Could plasma combustion improve fuel efficiency?

Three Los Alamos National Laboratory researchers are embarking on a series of experiments to test out the feasibility of a plasma combustion technique that could increase the fuel efficiency of engines.

In the technique, an electrical voltage is applied to a gaseous-phase fuel stream prior to combustion- turning the fuel into a plasma.

The idea has already produced excellent results with propane and the next step, according to the research team of Don Coates, Louis Rosocha and David Platts, is to try out the idea using a 5-10 horsepower, four-stroke twin cylinder engine fuel injected Cummins gasoline engine.

Kerosene, propane, gasoline and diesel fuel are all hydrocarbons, made up of the same basic chemical constituents but separated by the size of their individual molecules. The more efficient fuels, and therefore more highly refined and expensive kerosene and propane, consist of fairly small chains of carbon and hydrogen atoms, whereas the less efficient and cheaper fuels, gasoline and diesel, are made of long chains of molecules.

According to Coates, when electrodes attached at the spray nozzle of a fuel injector apply enough voltage to the fuel, energetic plasma electrons from voltage-induced breakdown of the fuel cause reactive species to be created, changing the basic chemical composition as the fuel becomes a plasma.

‘The plasma unit basically acts like a ‘cracker’ in a refinery, cutting the long chains of hydrocarbons into bite-size parts – the smaller the parts the better the burn – taking cheap fuels and making them combust like expensive ones,’ he says.

The three researchers believe they can construct such a ‘cracker’ device that is relatively simple, cheap and easy to retrofit to existing fuel injection systems.

‘This could also have a dramatic impact on the environment, with the reduction of combustion waste products, specifically nitrogen oxide. In the coming years, new federal requirements will force internal combustion engines to be cleaner and cleaner – this technology could be one way to achieve compliance with the regulations. And when you think of things like large jet engines running on diesel, there is a safety improvement as well; diesel will not explode like kerosene or gasoline, it’s low flash-point makes it much more safe to use,’ said Coates.

The idea of breaking down, or reforming, automotive fuel on a vehicle is not a new one though.

‘The main thing to remember is that there is a fixed amount of energy available in the molecular bonds of the original fuel (plus whatever energy is expended to break it down). In other words, there is no ‘new energy’ for the combustion process. Reforming fuel costs energy, so thermodynamically the process is actually less efficient,’ says Professor Rolf D. Reitz, a distinguished Professor of Engineering at the Department of Mechanical Engineering at the University of Wisconsin-Madison.

A potential advantage of reforming fuel, however, is that ignition processes may be more controllable with more reactive fuel species (important in a diesel engine), and flame speeds may be increased (in a spark-ignition gasoline engine). However, the benefits of this control might be fairly small since other (cheaper) methods are available to manipulate those factors.

‘Regarding emissions’, says Professor Reitz, ‘there may be advantages to reforming the fuel in this case because the reformate may have fewer high molecular weight species that escape the combustion process, and would be exhausted as unburned hydrocarbon.’

Ken Cowey, Technical Manager of Fuels and Lubricants at QinetiQ, also notes that this type of technology has been tried before both as a direct plasma and in the form of devices that ionise the fuel and oxidant before it enters the engine.

‘Although combustion technology of this type has some potential to improve the combustion in engines there are a number of fine balances that must be achieved before the technology becomes a practical on an engine,’ he says.

He concurs with Reitz that such a device would require energy to generate the plasma and dissociate the fuel and therefore any improvement in combustion must more than compensate for this parasitic loss.

‘In the case of jet engines, the combustion efficiency is already very high and any additional weight device would add weight and complexity to the aircraft,’ he says. He does, however, feel that the technology may be more applicable to car or truck engines, where a ‘cracker’ device may be able to reduce emissions such as nitrogen oxides which are of major concern for air quality.

‘Any improvement in combustion may also lead to better fuel consumption or power but this has yet to be proved,’ he adds.

QinetiQ are examining the use of plasma and related technologies as a route to power generation in a variety of applications. The company is currently working in a partnership to develop a small power unit based on the use of plasma to reform a hydrocarbon fuel, clean up the output stream and feed it to a fuel cell. This approach uses the efficiency of the plasma to break down the fuel and that of the fuel cell to efficiently generate quiet clean power.

It is anticipated that such a unit would find applications where remote power is required as well as auxiliary power for trucks especially those with chiller or freezer units. QinetiQ has also examined fuel and oxidant ionisation devices as a possible way to improve the burn in internal combustion engines. As for using the plasma technology to aid a switch from jet fuel to diesel for aircraft, Cowey feels that any such change would create more problems than it would solve.

‘Diesel fuel would freeze at the cruise altitudes of long haul flights and block off the fuel supply to the engine, the relight properties at altitude would also be very different. In Europe, and many other parts of the world, diesel fuel is a high demand product that is often in short supply and could not be substituted for jet fuel without creating a shortage elsewhere,’ Cowey says.

‘The bottom line,’ concludes Professor Reitz, is that ‘it’s an interesting idea, but it’s high risk research.’