The fuel mixture has a flash point; a temperature / pressure point at which,
instead of the steady controlled burn that we want, the remaining mixture
just explodes. This is detonation. The result is, that instead of a steady
pressure increase, we get a nearly instantaneous pressure increase in the
cylinder. This is analogous to hitting the whole inside the combustion
chamber with a hammer. Severe enough detonation can cause physical damage.
It is most often the piston rings and ring lands which suffer, because
they are the weakest pieces in contact with the combustion gasses.
Engine Knock Theory
Engine knock (also called ping) refers to the sound made
when the fuel mixture detonates. (This is NOT the same thing as pre ignition,
though pre ignition can CAUSE detonation.) The terms "knock" and "detonation"
are used interchangeably (even in most technical documentation) to describe
the event of fuel detonation. I will use "ping" to refer the sound produced.
Normally, your fuel mixture is supposed to burn smoothly, from the point
of ignition (the spark plug), to the edge of the cylinder. Imagine a flame
front, which starts at the spark plug and moves out in all directions at
a steady speed, until all fuel has been burned. The piston is moving this
whole time, still moving upward (the last of compression stroke) when the
burn begins, moving past TDC, and moving down again (into the power stroke)
as the burn finally completes. As the mixture burns, cylinder pressures
are increasing dramatically due to several factors:
Temperature is increasing due to combustion, and we all know that hot gasses
expand. (PV=nRT applies very nicely during the power stroke, since this
equation is specifically for closed systems.)
Liquid gasoline is being converted (via chemical reaction) to various gases,
and a gas is far less dense than a liquid. I'm no chemist, so don't
take the following as gospel. Basically, the hydrocarbon compounds
in the gasoline are combining with oxygen to produce CO and CO2 and H2O.
Other reactions occur also (such as nitrogen and oxygen from the air, combining
due to the heat and pressure to form NOx compounds) but they are side effects,
and are not relevant to increasing pressure (in fact, they may actually
decrease the temperature since most of these reactions absorb energy).
(Note the PV=nRT does NOT apply to chemical reactions or state changes.)
The spark almost always fires before the piston reaches top dead center
(this is the "timing advance" on the data logger), so until we reach TDC,
the cylinder pressure is also increasing due to shrinking volume (PV=nRT
I will throw this in here just for completeness, though it is not directly
relevant. Pre ignition just means that the fuel mixture ignited before
it was supposed to (before the sparkplug fired). This usually is caused
by some hot spot in the combustion chamber, such as glowing carbon deposits,
or an over heated sparkplug electrode due to incorrect type of plug or
a hot running (lean) cylinder.
The same burning process will occur as described above, but because
the process started early, the process will be farther along as we reach
TDC, our point of minimum volume. This means that the cylinder pressures
will be higher than they would have been, had the burn started at the correct
time. The higher pressure means it is more likely that the mixture will
The knock sensor is basically just a microphone screwed into the side of
the engine block. You can actually connect an audio amplifier to the output
of this sensor and hear the pinging caused by detonation (along with all
the other rattles, clatters, and bangs that the engine makes). The ping
does have a fairly distinctive sound, that I would describe as similar
to the "twang" that you can get from flexing a piece of sheet metal. The
ECU contains a filter circuit which tries to pick out just the sound of
the ping, and ignores everything else. The consensus on the digest is that
this circuit is not perfect (lifter clatter for example, is suspected of
getting through the filter and being seen as knock), but overall it does
a pretty good job.
This is what engine knock sounds like, directly
from the knock sensor.
There are three internal variables used by the ECU to represent the current
knock conditions of the engine: instantaneous knock, knock sum,
and octane. The pinging sound itself is a very quick, lasting
less than 1/10th of a second. Instantaneous knock tracks this. This
value is NOT loggable.
Instantaneous knock is responsible for immediately reducing ignition
timing when significant knock is detected.
Knock sum is a short term value, representing engine knock over the last
few seconds. It is derived from instantaneous knock. Knock
sum will climb very quickly when knock is detected, then will bleed down
slowly. The knock sum will go back to zero when you shift gears (I
suspect that this is triggered by the throttle position dropping below
a certain point, thus if you speed shift, your results will likely be different).
Here is an example of engine knock being detected a few seconds into
second gear (this is very mild acceleration). You can see the knock
sum (the green trace) slowly decrease over the next four seconds, then
goes to zero on the shift to third (seen by the throttle position and RPMs
Fuel Octane Rating
The octane rating of a gasoline is a measure of its resistance to detonation.
The higher the octane rating, the higher its flash point, thus the more
heat and pressure we can put on the fuel mixture before it detonates.
The knock sum value is used to derive a much longer term value, which
TMO calls octane. This is the knock value over the last 60 miles
or so of driving, and is meant to be representative of the quality of fuel
that you are using. This term is NOT presently loggable.
When the knock sum is 3 or below, the octane value is slowly increased.
Between 3 and 7, nothing is done to octane. Above 7, the octane value
is decreased. The octane value is one of the numbers that the ECU
uses when calculating ignition timing. In fact, (as I understand
it) the knock sum does not directly effect the ignition timing, but rather
changes the octane value, which in turn effects the timing.
