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?
Steve Cropley Autocar
2 mins read
7 April 2017

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


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7 April 2017
All the pollutant limits quoted above are I believe the maximum allowed under the current NEDC test regime. It would be interesting to know whether these limits will be revised upwards under the new WLTP test cycle. If the limits are kept the same, they will be even tougher to meet making all combustion engines and particularly diesels more complicated, heavy and expensive.

7 April 2017
Open fires and wood burning stoves produce up to 15 times MORE particulates than diesels, London should really ban them as well - it makes no sense to clamp down hard on diesels for particulates and let other sources carry on unhindered.

7 April 2017
Benzine - one of the most carcinogenic compounds known - is emmited by petrols, not diesels, Steve. And its not just burning petrol that releases it - smell some petrol and youre breathing in benzine.

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.

19 May 2017
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.

20 May 2017
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.

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