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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. 鈥淣obody had any
idea this was going on,鈥� 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鈥檚 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鈥檚 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. 鈥淣obody knows why that should be, but we鈥檙e going to try and
find out,鈥� 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.