Devices of this type include: Ecotek, Tornado Fuel Saver, Spiralmax, Vaporate, PICC; also Acetone as an additive

Petrol naturally exists at room temperature as a liquid. Surprisingly, the liquid form does not in fact burn in an engine. The fuel must first be vaporized before it can be burnt; liquid petrol burns very slowly, while an extremely rapid combustion is required in an engine. It is therefore very important to introduce the fuel to the intake manifold as a fine "mist" which can rapidly vaporize. Large drops of fuel tend to collect on cylinder walls, in the head gasket crevice, etc, and do not take part in the combustion process. This fuel is then released in the exhaust stroke as unburnt hydrocarbons, and represents both toxic emissions and worse fuel consumption.

That's pretty much what the makers of these devices claim, and it is quite correct. What they don't say is that modern fuel injection systems already deliver this extremely fine fuel mist, thanks to years of intensive development. The average drop size from a typical modern injector is around 0.1 mm, almost invisibly small, and so the vaporization is very good anyway. A device to increase atomisation might provide real benefits on an old carburettor-fuelled vehicle, but not on anything with fuel injection. The unburnt hydrocarbon emissions from a modern engine under normal crusing conditions are about 1 - 2% at most of the injected fuel, so even if all of this could be eliminated then the overall fuel saving would be tiny. (At full load engines tend to run rich - excess fuel - and in this case there are substantial quantities of unburnt fuel in the exhaust. But the engine spends so little time under these conditions that the effect on overall fuel consumption is tiny.)

One technology that undoubtedly does improve atomisation is the air-assist injector. This is a normal fuel injector surrounded by a shroud connected to atmospheric air. Air is sucked through fine slots in the shroud and blasts directly into the fuel spray:

This device is state-of-the art in fuel atomisation. The air enters the injector at several hundred metres per second and hits the fuel spray at the best possible place, right as it emerges from the injector. The result is much smaller fuel drops, which can give significantly reduced hydrocarbon emissions under cold running conditions. As a result, they are already fitted to many production cars.

OK, so what's the problem? Simply that the overall hydrocarbons benefit in the real world is quite small, and the fuel economy benefit tiny. I have made measurements on many different types of injector and even under laboratory conditions a fuel economy improvement on a warm or hot engine is almost undetectable. I have even tested a device that completely vaporizes the fuel on a heated surface*, eliminating fuel droplets altogether, and the economy improvement was tiny (1 - 2% at most). Now consider that most "atomisation improvement devices" fit well away from the point where the fuel injector is located, and ask yourself if they can really provide even these small benefits.

For even more conclusive proof, consider the case of cars fuelled by Liquid Petroleum Gas (LPG). This boils at -30C and so exists entirely in gas form in the engine. If poorly vaporized fuel were such a major cause of inefficiency, an LPG car should be much more economical since there are no fuel droplets at all to give an "incomplete" burn.

The 2004 Vauxhall (Opel) Astra 1.6 was available in the UK powered by either petrol or LPG. This is a factory-fit conversion, in which the ECU has been properly optimised for use on LPG, and the LPG version achieves 30.4 mpg on the standard test cycle. Because LPG is less dense and has a different calorific value to petrol (see appendix), the LPG figure is the equivalent of 39.2 mpg on petrol. Since the LPG car doesn't lose anything from poor vaporization, the petrol car must be worse than that - right? Actually, wrong - the petrol car achieves 42.8 mpg which is 8% better. Clearly the petrol car does not lose any economy at all through problems of atomisation or vaporization, and this is typical for a modern fuel-injected engine.

For more detailed information on one of these devices, see the Vaporate Case Study.

Some people have claimed that certain fuel additives - Acetone has been mentioned in this context - can greatly increase economy by making the fuel vaporize better. Exactly the same arguments as described above make this highly unlikely, quite apart from the potential danger to engine components from some "non-approved" additives. See the full Case Study for more details.

* As an aside, the idea that improved vaporization would save enormous amounts of fuel has been around for a very long time - at least since 1936 and Charles Pogue's "200 mpg carburettor". This device supposedly gave fantastic fuel savings by heating the fuel to help it vaporize. Well, I have personally tested the modern-day equivalent and the saving is a couple of percent at best.

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

If you found this page helpful, you may like to support my work. If you think I have made a mistake, or am talking complete nonsense, please take a moment to read the Response to Critics before saying so in public (on discussion Forums and the like).

Appendix: Energy content of LPG and petrol

LPG has a calorific value of 46.1 MJ per kg, and a density of 0.54 kg per litre.
Hence the energy content is 24.9 MJ per litre

Typical European petrol has a calorific value of 43.2 MJ per kg, and a density of 0.74 kg per litre.
Hence the energy content is 32.2 MJ per litre

So, assuming equal combustion efficiency, a petrol car would expect to travel 32.2 / 24.9 = 1.29 times further per gallon than an LPG car.

(These figures are approximate, since both LPG and petrol vary in composition, and hence energy content, throughout the world. For example, some figures from the USA suggest a factor of 104 / 79.5 = 1.31. This does not change the fundamentals of the calculation, though - the LPG car is still not significantly more economical than the petrol version.)