Test The Limits

The real-world problem with emissions tests

By Exa

April 12 2016

Emissions testing just got a whole lot more complicated than it already was for manufacturers – but it really doesn’t need to be.


The latest complication originates in Europe, where the EU parliament has just paved the way for adding the Real Driving Emissions (RDE) element to the forthcoming regulatory test procedures. This is where cars are tested out on the road with measuring equipment clamped to the tailpipe to find out what really is emitted under real-world driving conditions.


As the recent cheating scandal has made clear, new cars were never going to meet laboratory-set figures for NOx in the real-world. So the EU has controversially agreed to more than double the agreed limit for NOx when measured by the RDE element under the new Worldwide Harmonized Light Vehicle Test Procedures (WLTP).


However, there’s a reason cars are tested in laboratories – it’s the only physical environment that allows repeatable tests. Outside the lab there are too many variables. A small change in ambient air temperature, for example, can make a big difference to engine efficiency.


Wind is another. The car’s aerodynamics are highly affected by air turbulence, and wind can come from any angle. Also affecting the aerodynamics is the test RDE equipment itself. The portable emissions measurement systems (PEMS) used to monitor exhaust fumes are large, heavy devices that hang off the back of the car. They create energy-sapping turbulence in the one area that is important to improving fuel economy.


We believe the only way to test emissions to be fully repeatable, and to take into account the variables of  traffic, wind turbulence, and temperature is via digital simulation. In the U.S, the authorities already utilize simulation data from truck companies to calculate the aerodynamic effect that different tractor designs have on fuel economy and emissions.


They understand that makers would never be able to guarantee similar weather conditions on test days. The same thinking needs to be applied to cars. Digital simulation is the only way to achieve repeatable and comparable real-world figures, in a timely and cost-effective manner.