How the knock sensor works - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 12-22-2004
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How the knock sensor works

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I still can't explain why my truck is "peppier" on higher octane gas and I assumed it was the resistors. However, with the resistors REMOVED, or as much as 100k put in, there is no difference in timing.
I believe that you are seeing the influence of the knock sensor here. Forgive the long post but it is necessary to explain what I mean.

A knock sensor monitors vibration in the engine and reports this information to the PCM. The PCM is calibrated to react to a range of vibration that is associated with the frequency and intensity of the ringing in the engine assembly associated with audible detonation. The physical location of a knock sensor on the engine should be chosen to allow, as nearly as possible, equal sensitivity to knock in all cylinders. The PCM calibration is carefully adjusted to insure detection of detonation while ignoring other background engine noise. Inclusion of a knock sensor and its calibration strategy helps to insure that the timing will be optimized despite variations in fuel octane, changes in ambient temperature and humidity, formation of combustion chamber deposits and compression changes caused by high mileage engine wear.

In late model Ford engines, the knock sensor is allowed a range of influence, both above and below the corrected base timing value. A typical number for this range is plus or minus 6 degrees for a total range of 12 degrees.

The base timing table is calibrated using 87-octane fuel. When the engine begins running above idle, the ignition timing is initialized at a number equal to the current 87-octane base table value plus any spark adders from associated scalars or tables (positive or negative). An example of a spark adder would be a table of values that adds spark at different speed/load cells while the EGR is active. Other adjustments to spark timing are available in the calibration for corrections based on intake air temperature, coolant temperature, barometric pressure, etc. As in the case of the IAT sensor on late model engines, these tables or adders may be 'zeroed out' and not used, at the discretion of the calibrator. (Note: Spark tables are defined in terms of RPM and 'load'. Although load is a composite number based on several parameters, it can be thought of as proportional to throttle opening for purposes of this discussion.)

If knock is detected while running above idle at the corrected base value, the timing jumps back immediately by a calibratable amount, typically -3 degrees. If knock continues to be present at this new advance value, the timing will jump back 3 more degrees in the negative direction. The total negative deviation from the "correct" laboratory value is usually limited to 6 degrees.

If, on the other hand, the engine at its current base spark value shows no knock, the timing is steadily advanced at a calibratable rate (typically 1 degree/second) until knock is detected. Then the immediate 3-degree jumpback occurs as many times as necessary to eliminate the knock, again limited to -6 degrees from base.

The ignition advance remains in this continuous tug-o'-war between this slow 1 degree advancing and immediate 3 degree jumpback. The goal is to provide an engine free of audible knock while maintaining the highest possible spark advance to provide maximized power and fuel economy. Use of a fuel with octane higher than 87 can provide a small but noticeable increase in power and fuel mileage.

There are certain RPM vs. load points in the table where the full 12 degrees of ignition adjustment by the knock sensor strategy is not available. At high load and/or high RPMs, gasoline engines typically have maximum spark advance values that are detonation-limited requiring that they run a base value that is below optimum. In these table cells, there is an advantage to allowing the spark to advance when possible.

At lower RPM-load points, the base value provides the optimum spark advance and any additional advance would reduce power rather than add it. Therefore, there is a clipping table which overrides the other calculations to define the maximum spark advance allowable at any given RPM vs. load point. So, at lower RPMs and engine loads, the range of influence for the knock sensor may be limited by the clip to as few as 6 degrees, all negative.

Side note:
The above strategy applies to conditions above idle. When the engine is idling and there is normally no possibility of spark knock, the ignition timing goes into a different mode that does not involve the knock sensor. At idle, the timing advance is used to help control the idle speed. The Idle Air Controller (IAC) provides the slow, coarse adjustment and the ignition timing is the fine adjuster, reacting quickly to correct small RPM errors from the desired idle speed.

Last edited by V8 Level II; 12-22-2004 at 06:10 PM.
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Old 12-22-2004
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Thanks, Bob! That's a great explanation! :)

I suspected something to that effect, but I've not investigated it as you obviously have.
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Old 12-22-2004
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Not all engines have knock sensors. Don't know about any specifics though. Even when engines do have them it seems getting them to work properly is almost a black art. That's what I hear anyway.

As far as IAC goes, Ranger is one of the few left that still has one. I'm sure that by '09, or even sooner, it will be ETC. I'm gonna miss cables, I've always prefered them, though the ETC systems are getting more transparent.
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Old 12-22-2004
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Quote:
Originally Posted by Dave and Julie
As far as IAC goes, Ranger is one of the few left that still has one. I'm sure that by '09, or even sooner, it will be ETC. I'm gonna miss cables, I've always prefered them, though the ETC systems are getting more transparent.
ETC can be a good thing for emission control and integrates well with traction control systems. Personally, I'll take a good old fashioned throttle cable.

