Steve Walker
26/04/2011 08:10 | By Steve Walker

CO2 and other car emissions explained

CO2 and other car emissions explained (© PA)

As motorists, we all know a little bit about vehicle emissions. When we drive our cars, emissions are what comes out of the exhaust pipe and the quantity that's expelled has a big role to play in determining the amount of tax we pay. This much we know, but there's more.

It's tempting to think that car exhaust emissions and carbon dioxide (CO2) are one and the same thing. CO2 certainly gets top billing but there are other less well-known substances contained within the cocktail of gasses chugging out of our cars.

These can also have harmful effects and the ways in which government legislation seeks to control them could have a significant impact on the kinds of cars we all drive in the future.

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CO2 (© PA)

Carbon dioxide is widely believed to be a major contributing factor in global warming and the European Commission has laid down targets designed to limit the amount of it being released into the atmosphere.

That's the main reason why the VED road tax and BIK company car tax motorists pay is calculated based on the amount of CO2 their cars emit. Models with lower CO2 emissions also attract lower rates of the one-off showroom tax on new cars and if a car has very low CO2 emissions it can avoid paying the London congestion charge completely.

There's another important way that low CO2 can pay off too. The amount of CO2 released by a car is directly linked to the amount of fuel it's burning. Cars with the lowest CO2 outputs are also the most fuel efficient and with fuel prices being what they are, using less fuel can mean more big savings.

Global warming (© PA)

CO2 is measured in grams per kilometre. That's the weight in grams of the carbon dioxide released per kilometre driven. The official CO2 rating that's attributed to new cars is an average figure that's calculated in the same way as a car's official combined fuel economy.

A car will emit more or less CO2 depending on where and how it's driven so its impact on the environment could vary but only the official figure is used for taxation purposes.

With such big financial savings up for grabs by choosing low CO2 cars it's no surprise that the gas has become engrained in motorists' thinking over recent years. But there's more to car exhaust gases than you might think.

Nitrogen oxides, carbon monoxide, particulates, hydrocarbons and more

Harmful emissions (© PA)

This is where things begin to sound suspiciously like a chemistry lesson but stay with us. CO2 is by no means the only potentially harmful substance emitted by modern cars with internal combustion engines.

An analysis of a vehicle's exhaust output will also turn up varying levels of carbon monoxide, nitrogen oxides, sulphur dioxide, polycyclic hydrocarbons, particulate matter and other substances that aren't particularly beneficial for the environment or the people who breathe them in.

Nitrogen oxides (NOx) and sulphur dioxide (SO2) contribute to acid rain while carbon monoxide (CO), hydrocarbons (HC) and particulates (PM) are linked to a range of respiratory health problems. Whereas the amount of CO2 emitted by our cars is heavily publicized, emissions of these other substances have a far lower profile.

They are controlled though. Since 1993, car manufacturers have had to conform to a series of European emissions standards which set maximum limits for harmful substances emitted by passenger cars.

The limits vary for petrol and diesel cars and have grown progressively tougher over time. The Euro 5 standard came into force in 2010 and manufacturers are currently striving to meet the Euro 6 standard due in 2014.

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Car exhaust emissions (© PA)

Now you might think that a car which has low CO2 emissions will also produce low levels of these other harmful exhaust gases but that's not always the case. In reality, models with low CO2 outputs and high fuel economy can be amongst the highest emitters of these other undesirable substances (see the table below). This puts the car buyer looking to do their bit for the environment in a bit of a spot.

Information on CO2 emissions levels for new cars is widely available in the UK market. Every new vehicle in showrooms is required to display the Vehicle Certification Agency's point of sale sticker which clearly shows its CO2 emissions and fuel consumption but details of other emissions aren't included.

If you want to know how much NOx or CO your car is pumping out, you can visit the VCA website and search for that specific model. Most manufacturers publish the figures for their cars but they aren't given anything like the same billing as the CO2 emissions ratings, partly because low NOx emission don't bring anything like the same tax benefits as low CO2.

Different markets, different priorities

Fuel pumps (© PA)

In Europe, as far as vehicle emissions are concerned, CO2 has been king but that's changing. The most recent Euro standards are bringing the allowable levels of harmful emissions closer to what's been required in the US and Japanese markets for some time.

The European focus on controlling CO2 has helped boost the popularity of diesel engines and they currently account for roughly 50% of the market in Western Europe. A diesel will usually be more fuel efficient and emit less CO2 than its petrol counterparts but it will also tend to produce more of the other nasties that exhaust gases contain.

In the US and Japan, sales of diesel cars are growing but they remain far lower than in Europe, partly because they need to be fitted with expensive exhaust after-treatment technologies in order to meet the tough local emissions standards for NOx, CO and particulate matter. Historically, it's been far easier for manufacturers to meet these standards and keep prices affordable using petrol or hybrid power.

Emissions from industry (© PA)

As the Euro emissions standards get tougher we're likely to see a similar effect in the domestic market. Manufacturers are already predicting that diesel engines with the advanced technology needed to meet tightening emissions standards could become too expensive for use in smaller cars.

Compact petrol engines look like becoming more popular at the affordable end of the market in future but the latest clean diesel engines should continue to make sense in mid to higher-priced models. Indeed, the latest clean diesels are starting to get a foothold in the US with manufacturers like Volkswagen and BMW reporting that sales of their diesel models are on the up.

CarFuelMPG (combined)CO2 (g/km)CO (mg/km)NOx (mg/km)
Citroen C1 1.0i 3-doorpetrol61.410634019
MINI One D 1.6diesel74.399265173
Ford Focus 1.6 Ti-VCT 125PS 5-doorpetrol47.913634251
SEAT Leon 2.0 TDI 140PSdiesel58.9125349133
Audi A6 2.4 V6petrol29.123160417
BMW 535d Gran Turismodiesel42.2175342130

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