Crankless piston engines could deliver greener motoring

| Transport

Sam Cockerill of Libertine

Electronic control of piston motion and position provides the capability of removing mechanical constraints on engine efficiency.

UK company Libertine is breaking the mould in small scale power generation and internal combustion engines with its digital piston motion control technology that removes the constraints on piston motion usually associated with mechanical power trains including crankshafts and flywheels.

The technology is the one area of engine management that has so far eluded electronic control and could take the piston engine out of the suck-squeeze-bang-blow era into a new generation of high efficiency and low power train losses.

To understand more about this technology, Environmental Engineering’s Jonathan Newell had a very revealing Q&A session with the CEO of Libertine FPE, Sam Cockerill.

Q. A lot of people will find it conceptually difficult to visualise an internal combustion engine decoupled from the crankshaft, flywheel and gearbox that we’ve all become so familiar with. How would you help them overcome this conceptual difficulty?

A. In linear free piston engines, the piston also travels through the cylinder but it isn’t driving any rotating mechanical components. Instead it creates electrical power using linear generators. Without the mechanical constraints of a conventional piston engine, the motion profile can be controlled electronically and can therefore be optimised for operating conditions. This makes it very efficient. The engine is also smaller and cheaper than its conventional counterpart.

Q. Are there any operating examples?

A. An example of one we’ve supplied is being used by Bristol University which operates as a Rankin cycle test rig for them. It is a two cylinder expander 1.5 metres long.

Q. What do you mean by an expander?

A. Generally, the cylinders can be used as combustion chambers like in a car engines or expansion chambers like in a steam engine. Depending on how they’re configured and built, they can be used in either application. What we supply is gas cycle agnostic.

Q. The piston engine as we know it has been around for a long time. How ready do you think the automotive industry is for such a paradigm shift?

A. The piston and cylinder has such an enduring nature because it’s a hugely efficient energy conversion technology, moreso than gas turbines and other rotary engines, especially for smaller power outputs. This is one reason why the automotive industry continues to embrace it but manufacturers aren’t averse to change. If we look at the four main aspects of engine control, that is fuel, ignition, air and motion, the industry has made huge progress over the decades in improving fuel delivery and ignition through electronic control. In terms of air flow, the mechanical induction stroke has been decoupled from the crankshaft motion through the use of digital valves and variable boost pressure. The final barrier is motion control and that’s where Libertine comes in.

Q. Hybrid vehicles seem like an ideal application for linear free piston engines generating electrical energy. What would such an engine mean for the creativity of vehicle designers? Could we see long cylinders forming part of the chassis? Could there be an engine for each wheel and will the concept of an engine bay be a thing of the past?

A. There are a lot of possibilities. I’m not sure you’d want to put an engine on each wheel although it’s possible. It’s more efficient to have a central engine and have an electric motor on each wheel. However, you could have an additional engine to add into the configuration for different modes of driving. Regarding the overall system, with the linear engine concept, it no longer matters where things go in the car. There’s no mechanical chain to constrain things and each component is functionally discrete and so can be calibrated individually. This is very different from what happens in the automotive industry now where the tuning or calibration of the engine has to take the overall drive train system into account.

Q. Could you elaborate on that?

A. Building a car on this concept would be similar to how personal computers are constructed now. The provider of each element in the system provides functional assurance so you select the elements you want and assemble them into a computer that functions how it is expected to. This modular approach shortens development times and costs resulting in more choice for consumers. A similar shift in approach has already happened in the bicycle manufacturing industry which now has a much more integrated approach to component and subsystem design through the standardisation of interfaces. This also creates bigger aftermarket opportunities.

Q. The acquisition and processing of so much analogue data must have been a significant challenge, how did you overcome this?

A. The availability of development and control platforms like LabVIEW has made it possible for us to take on such a challenge, especially as a small start-up company, without having to write bespoke code.

Q. What kind of fuels can be used in such an engine?

A. There are no limitations on the kinds of fuels used in the engine. The way the industry has developed has led society to a high dependence on fossil fuels. In standard engine technology, the emergence of bio fuels introduces a variability in fuel quality which results in more emissions and less power. However, with digital piston motion control, the compression ratio can be varied depending on the fuel used. Varying the compression ratio on crankshaft coupled engines can only be achieved by changing the engine geometry which can’t be done during its service. On a linear free piston engine, it can be achieved by changing the piston motion profile electronically.

Q. So would this be achieved using presets selected by the driver depending on what fuel is used or would it be automatic?

A. Within certain parameters, this is an automatic process based on the detection of the fuel composition. This assumes that a certain type of fuel is used persistently but the quality of that fuel varies. An example is bio fuel. Bio gas can vary on a daily basis and whereas existing fossil fuels are highly refined and meet exacting specifications, there will be no such need for high degrees of refining for bio fuels if the engine can cope with such variability.

Q. Thank you Sam and I hope we can all benefit from the arrival of hybrid cars configured to owners’ specifications with no engine bay and running on bio fuels thanks to your technology.

More information on Libertine’s digital piston motion control can be found at the Linear Power 2015 event being held in Brighton, UK on 7-8th September 2015.

Jonathan Newell
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