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Variable Cam Control Tuning: ECU Control System

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ECU Control System

04.55

00:00 - Aside from the mechanical requirements of the cam control system, we also need to consider the ECU and how it will actually control it.
00:08 Of course the requirements here will vary depending on the type of cam control so let's start by discussing a continuously variable cam control system.
00:17 We've already discussed most of the components that go into the system but in this module we'll dive a little deeper into the software in the ECU.
00:25 Some of this will be specific to the ECU brand but we'll try to cover the high level concepts that are generally applicable to all systems.
00:34 The key to cam control is the cam position input and a trigger mode that will almost always be specific to the individual engine.
00:41 This will allow the ECU to monitor cam position once the inputs are set up properly as we've already discussed.
00:48 In most instances though we'll still need to calibrate the cam position which you could think of as telling the ECU where the 0 point for the cam position is relative to the engine position information it's receiving.
01:01 I liken this to setting the base ignition timing when setting up an ECU where we align the timing values that we see on the laptop screen with the actual timing we're seeing with a timing light at the crank pulley.
01:13 What we're normally trying to do is adjust the cam position or offset value in the ECU so that the ECU is displaying a cam position of 0 when it's in it's natural resting position and the control actuator is disabled.
01:27 Once the ECU understands the cam position it's able to then control it and this is where the actuator comes in.
01:35 The ECU will simply provide a high frequency square wave signal to the actuator and by varying the duty cycle the oil flow to the cam pulley can then be manipulated.
01:45 Understandably this needs to happen very fast to get the level of control we need and typically the solenoid will operate at approximately 200-300 Hz or in other words the ECU is switching it on and off between 200 and 300 times every second.
02:01 The duty cycle refers to the amount of time that the solenoid is in the on position vs off.
02:07 We might find for example that with a duty cycle of about 50% where the solenoid is on and off for equal amounts of time, that the cam position will remain relatively constant.
02:19 As we increase the duty cycle the cam may be advanced and the higher the duty cycle or the more time the solenoid is in the on position, the faster the cam will advance.
02:28 Conversely, if the duty cycle is reduced below 50% we may find that the cam will retard and again, the lower the duty cycle the faster the cam moves.
02:38 In this way, by manipulating the duty cycle 100s of times per second, the ECU can advance, retard or maintain the existing cam position.
02:47 The key to all of this however is the PID control algorithm which defines how the ECU will respond to an error between the cam position and the cam target.
02:59 PID control algorithms deserve their own separate module so we won't go into them here in detail but sufficed to say that the tuning of the PID gains is the key to accurate and fast cam control.
03:11 Now that we've got everything in place for control, we just need to tell the ECU what to do with the cam position and here we'll simply have a 3D target table for cam position.
03:20 The axes of these tables are important though particularly for aftermarket standalone ECUs that operate on the speed density principle.
03:29 What we want to do is make sure that we're using the same load axis on the cam target table as we've used for the main fuel or VE table.
03:38 The reason for this is that when we move the cam position, we're affecting the volumetric efficiency of the engine and we need to be able to accurately define the VE when we're tuning the fuel table.
03:50 If we use the same load input, let's say MAP then we can be certain the our cam position should always be the same for a given combination of RPM and manifold pressure.
03:59 This wouldn't always be the case if for example we used throttle position as the cam target load axis and MAP for the fuel or VE table.
04:08 This can get us into a situation where we will have small inconsistencies in our actual volumetric efficiency which will show up in errors in our air/fuel ratio.
04:18 Due to limitations of the oil pressure at idle and low RPM, as well as considerations around oil temperature, we'll likely find that the ECU may provide the option to lock out cam control below a certain RPM and engine coolant or oil temperature.
04:32 We'll discuss the reasons behind this as well as some of the problems this can create in the next module.