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MoTeC M1 Software Tutorial: Closed Loop Fuel Control

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Closed Loop Fuel Control


00:00 If you have taken the opportunity of fitting an LTC or PLM to provide a lambda input to your M1 ECU, you can take advantage of the closed loop fuel control system.
00:11 This is useful when calibrating the engine efficiency table to prevent the engine running excessively rich or lean as you move through the table, but you can also leave it active permanently to account for any minor discrepancies between the mixture aim and the measured exhaust lambda.
00:28 We can configure this feature through the ‘Closed Loop Fuel’ worksheet and obviously we want to start by enabling the closed loop fuel control system here.
00:38 The first setting in the list is the ‘Fuel Closed Loop Period’ table which is a two dimensional table of closed loop period versus exhaust mass flow.
00:48 This table defines how long the system will wait before updating the closed loop trim and it is there to ensure that the result of any trim change has had time to be registered by the lambda sensor.
01:01 At low exhaust flows which we may see at idle, it will take longer for any change in fuelling to affect the displayed exhaust lambda reading.
01:10 For this reason we want to use a longer period in these situations.
01:15 At higher rpm and load, the exhaust flow increases and hence the lambda sensor will respond faster to changes in closed loop fuel trim.
01:24 At higher exhaust mass flow, the period can be reduced to one hundred to five hundred milliseconds.
01:30 This table needs to be adjusted by monitoring how the closed loop system is performing.
01:36 If the period is set too low, this can result in unstable control and oscillation, while a period that is too long will give slow response to any lambda error.
01:48 The best way to set up this table is to log the exhaust mass flow or look at the live values for it so we can get a good range of numbers to work with in the table.
01:58 What we want to do is enter the exhaust mass flow at idle as the far left point in the table, and the exhaust mass flow at wide open throttle and high rpm as the far right.
02:08 This will give the table the right range and resolution for your particular engine.
02:14 We are going to move out of order here and the next parameters we will look at are the ‘Closed Loop Control Trim Minimum’ and ‘Maximum’ which will limit the amount of control that the closed loop fuel control system can apply.
02:27 The closed loop fuel control system isn’t a bandaid to fix a poorly calibrated efficiency table, and under normal use I prefer to use a limit of plus or minus ten percent.
02:38 If you are frequently seeing trims outside this range then you should probably revisit the efficiency calibration.
02:46 If you want to use the closed loop fuel control system to aid initial calibration though, you may choose to give the system more control - perhaps as much as plus or minus twenty five percent to help prevent excessively rich or lean mixtures while beginning your calibration.
03:01 By default the trim limits are a single value, but you can configure these as a two dimensional or three dimensional table to adjust the control of the closed loop system through the engine speed and load range if desired.
03:15 If we move back up the list of parameters, we have a couple that relate to diagnostics.
03:21 The ‘Fault Delay’ parameter is a measure of how long the closed loop trim must remain at either the positive or negative trim limit, before a Sensor Limit fault is registered.
03:33 This needs to be approached carefully as if your trim limits are quite tight, you may inadvertently trigger this diagnostic error.
03:41 This is the most common reason for closed loop not to work so it is worth spending the time to configure the maximum and minimum tables to define sensible trim amounts.
03:53 For example you might choose to leave closed loop disabled at idle and low engine speeds, or you could leave it on and allow it more room to trim in these regions as a 10% error at idle is quite common but not at full load.
04:07 Making the delay large will also mean that the trim must sit at the min or max limit for longer before it results in an error.
04:17 Next we have the ‘Temperature Threshold’ which defines the coolant temperature above which the sensor may be detected to be faulty.
04:25 Moving down, we have the ‘Fuel Closed Loop Reset’ value which the M1 uses for integration between the Quick Lambda and closed loop control.
04:33 This value should be left alone.
04:37 Next we have some parameters which allow the us to control the operating range of the closed loop control.
04:43 Specifically we can choose an engine speed and manifold pressure below which the closed loop control system will be disabled.
04:51 One of the critical parameters with tuning the closed loop control system is the ‘Control Trim Gain’.
04:58 This defines how aggressively the closed loop control will respond to an error.
05:04 A value of 100% here means that the entire error will be corrected in a single trim update.
05:11 While this sounds like the perfect scenario, this can result in the system becoming unstable.
05:17 A value of sixty to eighty percent is a good starting point.
05:21 It’s also important to understand that this parameter and the closed loop period table both influence how well the closed loop control will work.
05:31 If we move down, we now have the ‘Closed Loop Control Trim Average Gain’.
05:37 The closed loop system uses an average gain which continuously tracks the closed loop control trim and is used when the ECU is recovering from transient conditions.
05:47 The ‘Trim Average Gain’ value defines how aggressively the Trim Average will step towards the current closed loop trim value and the ‘Trim Average Decay’ defines how quickly the Trim Average will decay after shutdown.
06:01 You can think of the ‘Trim Average’ like the long term fuel trims in a factory ECU.
06:07 The last parameter we have is the ‘Warning Mode Diagnostic’ which we can use to enable a warning if the system is deemed to be faulty.
06:16 If you are using closed loop fuel control, you will see the current ‘Closed Loop Fuel Trim’ value as well as the ‘Trim Average’ displayed in the time graph.
06:25 A more useful display though is the closed loop control trim bar graph which acts as a quick visual reference of the current trim value being applied.
06:33 This bar graph is also repeated at the bottom of the Fuel worksheet in the Tuning workbook for a quick reference while calibrating the efficiency table.
06:42 Remember that even when the ECU is running in closed loop mode, we can still use the quick lambda function to tune the efficiency table.
06:50 In this case the ECU will apply the current trim value to the efficiency table and zero the closed loop trim.

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