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New WLTP/RDE fuel economy and emissions tests could still be unrealistic

New WLTP/RDE fuel economy and emissions tests could still be unrealistic
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Steve Fowler

2018-01-25 23:00

Mercedes says new WLTP and RDE economy tests won’t fix problems of existing NEDC tests not replicating real world driving

Real world emissions testing

The new WLTP/RDE (World Harmonised Light Vehicle Testing Procedure/Real Driving Emissions) tests for car fuel economy and emissions coming into effect this year are still unlikely to match real-world figures, according to Mercedes.

Speaking exclusively to Auto Express, Mercedes’ Dr Christoph Höhmann, in charge of Certification of Emissions and Consumption in the EU, said, “It’s not for us to introduce WLTP, but we stand behind it.”

Real world emissions tests: everything you need to know

Asked if he thought the new testing regime was the right thing to do, he continued, “It’s better than the old one. But if the expectation is that everything will be fixed, then in that respect it’s not right.”

Dr Höhmann went on to give an example of the new RDE tests, which includes on-road analysis of emissions with a PEMS (Portable Emissions Measurement System) device. “The framework [for RDE] does not say when you have to shift gears or whether you should be using items like the air conditioning or heated rear screen during the test,” he told us. “The air con could have a huge effect, but you don’t have to turn it on [in the test] and you will probably have it on in the real world.”

Every new model introduced since September 2017 must have been put through the WLTP testing programme, but every car registered from September 2018 – including existing models – is expected to have been tested and to have had its new figures made available to buyers.

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The new programme aims to provide a more real-world figure for consumers to base their buying decisions on, with the test split into two parts – the laboratory WLTP test and on-road RDE assessment. 

How will the WLTP and RDE emissions tests work?

Unlike the old NEDC (New European Drive Cycle) tests that were introduced in 1992, WLTP runs over a longer distance and time, with greater extremes of speed and variation in temperature to supposedly give a more realistic guide to fuel economy and emissions.

WLTP will provide figures for fully electric and hybrid models and will also provide different results depending on options fitted to cars, although many of the figures will be as a result of computer calculations rather than actual tests. This should be reflected in online car configurators, with manufacturers showing the effect on economy and emissions when extra equipment is added.

Every car maker is in a race to get every model certified under the WLTP protocol before September, or it will be illegal to sell the car under EU law. However, due to most countries, including the UK, basing taxation on old NEDC data, manufacturers will still have to provide those figures, too. That doesn’t necessarily mean an additional test, though – the EU has developed a computer programme, known as CO2MPAS, to replicate NEDC figures from WLTP tests. But there is still the option to retest under old NEDC conditions if the manufacturer disputes the CO2MPAS calculation.

Audi Q3 - exhaust

The new Mercedes CLS, on sale now and arriving in April, is the first Mercedes-Benz to be certified according to WLTP under the Euro 6d?TEMP emissions standard. That means the diesel version will be one of the first cars to avoid the increase in VED rate for diesels being introduced in the UK from April 1 this year. 

The WLTP regulations run to over 700 pages, with Mercedes estimating that the new regime takes around twice as much effort as testing under NEDC. To avoid any cheating, all WLTP tests are overseen by an independent body, while 50 per cent of the RDE tests have to be carried out by an external organisation. Additional independent tests will also be carried out, although manufacturers won’t know when or where these spot checks will be done.

Now check out our list of the most economical cars on sale...

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