Piston motion is the final major engine parameter that remains to be digitally controlled by the ECU rather than mechanically governed. Ignition and fueling went digital in the 1990s; systems for digital air provided by electronic boost and valve control have appeared already; only piston motion is still mechanically governed in modern engines, because of the persistence of the crankshaft in engine design.

—Sam Cockerill, Libertine CEO

In addition, it is simpler and is easily scaled to suit applications from 1 kW_e to more than 100 kW_e. Portable generators based on Libertine’s technology show a saving of up to 80% in package size and weight compared to the most efficient systems currently available.

Libertine’s linear machines will be incorporated into research engines at academic institutions for the study of advanced combustion methods such as homogeneous charge compression ignition (HCCI).

For engine designers accustomed to the inefficiencies associated with piston motion that is constrained by a crankshaft, it requires a fundamental shift in mind-set to appreciate the possibilities offered. Linear free piston engines can capture a much greater proportion of the energy contained in the fuel burnt, not only because the linear gas expander extracts electrical power directly from the expansion stroke, but also by optimizing the motion of the piston to improve combustion efficiency.

—Sam Cockerill

The sinusoidal motion of a piston in a conventional engine is rarely questioned because it is the inevitable result of crankshaft and connecting rod geometry, but the movement of the piston at the top of its stroke is far from ideal for optimum combustion and power generation. Libertine’s linear free piston technology gives researchers the freedom to explore advanced combustion strategies, such as HCCI, with independent control of piston position, reduced heat losses and a simple mechanical arrangement.

Improving the efficiency of a conventional crankshaft-coupled piston engine by varying the expansion ratio or compression ratio to suit different operating conditions typically requires a variable geometry connecting rod or combustion chamber, which leads to impractical levels of mechanical complexity. Our linear free piston technology permits the ECU to continually optimize these ratios by varying the piston motion, which can even be changed cycle-by-cycle if required. It really is that simple.

—Sam Cockerill

Libertine says that it’s patented technology overcomes several issues which have challenged linear power system developers in the past: motion control, system losses and component costs.

By using a novel high performance electrical machine architecture, coupled with a relatively long stroke with a small bore, the moving mass of the electrical machine is greatly reduced which improves control of piston motion and minimizes the combustion chamber’s surface-to-volume ratio at Top Dead Center, reducing heat loss. The long stroke also makes the system much less sensitive to positional error, simplifying the task of high speed control.

The stationary element of the electrical machine forms part of the cylinder bore, eliminating the need for sealing rings and reducing friction losses. The design architecture uses simple extrusions and pressings, combined with electrical winding and potting methods suitable for volume manufacture in order to minimise costs.