Digital piston motion control could mean new era in combustion engines

Libertine has built a demonstration prototype of its revolutionary linear electrical machine for what it is calling ‘Digital Piston Motion Control’ in free piston engine applications.

This technology will allow researchers to develop future combustion engines free from the piston motion constraints dictated by the crankshaft. Already validated at proof-of-concept level, Libertine’s new hardware uses production-feasible component designs suitable for automotive hybrid powertrain modules, such as range extenders.

“Piston motion is the final major engine parameter that remains to be digitally controlled by the ECU rather than mechanically governed,” said Libertine’s CEO, Sam Cockerill. “Ignition and fuelling 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.”

Unlike conventional engine-generator combinations, which use a crankshaft and mechanical powertrain to rotate an electrical machine, Libertine’s technology uses a linear electrical machine to generate power directly from the piston’s motion. The linear generator can also be used as a motor to apply a variable force to the piston, and this approach permits each piston’s velocity, motion profile and compression and expansion ratio to be optimally controlled via the ECU to accommodate startup, transient and flex-fuel operation. This flexibility offers significant improvements in combustion efficiency and has the potential to produce a third more power from the same fuel input.

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

Libertine’s linear machines will be incorporated into research engines at leading 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,” explained Cockerill. “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 optimising the motion of the piston to improve combustion efficiency.”

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,” said Cockerill. “Our linear free piston technology permits the ECU to continually optimise these ratios by varying the piston motion, which can even be changed cycle-by-cycle if required. It really is that simple.”

Libertine’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 minimises the combustion chamber’s surface-to-volume ratio at Top Dead Centre, 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.

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