Many fuel "saving" products claims that, even on reasonably modern engines in good condition, it is common for 15 or even 20% of the input fuel to emerge unburnt from the engine (to then either escape into the atmosphere as pollution, or be burnt in the catalyst without providing useful work). Clearly, if this large unburnt fuel fraction can be reduced, then fuel economy can be greatly improved.

In fact, any modern engine in good condition and at normal operating temperature emits only about 1 - 2% of the input fuel as unburnt hydrocarbons, even before passing through the catalyst. I have studied test data for literally hundreds of engines over the past 10 years and this is absolutely consistant for all engine types. Only engines that are very old (say more than 50 years) or in serious need of maintenance/adjustment would waste anything like 15 - 20% of their fuel in this way.

This can easily be proved by looking at exhaust emissions regulations applying to cars without catalysts. Some appropriate examples would be the Australian ADR27 (1974 - 1976) and the European ECE R15/02 [page 37 of this very large document] (up to 1979). Both are based on the 4 km long ECE 15 test cycle, and impose limits of around 10 grammes of hydrocarbons per test.

Now, a vehicle with an urban fuel consumption of 35 mpg (8 litres/100 km) would use around 0.32 litres of fuel during this test, which given the density of petrol (around 0.75) equates to 240 grammes. So the maximum allowable amount of unburnt fuel released during the test is 10 / 240 = 4.1% - and remember this is the maximum; most vehicles, even those sold in the 1970s, would have an unburnt fuel figure well below this. Since then, limits have continued to fall.

Another way of looking at it is to consider the maximum hydrocarbon concentration (ppm) in the exhaust. The UK's in-service emissions test (the MOT test) specifies a maximum limit of 1200 ppm (parts per million) of exhaust hydrocarbons. To calculate this as a proportion of input fuel, we need to know the concentration of hydrocarbons in the input fuel/air mixture.

As a simple estimate, assume that the fuel used is propane (C₃H₈). This burns with air in the ratio 15 parts air to 1 part propane (by mass); since the density of propane is 2.0 kg/m3 while air is 1.3, that means 23:1 by volume. So the input fuel/air mixture contains around 45 000 ppm of hydrocarbons, which means that the MOT test limit of 1200 represents 2.7%. Again remember that this is a maximum limit, above which the vehicle may not be used on the road; most vehicles have very much lower emissions. Of course this analysis is highly simplified, since the characteristics of petrol and propane are different - but not that different.

This observation - that the unburnt hydrocarbons in the exhaust represent no more than one or two percent of the input fuel - also leads to another important point: Further reductions in emissions, while obviously good for the environment, do not automatically imply a significant improvement in fuel economy. Various fuel "saving" devices point to reductions in emissions - say from 100 ppm to 50 ppm - as clear proof of improved fuel consumption. Since even 100 ppm unburnt hydrocarbons in the exhaust equates to about 0.02% of the input fuel, clearly this is not the case! Additionally, emissions measurements naturally show quite a lot of variability from test to test, especially given changes in parameters such as catalyst temperature (after a long drive, the "cat" will be hotter and so better at removing the toxic emissions). For these reasons, the US Environmental Protection Agency's guide to testing fuel "saving" devices specifically excludes emission measurements - the Inspection & Maintenance check - as evidence for the effectiveness of such products.

Some commentators have observed that this analysis neglects the effect of fuel that escapes the main combustion process (for example because it is trapped in the head gasket crevice) then emerges near the end of the working stroke and burns as the exhaust gas exits the cylinder. This fuel does not show up as unburnt fuel in the exhaust, but on the other hand makes little contribution to the useful work output of the engine. Data for this effect is not easy to come by, but the few reports I have seen suggest it is of similar magnitude to the unburnt fuel (so the 1 - 2% figure mentioned at the beginning of this page could perhaps be raised as high as 3 - 4%; still well below the 15 - 20% that the fuel "saving" device makers typically claim).

Also, don't forget that this "late burnt" fuel causes an increase in exhaust temperature. If a large reduction in "wasted" fuel really could be obtained, it would cause an equally large reduction in exhaust temperature. The catalyst would stop working (it needs to be hot), and emissions of toxic pollutants would rise dramatically. So arguably, even if it were possible to save a significant amount of fuel this way (which it isn't), the increase in pollution means it would not be a good idea.

Another suggestion is that the unburnt fuel goes past the piston rings and ends up in the engine sump. Well, certainly it is true that immediately after a cold start, when the intake system is cold and the fuel does not vaporize properly, a significant amount of petrol does find its way into the sump. But as the engine warms up, the petrol boils off again and passes through the crankcase breather system into the intake manifold, where it merges with the incoming fuel/air mixture and is burnt in the usual way. Clearly, there can be no large net flow into the sump, otherwise petrol would soon be pouring out of the dipstick tube!

(Having said that, on vehicles that are only ever used on short journeys and so never warm up properly, oil dilution by petrol can be a serious concern. That's why such vehicles should be taken for a good "thrash" every so often, and have their oil changed more regularly than normal. But even then, the actual amount of petrol involved, as a fraction of the amount burnt, is still tiny.)

Please also read the general comments on fuel "saving" devices, if you have not done so already