Hi Ron,
I want to follow up on this discussion on knocking, as there seems to be multiple effects that can cause it.
First, for the benefit of others who may not know what is behind knocking let me mention that, as you know very well, knocking is caused by "autoignition" of the air-fuel mixture in a hot spot of the cylinder away from the spark. The two pressure waves associated with the two combustion waves collide and these pressure waves generate sound as they collide with the cylinder walls. It is this sound that is called knocking. Thus the most important physical reason for knocking is the existence of two separate points where ignition takes place (and that the ignition does not in general take place at the same time at these points).
<High mileage cars can begin to knock due to a carbon build-up in the cylinders, which increases the compression ratio, and thus the need for a higher octane fuel. But this is not as common as it was years ago. >
You say here that carbon buildup causes knocking because it increases the pressure ratio. I am trying to understand why? It seems that it could happen because the cylinder volume becomes smaller and thus a higher temeperature exists at the end of the compression stroke. However, there maybe a more compelling reason. Lets say that the block is made of aluminum which is a very good conductor of heat. If this wall is then full of carbon residue, the carbon layer acts as an insulator. The conductivity of carbon is about 1.5 Watts/m C and that of aluminum is 170 Watts/m C. Thus to get rid of the same amount of heat from the engine block to the coolant, the inside surface temperature of the carbon is larger than would be the temperature of the cylinder surface that is free of carbon. My understanding is that, it is this hot carbon that promotes autoignition and thus knocking. What do you think?
<But the most likely cause IMO would be a problem in the EGR system (Exhaust gas recirculation). This system is used to dilute the fuel/air mixture with a small amount of exhaust gas, to lower the combustion temperatures in the cylinder. This helps prevent the formation of oxides of nitrogen and lowers harmful emissions. A carbon build-up can restrict the passages and reduce the flow of exhaust gas into the intake system. Or the system could be malfunctioning due to broken vacuum hoses or a faulty EGR valve or switching solenoid.>
The general issue that you talk about above is basically running the engine off its design condition. Since autoignition takes place most easily at some optimum air-fuel mixture, then the wrong air fuel ratio is likely to be a problem, and as you mention, the EGR system's poor condition contributes to it. Would you agree that running an old car up a hill also leads to the same kind of effect. That is the engine is not powerful enough because of leaky rings to operate at its design condition, and thus a wrong air fuel ratio is the result of the leakage and hence autoignition takes place.
Thanks for starting this thread. I will read it regularly.
with my best, Seppo
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