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Today we're talking about the number 1 killer of boosted engines. Knock. We are going to understand what it is, how ti differs from pre-ignition, how it correlates with other things in your engine and of course how to avoid it and how to control it.
So what is knock? The simplest explanation of knock is that it's abnormal combustion, but to truly understand it we must dive deeper than that and to understand what is knock we must have a good understanding of what's actually happening inside the engine.
So let's imagine our cylinder is on the compression stroke. Both the intake and exhaust valves are closed and the piston is traveling upward and compressing the air fuel mixture inside the cylinder. Now the spark plug will actually fire before the cylinder reaches it's top most position of travel or top dead center. The spark plug must fire before the piston reaches TOP DEAD CENTER because the goal is to ensure that maximum combustion pressure builds up by the time the piston starts traveling downward. So how do we ensure that maximum pressure builds up when the piston is right past TDC? By firing the spark plug before the piston reaches top dead center. This is called ignition advance. Firing the spark plug earlier accounts for the piston speed and gives enough time to the combustion to build maximum pressure right on time.
So in the beggining of the video we said that knock is abnormal combustion. Now normal combustion inside a gasoline or petrol engine occurs like this: air and fuel is compressed inside the cylinder. The spark plug fires and initiates the combustion. Combustion spreads out evenly from the spark plug until all or most of the air and fuel is burned off.
Knock occurs like this: Air and fuel is compressed inside the cylinder. Spark plug fires and initiates the combustion. Now as the combustion spreads it exerts pressure onto the still uncombusted air and fuel. At this point one or more pockets of uncombusted air and fuel spontaneously ignites before it's reached by the flame front traveling from the spark plug.
Now knocking is also called detonation. Detonation is a combustion process characterized by a super-sonic flame front. Meaning that it travels faster than the speed of sound which is 343 meters per second. Detonation creates destructive shock-waves that have the potential to damage everything around them. When a pocket of air and fuel self-ignites it actually detonates and it's these shock waves the cause a brief but intense spike in cylinder pressures and create the characteristic knocking sound. If knocking is strong enough or persists long enough it will damage or destroy engine components.
In contrast to this normal combustion is not detonation. Normal combustion is something called deflagration. Deflagration is characterized by a sub-sonic flame front meaning that it travels below the speed of sound. Deflagration increases pressures inside the cylinder in a gradual and controllable manner without unpredictable pressure spikes.
Ok so that's knock. What's pre-ignition? Knock and pre-ignition differ from each other in their timing. As we have seen knock occurs AFTER the spark plug fires, but pre-ignition occurs BEFORE the spark plugs fires. Pre-ignition happens while the piston is still moving upward on the compression stroke. The air fuel-mixture ignites spontaneously before the spark plug fires. In most cases pre-ignition can be more destructive because it happens on the compression stroke.
So why do knock and preignition occur? The simplest and most common answer is because temperatures inside the cylinder get too high. Why is that a problem? It's problem inside a gasoline engine because a gasoline engine compresses both air and fuel. If temperatures gets high enough it can lead to spontaneous ignition of the air fuel mix and we get detonation. The actual ignition source for pre-ignition is in many cases some sort of a hot spot which can form at a carbon deposit, spark plug tip or a sharp and protruding edge in the combustion chamber. The overly high temperatures can heat the hot spot until it glows red hot and this can lead to pre-ignition or knock.
Back in the day the safeguard against knock was to run a low compression ratio and leave a big safety margin. The reason behind this is that there wasn't any active knock control. If knock occurred the engine had no way of responding to it. But today things are very different.
A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard Caldwell
Pepe
Brian Durning
Brian Alvarez
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#d4a #boostschool #knock
1 июн 2024
