pcard

Can anyone help me clear up what I think is a contradiction. All of the following assume the spark advance to be a constant.

Given:
a too lean fuel mixture can cause pinging in an engine under load, at a given spark advance and rpm.
Therefore:
increasing the fuel air ratio should decrease the pinging.

However, here is the contradiction.

Assume One:
pinging is a result of the combustion occurring too soon in the cycle
Assume Two:
a higher fuel air ratio has a higher combustion rate
Therefore:
increasing the fuel air ratio should increase pinging.

Am I missing something? There is something about "being under load" that I do not get.
Also, am I correct in thinking that if I open the throttle at cruise, in order to pass or go up a hill for example, I then put the engine under load, and the result is a leaning of the fuel air ratio, and this is what causes the pinging? (Assuming the case where spark advance and rpm remain constant)

Thanks, Peter

Relevant reading: http://mb-soft.com/public2/engine.html

rcrac3r

IMO you might want to check your timing first to make sure it is set correctly. I would start with that first before adjusting air/fuel mixture.

MDchanic

Yup.

Speed of combustion is related to pressure and heat. The more of either one, the faster the mixture burns.

Pinging is caused by detonation, which is when bits of the fuel begin to burn before they are ignited by the spark plug.

In a properly performing engine, with every ignition cycle, the spark plug ignites the mixture, which burns with a uniform flame front, from the spark plug to the edges of the combustion chamber, the heat from which creates a smooth pressure pulse, which pushes the piston down in the cylinder.

With detonation, there are flame fronts from the outer areas of the combustion chamber moving toward the flame front from the spark plug, and they collide and create sharp, disorganized shock waves.

The same things that increase the speed of the flame front, pressure and heat, also increase the likelihood of detonation (If I am not mistaken, a lean mixture can encourage detonation mostly because it is easier to bring to a higher temperature faster than a rich mixture - the excess fuel cools the mixture).

The car's timing attempts to fire the spark plug at the right time, so that the flame front can be at its proper propagation point at the right point in the cycle (it needs to begin before TDC and fully spread out right after).
Increased RPMs mean less time for the flame front to move, so the spark needs to occur earlier as the engine turns faster (thus: mechanical spark advance).
Increased pressure means that the flame front will travel faster (and detonation will be more likely), so the vacuum advance makes the spark happen later under full throttle (when the wide-open throttle allows in air at full atmospheric pressure, instead of at less than atmospheric pressure, ie: a vacuum), and earlier under part-throttle.

When the engine is under load, the pressures in the combustion chambers are higher (because you are compressing atmospheric pressure 10x, rather than compressing 15 inches of vacuum 10x), and the mixture is hotter, so the spark travels faster, and the fuel is also more inclined to detonate on its own.

Did that make sense?

- Eric

pcard

Thanks Eric - if I can give that back to you to make sure I understand:

Under load the pressure and therefor the temperature in the combustion chamber is higher. This is a result of the fuel air mixture being denser on intake because of the increased pressure in the manifold. This increase can cause uneven ignition outside of the normal combustion front.
In addition there will be less cooling from the fuel if in a lean situation. This would lead to increased temperatures and therefore increased effective compression ratio (due to the change in specific heat ratio of the mixture) and so higher pressures and again uneven ignition.

If that is accurate, it makes a lot of sense.

The physics of this is interesting.

Thanks Eric!

MDchanic

Yes. You can have a single "hot spot" that catalyses detonation, or a bunch of different spots that break out.
Also, they don't strictly have to detonate before the spark plug sparks - even if it's a short time later, the two flame fronts can still collide.


I think conceptually that you could say that, but, in terms of the nomenclature, using the term "changing the effective compression ratio" in this way would be considered inappropriate.

But yeah, it sounds like you got what I said. Now to see whether anyone has any corrections.

- Eric

pcard

RC - I was not looking to solve a pinging issue, just wanted to understand what was going on, and why adjusting the timing would be the place to start.

Thanks, Peter

MDchanic

Okay. Basically, provided you are running gasoline with enough "octane" that it won't detonate under normal operating conditions, at a normal operating temperature, then advancing your timing (making it happen earlier) will, after a certain point, make the engine more likely to ping, as the flame front will have increased pressure too much before the piston is at TDC, so that as it is increased that much more by the time it reaches TDC, it will detonate.
Retarding your timing (making it happen later) will create a situation where the flame front is propagating through a low-compression mixture as the piston is already descending, adding no power, and creating an incomplete burn, with a gassy-smelling exhaust.

- Eric

pcard

Most of us know what serious pinging sounds like. What I wonder about is if my engine could be right on the pinging point, or pinging just slightly, and I would not be able to hear it. That could not be good for its long term health surely.
Aside from installing a knock sensor system, is there any other way to determine how close you are to pinging?
If I hear pinging my practice has been to back off two degrees and test. Repeat until no pinging heard. But is that enough?

MDchanic

I have seen this question as a source of some debate in the past, though not here (as far as I recall).

There is a school of thought that says that "silent detonation" can be deadly, and another that says it can't.

I am agnostic on the subject, but have not heard of cast iron American engines having problems with this (though aluminum motorcycle engines may be another story).

- Eric

pcard

Thanks Eric. I imagine that even if it is not too damaging "silent detonation" would be less efficient.

Peter