Best spark plug gap?

Hello,

Since gasoline engines will soon be in the majority again (we already have the 2.0 TSI, and starting around next March, we'll also have the 1.4 TSI CAXA as a successor to the ASZ Polo), I'm going to focus on spark plugs again, specifically the issue of electrode gap.
The search with that input yielded exactly 1 practical result:

/viewtopic.php?t=26786&highlight=electrode_spacing&start=26

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. . . the electrode gap was excessive, almost 2mm on some of the spark plugs. According to the previous owner, they had not been replaced, meaning they had been used for at least 100,000 km. It explains the poor cold start behavior and, above all, the burnt distributor cap, because, if I'm correct, an excessive electrode gap leads to an increase in the ignition current.

According to my understanding of spark ignition physics, the volume of the heated or ignited mixture increases with the electrode gap, meaning the mixture is ignited more effectively – as long as the high voltage is sufficient to bridge the electrode gap. Unfortunately, as the distance between the electrodes increases, so does the requirement for high voltage.
As far as I know, the biggest problem with cold starts in older vehicles with distributor-based ignition systems (with a fixed dwell angle) was this: When the battery is very weak and the starter motor is struggling, the resulting high voltage drops significantly, and may not be sufficient to generate a spark.
However, once the engine started and the battery voltage returned to its normal range, the high voltage was sufficient to ensure a safe ignition.

More recent, electronically controlled ignition systems (with variable ignition timing based on primary voltage and engine speed) no longer have this problem to a large extent. In other words, the high-voltage supply is practically always well within the safe operating range, meaning that the electrode spacing could be significantly increased without immediately encountering cold-start problems.
However, as the distance between the electrodes increases, the high-voltage stress on the insulation also increases, because a higher voltage is required to cause a spark discharge.
Furthermore, the duration of the spark discharge is reduced because the larger spark volume consumes the available ignition energy more quickly.

Despite this, my experiences in the 1980s with aftermarket ignition coils and increased electrode gaps, exceeding the OEM specifications by tens of thousands of kilometers, were always positive: less hesitation at part throttle, slight fuel consumption advantages, and even, in some cases, better starting performance.
That's why, back then, I wondered why these vehicles didn't have a larger electrode gap, for example 1.2mm instead of the usual 0.8mm.

Can someone here answer the question about the current gasoline engines?

Thank you .

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Despite this, my experiences in the 1980s with gasoline engines using electronic coil ignition systems and increased electrode gaps, exceeding OEM specifications by -many kilometers, were always positive: less part-throttle surging, slight fuel consumption advantages, and even better starting performance. That's why, back then, I wondered why these vehicles didn't have a larger electrode gap, for example 1.2mm instead of the usual 0.8mm.

Larger distance = later ZZ (zero crossing), because the voltage has to rise for a longer time.....therefore, I wonder where the energy saving comes from .

vwSchrauber wrote:

A larger gap = later spark timing, because the voltage needs to build up for a longer time..... Therefore, I wonder where the fuel efficiency advantage comes from.

Probably due to the "more powerful" mixture ignition, which is clearly noticeable, for example, by the reduced partial load fluctuations.
If the initial spark advance is steep enough to not be offset by later ignition, a measurable and explainable reduction in fuel consumption can certainly occur, in my opinion.
And in the waning era of mechanical (already contactless) distributors, it was even possible to compensate for a timing error caused by distance with adjustments using spark plugs .

Here's a little more about the ignition delay caused by a larger electrode gap:

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It only takes 30 microseconds to increase the voltage in the secondary winding of the ignition coil to 15,000 volts or more. http://www.kfz-tech.de/Oszilloskop3.htm 15kV, as far as I know, is a common range for the breakdown voltage at the electrodes.
If my calculation is correct, the crankshaft rotates approximately 1.1 degrees during the high-voltage build-up phase (30 microseconds) at 6000 rpm.
If the electrode gap is increased by 50%, the time it takes for the high-voltage build-up until sparking will also increase by approximately 50%. This would result in a phase shift of about 0.6° KW at 6000 rpm - in my opinion, this is not a significant amount that would cause a noticeable reduction in efficiency. It could cause increased fuel consumption of the engine.

Hello!

During a training session for mechanics hosted by Beru, someone asked why, for example with Renault vehicles, ignition coils seem to fail so frequently.
The reason was that the burnt-out candles created too large a gap between the electrodes, and the long spark distance overloaded the ignition coils.

When I think about it, that could very well be the case; it's not uncommon for cars that aren't regularly maintained to experience such problems.
And where the spark plugs are as old as the car itself.

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The answer was that the burnt-out candles had too large an electrode gap, and the long spark distance overloaded the ignition coils.

One could also argue that the spark plug reserves are simply too weak for the increased spark gap.

Nowadays, when a car won't start, it's not usually the battery that's drained, but rather the starter motor that fails. Is the battery now too strong, or is the starter motor too weak ?

Autoservice wrote:

One could also argue that the spark plug coil reserves are simply too weak for the increased spark gap.....

More precisely, the high-voltage insulation is likely only designed to withstand voltage peaks similar to those with a normal electrode gap, and will not last long under these conditions.

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Nowadays, you don't usually drain a battery completely if a vehicle won't start; instead, the starter motor burns out...... Is the battery now too strong, or is the starter motor too weak

In my opinion, the battery capacities available today are mostly so high that you can easily damage the starter motor without recharging.
With the 40Ah battery cubes that were common 30 years ago (before the proliferation of electronics and excessive quiescent currents), it would certainly be more difficult to damage today's starters.