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Emission measurement on ancient diesel: driving bans for vintage cars pointless?

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D he W 123 series is the best-selling from Mercedes. It is considered to be particularly solid, and its models are correspondingly durable and popular. This is exactly why the Mercedes-Benz W123-Club has existed since 1996, which has been run by the manufacturer as an official brand club since 1998. A long-time club member drives a W 123 (240 D) from 1982.

At that time, a Euro emissions standard was far from in sight. The club member's Mercedes limousine has 343,000 kilometers on the clock and an oxidation catalyst from Oberland Mangold in the exhaust system, which probably cleaned it up in accordance with Euro 1 in the 1990s. The catalytic converter only oxidizes hydrocarbons (HC) and carbon monoxide (CO), it has no influence on nitrogen oxides (NOX). But they are part of the emissions, especially from diesel vehicles. And they are harmful to health, which is why diesel engines have fallen into disrepute, especially since the VW emissions scandal, and are threatened by driving bans in many places if they do not meet the Euro 6 emissions standard. This has been in effect since 2014.

State-of-the-art emission measurements on a vintage car

The assumption: If six-year-old diesels are supposed to be too dirty for the city, then 38-year-old oil burners like the one would have to be 240 D can be the purest environmental disaster. An assessment that classic car drivers face from many angles. The club's 240-D driver works for a car manufacturer, also deals with diesel exhaust emissions and does not share this assessment. To substantiate this with arguments, he did what he does in his job with his oldie: measuring emissions. In accordance with the latest legal requirements, i.e. also in real road traffic (Real Driving Emissions, RDE) with mobile measuring devices (Portable Emission Measurement System, PEMS).

The club initiated an RDE ride at the service provider AKKA, and in autumn 2019 the W123 enthusiast drove his only 72 hp diesel with a 70 kg PEMS from AVL on the trailer hitch at 8 degrees through the Remstal, among other places. 'To achieve 120 km /h on the B29, it took 70 percent pedal travel, and despite full throttle I struggled to make the 60 km /h for the RDE cross-country trip on inclines,' the driver then reported. This certainly had no moderating effect on emissions. But theRDE-compliant route succeeded according to the specifications.

The RDE round brought the old Mercedes diesel to its performance limits.

Ancient -Diesel no dirtier than (many) EU6 diesel

The result of two calibrated trips: The W 123 emitted an average of 808 mg NOX per kilometer. It is only slightly above a 34 years younger, similarly sized, but around twice as powerful Mercedes CLA 200 d and in the solid midfield of the measurement results that auto motor und sport has regularly determined together with the experts from Emissions Analytics since the diesel scandal - under well comparable conditions: according to the RDE specifications, monitored by a measurement engineer on board, with PEMS, in real road traffic.

auto motor und sport
There are modern diesels that emit more NOx than the 240 D from 1982.

The suspicion that vintage cars are particularly harmful to the city air with high NO X emissions would be invalidated. Added to this is the low average annual mileage of such old cars. Their banishment from the cities would only bring an improvement for the city air if they were replaced by a completely new Euro 6d Temp diesel. At 808 mg /km, the 240 D, like many early Euro 6 diesels, is ten times above the limit of 80 mg /km. So it does not speak for it that it is particularly clean, only that it is not dirtier than many much younger diesels.

That begs the question: Why are early Euro 6 engines in terms of emissions so bad? There are two for thatReasons: One is of all things technical progress. With charging and direct injection, the output increased (with the CLA 200 d compared to the 240 D by around 100 percent), while fuel consumption fell (by around 23 percent) despite increased comfort and better safety, which increased the weight by around 15 percent. However, the engines have become more efficient due to the significantly higher combustion pressure - which in particular increases NOx emissions. This, too, can be got under control with technical effort, as the very modern diesels prove, which undercut the NOx limit value by a factor of four to ten.

Limit values ​​versus measurements on the test bench and in practice

The fact that this has only recently been working is due - second reason - to emissions legislation. As the graphic shows, the statutory NOx limit fell rapidly from 2000 to 2014. However, real emissions did not go along with the degression. Because they weren't measured. Rather, the limits only had to be adhered to on the test bench under laboratory conditions in the less demanding NEDC. The car manufacturers developed towards this - as well as more power and efficiency.

The CO2 balance of the diesel did that good, in practice even more than on paper. It was the opposite for NOx emissions. When VW was exposed to the exhaust gas fraud, the discrepancy between laboratory limit values ​​and real emissions came to light. RDE measurements only became mandatory with the introduction of the Euro 6d-Temp emissions standard.

auto motor und sport
The development of limit values ​​and real measured values ​​is phased in opposite directions.

These also spat out - as a by-product, so to speak - the aforementioned high NOx values ​​of earlier Euro 6 vehicles. The current limit values ​​can almost only be achieved with complex exhaust gas aftertreatment using SCR catalysts and urea injection, in some cases even two SCR systems are used. Modern high-performance diesels often surpass their prechamber predecessors à la 240 D in terms of raw emissions as much as they do in terms of performance and efficiency.


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