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Diesels: Have They Been Falsely Condemned?Back

Whether or not to buy a diesel is a fiercely divisive topic.

Conflicting evidence and changes in public opinion have led to huge speculation about the fate of diesel cars in the UK, and whether or not they have a future on our roads.

Although both petrol and diesel engines produce harmful emissions, nobody can seem to decide which ones are the worst, or how best to tackle the problem.

The European emission standards were introduced in 1993 to regulate emissions of all new vehicles, enforcing increasingly strict standards year on year. The latest Euro 6 standards came into force in September 2015. The aim is to reduce emissions of nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM) and hydrocarbons, which will in turn have a knock-on effect of reducing CO2 emissions.

Diesel vehicles are currently facing bad press, and the early 2000s policy to improve manufacturing and promote diesel ownership appears to have backfired. New evidence suggests that diesels are in fact much more harmful than previously thought, particularly in regard to NOx production. The recent VW Group emissions cheating scandal has only added fuel to the fire.

However, Mike Hawes, Chief Executive of the Society of Motor Manufacturers and Traders (SMMT) thinks that new diesel cars have been unfairly tarnished with the same reputation of their predecessors, and that “Euro 6 diesel cars on sale today are the cleanest in history”. There is concern that biased opinion may discourage drivers from purchasing new Euro 6 models, when in fact they deliver similar reductions in pollutants as their petrol counterparts.

So have diesels been falsely condemned?

Here are some key facts on the topic.

1. Diesel vehicles are not the main source of urban NOx.

A 2014 Transport for London report shows that although vehicles contribute to around half of London’s total NOx emissions, diesel vehicles account for only 11%. Heating of homes and offices is the worst offender, accounting for 16%.

2. Euro 6 emission standards are actually almost the same between petrols and diesels.

The latest requirements for Euro 6 petrol and diesel vehicles are summarised below. Permitted NOx emissions are only fractionally higher in diesels, whereas CO emissions are markedly lower. PM is argued by some to be one of the most harmful pollutants, and must be equally low in petrols and diesels.


Pollutant (g/km)












3. If all Euro 5 diesel London buses were changed to Euro 6, total NOx emissions for London would drop by 7.5%.

A real-world emissions test of the 159 London bus route showed a 95% drop in NOx emissions when replacing a Euro 5 model to a Euro 6. If this change was rolled out to the entire London fleet, an overall 7.5% drop of total NOx emissions would be observed, drastically improving the air quality in the capital. This data suggests that the latest Euro 6 technology really works, and lower emissions are capable even in diesel models.

The main problem when it comes to determining the worst offender, be it petrol or diesel, lies within how emissions are currently tested for.

Lab testing of emissions has thus far proven to be biased and inaccurate, but this will finally be challenged later this year. September will see the implementation of a brand new on-road emissions test, which will assess how emissions are affected by speed, congestion, driving style and road conditions.

Evaluating real-world emissions figures will give greater insight into the differences between petrol and diesel cars, and may put an end to the debate once and for all.

Posted by Maddie Parker on 12/04/2017