Aftermarket companies are already making money by tweeking the factory calibration for improved throttle response at the expense of emission compliance.

Believe this - lawyers are going to have a field day with ETC. It makes idle speed control and throttle control even more of a mystery to the public who make up our juries.
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Old 12-22-2004
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I know mine doesn't have a knock sensor....but then again the truck does seem to have more power throughout the RPM range when using 93. Also, pinging is non-existent.
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Old 12-25-2004
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"Fly by wire" is the way to go, even for performance -- IF you can get to the software.

Most everything in the steel mill has been converted to multiplexed controls via computer automation. The operator is in fact "suggesting" operations to the system which is deciding how to carry them out, and overriding commands that might cause damage to the system or the people around it.

Our rolling mill uses hydraulic cylinders capable of pressing with a force of 3000 tons (no that's not a misprint). The main mill drives are 5000 and 6000 HP (at just 40 RPM, calculate the torque, lol). Our melting furnace modulates the melting current at power levels up to 55 megawatts. Our caster has to precisely control the exposed, thin shell of metal over a molten center (sounds like candy) using water sprays. Too little an you could have a "breakout" with tons of molten metal pouring everywhere -- too much and you end up with unusable scrap or a jammed caster (8 story tall machine and very time consuming to disassemble). So you can see where a fully programmable system can be REAL useful to prevent tragedy.

Properly constructed, a fully programmable engine control would allow the complete optimization of the engine for any target conditions, whether that be efficiency, emissions, or power output. Sadly, optimizing for one is often at the expense of the others.

Last edited by n3elz; 12-25-2004 at 11:55 PM.
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Old 12-26-2004
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Quote:
Originally Posted by John Moorehead
I know mine doesn't have a knock sensor....but then again the truck does seem to have more power throughout the RPM range when using 93. Also, pinging is non-existent.
98~2000 Rangers w/3.0L were way oversparked and most eventually exhibit knock. That's why they like and respond to high octane fuel. The knock "fix" for these years was a replacemant coil and recalibrated PCM. If the ping persisted after that, there was a service bulletin procedure to reduce the spark in stages using an NGS tester.

http://rockledge.home.comcast.net/Ra.../3LPingTSB.pdf
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Old 12-27-2004
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Quote:
Originally Posted by n3elz
"Fly by wire" is the way to go, even for performance -- IF you can get to the software.

Most everything in the steel mill has been converted to multiplexed controls via computer automation. The operator is in fact "suggesting" operations to the system which is deciding how to carry them out, and overriding commands that might cause damage to the system or the people around it.
I can speak to that as well, but from a different perspective. The National Air and Space Museum at the Smithsonian in Washington, DC is one of our customers. They have a Zeiss VI-a star projector that was given to the United States by the West German government in 1976. The thing is absolutely priceless!

I suppose a touch of background is needed: a star machine is a (usually) large device w/ a bunch of little lenses and lamps that projects little lights onto a dome. The lights are organized (very carefully) to model the night sky. Additional elements are added for plannets in our solar system, the moon, and the sun. The motion of the spinning of the earth and rotation of plannets around the sun is emulated by moving the device. Most traditional star machines are driven by electric motors w/ complex gearboxes to drive each axis precisly. Gearboxes introduce backlash which becomes worse as the gearbox ages and wears. Also bear in mind that projectors like the Ziess VI-a wheigh quite a bit! Upwards of 1 ton!

So imagine having a switch directly wired to the drive motor for one of the axises of your priceless Zeiss VI-a. You, or your dumbass operator ramps the machine up so it is simulating dirunal motion at a very high rate so you can demonstarte how the sun moves throughout the year. Suddenly you either reverse the switch on purpose, or accidentally! Well, your priceless Zeiss VI-a star projector which was a gift from the now defunct West German government is .. well, trashed!

For this reason your friends at Sky-Skan have developed a box that interprets your control commands and ramps sudden motions down into more gradual accellerations. It also accounts for variable gear backlash and other parasitic elements though a fairly simple adjustment process done by your qualified Sky-Skan technician durring installation.

Okay, so I've been reading too much of my companies' marketing crapola!
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Old 12-27-2004
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Not at all!!! That was a great description of one of the fundemental benefits of automated motion control! :)
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