Abdul Rashid Abd Aziz.