In Part 1 of our discussion about Real Driving Emissions, we covered the shortcomings of traditional fuel economy and emissions testing methods using chassis dynamometers and fixed driving cycles. In Part 2, we will talk about ways in which simulation can replace the need for costly on-road testing, especially upstream in the product line development cycle.
The Traditional Approach
The way in which a driving cycle is performed on a chassis dynamometer already aims to simulate on-road behavior by imposing a load onto driven axles using rollers, typically connected to electric machines. Road loads (aerodynamics, inertia, etc.) have to be simulated by the dynamometer since the vehicle is actually stationary during the entire test. The test therefore only approximates on-road operation of these vehicles.
Numerical simulation offers a cost- and time-effective alternative to chassis dyno testing given that we have access to highly predictive and fast running engine and vehicle models. The preferred approach for this type of simulation is to dynamically target a chosen driving cycle via a controller or driver template. This method limits simulation vehicle performance to the maximum engine output and will reveal shortcomings of the powertrain or control strategies, in contrast to kinematically imposing the vehicle speed and back-calculating the engine load required.