How much fuel gets burnt in the engine? I say at least 98 per cent, the miracle technology / snake oil salesmen say 85 – 90 per cent. Who is right? Me, of course and I can prove it, not with pseudo scientific mumbo jumbo, but by analysing what the MOT emissions limits mean for engine efficiency. It involves a little basic science and arithmetic, but nothing more than you covered at secondary school (even if you didn’t pay much attention!)

The evidence that the snake oil men are talking nonsense lies in the MOT test exhaust limits for Hydrocarbons, or HC. Fuel is a compound of hydrogen and carbon, generally referred to as HC. When it burns it combines with oxygen to produce carbon dioxide (CO2) and water (H2O). Engines aren’t perfect, so not all the fuel burns. Any unburned fuel leaves the engine as HC with the rest of the exhaust gas. You don’t want HC in the atmosphere because it causes all sorts of smog and health problems. In other words, it’s a pollutant, so on modern cars it’s converted to CO2 and water by the catalytic converter. But cars made before 1992 didn’t have catalytic converters, so the unburned HC did come out of the tailpipe and the MOT test measured it and set a limit for it. The limit is 1,200 parts per million – a higher figure means the car fails the MOT and cannot be driven on the road.

Now we need to work out how much HC goes into the engine. It is mixed with air in a ratio of about 1 to 15 parts by weight (or, strictly speaking, mass). That ratio (known as the stoichiometric ratio) means that there is just the right amount of oxygen to burn all the fuel. Now, 1,200 parts per million is a measure of volume, whereas the 1 to 15 is by mass, so we need to convert the mass to volume. A cubic metre of air weighs just under 1.3 kg and a cubic metre of petrol vapour weighs just over 2 kg. (That figure is actually for ethanol vapour, but it’s virtually the same for petrol vapour). A bit of simple arithmetic shows that the 1 to 15 ratio by mass becomes 1 to 23 by volume. Expressing that ratio as parts per million gives us the following:

For every million volumes of petrol and air drawn into the engine, 41,667 of them will be petrol and 958,333 of them will be air. (The volumes can be cc, litres, cubic inches, whatever you like so long as it’s a volume and there are a million of them). In the exhaust there must be less than 1,200 volumes of unburned fuel. 1,200 is just under 3 per cent of the 41,667 that went in, so a minimum of 97 per cent of the fuel must be burnt in the cylinders for the car to be allowed on the road.

Don’t forget that 1,200 is the maximum. Most cars – even from twenty years ago – were well within that limit, in fact a quick flick through an old car data manual shows that the exhaust HC specification for virtually all cars by 1990 was 300 parts per million, giving an unburnt fuel figure of 0.7 per cent, or to put it the other way round, more than 99 per cent of the fuel is burnt in the cylinder. Now bear in mind that a petrol family car from 1990 would typically do about 30 mpg whereas today we expect about 45 mpg or more and you get an idea of how efficiently modern engines burn the fuel. (I accept that there’s some carbon monoxide in the exhaust as well, which is partially burned fuel, but even adding that on still won’t get anywhere near the claim that 10 to 15 per cent of the fuel is wasted.)