Re: Knock sensors

[rdh@sli.com (Robert D. Houk)]

   > A 'ping' is the predetenation of fuel,..it is igniting BEFORE the spark 
   >arrives (potentialy causing damage blah blah blah...)
   >So if a knock sensor detects a ping, it will retard my ignition timing... 
   >Now if Im not mistaken, my whole problem is that it ignited too early, 
   >why retard it? What does it gain? Am I missing something here? 

   Because you need to get the combustion chamber temps down, Advancing the 
   timing increase the combustion chamber temps, to much temp or to low a 
   octane (Think Flashpoint) the more knock.

I wasn't really aware of distinguishing between "detonation" and "pre-
ignition", but I suppose it's useful.

Detonation is just what it sounds like - an explosion. Bad bad news for
the ["soft" alloy] piston and cylinder head.

Proper combustion is a process wherein the air/fuel burns in a
controlled (and relatively time-consuming) way, applying "reasonable"
pressure on the piston to perform "work" (ultimately, moving that
Audi/etc.). Apply too much pressure and the engine turns into shrapnel
. . . (if you're lucky, you only shred your head gasket).

This combustion process is a [very] complicated function of temper-
ature, pressure, fuel, oxidizer, contaminants, and whatnot. "Speed of
combustion" has a positive relationship to pressure -- higher pressure
will result in higher speed of combustion. This is why idle/high-
vacuum needs lots of "advance" -- it takes longer to burn at low
pressure so the ignition fires "earlier" in the cycle so that by the
time the piston gets to TDC and starts moving downwards again, there
is some decent com bustion going on to exert pressure and thus perform
"work". However, get the timing too advanced, and the pressure peak
comes before TDC and will attempt to run the engine backwards. I've
actually seen "really high" compression gas engines diesel and run
backwards this way. Ugly...

Detonation is when the mixture essentially burns instantly (aka
"explodes") when the pressure builds up too fast, accelerating the
combustion pro- cess. This can happen in a number of ways. One way is
when some red-hot spot (carbon deposit, exhaust valve, etc.) "pre-
ignites" the mixture so that the pressure builds up (e.g., before TDC,
thus trying to accelerate the engine backwards) way too soon -- e.g.,
the engine is still further compressing the mixture as it is burning
and raising its own pressure.  Another way is simply dieseling -
spontaneous combustion as simply compressing the mixture provides
the heat needed to ignite it. A third way is where the increasing
pressure of the so-far-smoothly-burning mixture causes spontaneous
combustion of assorted bit'n'pieces of the mixture along the
propagating pressure/wave front (you could conceivably get a whole
bunch of very small detonations this way...).

Similarly, "lugging" the engine (trying to run it too slow under
power) is just supplying so much fuel/air that the piston cannot be
accelerated fast enough to keep the mixture from "detonating" as the
pressure builds up faster than the piston expands the volume of the
combustion area,and is VERY HARD on the engine!

As an aside, a really graphic demonstration of the pressure/rate-of-
combustion can be demonstrated via "gunpowder". Modern "gunpowder"
is considered a "propellant" and not an explosive. Obviously, firing
a gun produces a *VERY FAST* combustion of the gunpowder -- in the
sub-millisecond range as the pressures reach 50,000 PSI. Yet if you
pull the bullet and just pour the powder into a small pile on the
ground, it is hard to get it to burn! You can easily extinguish a
match by stuffing it into the gunpowder. Once you do get it to start
burning, it just sits there and fizzles for awhile, a coupla seconds
at 14 PSI . . . [Obviously, I am not recommending that everyone go
and start experimenting with burning piles of gunpowder! It's still
dangerous stuff! Be careful out there, people!]

Back to the program at hand . . .

Under "normal" conditions, at high revs, with normal timing developing
normal power strokes, a fair amount of combustion is *STILL* occuring
in the gases being pushed out the exhaust valve(s) and down the ex-
haust pipe (and beating on the turbo's turbine blades; Whee!), so the
combustion process is, like I said, a fairly time-consuming proposi-
tion. Nowhere near 100% of available work effort is extracted from the
mixture to push down on the piston and deliver useful work.

The higher the pressure developed as the piston moves down past TDC
(Top Dead Center), the more work performed. The earlier you fire the
spark plug ("more advanced timing"), the sooner the combustion process
get started, and the higher the pressure as the piston moves past TDC
(i.e., "advanced" timing develops "more horsepower"). Until such time
as the pressure passes the knee in the curve and moves into detona-
tion, turning your engine into a grenade.

So, rule of thumb: more advanced timing means more horsepower, right up
until you destroy the engine from runaway detonation. Obviously, the
closer you can run to the point of detonation, the more power you can
extract from the engine (and the more efficient the consumption of
gasoline). Completely "static" systems run relatively retarded igni-
tion timings so they will be in the safe area under worst case condi-
tions. More sophisicated (engine computers, etc.) systems use "knock
sensors" (microphones hooked up to the engine) to detect very early
"mini" detonations (when they're still "harmless", as it were), and can
continuously adjust (advance and retard) the timing to try to hang out
right on the hairy edge of combustion/detonation. Using a higher-octane
gas allows the computer to advance the timing further before detonation
and thus "extract more power" per gallon of gas. (This is why "premium"
gas can "generate more power" in a knock-sensor-equipped smart engine-
computer car, and why pumping premium into a car that isn't setup to
advance the timing to take advantage of it is a total waste of dollars;
the gas itself isn't more powerful, but it allows the engine to run in
a more powerful regime/closer to the edge/more efficient manner.)

					-RDH