As I understand it, pinging typically occurs because the fuel preignites in the cylinder, due to high pressures and heat in the combustion chamber. I have always heard that one way to reduce pinging is to retard ignition timing, but I don’t understand why that is. If you retard the ignition timing, the piston will travel upwards even further before the fuel is ignited - won’t this lead to even higher pressures and heat, which would increase the risk of preignition?
It seems to me that the logical choice would be to advance ignition timing, which would let you ignite the fuel before it gets compressed to very high pressures and has the chance to preignite.
If the pinging is from firing too soon. Retarding the timing makes it fire a little later.
You’ll have to explain what you mean by “firing too soon.” As I understand it, pinging is caused when something other than the spark ignites the fuel/air mix.
It’s because the explosion that occurs is not something that occurs instantaneously and is over with. The explosion takes place before TDC and continues on through TDC for a short time afterwards.
Ideally, the peak of the combustion should occur right when the piston is about move on the downstroke.
Pre-ignition means the peak happens before the piston reaches TDC. In turn, the premature peak of the explosion is trying to force the piston back in in the direction that it came from.
Retarding the timing means a later firing spark plug which in turn moves the peak of that explosion further on down the road, to use an analogy.
That’s in simple terms and in a nutshell it’s all about the length in time of the explosion, even if it is in milliseconds.
If the piston is traveling upward and meets an exploding charge there will be a great deal of stress in the combustion chamber. That is what happens when the timing is overly advanced.
Retarding the timing reduces the dynamic compression ratio. This reduces the pressure on the gasoline air mixture, so it reduces/eliminates preignition.
ok4450’s explanation of firing before the piston is tdc and ready to go down again, therefore attempting to force it to go back down before it’s ready,
Fully explains what blasted a hole clean through the entire top of one piston in my 80 Bronco 351M.
Allowing any pinging to continue is something I now never do.
Thanks for the explanation. I’ve just always been comfused by people who state, as tardis does below, that retarding the timing reduces the compression ratio, because isn’t the air/fuel mixture even more compressed if you allow the spark to fire at a later point in the combustion cycle?
In addition to the other good explanations, this might help - timing is advanced (through both vacuum and centrifugal (engine speed) means - to try and match the peak pressure from combustion to the optimal position of the piston. Get it too early, and the ping results from the combustiion pressure wave hitting this piston as it’s still moving up, get it too late, and the piston’s already moving down as the peak combustion pressure occurs, reducing power output. Hence the advice to ‘advance it untill it pings under load, then back off a little’ for pre-computer cars. This is now done by the combination of the computer and knock sensors.
If you want to read more look at the differences between pre-ignition and detonation, both conditions will cause a “pinging” noise but are not the same thing. Detonation is the event caused by excessively advanced ignition timing, or the use of fuel with too low an octane rating.
Since many cars of 2010 vintage (and earlier) cannot have their timing manualy set, advanced ignition timing is an issue faced by fewer and fewer cars. I for one liked it when distributors became “fixed” on the models I commonly worked on, no requests to simply “check my timing”. When a customer shows up at your door with a timing check request it usually leads to other issues that take up your time without compensation.
The story gets even longer if we tell some more boring stories. Timing that is too far advanced will cause an engine to run hotter. If you worked at a gas station in 1966, you would sometimes see the average mechanic come in with his own car after he was on the highway for a while. He would borrow a wrench so he could retard the timing and stop the pinging that happened every time he stepped on the gas pedal to pass a car or go up a hill.
He had done work on the car and reinstalled the distributor without adjusting the timing. There was some laziness involved or he didn’t have his timing light at home. His engine was running hot and that was causing the pinging. Retarding the timing will cause the engine to cool down a bit and stop the pinging.
If the timing is too far advanced, the spark itself can cause the pinging. If the engine is too hot or the fuel has too low of an octane rating, pre-ignition can cause the pinging. If there is carbon buildup anywhere in the combustion chamber, the rough ends of the carbon will get red hot and cause the pinging.
