Engineers at Sandia’s Combustion Research Facility and the Technical University of Denmark have discovered a new way to see and photograph pollutants in car engines. By understanding when – and how – soot forms inside engines, researchers can cut harmful emissions at the source.

Traditional engines work by pulling petrol and air into a cylinder, compressing it with a piston and igniting it with a spark. The resulting explosion forces the piston down, producing power. In a bid to clean up their cars, many manufacturers are adopting low emission, ‘direct injection’ fuel systems. Instead of mixing the air and fuel beforehand, nozzles spray petrol under high pressure directly into the cylinder. This burns less fuel with each explosion, giving better fuel economy and lower carbon dioxide emission per mile driven.

A sooty solution

This method of combustion, however, produces soot. Made from unburned carbon in fuel, it is one of the biggest causes of air pollution worldwide. Unlike the characteristic black smoke from older diesel cars, soot from gasoline direct injection engines is much smaller. This makes it invisible to the naked eye.

The team have invented a new optical device to take snapshots inside the engine itself. Extreme pressure and high temperatures make it a particularly challenging environment. One unusual phenomenon that makes imaging so difficult is known as ‘beam steering’. This is where light seems to ‘bend’ as it passes through different refractive indexes and is often seen as a ‘mirage’ on roads in summer. As the hot pavement heats the air above it, the refractive index changes. Light then changes direction as it passes from cool to warm, giving the impression of water on the road. In the same way, light in a combustion chamber passes through different pressures and temperatures.

Lights, camera, action!

To combat the hostile environment, engineers developed a new lighting rig with a custom diffuser and LED lights. Once the flame is captured on camera, researchers can calculate the amount of soot produced as gasoline explodes.

The technique measures the intensity of LED light as it passes through the chamber. As light enters the combustion chamber, it is absorbed or scattered by soot particles. By measuring how much the light dims, researchers can work out exactly how much soot is present.

Vehicle manufacturers must be able to meet clean air laws, while maintaining fuel economy for customers. To do so, they must cut the amount of soot produced, but they can’t do it alone.

The Engine Combustion Network was set up by the study’s Lyle Pickett and brings together researchers from across the world. Pooling ideas and data, they have already provided 20 years of research in a fifth of the time. The challenge, however, is ensuring the quality of data. The study’s optical tool hopes to establish a standardised experiment. This should increase both the reliability and replicability of data.

“With so many researchers eager to participate, it is important to ensure that everyone contributes high-quality data acquired in a technically sound manner,” said Pickett.

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