Here is how Todd (the ECU master) describes octane:
Here is basically how the whole engine timing control feedback loop
There are two tables in the ECU. Both of them use RPM for one
coordinate, air mass in the other coordinate. One of the tables is
for the minimum timing the ECU will ever put out, the other is the maximum
the ECU will ever put out.
Imagine a 3-D space with the lower map on the bottom and the upper
map on top, overlaid on each other. So we have an X/Y/Z space where
X is RPM, Y is air mass, and Z is what I like to call "octane". When
the ECU needs to calculate timing, it traverses the X and Y coordinates.
Now imaging a "pole" that stands up from the bottom map and extends to
the top map. This pole length will change depending on which part
of the map you are traversing. Now take the "octane" value (0-100%)
and multiply that times the pole length. Then add that to the value
at the base of the pole.
If your "octane" is zero, you get the bottom map. If it is
100%, you get the top map. If it is 50%, it is somewhere halfway
up the pole.
Whenever knock-sum is below 3, the ECU starts climbing the pole (if
it isn't already at the top). It climbs very slowly. However,
whenever the knock sum gets above 7, it slips down very quickly.
Since it can go all the way to 43, it usually slides a little bit farther
than it climbs. Under steady state conditions, the ECU is always
trying to climb that pole. It doesn't intentionally try for knock.
In fact, if you are running, say, 118 octane, the ECU might spend all of
its time on the ceiling. But it can't go any higher, so it still
isn't getting any knock.
It is my suspicion that the top table is for, say 92 octane at 11psi,
and the bottom table is for, say 87 at 7psi. BUT - it seems that
people can still get value out of 110 octane fuel, so that would imply
that the top table might be for a higher octane. Or maybe that is
just a function of topping off the boost to 22psi.
There are other minor terms that affect the above timing, like whether
the car is warmed up or not, but they usually don't affect normal driving.
The biggest impact is if the instantaneous knock term that the ECU uses
to derive knock sum is above a certain amount, it will subtract a raw 4
or 8 degrees off immediately. BTW, this is a knock term you can't
How The ECU Controls Knock
The ECU uses three methods to reduce engine knock:
The ECU will reduce the timing advance. This is just the opposite of
pre ignition, i.e., the mixture ignites late, rather than early. Because
the piston will be farther along in its stroke at any point in the burn,
cylinder pressures will be reduced. A drop in timing advance is usually
very visible on the logger when significant knock occurs.
Richen fuel mixture
It is not intuitive, but a richer fuel mixture will burn slower and
cooler than a leaner mixture. Thus the ECU is able to reduce cylinder pressure
by adding additional fuel to the mixture. This can be seen on the logger
as an increase in the oxygen sensor voltage. This should also be visible
as an increase in injector pulse width, however this is usually difficult
This seems like an appropriate place to point out (in case it's not obvious)
that the pressure in the cylinder is exactly where the power comes from
that we get out of the engine. The higher the cylinder pressure, the more
force against the piston, and the more power we get. Since the ECU reduces
knock by reducing cylinder pressure, it also reduces the engine power output.
Close boost control solenoid
The ECU will close the BCS which will reduce the maximum pressure output
of the turbo (assuming a stock boost control setup). Less boost means less
fuel mixture going into the engine, which means lower cylinder pressures.
This cannot be easily seen on the logger. The logger does not monitor the
Boost Control Solenoid
The boost control solenoid is a safety device that the factory engineers
put in to protect the engine under extreme conditions. It does this
by closing the factory air bleeder, thus limiting boost pressure to about
9psi (assuming a stock setup). There are three things that I am aware
of which can cause the BCS to close:
(**This needs verified**)
The BCS never just flatly turns on or off, but instead pulses, with a varying
pulse width over the course of several seconds, to gently switch from on
to off, or vice versa (the exception is when turning the ignition key on
or off). If an LED is connected to your BCS, the first two cases
above are usually seen as a flickering of the LED. The third case
is more interesting: when the octane drops below the limit, the BCS does
NOT close right away. It will wait until the next time you start
the car. The BCS will then remain closed continuously until the octane
value rises above the limit. The BCS will then open, while you are
Very high air flow
When the ECU detects air flow beyond a certain point, it assumes that
something is wrong, and closes the BCS to try to protect the engine.
This condition is often seen at high RPMs with moderate engine modifications.
Short term engine knock (I don't know which value) can cause the BCS
to close for a few seconds, until the knock goes away.
If the octane value gets bad (low) enough, the ECU will close your
boost control solenoid for an unlimited length of time (until the octane
Bad Or Disconnected Knock Sensor
If the knock sensor is not putting out any signal at all, the ECU will
derive its own knock sum internally. The derived knock sum (in 1992
at least) is either 0 or 11. No other values are used. The
derived knock sum seems to be based on acceleration: if the RPMs are increasing
significantly, the knock sum goes to 11, else it goes to zero.
This is an example of a log with the knock sensor disconnected.