Spark timing vs timing light 1

[CPMach]

The question came up at work: Does the timing light flash as the current passes through the inductive pick up or does it flash as the current jumps to ground via the spark plug? We set timing at WOT at 6000 rpm fully loaded on the dyno. We set individual cylinders to read the same as cylinder #1. We adjust this timing by bending the reluctor inside the distributor for any cylinders that are differant. The question arises because as the cylinder pressure increases the resistance across the spark plug increases, thus slowing the actual ignition timing and perhaps delaying the signal to the timing light? Any thoughts?

 

[MikeHalloran]

The timing light is triggered by the spark current that is also jumping the gap at that instant.



Mike Halloran
Pembroke Pines, FL, USA

 

[Harry6888]

Beware - not all timing lights are created equally. Be sure to use a high quality light, maybe even test 2 or 3 against each other.

 

[CPMach]

We have checked multiple timing lights and found that it is not uncommon to see 1 degree variation. Let me add another twist: We had a customers engine that he had assembled. After he had adjusted the valve lash, he left an adjuster loose. We began to perform a timing loop to determine the best timing, while doing this we checked individual cylinder timing and found that cylinder 4 was 3 degrees retarded. We shut the engine off and rechecked valve lash, at this point finding the valve lash on cyl 4 had loosened up by about .030 due to the loose adjuster. After repairing this problem the cylinder 4 was rechecked and did NOT need the reluctor modified...the only change was a variation in cylinder pressure.

 

[MikeHalloran]

Now you've got me curious, especially if the engines in question are mechanically timed.

Conversely, if there's an ECU in the circuit between the reluctor and the spark coil, then all sorts of strange behaviors are possible, including retard of an individual cylinder in response to ... well, anything that the ECU can sense or infer.







Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

I think consistency is more important that the actual value. Use a decent light and use the same one all the time. Can you actually locate TDC to within a degree statically? How about actual running (dynamic) TDC? Keep in mind your harmonic balancer rotates torsionally if you're reading timing off its circumference and your crankshaft will have some twist as well. We work with rather large engines and the individual crank throws have been found to be out as much as four degrees as manufactured. If you want to be super-accurate with your timing you really need individual in-cylinder proximity sensors to find running TDC.

 

[IceStationZebra]

OP - "Does the timing light flash as the current passes through the inductive pick up or does it flash as the current jumps to ground via the spark plug?"

First, since electricity travels at near the speed of light there is virtually no delay as it travels down the wire and jumps the spark plug gap. If there was a measurable delay all spark plug wires would need to be the same length, just like diesel injection lines. And your timing light would show a slightly different timing depending on where you placed the pick-up along the spark plug wire.

Second, there is likely a very small delay in the timing light itself. The inductive coil needs a finite amount of time to build up enough energy to trigger the strobe circuit. It would be interesting to test two different timing lights on the same engine at the same time to see if there is a perceptible difference.

Third, I agree with Bribyk. You are worrying about something that doesn't really make a difference. Just be consistent.

ISZ

 

[CPMach]

We have used multiple timing lights to check the accuracy of our dyno timing light. We use the French-Grimes light which requires 12v to power it. It appears to be as good as anything available. The real reason for the question is more based upon cylinder pressure changing the observed timing. Is the cylinder to cylinder timing variation caused by a difference in cylinder pressures? I believe that his is partially true. If we were to build an engine that had 13 to 1 in a cylinder and 10 to 1 in another cylinder, as we fully loaded the engine are we going to see a variation in timing based on cylinder pressure only?

 

[Dan320]

Cylinder pressure will, at least theoretically, influence the voltage needed for the spark to jump the gap due to differences in gas insulation values due to pressure.
If there is a measurable difference in reality, I do not know.
Dan

 

[Bribyk]

I do have some rough numbers on this... When cranking and firing with "wasted" spark (firing on both compression and exhaust strokes, the difference in time that the primary side of a coil is energized is around 8 microseconds. At 6000 rpm this would work out to just over a quarter of a degree, which is also roughly the accuracy of decent microprocessor-based ignition systems.

 

[Dan320]

That would set the maximum timing difference related to pressure, then. (?)

Dan

 

[GregLocock]

B-man, great info. Silly question, could you use that to detect weak mixture, or misfires?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

It would seem so.

http://www.fs.isy.liu.se/~larer/Projects/main.html

Spark-Advance Control by Ion-Sensing and Interpretation
...The development is based on new sensors or improved interpretation of available sensor signals. One example is ionization current sensing which is obtained by applying a sense voltage on the spark plug when it is not used for firing. The sensed current depends on the ions created, on their relative concentration and recombination, on pressure, and on temperature to mention some of the more important factors. The signal is very rich in information but also complex to analyze.

 

[Lou Scannon]

A crude 0-scope?

 

[automatic2]

There is a time constant for sufficient voltage to build, required to 'fire' the plug. The constant is determinent on inductance/resistance, and includes the entire circuit. Rather than a timing light, you could use a current transformer and read the voltage and current on a dual trace scope. As mentioned, you are looking for consistency across cylinders, noting the voltage levels required to commence current flow. Variations in voltage will translate via the time constant, into degree variation. A variation over the speed range could imply gap resistance, whereas a consistent, but different voltage level compared cyl to cyl could imply a circuit problem.

 

[patprimmer]

Or a variation in cylinder pressure or a:f in various cylinders.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Bribyk]

Industrial ignition systems use this time difference to flag ignition issues such as bad secondary coil or deteriorating spark plug or even to run without a cycle trigger (cam reference).