Exhaust gas measurement of the future: aiming for a global standard (part 2/3)
WLTP stands for “Worldwide harmonized Light vehicle Test Procedure”. The name highlights that the new test shall be valid and applicable globally instead of just on European grounds. “Moreover the test procedure shall provide a realistic picture but still be reproducible”, explains DEKRA’s expert Erik Pellmann.
In the future, as a part of the type approval for cars, exhaust gas measurements will be complemented by the so-called RDE measurement on the road. (Picture: Dörte Neitzel)
“But a realistic picture doesn’t mean the same thing everywhere”, says DEKRA’s expert for exhaust gas measurements. What he wants to say is that the views and demands to a practice-oriented testing cycle are still widely divergent from Europe to Asia. For a long time, it wasn’t possible to reach an agreement regarding a single, global standard.
Mountian, valley and maximum speed
“A realistic testing cycle in Austria would include many mountain roads, in Denmark there would be none instead”, adds Volker Noeske, manager of the DEKRA Automobile Testing Centre (DATC). Also the maximum speed during a test is not the same everywhere: cars in Germany reach up to 130 km/h without being under full load. Indian cars instead may encounter their limits already at 100 km/h. This ended up in a compromise regarding the WLTP: There will be four distinct vehicle categories for the testing procedure, C1, C2, C3a and C3b. Depending on the mass-performance ratio another testing cycle will be applied. In Europe, for example, the testing cycle C3b will be applicable, which is designed for cars that can reach speeds above 120 km/h.
During WLTP, the car will be tested 30 minutes at the test bench and will drive for 23 km. In contrast to the NEFZ the test will take into consideration special equipments and the amount of quiescent current. “However, the WLTP will be, as its predecessor, a pure laboratory testing”, as Volker Noeske says. He says it with a knowing smile though, and places his hand on a car which has some sort of suitcase buckled on its trailer coupling. “In order to compensate for the deficits, RDE, wich is a real road testing procedure, has been added to WLTP.”
Example of a WLTP testing. (Picture: DEKRA)
RDE – exhaust gas measurement on the road
During RDE testing (Real Driving Emissions), a road cruise will be performed and the emission of nitrogen oxides (NOx) and carbon monoxide (CO) will be measured. A particle measurement for diesel as well as for petrol engines is expected to come along starting in 2020. The technician attaches a so-called PEMS device to the rear of the car to be measured.
PEMS stands for “Portable Emissions Measurement System”. The device looks like a medium-sized suitcase which contains the measurement technique and is connected to the exhaust system via a tubing. The measured values are transmitted from the PMS-device to a laptop. The laptop gathers information from numerous other sensors which are also attached to the car, for example a temperature sensor and an altimeter.
The testing needs time: for the attachment of the measurement system to the car to be tested, the DEKRA experts need about half a day. Additionally, the driving and the analysis of the results take their time. “That makes the RDE measurements considerably more complex than the examination on the test bench”, says Erik Pellmann.
Strict guidelines for the driving test
The engineers let us know that the RDE drives are never 1:1 reproducible. Nevertheless, they are standardized as much as possible, as there are numerous requirements to the surrounding area, the vehicle condition and the route”, says the expert for exhaust gas measurement and describes some of the testing conditions: First, the vehicle mass must not surpass 90 percent of the maximum load capacity, including the driver, co-driver, the measuring techniques and the power supply. The ambient temperature shall be between 0 and 30°C during the drive and the air conditioning in normal operation mode.
The route, with a scheduled duration between 90 and 120 minutes, shall contain an urban (34%), a rural (33%) and a part on the highway. Furthermore, the route must not be located over 700m above sea level. During the drive, the car has to span less than 100m in altitude. On the highway, the speed of the car has to be between 90 and 110 km/h minimum, but not faster than 145 km/h.
The average speed for the urban part shall be between 15 and 30 km/h. There have to be several planned stops of 10 seconds minimum. “If any unplanned longer breaks happen, the driving test must be replicated”, describes Erik Pellmann one of the possible pitfalls. As the testers know from experience, this scenario is not unrealistic. One of the experts got into a police control during a RDE test driving.
Comparable measurement results
As these detailed requirements of RDE still leave the possibility for considerable variance, the measurement results get normalized, which means they are made comparable. “A ride at 1°C and at an altitude of 600m is still not comparable to a driving at 29°C in lowlands”, the explains Erik Pellmann. Until 2020, the normalized RDE exhaust measurements are allowed to surpass the measured WLTP values by 2.1. From 2020 on, the reins become tightened and the factor drops to 1,5. “This should lower the pollutant emission even further”, says the DEKRA expert.
So the road is clear: exhaust and consumption measurements on the test bench will be continued, but the change of the testing cycle makes them much more realistic. This is supported by the real road drive, even if it is not possible to control it for every factor. “This regards the driving behaviour for example. A crucial factor is still sitting behind the steering wheel”, says the manager of the test centre Volker Noeske.
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