When particle accelerators hit the headlines, it's usually when they are used to probe big questions about the fundamental nature of matter, space and time. Less well-known, perhaps, is their use in more down-to-earth science – the kind of research that hopes to have an impact on our day-to-day lives.
I visited one such example around a 30-minute drive from Chicago. From a distance the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory resembles a giant, doughnut-shaped spaceship sitting in the middle of a large green field. Get much closer and you’ll find researchers trying to better understand how modern car engines work, which they hope will help find cleaner, more efficient ways of burning petrol and diesel.
Fuel injection has been the subject of research for more than 20 years, however the physics of sprays are still not very well understood. Basic discoveries still need to be made.
To try to shed some light on the matter, the Argonne researchers have turned to a smaller version of the Large Hadron Collider to see if it can help reveal details about fuel injection that haven’t been seen before. Like its much larger sibling at Cern, the circular particle accelerator at Argonne shoots electrons around its 0.7-mile (1.1-km) circumference at a tiny fraction below the speed of light. Each time one of 80 magnets spaced around this ring give the electrons a shove to keep them moving around, they emit X-rays that fire off at a tangent and are channeled into one of 35 laboratories.
In one of these labs, called “Sector 7”, Dr Christopher Powell, an engine research scientist at Argonne, stands amid a jumble of stainless steel machinery, coloured tubes, wires, and computer monitors. He is investigating the precise details of what happens when fuel injection systems squirt petrol or diesel into the cylinders which house the pistons that drive internal combustion engines. It's a process that lasts around one millisecond and occurs many times a second in a running engine. Understanding how the fuel is dispersed and mixed when this happens is crucial to making the burning process as efficient as possible.
The high energy X-rays Powell uses in the experiment exit the particle accelerator through a hatch before travelling along a steel pipe, being focused and then beamed into a model of a cylinder. A variety of injectors can be used, from standard ones used in today's cars to experimental designs.
This process allows the flow of droplets of fuel to be imaged in a way that is not possible with ordinary light or lasers, for example. Using visible light presents challenges similar to those of using car headlights in fog, which consists of droplets of water suspended in the air, just like a jet of fuel sprayed from an injector.
“You turn on the headlights and the light just reflects back into your eyes," says Powell. "You turn on the high beams to try to make it even brighter, and you get even more light back in your eyes.”
Visible light can highlight the outline of fuel spray but not any internal detail. X-rays, on the other hand, can pass through the relatively large droplets of fuel because their wavelength is much shorter than that of visible light. On a much smaller scale, some of the X-rays are absorbed by the atoms that make up the drops, making high-resolution imaging possible. “If you count how many X-rays are absorbed, you can count how many atoms there are,” Powell explains.
The result is a detailed moving image of a jet of fuel in the cylinder that can be presented in various ways, in black and white or in colour. Conventional X-ray sources could not be used because they do not generate enough X-rays to generate sufficiently clear imagery.
Powell shows me a video of what looks like some children’s building blocks being pushed together. It is actually a very close-up X-ray of the tip of a fuel injector, taken through the injector's steel body. It shows the individual components in action, with the valve moving to release the high-pressure fuel. Bubbles can be seen in the fuel supply. Powell says their presence was unknown until identified recently using this high-energy X-ray imaging technique.
“That was something interesting we discovered just a few months ago,” he says. “When the injector valve closes, gas is actually pulled back inside the injector. This is important for quality issues for an injector.”
In a real engine the gas in the cylinder being pulled back into the injector is likely to be made up of hot combustion products, which could be corrosive. Manufacturers are looking at these findings to see what impact this process could be having on the life spans of their injectors.
X-rays transformed medicine by allowing doctors to see inside living bodies. Powell and others hope that by using X-rays to probe the inner workings of engines in fine detail, they can be used to fine-tune the combustion process to increase efficiency and cut emissions, and as a result transform transport too.
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