Devices of this type include: Broquet, Fitch Fuel Catalyst, Prozone, Fuelstar, Fuelcat, Enviromax Plus, Vitalizer, Firepower, PICC, Euro FuelSaver

A vast array of usually tin-based products, either dropped in the tank or fitted in the fuel line, claim to improve the fuel quality and so improve power and economy. (NB This page is about "fit and forget" devices; for information on products to be added at every tank fill, see the fuel additives page.)

The first point to note is that tin is not generally regarded as an efficient catalyst for hydrocarbons. The "catalytic cracking" systems in oil refineries often cited by makers of these devices in fact use Zeolites, composed mostly of aluminium and silicon. The catalytic converters in vehicle exhausts use platinum, rhodium and palladium. Tin is not a significant component in either of these catalysts.

A second general point is that claims to have improved Russian fuel in the Second World War do not mean that the device will do anything to modern fuel. Refinery technology has moved on enormously in the past sixty years; modern petrol and diesel fuel is a carefully-engineered product almost ideally matched to the requirements of the engines it is used in.

Thirdly, the makers of these "catalytic" devices state with confidence that they change the properties of the fuel in some way, for example converting long-chain molecules to short-chain ones. Such changes could easily be detected using a technique such as mass spectroscopy. It therefore seems strange that the makers do not generally have any such measurements to show that the device actually works in the way they claim.

The claims for these "fuel catalysts" are wide-ranging and varied, but I will try and debunk some of the more common ones here.

A frequent claim is that it allows use of unleaded petrol in "leaded" engines, because the tin coats the valve seats and so prevents wear. This may perhaps be true, but the quantity seems far too low. These devices often quote a 200 000 mile life. In that time a car would use leaded fuel containing around five kilograms of lead. These devices do not weigh even a tenth of that, so you have to wonder if there is enough there to protect the engine. Also beware of testimonials along the lines of "I ran my leaded-only car on unleaded for five years with (insert name of device) fitted and there was no damage". Firstly, some engines had hardened seats anyway and only needed the lead to prevent knock. Secondly, a gently-driven car is unlikely to suffer significant valve seat wear on unleaded, especially if it has already built up many years' coating. (This is not a recommendation to put unleaded in your cherished classic, though!)

Another similar claim is that the device increases the octane rating of the fuel and so prevents knock, again allowing use of unleaded in "leaded" vehicles. This might perhaps be possible, but you would have to wonder why refineries have invested billions of pounds in Zeolite technology instead, if tin is so effective. Once more testimonials of many years' trouble-free running should be treated with suspicion. Many engines designed for 97RON leaded wouldn't knock on 95RON unleaded anyway, due to their design. Others might knock under some conditions but since "classic" cars tend to be driven fairly gently it would never be a problem.

There is also some anecdotal evidence that these devices do not provide as much protection against valve-seat recession as the makers would claim.

Turning away from "leaded" engines, these devices also claim a "better" burn in some way on all engines. The same comments apply here as to magnet systems:

A common claim is that it makes the fuel burn faster. Full details of the effect of burn rate on fuel consumption can be found on the turbulence page, but basically:

• Faster burning does not, even in theory, improve fuel economy significantly on modern engines (the burn rate is pretty close to optimum anyway)
• If the fuel really does burn faster, the ignition must be retarded to suit

Another claim is that the fuel in some way burns "better" or "more completely". But only about one or two percent of the injected fuel escapes unburnt from the engine (because it was trapped in the head gasket crevice, for example). The other 99% is totally broken down into smaller molecules, and then combined with oxygen to form water, carbon dioxide and carbon monoxide. Essentially all the chemical energy in the fuel is released as heat. How can the burning be any "better" than this?

The mechanism by which catalytic devices often claim to work is by converting long-chain fuel molecules to short-chain ones. It is of course true that petrol and diesel consist of many different molecules, ranging from large ones such as octane (C₈H₁₈) to small ones such as butane (C₄H₁₀). Longer molecules can in theory be broken down into shorter ones, though this process normally requires heat and pressure, as well as the presence of a catalyst. But even if the fuel "saving" device does break the molecules down, this does not imply improved fuel consumption or emissions.

