pamotorman wrote: ↑Mon Nov 06, 2017 10:47 am
ptuomov wrote: ↑Mon Nov 06, 2017 10:15 am
pamotorman wrote: ↑Mon Nov 06, 2017 10:06 am
since the engine is a heat pump the more heat retained in the chamber the more HP. this is the reason you need .5 more CR to get the same HP with aluminum heads vs CI because more heat is transferred into the water jacket with aluminum heads. this is the reason to coat the chamber to prevent the heat loss thru the heads
That's how it works if you're not knock limited, like with a diesel engine. However, how does it work with a knock-limited engine, say, 93-octane pump gas port-injected spark-ignition engine?
since you can use a lower CR to get the same HP with coated chamber aluminum heads would that not reduce the tendency for the engine to knock ???
My short answer would be "no"; my long answer is the rest of the post below.
What actually makes an engine knock / fuel to pre-detonate? Is it the static CR, the cylinder pressures, or the temperature?
My understanding is that it is the TEMPERATURE in the combustion chamber that causes the fuel to pre-detonate / combust from heat alone, without a spark; CR is just a very rough INDICATOR / approximator of what your combustion temperature is going to be.
A lower CR coated chamber should have the same tendency to knock at the same temperature as an uncoated chamber of higher CR, in my understanding.
Example: Let's say we have a fuel with an auto-combustion temperature of 250 degrees Celcius at stochiametric AFR. This scenario represent my understanding. We run this fuel in 4 identical engines with the only difference being whether the head is iron vs. aluminum and whether it's coated or not.
#1: Iron head with a 9.5:1 compression ratio reaches 250 degrees Celcius just a couple of crank rotation degrees prior to the spark igniting the mixture; we have pre-ignition and need to remove a couple degrees of timing to avoid pre-ignition at current temps (or drop the incoming air temp or coolant temp)
#2: Aluminum head with 10:1 compression ratio reaches 250 degrees Celcius just a couple of crank rotation degrees prior to the spark igniting the mixture; we have pre-ignition and need to remove a couple degrees of timing to avoid pre-ignition at current temps (or drop the incoming air temp or coolant temp) -identical situation to #1 despite different CR and materials as it's the combustion temperature of the air-fuel mixture that matters and determines when pre-ignition occurs
#3: Aluminum head with coated chambers with 9.7:1 compression ratio reaches 250 degrees Celcius just a couple of crank rotation degrees prior to the spark igniting the mixture; we have pre-ignition and need to remove a couple of degrees of timing to avoid pre-ignition at current temps (or drop the incoming air temp or coolant temp) -identical situation to #1 & #2 despite different CR and materials/coatings as it's the combustion temperature of the air-fuel mixture that matters and determines when pre-ignition occurs
#4: Iron head also coated (crazy I know) with a 9.5:1 compression ratio reaches 250 degrees Celcius MANY crank degrees prior to the spark igniting the mixture; we have pre-ignition and need to remove a LOT of degrees of timing to avoid pre-ignition at current temps (or drop the incoming air temp or coolant temp, or lower CR or get rid of the coating, or retard the cam, or....) -near identical situation but the max temp threshold is reached sooner so we need to pull more timing to prevent the temps goign over our threshold
Assumption: Temperature is the cause of pre-ignition / detonation of a given fuel; CR, DCR, all the other indicators have rough coorelation to pre-ignition /detonation
Or to say it another way: the temperature of the fuel being raised beyond the point at which the fuel spontaneously combusts is the CAUSATION of pre-ignition; the other factors are merely COORELATION to what we should actually care about.
(Thermal barrier coatings put more heat into the combustion chambers and this attribute makes pre-ignition / detonation more likely, BUT thermal barrier coatings create more even temps, reducing hot-spots and this attribute helps to reduce the likelihood of pre-ignition. If your pre-ignition was because of a hot-spot, thermal barrier coatings might help; if it's full-on detonation / spontaneous combustion of the air fuel mixture, thermal barrier coastings should make it worse.)
Unless we're talking about a magical unicorn "heat diode" coating and then I got nothing...
Adam