Ignition Spark theory questions...

What would be the typical symptoms (engine performance-wise) of a weak spark on a 2-stroke, say a CR125?

Or, does the spark really matter if the engine seems to crank easy and run quite well?

Likewise, are their performance advantages/disadvantages to having a stronger spark than normal?

NOTE: I'm an electronics engineering instructor and the electrical part of the engine is no problem for me (other than how that spark really relates to the engine side of things). I do find it a disappointment there are no voltage measurements noted in the service manual, only static resistance measurements. Resistance measurements sometimes don't reveal electrical problems, especially when high voltages are involved. I do have equipment that will allow me to measure voltages (even pulses and spikes) up to 50kV and feel--if this info was available--could make actual meaningful measurements while an engine was running. But, I do understand that most probably don't. Still, even most could measure voltages around the CDI and before the coil, I would hope.

The only symptom of a spark that is too weak is a failure to ignite the fuel mixture. This can manifest itself as a complete failure to start, intermittent misfiring, etc.

Spark that is too weak can fail to cross the gap at the spark plug for two main reasons. One is simple lack of voltage to jump the gap, and the other is because the electric charge is shunted across the insulator of the plug by some conductive deposit such as oil or metallic residue from engine oil or fuel additives.

In the latter case, it can become either a hard failure or an intermittent problem and is a matter of degree related mainly to four factors:

  1. The voltage of the spark discharge
  2. The rise time of the spark discharge
  3. The conductivity of the fouling deposit
  4. The composition and design of the electrode

Item 1 is obvious enough. If the voltage is high enough, at least some of the charge is likely to cross the gap in spite of there being some conductivity over the insulator.

Likewise, item 2. The faster the voltage rises, the less likely that the electrical current can be diverted from crossing the gap into what amounts to a U turn at the insulator. Electricity is, in the end, solid matter in motion, and has a certain inertia. A strong spark with a very rapid voltage rise is much more likely to cross the plug gap than to short across the insulator, even when the plug is fouled.

Item three is probably so obvious it needs no discussion.

Item four is the reason for iridium, platinum, gold/paladium, and other precious metal spark plug electrodes. Some believe incorrectly that these plugs offer an advantage by reducing the resistance of the conductor. It is true the these materials are more conductive, but the gains from that attribute are minimal. What makes them work better relates to the fact that a spark jumps from a sharp point more readily than a broad blunt tip. If you watch a spark jump from a standard plug, you will likely see that th espark is concentrated at the 90 degree edges of the electrode. Using noble metals allows the electrode to be made finer and stay sharper longer because those materials resist erosion by the combustion process much better.

All these things are of benefit only up to the point that the spark is able to ignite a burnable fuel charge every power stroke where one exists. Once you reach that level further improvements do nothing more for you.

The only accurate way of measuring spark output voltage is by means of an oscilloscope. With an engine running, the measured voltage will show only what the voltage rose to in the live circuit, the potential voltage being limited by the flow of current across the gap. The total potential voltage can be displayed by triggering the ignition event with the plug disconnected, so that current cannot flow. Ironically, whereas we seek to provide a great deal of spark energy to the system, what we look for on a scope is a relatively lower peak secondary voltage.

Thanks! That's a fairly comprehensive summary. So, if I understand you correctly, IF the spark is present every time (i.e. making it across the gap), then the strength of the spark is irrelevant?

I had thought of making some measurements with my oscilloscope on the coil's primary, but I have no reference for what should be a normal voltage reading there. Also, that waveform will be a pulsed waveform of some sort, so the most valuable data would be a peak voltage and wave shape. Again, I have yet to see any such data.

You will need a spark that can jump the gap and ignite the fuel under the conditions in the engine. getting a weak spark outside the the engine may not ignite something that is under compression and in basicly a turbulant windstorm. But getting the flame started at the right time is all it is recquired to do.

Although I surely do not know but would venture to wager that a low compression flat head engine or an older 2 stroke engine with a rounded dome head would recquire less voltage to cross the gap than a supercharged,high compresion,high swirl crossflow type of head or the 2 stroke with a squish chamber

Thanks! That's a fairly comprehensive summary. So, if I understand you correctly, IF the spark is present every time (i.e. making it across the gap), then the strength of the spark is irrelevant?

Almost correct, yes. As the post above states, the simple presence of a spark doesn't mean there will be one under a compression load. But once the spark is strong enough to be there each time and is "hot" enough to ignite the fuel reliably, a surplus beyond that point is just that, surplus.

Measurements at the primary will vary based on several things and as to wave form, you can probably find some comparisons of CDI and other types of primary wave forms on the web.

The oversimplification of spark is that the resistance at the gap of the plug is proportional to cylinder pressure. The higher the cylinder pressure, the higher the resistance at the plug gap. This means higher cylinder pressures require more voltage to cross the gap. So what if you put a 50kv coil on your bike that runs fine with 30kv? Your 50kv coil will discharge at 30kv because that is the votage required to form an arc.

If you notice, the higher the "performance" of the engine, the smaller the gap the manufacturer recommends. Chevy truck motors might speck a gap of .050" to .070". They have very low compression pressures and it takes little voltage to make the spark. A turbocharged engine or a high compression race engine will usually run in the .028" range because the higher cylinder pressures make it harder to form an arc with a wider gap. High voltage coils can push an arc across a larger gap under higher compression pressures. But... spark plug gap doesn't really translate to horsepower at the wheels so...

The oversimplification of spark is that the resistance at the gap of the plug is proportional to cylinder pressure. The higher the cylinder pressure, the higher the resistance at the plug gap. This means higher cylinder pressures require more voltage to cross the gap.

You might be referring to Paschen's law and the Paschen curve that relates breakdown voltage as a function of pressure and type of gas:

http://en.wikipedia.org/wiki/Paschen%27s_law

While it is true that breakdown voltage increases with pressure (above the Paschen minimum), it is more due to mean free path of electrons rather than resistance. Either way, 10:1 compression ratio (almost 150 PSI) significantly increases the required breakdown voltage.

I used to work at a high energy physics lab years ago. Like most of these experiments, very low pressure and high voltage (very high in some cases) was involved. If high voltage was on when the pressure got close to the Paschen minimum in the vacuum chamber, there would be some serious arcing and possible damage to critical components.

Edited by msiddalingaiah

All these things are of benefit only up to the point that the spark is able to ignite a burnable fuel charge every power stroke where one exists. Once you reach that level further improvements do nothing more for you.

This is what I always believed, although there's still a significant market for super duper ignition cables that claim to significantly improve performance with a "stronger spark". While poor cables can cause problems, all you really need is a decent connection with no cracks, short circuit, or leakage.

2 things not mentioned (I think)

1) Risetime is also important.

2) An O-scope and a chunk of wire wrapped around the plug lead can make misfires visible (but it's tough to ride that way)

Dave

2 things not mentioned (I think)

See post #2

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