Firstly, the precise blend of components of modern petrol (and indeed diesel) is quite carefully "tuned" to match the requirements of the engine. This even involves selling different petrol in summer and winter to compensate for differing temperatures! The proportion of the fuel that evaporates at different temperatures (the "boiling curve") is determined by the blend of high boiling point (long-chain) components and low boiling point (short-chain) components. If the proportions are altered, then the boiling characteristics of the fuel will change. The likely effects are either poor cold starting or poor hot starting, with increased emissions in each case.

Secondly, short-chain molecules do not generally produce significantly more energy when burnt. The calorific values of most hydrocarbon fuels are around 44 - 46 MJ/kg, with smaller molecules producing only slightly more energy than larger ones. Claims that smaller molecules burn "better", "more completely", or "more energetically" are not supported by experimental data (consider, for example, the fuel economy of LPG vehicles).

Some such products also claim a cleaning effect.

Especially surprising is the fact that most makers of fuel "catalysts" claim their products are equally suited to petrol and diesel engines. Petrol and diesel are quite different; the combustion processes in petrol and diesel engines are quite different; and the qualities that make a "good" petrol are not the same (indeed in some cases are exactly opposite to) the qualities that make a "good" diesel. Even if we assume that a catalyst to produce "better" petrol could be devised, and also a catalyst to produce "better" diesel, for these to be one and the same thing seems highly implausible to me.

Some commentators claim that the various catalysts work very well in theory, but the evil oil companies specifically add products to their fuel to "disable" them. Even if true (which would be easy enough to prove by carrying out scientific tests on the product using an alternative fuel), you have to ask why these catalytic products are not simply sold in other countries where the fuel blend is different.

In terms of specific products, the Broquet device was tested in 1994 by the UK consumer group Which?. They ran three different cars at three different test speeds on a rolling road - as standard, with the Broquet fitted, and with it removed again. (This is not quite as satisfactory as "proper" cycle-based rolling road tests, but far better than uncontrolled on-road measurements.) The results were as follows:

Citroen AX 1.1

Speed (mph)	MPG std	MPG with Broquet	MPG back to std
30	69	70	71
50	54	56	56
70	40	40	41

Rover 620i

Speed (mph)	MPG std	MPG with Broquet	MPG back to std
30	49	52	53
50	41	42	43
70	32	33	33

Mitsubishi Galant 1.8

Speed (mph)	MPG std	MPG with Broquet	MPG back to std
30	55	57	58
50	46	46	47
70	35	35	34

It can be seen that the improvements after fitting the Broquet are extremely small, and in most cases the benefit remains after the device has been removed. Which? believed that this was because the cars were relatively new and still running-in - economy is therefore improving as the engine friction reduces. If the improvement were really due to the Broquet, the economy should have returned to "baseline" after it was removed.

Broquet's response to this was that the tests were "unrepresentative" as they had used "modern petrol-engined cars". But remember that these cars were built in 1993, and so today would be classed as 11 years old - yet nowhere on Broquet's web site does it state that small (or zero) economy gain would be expected on post-1992 vehicles. (Note to US readers: these are imperial gallons, so the MPG is about 20% better than US MPG.)

I would like to post the whole Which? article here, but cannot as it is copyright.

Another "catalyst" product, which has recently (February 2008) become quite high profile on the Net, is Firepower. Their explanations of how the product "works", which mostly contradict known engineering principles, are fairly typical for fuel "catalysts", but the company selling them rivals even Ethos for impressive-sounding but implausible and sometimes demonstrably false commercial claims.

Interestingly, certain makers of both catalyst and magnet-based fuel "saving" devices claim that they were used by the RAF during World War 2. Amazing that the British armed forces should have found not one, but two, miraculous fuel-saving devices; even more amazing that they have apparently now "lost" both of them. (Since getting fuel to the front line is a major logistical problem, the armed forces are more interested in fuel consumption than you might think.) A sceptic might wonder how much truth there is in either claim.

Use in other applications: my field of expertise is engines for cars and light trucks, and that is the central subject of this Web site. Many people claim benefits from their "fuel catalysts" in very different applications such as large marine engines and industrial burners; I remain sceptical of these claims, but cannot comment definitively on them as I have little experience with such applications, and there are significant differences to car / light truck engines.

(NB AdBlue is a quite genuine additive that will be used to enhance catalytic reduction of NOx emissions on many future diesel trucks. Negative comments on this site regarding fuel additives and "catalysts" do not in any way refer to AdBlue, which is only of use on engines specifically designed for it.)

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).