Devices of this type include: Fuel Saving & Power Push, Fireball Ignition, Motorspark, "Cryogenically treated" spark plugs

Petrol engines rely entirely on the spark to ignite the fuel/air mixture and provide power to drive the vehicle. If this spark is unreliable - for example, if it is too weak to always jump the plug gap - then the engine will misfire. This misfire represents fuel simply passed down the exhaust pipe unburnt.

So far, so good. In the "bad old days" of contact-breaker ignition, devices to improve ignition energy could improve spark reliability, especially under cold or damp conditions. You could see real improvements in fuel consumption and / or power. But modern electronic ignition systems are extremely powerful anyway and provide at least 99% ignition reliability. If they didn't, the emissions of unburnt fuel would far exceed the amounts allowed by emission legislation. Adding more ignition energy might perhaps provide an absolutely immeasurable improvement in fuel consumption, but at the expense of increased spark plug wear.

One particularly interesting concept is the Meissner Spark Intensifier (invented in the early 1900s) and many similar re-inventions since. Put simply, this is a small air gap in the HT lead between the coil and the spark plug - devices that require you to cut the HT line are likely to be of this sort. The Spark Intensifier may perhaps give better igntion reliability if spark plugs are heavily fouled, but this is very rarely the case on any modern engine. In any case, simply replacing the plugs would be a better (and usually cheaper) solution to this problem.

Fundamentally, once the mixture is lit, it's lit. A fire doesn't burn hotter because you use a bigger match to light it; a bigger spark won't make the fuel/air mixture burn any hotter either. I have carried out many engine experiments involving changing ignition energy, and while there are some marginal benefits in extreme conditions (cold, damp, etc), the overall effect on power and fuel consumption on any modern engine is negligible.

Some devices also claim to make the mixture burn more quickly. Even if this were possible (and it is most unlikely), a faster burn does not usually translate into better fuel efficiency. See the turbulence page for more information. Indeed, a more rapid burn might actually be damaging to an engine because it could promote knock unless the ignition timing is retarded appropriately.

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

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