After three days of driving on the Air Force Base where the speed limit was 35 if you were lucky, the carbon would build up and then act up when you got off the base and tried going up the hill at 50 MPH. The pinging was bad. I would then blow the carbon out by flooring the pedal and shifting to 2. A black cloud would appear behind me and the pinging would stop until I drove on base for a couple of days. Then I would repeat the whole procedure.
So you can see that you can’t stop preignition by advancing the timing. It would only make it worse. In other words, preignition can’t cure preignition.
You stated "As I understand it, pinging typically occurs because the fuel preignites in the cylinder,"
The fuel pre ignites because the spark happens too soon, retarding makes the spark happen later. We could bet into why a higher octane fuel reduces ping, but that was not your question.
TDC is top maximum height of a piston in it’s range. Cars typically are timed ahead of top dead center as the maximum power from the explosion is reached shortly after ignition.
No, pinging is not caused by “spark happening to soon.” It’s caused by the fuel/air mixture detonating spontaneously before the spark plug fires.
Diesels cause explosion by compression. The only spontaneous ignition in gas engines is due to carbon buildup causing post ignition as the compression ratios are different in my experience.
Here’s what happens to cause “spark knock” pinging:
First it happens after the spark, as opposed to pre-ignition or detonation.
Normally, when the spark plug fires the flame spreads from the spark plug out to the farthest reaches of the combustion chamber.
The fuel-air mixture is swirling around, which helps speed up the spread of the flame.
When the flame has spread part way the remaining unburned fuel-air gets compressed and heated by the nearby flames and expanding gases (or red hot carbon deposits).
Given enough heat, pressure and time the fuel-air mix can spontaneously burst into flame.
This happens to be how diesel engines work, except the fuel is gradually introduced to air that is already hot and compressed.
This spontaneous burn happens much more quickly than the normal flame spread, making a ping noise and hitting all surfaces of the combustion chamber like a hammer blow.
Octane rating is the fuel’s resistance to this spontaneous combustion.
If the spark is too soon before TDC the pressure and temperature builds too high during the flame spread because the piston is still going up.
Anything that speeds up the flame spread and reduces the time unburned fuel-air spends at high pressure and temperature reduces the tendency to ping:
More swirl, higher RPM, small compact combustion chamber, centrally located spark.
There is an upper limit to gasoline engine bore size due to flame spread time and spark knock.
Spark Knock: The spark ignites the charge. As the flame front advances, the remaining, unburned charge detonates with explosive force instead of burning at a constant rate.
Detonation: Entire charge self-ignites and burns in an uncontrolled pattern…
Retarding the spark reduces the pressure that causes the charge to become unstable and detonate…
True detonation is caused by a lack of octane or increased compression ratio caused by combustion deposits. Sometimes, these deposits can glow, creating a glow-plug that pre-ignites the charge…Again, uncontrolled combustion…
Today’s engines, by design, make it very difficult for spark knock or detonation to occur…
I replace rings and bearings on a 1955 John Deer tractor. Displacement was 305cid,from two cylinders. Most likely the compression is pretty low in these engines. the pistons were darn big(had to be close to 6"). Never drove a tractor before this one and my client was in his 80’s and was not doing test drives anymore, well first time out of the gate with this thing and I about flipped it over.Old man laughed pretty hard. The old guy bought this thing locked up and I had to use a 12lb sledge hammer to pound the pistons out as they were froze to the bore, took hours to pound them out.
That’s a big bore, but no gasoline engine could have a bore as big as this diesel:
Is that one of those ocean liner engines? Forgot to say, those John Deer’s were know as “Jonny Poppers” because of the odd engine sound. I am not sure but I don’t believe the crank was made so the firing was exactly opposite of each other. Someone has to have heard a Jonny Popper before.
Yeah, looks like one of those ship engines. Use them a lot on cargo ships, too. The fuel they use is the next thing to tar, would be solid if cold. Cheapest liquid fuel out there. Air pollution from these ships is becoming a political hot potato.