Exhaust gas measurement of the future: ready to go! (part 1/3)

Since Volkswagen has admitted software manipulations during the type-approval for some vehicles, exhaust measurements in general are under debate. Earlier than the scandal, the European Union decided that the actual test cycle shall be replaced in barely two years time. During a practical workshop, DEKRA has shown how the tests are currently performed and which criteria will be applied in the future. The aim is to introduce a global standard that takes into account different vehicle categories and real road use.

Abgasmessung, NEFZ

DEKRA is carrying out exhaust gas measurement following the actual European standard NEFZ. (Picture: Dörte Neitzel)

Gradually, the tyres of the black BMW are setting the bench rollers in motion. The man sitting in the wired car is keeping his eyes fixed on the monitor installed in front of the windscreen. Occasionally he accelerates, brakes, engages and shifts. After 20 minutes, the driver’s door opens again – the test is completed. In the DEKRA Automotive Test Centre (DATC) of Klettwitz in Brandenburg it is time to say „Next please!“. At a workshop, DEKRA is demonstrating the exhaust gas measurement, which makes part of the type approval of new car models.

The measured values shall be below the EU limit value; otherwise the car manufacturer doesn’t obtain the authorization. “This test is a must”, says Volker Noeske, manager of the DATC. “We are following the up-to-date European standard NEDC.” The acronym stands for “Neuer Europäischer Fahrzyklus” (New European Driving Cycle, NEDC). Although it isn’t all that new according to Noeske: “We have been applying this standard since the mid 1990’s.”

NEFZ: Gas exhaust measurement under laboratory conditions

The NEFZ measures nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2) and the power balance of hybrid and electric cars. The test runs exclusively on a chassis dynamometer in the laboratory under reproducible conditions, according to Noeske. This allows a comparison of the results.

Before a test can start, two conditions must be fulfilled: as for cars without all-wheel drive only the two rear wheels are moving on the test bench, DEKRA’s staff member has to determine the driving resistance first. Time to move to the testing ground. During the so-called coast down test the test driver speeds the car up to 130 km/h and lets the car coast to stop. In this way he can calculate the driving resistance from the rolling and engine resistance, the drag and the mass inertia of the car. The calculated value will then be set at the chassis dynamometer.

Perfectly conditioned

Second step of the preparation: After the coast down test the testers park the car to be examined in the conditioning hall for six to 36 hours at a temperature of 20 to 30 degrees. This procedure allows to “condition” all parts and liquids of car to be tested at the right temperature.

The actual measurement takes 20 minutes and follows a rigorous procedure regarding speed, accelerations and stops. The inner city driving profile (phase 1) with a speed-up up to 50 km/h takes two-thirds of the time. The extra-urban driving profile (phase 2) includes accelerations up to 120 km/h. The average speed is approximately 33 km/h.

Abgasmessung NEFZ

The driving cycle of the NEFZ. (Picture: DEKRA)

“Long in the tooth”

The NEFZ is a matter of debate among the expert community not only since the manipulations at VW. „The procedure has gotten long in the tooth and is not practical enough“, criticises Erik Pellmann. A glance at the distinct phases clearly shows what DEKRA’s expert wants to say: The drives are too uniform and are characterized by almost constant and very slow speeds.

Moreover, the maximum speed of 120 km/h is achieved only for a few seconds. “Due to this driving cycle layout, the systems of start-stop and intelligent generator control get a bigger impact than they have in reality”, Pellmann explains. Of course the car manufacturers utilize the legal scope for their benefit: As the expert explains, they are sending the lightest version of a car model o the test rig. Additional electronics packages or extra equipment remain unconsidered during the testing. Regardless of the topic of VW and the manipulation, there is consensus of the deficits in the testing method; the days of the NEFZ are numbered. The new testing cycle WLTP is expected to replace the European driving cycle from September 2017 on.

In the second part of our series you will learn how the new testing cycle will be implemented by DEKRA.

Part 3/3: Interview with Clemens Klinke, board member of DEKRA SE, about manipulation, tests and increasing costs.

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