Environmental Engineering's Andy Pye examines the implications of having digital motion control of internal combustion engines rather than being mechanically governed.
Libertine FPE has built a demonstration prototype of what it is calling “Digital Piston Motion Control” for free piston engine applications. It dispenses with the crankshaft and other mechanical powertrain components in small power generation systems.
Pistons without cranks
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 their 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.
The free piston engine offers a number of advantages over a traditional reciprocating engine coupled to a rotary generator: firstly, the powertrain from piston to generator is dramatically simplified. Piston side loads are virtually eliminated and this, together with the elimination of crank and connecting rod journal bearings, allows the lubrication system to be downsized or removed.
Secondly, although 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, the movement of the piston at the top of its stroke is far from ideal for optimum combustion and power generation.
In the free piston engine, the piston is no longer constrained by the crankshaft to follow a fixed sinusoidal motion over time, so this motion can be tailored according to the fuel being used and the required output at any given moment. This is especially important for flex-fuel and dual-fuel vehicle engines, and also for biogas power generators which see changing fuel composition.
Thirdly, large over-expansion cycles are made possible in small engine formats. Whereas in crank engines the piston’s stroke is defined by the crankshaft, in a free piston engine the piston motion can be considerably extended, allowing over-expansion of burned gases to extract the maximum possible work before these are released.
Free piston engine development
The free piston engine is not a new concept – it was first conceived in the 1920s and various forms have been used successfully in applications such as air compressors, gas generators and hydraulic pumps. General Motors developed an automotive free piston engine in 1957 drawing on a concept developed by SIGMA in 1944.
Other groups currently active include Sandia National Laboratories in the US, the Czech Technical University, West Virginia University, Stockholm’s Royal Institute of Technology (KTH) and Australia’s Pempek Systems. Sandia registered a US patent for a free piston engine (US 6199519 B1), characterised by high thermal efficiency and low NOx emissions and is particularly suited for generating electrical current in a hybrid automobile.
Sandia’s interest is backed up by extensive research and test facilities. Researchers are investigating how burning droplets of fuel are generated and behave, using 3D measurement techniques based on digital in-line holography (DIH). The work is designed to explore how to avoid catastrophic fires in road accidents or space rocket launch pad failures.
“Major investment by OEMs and leading universities across the globe over the past decade demonstrates a broad recognition of the benefits of free piston engines,” explains Libertine CEO Sam Cockerill. “We can provide ready-made research engine hardware to advanced technology groups, delivering the digital piston motion control necessary.”
Overcoming the motion control barrier
It is motion control that is the foremost reason why free piston engines have not achieved widespread adoption in automotive or static power applications. Multiple pistons must be accurately positioned and synchronised in the absence of the crankshaft, otherwise the compression rate and ratio will vary leading to combustion variations and potential misfires. And if pistons are not synchronised, the engine will not be electrically and mechanically balanced, leading to harsh vibrations and power spikes.
Other factors which have held back development include the control of tailpipe emissions and the cost of solving new design challenges. General Motors have also identified piston lubrication, cooling and wear as further issues requiring attention.
“Piston motion is the final major engine parameter that remains to be digitally controlled by the ECU, rather than mechanically governed,” continues 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. Our patented design architecture overcomes the challenges of control, complexity and system losses that have previously held back commercialisation.”
Libertine’s approach permits each piston’s velocity, motion profile and compression and expansion ratio to be optimally controlled via the ECU to accommodate start-up, transient and flex-fuel operation.
“For engine designers accustomed to the inefficiencies associated with piston motion that is constrained by a crankshaft, it requires a fundamental shift in mindset to appreciate the possibilities offered,” adds 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.”
Libertine’s linear free piston technology also gives researchers the freedom to explore advanced combustion strategies, such as homogeneous charge compression ignition (HCCI). HCCI designs achieve gasoline engine-like emissions with diesel engine-like efficiency and ultra-low NOx and particulate emissions without a catalytic converter. However, unburned hydrocarbon and carbon monoxide emissions still require treatment to meet automotive emission regulations.
“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.”
Applications for free piston engines
It is significantly simpler and easily scalable to suit applications from 1kWe to over 100kWe. 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.
By including a library of lab-validated optimisation tools as a ‘developer toolkit’ with the hardware, Libertine makes it possible for interested partners to simultaneously optimise combustion, electro-mechanical and thermal design parameters. The flexible, modular construction means the technology is readily scaled to suit different client research applications.
Research teams already using Libertine’s linear power systems include the University of Brighton and the PETRONAS-backed UTP in Malaysia. The project at Brighton, part-funded by the UK government through Innovate UK, uses a pair of Libertine’s linear free-piston expanders in an ethanol Rankine cycle to extract energy from a hot gas source which represents the flow of a vehicle’s exhaust under a range of steady-state conditions.
The results from rig tests are expected to confirm the system’s potential to convert the high grade heat in the exhaust into electrical power, which can contribute to either powertrain or auxiliary loads. By providing physical test results to validate simulation tools, the data will allow Libertine to model specific applications with high confidence.
The test rig and linear free piston expander technology will form a centrepiece at Linear Power 2015, billed as the world’s first technology forum for linear power systems technology researchers and application developers. “Visitors to Linear Power 2015, to be held in Brighton in September, will be able to see the system working in the lab, review the results and discuss potential applications,” says Cockerill.