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MoTeC M1 Software Tutorial: Fuel Pressure

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Fuel Pressure


00:00 Through the course so far we have discussed the importance of fuel pressure on the accuracy of the M1’s fuel model.
00:06 Now that we have moved into the actual tuning portion of this course, I just want to reiterate why this is important and how it will effect your tune.
00:15 More importantly we will see how correctly calibrating the M1 will give us some advantages if we make changes to the fuel pressure or the fuel system at a later stage once the engine is tuned.
00:27 First we need to understand that the amount of fuel an injector will supply is dependent on the fuel pressure, or more importantly, the differential fuel pressure.
00:37 By differential fuel pressure, I mean the difference between the pressure of the fuel in the rail, and the manifold pressure acting on the pintle side of the injector.
00:46 To illustrate the importance of differential fuel pressure, imagine we have an injector with 3 bar of fuel pressure acting on it and it is injecting into atmospheric pressure of 100 kPa which is 0 bar gauge.
01:00 This gives a differential pressure of 3 bar and the injector will flow whatever it is rated to flow at that particular pressure.
01:08 If we now raise the boost pressure in the manifold to 3 bar, the differential fuel pressure becomes zero since the manifold pressure now equals the fuel pressure.
01:18 In this case, no fuel will flow through the injector.
01:22 This is an extreme example but it illustrates the idea.
01:26 Depending how your fuel system is set up, you may have a fixed fuel pressure that is constant regardless of manifold vacuum or boost, or you may have a fuel pressure regulator referenced to manifold pressure, where the differential fuel pressure should remain relatively constant.
01:43 Here we will quickly go through the various fuel pressure settings and this is going to double up on some settings we have already visited during the course.
01:52 We will also deal with how to adjust the calibration if you change the fuel pressure or the way the regulator is set up.
02:00 Let’s start by heading to the All Calibrate worksheet and I’m going to type in Fuel Pressure to bring up a list of parameters related to fuel pressure.
02:09 The first parameter we need to adjust is the Fuel Pressure Sensor Type, and this will depend on the type of sensor you are using.
02:17 The most common is a Gauge Sensor which will read relative to ambient pressure.
02:23 If you are using a sensor, it’s important to set this correctly as it will influence the displayed differential pressure.
02:30 Next we need to tell the ECU if the fuel pressure regulator is referenced to the inlet manifold or to ambient pressure.
02:38 This lets the ECU know if the fuel pressure will increase or decrease in line with manifold pressure or if it will remain fixed at a constant pressure.
02:48 This setting will depend on how your fuel system is plumbed.
02:52 A modern return less system would be set to ‘Ambient’ while a conventional fuel system with a return will be set to ‘Inlet Manifold Pressure’.
03:02 Lastly here we need to ensure the default fuel pressure value is accurate.
03:08 Particularly if you aren’t using a sensor, the ECU will use this value to determine where in the injector characterisation the ECU is operating.
03:17 It therefore needs to be set to match the actual fuel pressure.
03:21 If you are using a sensor and it goes into fault, the ECU will use the default value so again it’s important to be accurate.
03:30 Provided you have correctly configured these settings, you can go ahead and tune the engine.
03:35 The advantage of the M1 fuel model is that if you have tuned the engine accurately, we can now make changes to the fuel system without needing to retune the engine.
03:45 For example we can fit an aftermarket fuel pressure regulator and adjust the fuel pressure up or down.
03:51 If your engine runs a return less fuel system with a fixed fuel pressure, you can also go ahead and change this for a manifold pressure referenced fuel pressure regulator.
04:01 As long as you make the relevant changes in the calibration, the ECU will track the changes to the fuel delivery and the mixture will still track your target.
04:11 Let’s start by looking at how to address a change in fuel pressure.
04:16 Let’s say that you have completed a calibration, and the fuel pressure has been set to three bar but now you want to raise the fuel pressure to four bar.
04:24 If you have a fuel pressure sensor fitted, you can simply make the change to the fuel pressure and the ECU will change the fuel delivery accordingly.
04:32 As a matter of course though we also want to change our default value for the fuel pressure.
04:38 Remember we need to do this incase the sensor goes into fault, but also if we don’t have a sensor fitted, this is the value the ECU will use.
04:47 To make this change, we can go to the ‘All Calibrate’ menu and start typing ‘Fuel Pressure Default’.
04:53 Now we can move down and adjust the default value from three bar to four bar.
04:58 If at the same time we have moved from a return less fuel system to a manifold pressure referenced regulator, we have one more job to do.
05:07 We need to head back to the ‘All Calibrate’ menu and change the ‘Fuel Pressure Regulator Reference’ from ‘Ambient’ to ‘Inlet Manifold Pressure’.
05:16 So this should illustrate just how easy it is to make what would normally be a fairly fundamental change to the fuel system, and easily accommodate that change with M1 Tune.
05:27 This also gives some insight into the advantage of the M1’s fuel model and why it is a benefit since we don’t need to adjust the tune after making these changes.

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