Conformity in All Situations

Radical change in emission legislation: the rather steady-state NEDC is now being followed by the dynamic WLTC on the emission roller dynamometer. Vehicles must also prove under real-world conditions that they reliably meet all limit values – but real driving emissions (RDE) can only be ascertained in real-world traffic and with mobile measuring equipment (PEMS). IAV takes the extremely challenging aspects of RDE and PEMS into consideration in all phases of the development process.

Whereas the main focus for the diesel engine is currently on nitrogen-oxide emissions, the developers of gasoline engines are now chiefly concerned with carbon-dioxide and particulate emission. To date, all emissions have been measured on the test bench, with tests being based on the New European Driving Cycle (NEDC). This is changing: the rather steady-state NEDC at a maximum speed of 120 kilometers an hour is being replaced with the Worldwide Harmonized Light Vehicles Test Cycle (WLTC) on the dynamometer which, depending on engine output, gets up to a maximum speed of around 130 kilometers an hour.

But the implications of introducing RDE are even more serious. The top speed here is a far more realistic 145 kilometers an hour. The greatest challenge comes with the test conditions: In RDE, vehicles are required to demonstrate in real-world traffic around town, across country and on the freeway that they reliably meet all limit values throughout their entire operating range.

Currently, a conformity factor (CF) of 2.1 is in place. The floating mean value for emissions measured in road traffic with PEMS (Portable Emissions Measurement System) must not exceed the limit values by a factor of more than 2.1.

The CF takes into account the more stringent measurement conditions: whereas the highly sensitive measuring equipment on the test bench works under ideal conditions, it is exposed to many disturbing influences in the vehicle, such as constant vibration and emissions from other vehicles. Yet, from 2020, the CF will fall to 1.5 when RDE will be compulsory for new-vehicle homologation. An RDE drive is only valid if the CO2 emissions produced while driving and in relation to road speed meet the legal requirements.

“Here, in theory, there can be an infinitely large number of applicable cycles”, says Andreas Rosenek, head of the Gasoline Engine Calibration department at IAV. “Added to this are widely varying external influences on the results: the driver, the gearshift program, outside temperature, altitude and traffic situation all have a huge impact on emissions. We are now facing the major challenge of reliably validating the results.”

To get as many valid RDE drives as possible, IAV uses a tool that constantly tells the driver on a notebook monitor the percentage of requisite valid measuring points already reached. In future, it will also be linked to Google Maps to avoid unnecessary congestion and, in this way, not exceed the prescribed maximum measurement time.

RDE is having a major impact on all phases of the development process in general. Even during the concept phase, developers need to perform test bench measurements and carry out simulations to gage whether the drive system is likely to meet RDE requirements later on. “To do this, we use data-based engine models, a catalyst model, such as Axisuite, and a vehicle model to predict whether the concept is set to pose problems”, says Dr. Karsten Röpke, head of IAV’s Calibration Methods department.

While calibrating, the process must be made RDE-compliant on the engine test bench, on the chassis dynamometer and with PEMS measurements on the road. “In particular, this involves scrutinizing and calibrating those phases that may lead to emission peaks and higher emissions, such as mixture pre-control, catalyst clearing and acceleration phases. The quality of the lambda controller also plays a vital part in emissions”, Rosenek says. In this phase, measurements are performed on the road and on the chassis dynamometer.

In subsequent validation, measurements need to show that vehicles really do meet RDE specifications under all conditions. “This will involve developing a methodology based on as few measurements as possible”, Dr. Röpke adds. “For this reason, we are identifying particularly critical maneuvers as well as all influencing variables and the impact they have.” Other measurement data – e.g. from endurance and test drives – could also be channeled into evaluation. This can produce a huge volume of data which IAV manages and evaluates in specially developed database systems.