Fuel claims polarise opinion

A scheme to boost fuel economy by reducing viscosity has met with scepticism from academics
Michael Pollitt
guardian.co.uk, Thursday November 27 2008 00.01 GMT


Everyone would like to get more mileage out of their car. Professor Rongjia Tao, of Temple University in Philadelphia, reckons he can increase diesel fuel economy by up to 20% by applying an electric field to the fine spray of fuel that is injected into an engine's combustion chambers, where it ignites and powers the pistons.
Tao says that the electric field makes the droplets smaller, leading to better combustion. There's no doubt that smaller droplets will burn better and produce more energy. The question is, can the electric field really make that happen?
Troubled waters
Water has a low viscosity while oil's is higher - that's why water flows more easily. In 2006, Tao and a colleague demonstrated that an electric field (electrorheology) and a magnetic field (magnetorheology) temporarily reduced crude oil viscosity.
Taking this idea forward, Tao thought about refined automotive fuels such as diesel or petrol. "I realised in physics it was known that, for droplet formation, viscosity plays a very important part."
The right atomisation of fuel before its combustion is a vital step. Tao says most harmful emissions come from incomplete burning, so reducing fuel droplet size increases the surface area for burning, which leads to more complete combustion. A finer spray is possible by boosting fuel line pressure, or using electrostatic atomisation, where the fuel droplets are negatively charged.
In a recent paper, Tao points out that high-pressure (100 bar) solutions require "substantial changes of the fuel lines in vehicles, because current gasoline vehicles can only sustain a fuel pressure less than three bar". (One bar is standard atmospheric pressure.)
But experiments by Tao and his colleagues showed that smaller diesel fuel droplets were formed when a strong electric field was applied to reduce the viscosity just before atomisation. The fuel passes through an electrically charged tube where two internal meshes produce an electric field of 1 kV/mm between them. It then flows to the fuel injector for atomisation. "The average viscosity reduction is about 9%," Tao says.
For diesel fuel, the number of smaller droplets increased (some are smaller than five microns) when the fuel device was switched on. There was less of an effect with petrol. A diesel engine trial showed a 5.5% improvement in economy but, following modifications, six months of road testing gave a 20% economy boost for motorway driving and between 12% and 15% in city driving.
Will a simple low-powered device (it uses less than 0.1 Watt) really achieve all that? After all, similar claims by fuel magnet manufacturers (which put a household-strength magnet around the fuel line) have been debunked. So the Guardian sought opinion from three academic experts. One didn't believe Tao's research, saying "this does not stand up to its claims".
Dr John Shrimpton of the University of Southampton agrees with Tao's conclusion that a fuel-saving technology could be useful, but says the physical explanation of a viscosity decrease is "full of holes".
"The viscosity change is I think due to electrical forces present in the fluid biasing the viscosity measurement - poor experimental method. The viscosity will appear to decrease, but it is not decreasing," says Shrimpton. "A much better explanation for the improvements in fuel economy is the improvement in atomisation, spray dispersion and fuel-air mixture preparation in the engine due to the electric charge on the liquid drops."
Shrimpton co-authored a paper on the dynamics of electrically charged transient evaporating sprays, which offers a detailed mathematical understanding. This leads him to think that a reduction in surface tension might be a factor with Tao's work.
Brunel University Professor Wamadeva Balachandran, who has spent many years working in electrospray atomisation thinks there's a "completely different explanation". He queries the use of mesh rather than fine charged needles normally used for electrospray atomisation.
Attractive technology
"The other big problem I have is that they have used extremely old techniques to measure the [droplet] size distribution. This is something I used 25 years ago when there were no laser techniques available," says Balachandran.
However, he agrees with Tao that high-pressure fuel systems are a difficult way to improve economy.
"This is why the electrospray technology is attractive," he says. "I've spent 20 years of my career messing around with electrosprays and we haven't got a device we can use in a motor car."
Temple University has applied for a patent on Tao's work and has licensed the technology to California-based Save The World Air Inc. There are now plans to test the fuel device on diesel-powered trucks with hopes of making fuel efficiency improvements of between 6% and 12%. But for those who bought a much-hyped fuel magnet to save money, it might seem like déjà vu.