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Car emissions testing explained: what you need to know in 2018

Car emissions testing explained: what you need to know in 2018

Vehicle emissions testing is big news, what with the 2015 Volkswagen ‘dieselgate’ scandal that illustrated how at least one car maker was able to beat the system undetected for years, and evidence showing wide discrepancies between results obtained from laboratory testing and those obtained on the road. 

Then, in the November 2017 Budget, drivers were made aware of a little-known emissions test, called RDE Step 2, that, if their new diesel car doesn’t pass, will mean them paying extra road tax from April 2018. 

All of this on top of an emissions-based road tax system that was replaced in April 2017 but which still applies to cars registered prior to that date, the new £10 T-Charge applied to the dirtiest vehicles entering the London congestion zone and the rash of manufacturer scrappage deals aimed at cars that don’t conform to particular emissions regulations – and linking all of them: emissions testing. 

So what are these emissions tests and what new ones can we expect? Read on to find out…

Why and when is emissions testing carried out?

Before an all-new petrol and diesel model, as distinct from just a new car, can be approved for sale in the EU, its exhaust emissions must be tested to ensure they satisfy air quality standards. A variety of pollutants are checked, chief among them CO2, NOx and particulates, which are microscopic particles produced by diesel engines.

There have been a series of standards since the first one, known as Euro 1, was launched in 1992. Each one has been stricter than the last. Currently we’re at Euro 6, launched in 2014.

It’s much stricter even than Euro 5. A so-called second-stage Euro 6 regulation that is stricter still was launched in September 2017.  

Who does the emissions testing and what do they do?

Car makers do the testing but they are witnessed by government agencies to ensure they are carried out correctly. To make sure a car maker can't present a doctored vehicle, test cars are taken at random off the production line. To ensure consistency between different models, accurate records of each car’s tyre pressure and fluid levels, and the air temperature in the test area, are kept.

The test car is placed on a rolling road, which allows the engine to drive the wheels without moving the car, inside a laboratory. The car is put through a series of prescribed tests during which emissions readings are taken by gas analysers located in the car’s exhaust stream. Every type of engine, gearbox and wheel size combination is tested. 

What is the emissions test?

Until September 2017 it was the New European Driving Cycle (NEDC). This test could be performed on a flat road in windless conditions, or on a rolling road in a laboratory. It was devised in the 1980s, so has been used to check new models conform to Euros 1 to 6. 

However, even by the late 1990s it was clear it wasn’t fit for purpose. Scientists had noticed discrepancies between its results and those obtained on the open road. The test was criticised for not being demanding enough while car makers quickly learned how to exploit loopholes in it. 

So, in the wake of ‘dieselgate’, in September 2017 a completely new test, called the Worldwide harmonised Light vehicle Test Procedure, or WLTP for short, was devised to replace it. 

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How does the new WLTP test work?

At first glance, it doesn’t look that different from the old NEDC test. It’s carried out on a rolling road, for a start. However, according to the EU legislators and car makers that devised it, the new test cycle includes higher speeds, more dynamic and representative accelerations and decelerations, and stricter vehicle set-up and measurement conditions than the NEDC.

As a result, they say, these elements make the WLTP test more accurate than the old NEDC test.

When does the WLTP test come into force?

It came into force on all-new models in September 2017 and will apply to all new cars from September 2018.

So the new WLTP test is totally representative of real-world driving?

Not exactly. There will still be discrepancies between results achieved in a laboratory and those achieved on the road. It’s why, along with the new WLTP lab test, another, called the Real Driving Emissions (RDE) test has been devised that takes place on public roads. The authorities say it complements the WLTP test. It does not replace it.  

How does the RDE test work?

As before, the car is taken straight off the production line at random. It’s driven on pubic roads for 90 minutes along a prescribed route and in conditions that satisfy a range of criteria including:

  • Low and high altitudes
  • Year-round temperatures
  • Additional vehicle payload
  • Up- and down-hill driving
  • Urban roads (low speed)
  • Rural roads (medium speed)
  • Motorways (high speed)

How are the exhaust gases measured?

A device, called a Portable Emissions Measuring System (PEMS) that looks like a complex mass of tubes and pipes is fitted to the back of the car and attached to the exhaust pipe.

As the car is driven along, this device analyses the exhaust gases. The weather conditions and the car’s precise location at all times are monitored by a weather station and global positioning system incorporated in the PEMS. 

When does the RDE test come into force?

It comes into force in two stages, called Steps. RDE Step 1 comes into force on all-new model types from September 2017 and on all new cars from September 2019. RDE Step 2 comes into force on all-new model types from January 2020 and on all new cars from January 2021.

What’s the reason for the ‘Steps’?

They’re a way of tackling the gap between the emissions figures obtained in the laboratory and those obtained on the road. The RDE is a tough test and it will take car makers time to develop emissions systems able to cope. Step 1 gives them a breather by permitting a larger discrepancy while Step 2, which comes into force a bit later, narrows the gap substantially.

Why was the RDE test criticised after the November 2017 budget?

In the November 2017 budget it was announced that diesel cars registered from 1 April 2018 and not complying with the RDE Step 2 regulations will, from that month, move into the next road tax band, although only in their first year.

The problem is that very few new diesels are likely to satisfy the regulations so soon. Car makers will need longer to develop Step 2-compliant emissions controls meaning most new diesels will attract a higher rate of road tax in their first year.