Currently reading: Diesel engines: what comes out of your car's tailpipe?
Diesels are getting a bad rap in the press, but why are the government clamping down on emissions so forcefully?

Carbon dioxide (CO2) makes up the vast amount of emissions from your car’s tailpipe, and this has formed the basis for car taxation policy in recent years and has therefore received the most publicity. New car CO2 emissions in Europe have to average 95g/km by 2021.

Carbon monoxide (CO) is another wellknown pollutant from a car’s exhaust, one that contributes to smog but one that catalytic converters have helped to reduce from cars considerably. EU6 emissions regulations place limits on CO at 0.5g/km for diesels and 1.0g/km for petrols, which produce more of it.

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Nitrogen oxides (NOx) — which include nitrous oxide (N2O) and nitrogen dioxide (NO2) — is what hit the headlines, with VW cheating the emissions testing system in the US and beyond by making the cars produce up to 40 times less NOx when undergoing emissions tests than in the real-world. AdBlue and other aqueous urea solution systems are designed to reduce NOx emissions, which are limited to 0.08g/km in diesels and 0.06g/km in petrols.

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Particulates — tiny particles of solid or liquid matter — are produced mainly by diesel engines, and in trace amounts in petrols. They range from plain old soot to much more toxic substances and are a major health concern that particulate filters have sought to reduce. EU6 caps these at 0.005g/km.

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Hydrocarbons (THC and NMHC), including benzene and isooctane, are also in exhaust gases. These are effectively unburnt fuel molecules and many are highly toxic. Other emissions found in small amounts include sulphur dioxide (SO2), nitrogen (which isn’t harmful) and water vapour. 

Diesel engines: your questions answered

Steve Cropley

Steve Cropley Autocar
Title: Editor-in-chief

Steve Cropley is the oldest of Autocar’s editorial team, or the most experienced if you want to be polite about it. He joined over 30 years ago, and has driven many cars and interviewed many people in half a century in the business. 

Cropley, who regards himself as the magazine’s “long stop”, has seen many changes since Autocar was a print-only affair, but claims that in such a fast moving environment he has little appetite for looking back. 

He has been surprised and delighted by the generous reception afforded the My Week In Cars podcast he makes with long suffering colleague Matt Prior, and calls it the most enjoyable part of his working week.

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ericmorton 20 May 2017

Diesel Engine

Vehicle emission is now a serious issue. We all know the fact that our vehicle produces carbon monoxide in large amount which is very much harmful to the atmosphere. Most probably due to bad fuel or any dust in the engine oil we have face large amount of carbon monoxide problems. So, it is quite better to maintain our vehicle in good condition with good fuel. So that it produce less smoke from the tailpipe.
dipdaddy 19 May 2017

it was lies to begin with and feeding the corporates

I sometimes feel that the corporate companies are behind encouraging the governments to push us to go for diesels. The demand has to come from somewhere in order to invest in the product and marketing. I can't believe for one second that the scientists (if there were any to begin with) didn't analyse and see the dangers of diesel fumes and various gases coming from it. Any baboon (scientist!) could have seen these harmful gases, realize their effects and if this multiplied by 50,000 or more then what bad effect it could have on people and environment.

It seems manufacturers, oil companies, the governments have pocketed a lot of money from this diesel boom, and now the government has an excuse to bump up taxes on diesels for being harmful now? i wonder what these scientists are being paid stay quiet?

Someone be it a corporate or government institute or an associate organisation needs to be held accountable for this. if the evidence shows that they knew in hindsight about these harmful gases then they should be punished. I'd rather prefer an media outlet do some investigative journalism into this than just point out the obvious on what we already know about few months back.

Neil2129 7 April 2017


Surely the greatest gas being emitted from vehicle tailpipes is Nitrogen, given that the air being taken in is around 78% Nitrogen, and most of that is unaffected by the combustion process. There is no oxygen created in the engine, so the amount (in molecules) of CO2 emitted cannot possible exceed the amount of O2 ingested. Taking account of the increased mass of CO2 vs O2, the maximum percentage by mass that CO2 can possibly achieve is around 28%, while Nitrogen still makes up around 70%.
OK, it isn't a pollutant, but it is still emitted.