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Is there a case for targeting diesel air quality measures?

Would a targeted response based on real world emissions data deliver less disruption to car owners and the car industry while still achieving clean air goals?

Firstly it's useful to have an idea of nature of the problem with diesel car emissions.  The real world emissions of different new diesel cars and vans vary wildly.

Lets have a look at chart below from the ICCT in September 2017. It shows the real world emissions of new cars that should conform to the Euro 6 standard NOx emissions standard of 80mg.

The chart shows that at the 2 extremes around 10% of Euro 6 cars actually comply with the legal Euro 6 limits when they are driven, while the worst cars are 24x over the limit. Most cars are in between and all are referred to by the car industry as "clean Euro 6 diesels"

  • Using the right engine and clean up technology like adblue these diesels show a diesel can be clean.
  • The only doubt concerns whether these pollution reduction systems are as effective in urban traffic or in the first 7 minutes of a journey and whether they are programmed to switch off by carmakers. Many cars tested before the new Euro 6 RDE test switch off emssions control equipment below a fixed air temperature usually between 10c and 17c according to the car. Software fixes can adjust this.
  • Just over 10% of Euro 6 diesels comply with legal limits on the road and nearly 9 in 10 don't!
  • When car makers realised what the effect of real world emission tests would have on the results they lobbied for a conformity factor.
  • Effectively this is like knowing you're going to flunk your new school exams and paying the exam board to call it a pass. They chose a factor of 2.1 so in Euroland it's like a 30% exam fail being an impressive 63% pass when your results arrive.
  • Being fair even a 2.1 factor on a real world test is very demanding for European diesels. Only 26% of Euro6 would pass - leaving 74% that wouldn't. Passing will be expensive and difficult to justify on smaller cheaper cars.
  • Cars in this group have emissions of just under 3 petrol cars.
  • This group has emissions of between 2.1 and 10x the legal Euro 6 emissions limit when on the road.
  • 61% of diesel cars on sale fall into this band and 87% of all diesel cars emit under 10x the legal limit!
  • These cars emit the equivalent of between 3 and 13 petrol cars for toxic NOx.
  • These 13% of diesel cars fail Euro 6 in an extraordinary way. There is really no excuse for these cars to continue on sale or to be exempt from urban diesel bans or charges.
  • They emit between 10 and 24 times the toxic diesel NOx limits on the road or as much as between 13 and 32 petrol cars.
  • These cars should be targeted for diesel scrappage schemes. Many in this group are worse than an average Euro 5 diesel.

How many dirty diesels should you target?

A selective approach means some diesels cleaner diesels would be allowed to drive without penalties in urban areas. Allowing diesels than "only" fail Euro 6 by 2.1 times still means measures applying against 74% of diesels while the diesels you allow still emit as much as 3 petrol cars worth of emissions!

Fail 2.1 x
Euro 6

Fail 4x
Euro 6


Fail Euro 6 by
more than 10x

Selecting only dirty diesels for air quality measures would make more sense if there was a roughly equal number of clean and dirty diesel models. But if you define a clean diesel as being able to operate within it's legal 80mg/km NOx limit on the road (itself a higher limit than petrol cars) then only 10% would pass. Models passing the new EU tests from September 2017 will do better but the dirty current models are will remain on sale till 2019. So the question it economic to help really clean diesels continue when there are so few of them to save?