Diesel engines: what comes out of your car's tailpipe?

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