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MoTeC M1 Software Tutorial: Engine Overrun

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Engine Overrun


00:00 If you want to disable fuel injection when the engine is in over run, this can be achieved through the ‘Engine Overrun’ worksheet in the ‘Engine Systems’ workbook.
00:10 The aim of this feature is to disable the injection when the throttle is closed and the engine speed is sufficiently high to suggest the engine is in an overrun condition.
00:21 The parameters need to be adjusted to ensure that the overrun mode is entered only when it is required, and that the engine will transition smoothly back to normal operation.
00:31 We can start by setting the ‘Overrun Mode’ to ‘Enabled’ to turn overrun control on.
00:37 When the engine is operating in overrun mode, there will be an excess of oxygen in the exhaust system that will affect the lambda readings.
00:45 For this reason its important to make sure that the ECU doesn’t attempt to use closed loop fuel control during this time.
00:52 The ‘Closed Loop Recovery’ parameter also specifies how long the closed loop control will remain inactive after injection resumes.
01:01 Next we have ‘Overrun Delay’ which specifies how long the engine speed and throttle position thresholds must be exceeded for before the ECU enters overrun mode.
01:11 A typical value of 1000 milliseconds works well to prevent the ECU going into overrun mode every time you change gear or lift off the throttle.
01:21 Now we have the engine speed and throttle position thresholds and hysteresis values.
01:27 These are used to define the conditions where overrun will become active.
01:32 Generally we want to set this to be active around five hundred to eight hundred rpm above our normal idle speed, and at a throttle position that indicates closed throttle.
01:42 You do need to be careful with the throttle position as most sensors will vary slightly in their output, so a value of zero probably isn’t going to work well as occasionally the throttle position may read zero point one to zero point six for example, when the throttle is actually fully closed.
02:00 At the same time you need to make sure that the threshold setting isn’t too high or the overrun mode may become active when the engine is being operated at very light throttles.
02:11 We also have the option of retarding the ignition timing when the engine is in overrun if desired.
02:17 The target ignition angle when in overrun mode is set by the ‘Ignition Timing Target’ parameter here.
02:24 Typically this will be a more retarded value than the normal ignition timing the engine will be running.
02:30 We can control how aggressively the ignition timing is retarded and how quickly it is reintroduced using the ‘Retard Rate’ and ‘Advance Rate’ parameters.
02:40 We can monitor the operation of the ‘Engine Overrun’ function in the channels list below.
02:45 In Particular you will want to monitor the ‘Engine Overrun State’ which will tell you what the function is doing at any time.

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