Diesel Emissions - What does it all mean?

Emissions from our vehicles are frequently discussed, by everyone from journalists to protesters to politicians. Unfortunately they are very commonly massively misunderstood, even by those making legislation or campaigning one way or the other.

We have put together a good explanation of the different types of exhaust emissions from your vehicle, what technologies exist to handle them, and how they work.

CO2 - Carbon Dioxide

This is probably the most commonly discussed Diesel exhaust emission. 

CO2 is a very common greenhouse gas, this means that it contributes to global warming.

It is however non-toxic. CO2 is the bubbles in Coca-Cola or a pint of beer.

As a result it does not much matter where specifically CO2 is emitted, CO2 will not harm the local air quality.

In the UK the CO2 figure appears on the V5 and is used as part of taxation criteria for most vehicles.

CO2 figures are directly connected to fuel economy, effectively producing a lot of CO2 requires combustion of a lot of fuel.

PM - Particulate Matter (Soot)

Particulate matter is a fancy name for exhaust soot. The black soot that we typically associate with "dirty" diesels is Particulate Matter (PM).

PM damages local air quality, and high levels of PM have been linked with respiratory illnesses.

In order to reduce PM the primary change to modern Diesels has been the fitting of DPF's.

Check out our DPF article here:

DPF (Diesel Particulate Filter) systems

In the UK the annual MOT tests for PM by measuring exhaust opacity. The UK MOT does not check CO2 or NOx directly.

NOx - Oxides of Nitrogen

NOx are a collection of compounds of nitrogen and oxygen. They are formed at extreme temperatures inside a Diesel engine, where normally inert Nitrogen can fuse with Oxygen.

They are harmful for a variety of reasons:

  • They are a greenhouse gas (contribute to global warming).
  • Smog and low level Ozone is caused by NOx.
  • They are linked to respiratory illnesses and are suspected to be carcinogenic (cause cancer).
  • Acid rain is caused by NOx. 

Many cities now have low emissions zones, leading many motorists to be confused as to why they are being told that a car which has low CO2 emissions has "high emissions".

EGR and SCR (AdBlue) systems are both primarily designed to reduce NOx.

We have articles about those here:

SCR (AdBlue) systems

EGR Systems

Because of these reasons it matters not only how much NOx and PM are emitted, but also where they are emitted.

In the middle of nowhere, a small amount of NOx and PM don't have a substantial local effect,

Arguably it has been short sighted of successive UK governments to tax motorists based upon CO2, as given that fuel is already taxed, there is already a tax that is directly linked to CO2.

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