Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - Article: be skeptical of gadgets/additives promising "more complete combustion"

A number of "magic bullet" gimmicks and additives claim to improve engine efficiency by causing "more complete combustion".

Their suggestions that there are big MPG savings ripe for the picking imply that a significant amount of unburnt fuel is being spat out of the engine, wasted.

On this topic, an article from the Times Online:

Can I get more efficiency from my engine?
Claims that certain devices can improve power, fuel consumption or emissions by improving the efficiency of combustion are complete nonsense, says Times Online's technical expert Tim Shallcross

The author looks at emissions of unburnt hydrocarbons (HC), and walks the reader through the mathematical explanation of how, even with cars not equipped with catalytic converters, the UK's maximum legal limit for HC in the exhaust (above which the car fails the test and can't be driven on the road) is 1200 parts per million, or just under 3%.

Quote:

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 - (source)

The Times article is a good companion to the one at fuelsaving.info: "Background: unburnt fuel fraction in modern petrol engines".

It addresses the same question but goes into further detail, also discussing the questions of:

- fuel burning after it's expelled into the exhaust manifold
- fuel that may pass the piston rings and enter the crankcase/oil

Both are worth a read: