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171 While writing a ludicrously fefl large cheque for the road tax on my BMW M3,1 got thinking about C02 emissions. At 290g/km, could a single long journey really produce my own body weight in C02?

The official combined-cycle fuel consumption figure for my car is around 23mpg,so by my calculations 23mpg= 37km/gallon = 8.1 km/litre. Since petrol weighs around 750g per litre, the weight of the petrol that my car burns in one (combined-cycle) kilometre is only 92g. Where does the other 198g come from? Surely the weight of the C02 in my petrol cannot exceed the weight of the petrol? - James Rush

Your car really does chuck out a higher mass of C02 than it consumes as fuel, and of course you can force even more out of the tailpipe with a little 'spirited' driving.

But it's not magic. Petrol and diesel are known as hydrocarbons because they are mace principally of hydrogen and carbon, but a molecule of C02 is mostly made of two oxygen atoms, and diese come from die vast amount of air an engine munches through. In fact, a petrol engine uses over 14 grams of air for each gram cf fuel it consumes. In your case that works out as something in the region of I .4kg of air per km, which is about I 100 litres.

Oxygen is surprisingly heavy stuff. The two atoms in a molecule of C02 weigh over five times more than the carbon atom they are attached to. However, in your calculation you have used the entire weight of fuel to get 92g/km, buc only about 50g of that fuel is carbon, the rest is hydrogen, which is mostly exhausted as water (H20).

Also, not all the carbon is turned to C02. Some fails tc bum and exits as hydrocarbon - this is the HC bit on your MCT emissions test - although catalytic converters turn HC into more C02 and a little bit of water.

It is quite astounding just how much C02 a car throws out. A 90-litre tankful of petrol turns into 200kg of greenhouse gas, so every five fill-ups results in a tonne of the stuff being blasted into our atmosphere So make sure it was worthwhile - RH

Milltek Sport ¬°PMj official fitting centre

The mass of C02 emitted by a car is many times the mass cf the fuel you put in -you can thank air for that

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