Plug gaps, small or big?

[BrazilianZ28Camaro]

Wouldn't the greater gap reduce the spark duration per cycle?

If so, maybe you can fine tune the duration of the spark versus the spark "range" via plug gap;

[MadBill]

A simplified explanation is that the larger gap drives up the voltage requirement and results in a higher energy release from the initial arc.

[dieselgeek]

Wouldn't the greater gap reduce the spark duration per cycle?

If so, maybe you can fine tune the duration of the spark versus the spark "range" via plug gap;

CDI sparks are *very* short duration compared to an inductive ignition. (I prefer inductive, especially in forced induction applications - and eliminating the distributor altogether).

MadBill, you're an engineer, would you expect the increased ignition voltage requirements of a bigger gap to cause more drag from the alternator? (in an application that uses one, that is).

[BigBlocksOnTop]

I would think with the turbulence in the chamber that the electric arc actually loops out and away from the electrodes. The bigger the gap the bigger the arc loop!

[MadBill]

Wouldn't the greater gap reduce the spark duration per cycle?

If so, maybe you can fine tune the duration of the spark versus the spark "range" via plug gap;

CDI sparks are *very* short duration compared to an inductive ignition. (I prefer inductive, especially in forced induction applications - and eliminating the distributor altogether).

MadBill, you're an engineer, would you expect the increased ignition voltage requirements of a bigger gap to cause more drag from the alternator? (in an application that uses one, that is).

Actually, I'm a mechanical technologist, but my smattering of electrical knowledge suggests that while some ignition systems have pretty hefty demands (as much as 20 A.), the energy released at the plug gap is pretty small, a few hundred millijoules for even an 'arc-welder' system and so any difference due to gap size would be undetectable.

Re a possible "arc loop", this is a factor. I understand the arc can even be blown out by extreme mixture motion.

[David Redszus]

The size of the spark plug gap has three effects to be considered.

The larger the gap, the more voltage is required to jump the gap and to ionize the air space. If sufficient reserve voltage is available, a larger gap can be used. If not, the gap size must be reduced.

The larger the gap, for a given reserve voltage, the shorter the spark duration. If the gap is reduced we can increase the length of the spark duration, but at some point, it no longer makes any difference.

The larger the gap, the more advanced the equivalent ignition angle. The size of the initial flame kernal is determined by the size of the spark plug gap. If the gap is increased in size, the flame kernal is larger, giving the same effect as if it had been ignitied earlier. In most cases, the range of gap sizes available will have no more than a 3-4 degree effect on equivalent ignition timing. If slightly advanced timing can improve engine performance, then a larger gap is beneficial. If there is already too much ignition timing (or an excessively fast burn rate) then a larger gap will be non-beneficial and perhaps even destructive.

Now we have some trade-offs to consider. Which is more important to the performance of a given engine: longer spark duration or advanced ignition timing?

If spark duration is adequate (a spark lasting until TDC) and ignition advance timing is correct at each throttle and engine speed, then it becomes quite difficult to extract more power by use of spark plug gap. Now things like electrode tip heat range and tip temperature, squish velocity prior to TDC, chamber temperature and pressure emerge as dominant variables.

[BrazilianZ28Camaro]

CDI sparks are *very* short duration compared to an inductive ignition. (I prefer inductive, especially in forced induction applications - and eliminating the distributor altogether).

Correct , but CDIs like MSD do decreasing mult sparks per cycle up tp 3000 rpm.

Above that there's only one spark per cycle, with out the risk of coil saturation , like a indutive system.

Maybe, a larger gap can "deshuroud" the flame kernal at its starting point and maybe, promoting a faster burn with less ignition advance?