New Scientist | Evolution of the Car

New Scientist | Evolution of the Car: "By Jeff Hecht

ENGINEERS have been barking up the wrong tree in their efforts to make diesel engines run cleaner and more efficiently.

A new X-ray study has revealed a type of supersonic shock wave that no one has seen before in the high-speed fuel jets used in diesel cars. 'Nobody had any idea this was going on,' says Jin Wang of the Argonne National Laboratory in Illinois. Engine designers will now have to scrap their old models of fuel mixing and combustion.

In a diesel, fuel ignites spontaneously when it is injected into the combustion chambers. The way the fuel mixes with air is crucial to how it burns, so knowing how the fuel sprays out is important for boosting efficiency and reducing pollution.

Researchers use light scattering from fuel droplets to profile the shape of the injected fuel jet. However, droplets in the jet scatter light many times, obscuring what's going on.

To see through the haze, Wang and his colleagues tried using a single-wavelength X-ray source and a high-speed detector that recorded an image every 5 microseconds. They sprayed standard diesel fuel, mixed with a caesium compound to enhance its X-ray contrast, into a chamber containing the inert gas sulphur hexafluoride to stop it combusting. As the jet moved through the gas, they took a series of pictures.

The team's research was funded by the automotive systems company Robert Bosch in Stuttgart. To simplify measurements, they used a modified version of a standard fuel-injector nozzle that had only one hole rather than the usual five or six.

They found that 90 per cent of the fuel was concentrated in a thin jet behind the V-shaped shock wave, with the densest concentration of fuel right behind the shock front. And while the gas in the chamber slowed down the leading edge of the fuel jet, the trailing edge moved several times faster, at supersonic velocity. As the tail end of the fuel jet caught up with the leading edge, most of the fuel became concentrated in a blob just behind the point of the shock cone. 'Nobody knows why that should be, but we're going to try and find out,' Wang told New Scientist.

The finding will send fuel efficiency researchers back to their drawing boards, says Oleg Vasilyev, a fluid dynamics specialist at the University of Missouri in Columbia. Fuel distribution in the initial jet is critical to how the fuel spreads through the chamber to be burnt. A better understanding could lead to new injector nozzle and chamber designs that improve fuel efficiency and reduce pollution, says Vasilyev."