Now that the OP has been answered I hope you all will contribute to fight my own ignorance.
It is my understanding that pinging in an engine comes from pre-ignition. That is, the fuel-air mix in the cylinder is compressed to the point that it ignites without the aid of the sparkplug. Thus, ignition occurs as the cylinder is still on its compression stroke. This can cause damage since the cylinder is still being mechanically pushed toward the cylinder head when it suddenly has a tremendous amount of force pushing it in the opposite direction.
High octane fuel is designed to resist pre-ignition. It is formulated so that it can handle the increased pressures without igniting spontaneously thus allowing the spark plug to ignite the vapor mix a tthe correct time as designed.
The important takeaway here (apart from the fact that it happens prematurely) is that ignition is created via cylinder pressure and not a spark created by the spark plug. That’s why, traditionally, one way to determine if your engine needed high-octane fuel was to look at the engine’s compression ratio. High compression = high octane requirements.
So. Given the above, how does the computer that controls ignition compensate for pre-ignition? To my understanding changing the ignition timing would have nothing to do with knocking – the very process of pre-ignition is one that does not involve the spark plug. Hell, you could theoretically unhook the spark plug completely and still get ignition.
It seems to me that the only way to make a true non-pinging engine would be to design some sort of pressure relief valve so that, in the event a knock is picked up by the computer, a valve in the cylinder head is opened a tiny bit to relieve some of the pressure in effect making the engine a lower-compression one. That would crate a whole 'nuther mess of problems but since pre-ignition is caused my mechanical pressure the only solution that I can see, assuming the use of a fuel with insufficient octane, is to employ a device that reduces cylinder pressure. A mechanical solution to a mechanical problem, as it were.
So how does a computer reduce what is fundamentally a mechanical (or chemical, if you will) problem?
My understanding is all the computer is doing is trying to match the timing of the spark to the ignition that’s already happening anyways because of the compression.
Preignition happens when the mixture is ignited by a hot spot somewhere in the combustion chamber (typically a hot bit of carbon buildup) before the spark plug lights it. When this happens, the entire mixture burns before it’s supposed to. The problem is not pressure, it’s heat: if this goes on for more than a few cycles it can dump tremendous amounts of heat into the combustion chamber surfaces. The aluminum piston is usually the first victim: see “holed piston.”
Hot spots can also ignite parts of the mixture after the spark plug has started the main event, and if early enough, can cause overheating problems similar to preignition.
Both of the above are instances of deflagration - that is, “normal” combustion that proceeds at subsonic velocities. They do not directly cause knock/ping, and if they happen, your car’s knock sensors will be unaware. You are correct in understanding that if preignition is happening, retarding spark timing will do nothing to mitigate it (though spark retard may mitigate post-spark hot spot ignition).
Pinging/knock happens when unburned pockets of the mixture are at such high temperature and pressure that they become unstable. Acoustic perturbations moving through the mixture are enough to light it off, and that wave of pressure and heat accelerates to a supersonic shock wave, burning the remaining mixture at extremely high speed. This is an instance of detonation, the very same phenomenon that leveled portions of Beirut not so long ago. By the time the shock wave reaches the wall of the combustion chamber, it exerts extremely high local temperatures and pressures at its point of impact before reflecting and then ringing back and forth across the combustion chamber. It’s this ringing that is detected by the knock sensors, triggering the ECU to implement a delay in the spark timing so as to mitigate the conditions that have been causing the knock. Very light knock is not a problem (this used to be called “the sound of economy,” and meant your spark timing was advanced as far as it could safely go, providing the best available engine efficiency). But if heavy knock is allowed to continue for many cycles, it generally tends to hammer the same spot in the combustion chamber, gradually eroding material away and depositing a lot of heat that may lead to thermal distortion and/or melting.
I think I’ve heard that they measure octane ratings differently in Europe, so the numbers there are slightly higher than in the US. @Velocity has a Eurpoean car, although I would think they would update the American version of the owner’s manual to use American octane numbers. Unless Velocity is actually in Europe.
That it correct. The US ratings are an average of the Research Octane Number and the Motor Octane Number (R+M/2, as you see on the pump), and Europe’s rating is purely the Research Octane Number.
I drive a car with a stick shift. I will hear knocking when I am at a relatively low RPM, I am in one of the higher gears, and I step on the gas.
Example: I finally reach town and slow down to 30 MPH. I am still in 5 gear, but everything is fine. I then exit the town and give it some gas. The car doesn’t want to accelerate much because of the high gear and low RPMs, obviously. But I also hear knocking. I can make the knocking go away by going to a lower gear and getting the RPMs back up.
Limit the RPM. Since the problem is cause by etching or heating the cylinder walls, simply running the engine at lower RPM prevents destruction.
For high power engines, the instruction is to limit the power, which means that running high RPM is ok when the engine is unloaded. Power can also be limited by adjusting the turbo-charger: high power engines have independent control of the turbo charger.
Aircraft engines are often more finely tuned that car engines Perhaps someone who is accustomed to adjusting power and mix and cylinder pressure will comment.
That’s a different problem. That’s not knocking, that’s lugging. You have it on the edge of stalling because the engine power is having difficulty overcoming the tall gearing. The reason it happens with a manual and not an automatic is because the connection between the transmission and engine is entirely mechanical rather than a fluid coupling.
As an analogy, it’s like trying to start a bicycle from a dead stop in high gear. It’s extremely difficult at best, and impossible at worst. Do that to an engine and it will stall.
Only refuel using premium-grade unleaded gasoline with at least 91 AKI/95 RON (91 octane).
As a temporary measure, if the recommended fuel is not available, you may also use regular unleaded gasoline with an octane rating of 87 AKI/91 RON. This may reduce engine performance and increase fuel consumption. Avoid driving at full throttle and sudden acceleration. Never refuel using fuel with a lower octane rating.
To maintain the engine’s durability and performance, premium unleaded gasoline must be used. If premium
unleaded gasoline is not available and low octane fuel is used, follow these precautions:
Have the fuel tank only partially filled with unleaded regular gasoline and fill up with premium unleaded gasoline as soon as possible.
Avoid full throttle driving and abrupt acceleration.
Do not exceed an engine speed of 3 000 rpm if the vehicle is loaded with a light load such as two persons and no cargo.
Do not exceed 2/3 of maximum accelerator pedal position if the vehicle is fully loaded or operating in
mountainous terrain
In short, what has already been said. Not a big deal, fill back up with premium when you can, and take it easy on the throttle.
By downshifting and using a lighter throttle setting to achieve the same acceleration that you had in the tall gear at low RPM, you are relieving two of the conditions that are conducive to knock:
The lighter throttle setting reduces the peak pressures and temperatures in the combustion chamber. The pocket(s) of unburned mixture that were previously prone to detonation on a hair-trigger are now more stable, and can be combusted in the normal fashion without detonating.
By operating at a higher RPM, the combustion takes less time. This is because of in-cylinder turbulence, which scales with RPM and results in stretching/distortion of the flame front that speeds up the combustion process; that RPM-scaled turbulence is the reason an engine can both idle at 600 RPM and make power at 7500 RPM. The total time required for combustion matters for detonation because of a fuel property called ignition delay. Rather than copying and pasting, I will refer you to this post of mine last summer that explains the issue in detail:
Perhaps someone who is accustomed to adjusting power and mix and cylinder pressure will comment.