Additives Bad For Engine?

Do fuel additives have adverse affects on engines performance? They are marketed as fuel savers. It does save on petrol cost, but I am worried it may damage the car. How do these additives wok in lowering fuel consumption?

Reply:
Most fuel additives are either octane improvers or upper cylinder lubricants. The upper cylinder lubricants do help to lubricate the valve guides and to an extent the valve seats, as well as the upper part of the cylinder walls that are subjected to the heat of combustion.

The octane improvers sometimes contain small quantities of alcohol that may give a small improvement in performance. How much of performance improvement measured against the cost of the additive should tell you whether it is worth of cost. I do not think these additives would really cause any damage to the engine, although i would think that the emmission readings would be affected if the engine were put on analyser.