Recently, the idea of adding small quantities of Acetone to fuel (mostly petrol (gasoline), but diesel has also been mentioned) has attracted a lot of interest. (At the time of writing, "acetone gasoline mileage" gets over 20 000 hits on Google.) Given this high profile, it seems worthy of some theoretical examination.

The theory, so far as I can tell, is that it enhances the vaporisation of the fuel and so reduces the fraction that is wasted by escaping unburnt out of the exhaust or past the piston rings. While I have no specific knowledge of the effect of acetone on fuel vaporisation, it is nonetheless very easy to demonstrate that it cannot give a useful benefit via this mechanism. It is very well known that the vaporisation of fuel in a modern engine under normal operating conditions is already very good, and only about 1 - 2% of the input fuel escapes unburnt. Hence, whatever the properties of acetone, it is extremely hard to see how the economy benefit can be more than this relatively small amount.

There are, of course, many people who have tried putting acetone in their tanks and reported huge benefits. But you can find equally powerful testimonial evidence for a whole raft of devices that have been scientifically proven to be worthless. Fuel consumption measured on the road is subject to so many confounding factors - traffic, journey type, driving style, weather, etc, etc - that it is very, very easy to see changes of 10%, 20% or even more, even when the true engine efficiency has not altered at all. This page considers the general risks in determining fuel consumption via uncontrolled on-road measurements rather than scientific testing. Some people have tried very hard to eliminate or compensate for the confounding factors, but in the end it is impossible to control them all outside of a proper testing laboratory.

Many "supporters" of acetone have tried it themselves with apparently good results, and are scathing of the experts (like myself) who dismiss it on theoretical grounds without doing any testing. The point is, of course, that those same experts also recognise that any on-road testing they (or anybody else) do is essentially pointless, since it is virtually impossible to separate the true effect of the acetone from the "noise factors" - therefore, why go to the time and trouble of doing testing, when it can only produce an essentially random result?

Sterling Allan from Pure Energy Systems has done a lot of work on correlating results from people using acetone. I don't really agree with Sterling, but he certainly has his head screwed on a lot better than many of the people commenting in this field. Interestingly, Sterling reports that "typical" benefits are only 2 - 10%, and indeed his own carefully controlled testing ("Most Accurate Testing") showed no benefit at all. Given this, and the very large potential for errors in on-road measurements, it is not difficult to conclude that the "true" benefit may well be close to (or even exactly) zero, rather than the 30-plus percent frequently claimed.

Balanced against this fairly small potential benefit is the risk to engine components. Acetone is a powerful solvent that attacks many plastics, and while the majority of car components do seem to be resistant to it, such components are very rarely specifically designed or tested for acetone-resistance and so the theoretical risk is obvious. Those who have experimented with acetone say they have seen no problems, but without testing every plastic part of every vehicle, it's impossible to say for certain. Additionally, there may be specific risks to diesel engines - either due to acetone's tendency to mix with any water in the fuel (and then deposit that water, catastrophically, in the fuel pump), or since it may reduce the fuel's lubrication ability.

The source for this idea appears to be an article by Louis LaPointe. The first thing to say is that LaPointe appears to be honest and well-meaning, and (unlike the people promoting many of the other ideas mentioned in this website) I don't see that he gets any financial benefit from people using acetone. Nonetheless, many of his specific comments (reproduced below in italics) just don't seem to make sense to me, which then makes me wonder about the accuracy of the whole idea (quite apart from the theoretical observations above). Here are some of the implausible points LaPointe makes:

This article about acetone probably draws conclusions that Big Oil and the American Car Manufacturers do NOT want you to know. They suffer from unlimited corporate GREED. They want bad mileage. The worse, the better as far as they are concerned.

Conspiracy theorists have been making this kind of claim for decades, and it makes no more sense now than it did then. OK, oil companies can sell more oil if cars are uneconomical, so you can see the logic. But how does it possibly help car makers to produce uneconomical cars? Ford and GM are having to offer massive discounts - thousands of dollars - to sell their unpopular SUVs and pickups, while Toyota can charge pretty much what they like for the Prius. Right now, thanks to high oil prices, customers are desperate for more economical cars, and will pay over the odds for them. Suppose Ford could improve the economy of all its vehicles by 20%, and as a result customers would pay an additional (very conservative) $500 per car. On the 7 million or so vehicles Ford sells every year, that would equate to almost four billion dollars of extra profit, through improving fuel economy. Why would a "greedy" Ford choose to make cars people will not pay a good price for, if they could easily change things?

...engineers and researchers are nearly all in favor of acetone...

That's not what you see if you read comments on Newsgroups or discussion forums, where "professionals" in the automotive field are generally very sceptical.

There is a great little device [Scan-Gauge] available to check your exact gas mileage...we use the TRIP function to average the MPG at a steady 50 MPH both ways

The Scan-Gauge is certainly a handy gadget, but checking fuel consumption over a short distance at 50 mph allows many confounding factors (most notably tiny variations in speed) to significantly affect the result. Modifications to the engine (or fuel) can also reduce the accuracy of the Scan-Gauge. Again, check this page for general comments on the dangers of on-road fuel economy testing.

Alcohol in general is anti-mileage but the oil company stooges claim it is wonderful

There's full commentary on that claim here, but the point is simply this: alcohol is not, and never has been, claimed to improve economy. Rather, it was added to gasoline because it can reduce HC and CO emissions on older cars, and is nowadays widely used (especially in Brazil) simply because it is made from plants rather than crude oil. (Odd that "oil company stooges" would promote something that reduces crude oil use...)

Too much acetone will decrease mileage slightly due to adding too much octane to the fuel. Too much also upsets the mixture ratio because acetone (like alcohol) is a light molecule.

This is where I get very sceptical. If acetone really worked as claimed, I could imagine that adding more than a certain fraction gives no additional benefit. What I find very hard to understand is how adding more than about 0.1% actually makes matters worse. In the absence of any error bars or raw data, I am suspicious that this shape has just emerged from an (honest but misguided) attempt to fit smooth curves to essentially random results.

From theory, there seems no reason why excess acetone could be detrimental to economy. The mixture ratio is always optimised by the lambda sensor on any modern car, even with several percent acetone, so this will stay at the required setting. And there's no such thing in gasoline terms as "too much octane"; knock is always a bad thing, and the definition of "octane rating" is just "resistance to knock". Apparently even 10% acetone only adds about 3 points to petrol's octane rating (which fits pretty well with the idea that pure acetone is 150 octane), so 1% is only 0.3 points, which is absolutely insignificant. Short of assuming that acetone is a powerful flame retardant (and there does not seem to be any evidence for this), why should adding just 1% give a reduction in fuel economy of over 50% from optimum?

A four-cylinder engine is capable of producing better MPG than a six or eight. This is due to the longer TIME for combustion allowed by fewer cylinders for better efficiency. The longer time for the burning process in a four is twice that of an eight.

That's just nonsense. At any particular rpm, the pistons in an 8 cylinder engine take just as long to move through the power stroke as do the pistons in a 4 cylinder engine. Indeed, V8s typically run very "tall" gearing and so rev slowly at cruising speeds, so there is actually more time for the burning process. Four cylinder engines are typically more economical than eights, but this is due to lower friction (and also the fact that they are generally smaller in capacity) rather than anything to do with burning time. (After all, a four-litre V8 is just a pair of 2-litre I4s stuck together, as far as the fuel and air are concerned...)

Acetone helps the fuel become a vapor more easily inside the chamber and minimizes wasted fuel. All it does is offer total combustion of fuel in any kind of engine.

As explained before, modern engines give 98+ percent combustion, so there really isn't a lot of room for improvement there. The same point applies to LaPointe's repeated assertions that a very large fraction of the input fuel fails to vaporize and goes unburnt out of the exhaust - it's just not true on any modern engine in good condition.

Acetone allows gasoline to behave more like the ideal automotive fuel which is PROPANE...the oil industry have suppressed propane for many decades

This will come as a shock to the many thousands of motorists running their cars on LPG, the main component of which is propane (the other being the chemically similar butane). LPG is actively promoted by many oil companies, which would seem odd if they are "supressing" it. Propane certainly does vaporise very well in engines, due its low boiling point, but despite this it is no more efficient than gasoline.

Admittedly, none of the above analysis proves that acetone doesn't help fuel economy. But when the person initially promoting it is apparently mistaken on some fundamental points, you do have to wonder...

Having said all that, there is one possible way in which acetone might help an engine, and this may be why some users apparently report good results. Under some circumstances (for example if poor quality fuel is used), deposits may build up on the small metering holes in the fuel injectors and restrict the flow. This will tend to cause poor performance and "driveability", and on more modern (post-1996 in the US) vehicles the Check Engine light will probably illuminate. Acetone may be able to remove these deposits and restore normal operation - though there are much less "risky" ways to achieve the same effect, such as using proprietry injector cleaner or even a few tankfuls of a premium fuel every so often. But the vast majority of vehicles will not show this effect, and even on those that do, the effect on fuel consumption is likely to be small.

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