MoTeC M1 Software Tutorial: Knock Control
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Knock Control
08.49
| 00:01 | The M1 ECUs include an onboard digital signal processor that can take the input from up to four knock sensors and allow for accurate closed loop knock control to protect your engine from damage due to detonation. |
| 00:15 | To use this feature you will need to have at least one knock sensor wired to your engine, but it is preferable to have one per bank on V configuration engines or boxers, or one near the front of the block and one near the back on inline six cylinder engines. |
| 00:31 | V10 and V12 engines will give the best results with two sensors per bank. |
| 00:37 | While two knock sensors are better, they aren’t essential and excellent results can be achieved by using a single sensor and correctly adjusting the individual cylinder gains as we will discuss shortly. |
| 00:51 | Let’s move to the ‘Knock’ worksheet which is in the ‘Initial Setup 2’ workbook, and we will start with the ‘Knock Resources’ which is where we can configure how many knock sensors are installed. |
| 01:02 | As well as telling the ECU how many knock sensors are installed, we can also define which sensor each cylinder will use. |
| 01:10 | For example on an inline six cylinder engine, we would normally fit two knock sensors - One near the front of the block and one near the rear of the block. |
| 01:18 | In this case we would allocate cylinders one, two and three to ‘Knock Input 1’, and cylinders four, five and six to ‘Knock Input 2’. |
| 01:27 | This helps improve the knock control systems ability to accurately detect knock on all cylinders. |
| 01:33 | Now we will move up to the ‘Knock Details’ and we can start by enabling the knock control system by setting the mode to ‘Enabled’. |
| 01:42 | Next we have four knock frequencies that we can program, identified in slots A to D, these are 4 choices of centre frequency that the ECU will use to detect knock. |
| 01:54 | The frequency of knock will vary from engine to engine and by monitoring four frequencies simultaneously, the M1 allows us to very accurately find the best frequency to accurately detect knock. |
| 02:07 | While the M1 will monitor four frequencies simultaneously, it only uses ‘Knock Frequency A’ for the actual control. |
| 02:15 | This means that once you have assessed the knock frequency for your particular engine, you must enter this frequency into the parameter for ‘Knock Frequency A’. |
| 02:24 | If you have no idea where to start with the knock frequency, you can calculate a good starting point by using the equation eighteen hundred divided by three point one four multiplied by your piston diameter in millimetres. |
| 02:39 | The result of this equation is the knock frequency in kilohertz. |
| 02:45 | In general though starting with frequencies of eight, ten, twelve and fourteen kilohertz are a good starting point. |
| 02:54 | Often we find that the signal to noise ratio is better at the second order frequency, or in other words double the calculated knock frequency. |
| 03:04 | Once you induce a knock event and identify which frequency showed it up the best, narrow in on this range with numbers at similar levels. |
| 03:14 | Moving down, the next parameter is ‘Knock Activate Delay’ which defines how long in milliseconds the knock control will be disabled for after an ignition cut event. |
| 03:25 | Next we have the ‘Knock Threshold’ which is the table the M1 will use to determine if the engine is knocking or not. |
| 03:32 | We need to calibrate this table with the background noise profile of the engine when no knock is occurring. |
| 03:38 | Since the background noise profile will depend on engine speed, we can press the ‘A’ key to bring up the Axis Setup menu, enable the engine speed axis and then generate a table of engine speeds that will cover the range we want. |
| 03:53 | If you want, you could also enable the Engine Load Normalised axis to vary the knock threshold based on both engine speed and engine load. |
| 04:02 | When tuning the knock threshold table, we want to sample the knock levels throughout the engine speed range in a condition where no knock is occurring and set the knock threshold table just a little higher than this background noise to ensure the knock control system doesn’t register knock. |
| 04:19 | The knock channels output is proportional to cylinder pressure so the background noise will keep increasing until max cylinder pressure is achieved This setup needs to be done by testing and logging the noise profile of your engine, but the time graph function makes it very easy to see what is happening during a dyno run. |
| 04:38 | To make it really easy to see how the knock threshold relates to the recorded knock levels, I like to add the ‘Knock Threshold’ parameter to the time graph. |
| 04:46 | We can do this by right clicking on the time graph and selecting properties. |
| 04:51 | Next we can click on the group we want to display the knock threshold in and click ‘Add Channel’. |
| 04:57 | Use the search bar to look for knock threshold and double click it to add it. |
| 05:02 | Now you can press OK and the knock threshold is displayed. |
| 05:06 | Remember that if your package is locked, you will need to unlock it to do this. |
| 05:11 | You can unlock the package by heading to the ‘Layout’ drop down menu, selecting ‘Layout Editor’, clicking on the little padlock icon by the preconfigured workbooks, and clicking ‘Unlock’ The knock channels displayed on the live screen may not show a single knock event since the laptop screen update rate is only around twenty five hertz, so logging is the best way to see if a knock event occurred. |
| 05:35 | If you are running up to six thousand rpm then fifty hertz is the minimum frequency to log at to guarantee to see all single knock events. |
| 05:43 | To be certain though, I would recommend one hundred hertz for most applications. |
| 05:49 | The next parameter is ‘Knock Recovery Rate’ which defines how fast any knock retard will be removed if knock is no longer detected. |
| 05:58 | This is in degrees per second so a higher value will remove the retard faster. |
| 06:04 | The ‘Knock Trim Gain’ defines how aggressively the ECU will remove ignition timing based on how far above the knock threshold the knock level is. |
| 06:15 | The way this works is that the Gain value is a percentage of the knock level minus the threshold. |
| 06:22 | So if you have a gain of twenty percent, a knock level from cylinder one of ninety and a knock threshold of 60, the ECU would multiply the gain factor of twenty percent by the difference between the knock threshold and the knock value which is thirty. |
| 06:40 | The retard applied would therefore be twenty percent of thirty, which is seven point five degrees. |
| 06:47 | Next we have the ‘Trim Limit’ which clamps the maximum amount of retard that the ECU can apply. |
| 06:53 | You can see that all of these parameters here can be configured with two dimensional or three dimensional table if you want to alter the way the knock control system works in different areas of engine operation. |
| 07:06 | Now we have the ‘Knock Window Start’ and Knock Window Width parameters which define where we expect detonation to occur. |
| 07:13 | The default values have the knock window start at ten degrees after TDC and extend for sixty degrees of crankshaft rotation. You shouldn’t need to adjust these settings. |
| 07:26 | You can also set up the knock control to output a warning if you want. |
| 07:30 | This can be setup by changing the ‘Knock Warning Mode’ to ‘Enabled’, and adjusting when you want the warning to be active. |
| 07:37 | You can set up the warning based on either the amount of ignition retard currently applied, or at a certain knock threshold. |
| 07:45 | The last parameter we need to discuss is the ‘Knock Gains’. |
| 07:49 | These gains can be used to normalise the knock values measured from each cylinder so that their noise profile under normal knock-free operation is similar. |
| 07:58 | This lets the knock threshold be set tighter to the background noise. |
| 08:02 | and offers more accurate detection of light detonation events. |
| 08:06 | The gains can be used to account for the distance between a cylinder and the knock sensor affecting the measured noise level. |
| 08:14 | The aim is to adjust these to either raise the noise level of any cylinder that is low, or alternatively we can drop the noise level of any cylinder that is exceptionally noisy. |
| 08:25 | It should go without saying that configuring the closed loop knock control system should be done carefully as it requires the engine to undergo detonation in order to properly configure the system. |
| 08:37 | Detonation can damage your engine so you need to approach this with care. |
| 08:41 | I also prefer to use audio knock detection equipment to confirm when knock is occurring. |
