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Variable Cam Control Tuning: Cam Position Information

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Cam Position Information

05.51

00:00 - One of the key elements to continuously variable cam control is an input to the ECU that provides it with the information about cam position relative to the crankshaft.
00:10 The ECU will actually require some of this information even in a fixed cam engine so that it knows where it is in the engine cycle or in other words, which cylinder is firing at any time.
00:21 That information is required in order to provide sequential injection as well as direct fire spark ignition.
00:28 In conventional engines, this information is usually provided by an input placed on the camshaft or driven off the camshaft.
00:36 The reason this signal is taken from the camshaft is that as we discussed in the cam fundamentals section, the camshaft turns at half engine speed which means that the cam goes through a full 360° rotation per engine cycle.
00:52 If we had a single tooth pickup on the cam, this would be perfect because the ECU will receive 1 signal per engine cycle which allows it to know where it is in the engine cycle.
01:04 In reality, the synchronisation input may not consist of a single tooth and every manufacturer seems to have their own unique idea on what the optimal combination for their range of engines.
01:16 The principle however remains that the camshaft can provide what the ECU requires for synchronisation.
01:24 With a cam control engine, things are a little more complex as we now need to know not only where in the engine cycle we are, but also where the cam is located relative to the range of movement it offers.
01:36 While a single tooth on the cam could technically fit the bill, it gives the ECU a very low resolution signal which isn't going to offer good control.
01:46 What I mean by this is that with a single tooth, the ECU can only measure the cam position once every engine cycle and the actual cam position could quite likely move before the ECU sees the next tooth.
01:58 Adding more sample teeth seems like a sensible solution however as soon as there are multiple teeth it can then become impossible for the ECU to know how a single one of those teeth relates to cam position.
02:10 The typical solution to this is to use a trigger wheel with unequally spaced or a missing tooth trigger pattern on the cam.
02:18 This might for example consist of 4 equally spaced teeth around the cam circumference with one tooth then removed.
02:25 This would be referred to as a 4 minus 1 trigger input.
02:29 With this sort of input, the ECU can see the missing tooth gap because it's expecting another equally spaced tooth which doesn't occur.
02:37 The ECU can then use the location of the tooth that occurs after the gap as a reference to work out cam position relative to the input it's receiving from the crankshaft relating to engine speed.
02:50 This might sound complex but in practice it's quite straightforward and beyond some simple initial setup, as tuners we really don't need to know too much more about it.
02:59 To get a better idea of what this looks like to the ECU, let's have a look at an input capture from a MoTeC M150 ECU controlling a Toyota 1UZ-FE VVTi engine.
03:13 There's a bit to digest here but let's start with the basics which is the red trace labelled engine speed reference.
03:19 This signal tells the ECU what the engine RPM is and in this case it comes from a 36 minus 2 wheel that's fitted on the nose of the crankshaft.
03:29 You can clearly see the gaps in the red trace that correspond to the missing teeth on the trigger wheel.
03:35 This gap lets the ECU know when number 1 cylinder is at top dead centre but because the ECU sees this gap twice per engine cycle, it's not possible to know if the engine is on the compression stroke or the exhaust stroke.
03:50 To get this information, the ECU also has a separate engine synchronisation input which is the orange trace.
03:57 In the case of this engine, this is a single tooth and we can see that it occurs only once per engine cycle like we've already discussed.
04:05 Armed with this information, the ECU knows what the engine RPM is and where it is in the cycle at any time.
04:13 These are the basic inputs for operating the engine but they still don't define the cam position.
04:19 In the case of the 1UZ-FE, the ECU is provided with a cam position input for each bank of cylinders, given that this is a V8 engine.
04:26 Looking at the green trace, labelled inlet camshaft bank 1, we can see we have 3 teeth to the pattern.
04:33 Looking a little closer though we can see that the gaps between the 3 teeth aren't even and this is what the ECU needs in order to define the cam angle.
04:42 In this case, there are 2 gaps of 135° between teeth and then one of 90°.
04:48 The ECU can use the unequal tooth spacing and relate this to the engine reference trace to define the actual cam position.
04:56 The blue trace provides the exact same pattern and same information for the opposite bank of cylinders.
05:03 Since the trigger pattern is very dependent on the particular engine, you're almost certainly going to be choosing an engine specific trigger mode inside of the ECU software during your configuration and this means that the ECU is going to be expecting the trigger patten that the engine is providing and will inherently know how to decode it making our job as easy as choosing the right option.
05:25 In some instances you may be dealing with an engine that your ECU manufacturer has no trigger mode for.
05:32 This needs to be approached with care as some ECU manufacturers are open to adding new trigger modes to support different engines while others aren't.