How much soot is in car exhaust?
Posted by Christina Benjaminsen
New technology allows us to measure soot particles in the exhaust of diesel engines, reducing emissions from diesel-powered vehicles and improving their energy efficiency.
After three years of development, Norwegian scientists can now measure the soot content of a vehicle exhaust while it is actually on the road. The new sensor measures soot particles in the exhaust gas after it has passed the particle filter, rather than before. Project manager Andreas Larsson, a scientist at SINTEF, believes that continuous measurements of emissions will be in demand in the future. “Stricter emission limits for CO2 and other pollutants will lead to demands for better monitoring of vehicle exhaust gases,” he says.
Measures “scrubbed” exhaust
Measuring soot when actually on the road has been a major challenge, but by applying a principle known as thermophoresis, the scientists have managed to measure soot concentrations in clean, or “scrubbed” exhaust.
“Thermophoresis is a physical phenomenon that draws microscopic soot particles in the exhaust gas towards cold particles or regions in the gas. When soot particles collide with high-energy hot particles, they are forced towards colder, less energy-intensive regions. In other words, particles are transported from hot to cold parts of the system,” says Larsson.
The sensor itself is installed in the middle of the gas flow, so that it is exposed to the high-temperature exhaust gas. If the sensor is kept cold enough, solid particles in the exhaust gas will be attracted to the cold surface of the sensor, where they can be measured.
A further challenge was therefore to reduce the sensor temperature by 50 to 70 degrees relative to the hot exhaust, which can reach temperatures of as much as 100 to 300 degrees Celsius.
The scientists solved this problem by fitting a heat-conducting shield around the sensor. Between the heat-shield and the sensor is a heat-conducting air layer that prevents heat from reaching the sensor.
SINTEF patented the cooling principle in 2009. A good deal of research still needs to be done before it can be brought into commercial use, but so far, the results are promising. The primary field of application will be diagnosis of technical faults in vehicle exhaust systems, such as cracks in the particle filter.
The scientists believe that in the longer term, the sensor will also be able to lower vehicle energy consumption, and thus help to reduce emissions of the greenhouse gas CO2 and other pollutants such as NOx and particles.
Volvo Technology, which is a central partner in the project, has applied for a patent on the method together with Volvo Car Corporation.
Christina Benjaminsen has been a regular contributor to the science magazine Gemini for 11 years. She was educated at Volda University College and the Norwegian University of Science and Technology, where she studied media and journalism.