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EFI Tuning Fundamentals: Fuel Models

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Fuel Models

06.22

00:00 - Now that we've learned about the fuel table, I want to talk about what the numbers in the fuel table actually represent and this is going to depend a little on your ECU and perhaps how you've chosen to configure it.
00:10 In this module, we're still primarily discussing stand alone aftermarket engine management so this won't be applicable if you're reflashing a factory ECU.
00:19 The two most common fuel calculations include injection time, where the fuel table is directly defining the injector opening time, and VE or volumetric efficiency, where the table is actually defining the air flow into the engine.
00:33 We'll discuss each of these separately starting with injection time.
00:37 An injection time based fuel model can further be broken down with some ECUs directly defining the injector pulse width in milliseconds as the numbers in the fuel table, as you can see in this example here.
00:49 In this sort of ECU, the numbers are the desired injector pulse width before any background compensation such as air temperature or coolant temperature are applied.
00:58 The other way of dealing with an injection time based fuel model is to define a master injector pulse width as a base parameter and then the numbers in the main fuel table define a percentage of this value, as you can see in this example.
01:11 Let's say we had a master injector pulse width value of 16 milliseconds.
01:16 If we have a number of 50% in the fuel table, then the ECU will deliver a pulse width of 50% of 16 milliseconds, which would be eight milliseconds.
01:25 A value of 100% would give an injection pulse width of 16 milliseconds.
01:30 Actually there is a little more to it than just this though.
01:33 One of the advantages I see with this sort of fuel model, is that the ECU does some of the hard work for us in maintaining a consistent fuel delivery.
01:42 as engine loads or manifold pressure changes.
01:45 If you can remember back to the module on air density, we found out that it varies with manifold pressure.
01:50 All other things being equal, if we double the pressure in the manifold, we also double the air density, and hence we double the oxygen content in the cylinders.
01:59 What this means is that if we want to retain a consistent air fuel ratio as we double the manifold pressure, we need to double the amount of fuel being supplied.
02:08 With this sort of fuel table, this calculation is handled in the background and the ECU automatically adjusts the injector pulse width as manifold pressure changes.
02:18 With an ECU using this sort of fuel model, the numbers in the table represent the percentage of the master injector pulse width at 100 kPa before background compensations are applied.
02:29 Let's take our example where we had a master injector pulse width of 16 milliseconds.
02:34 Now let's assume that the number in our fuel table is 50% and we're operating at 100 kPa.
02:41 So the pulse width of course delivered to the injectors is eight milliseconds, 50% of 16 milliseconds.
02:48 Now we add boost and we reach 200 kPa of manifold pressure.
02:53 If the number in our fuel table is still 50%, now the injector pulse width will be doubled because we've doubled the manifold pressure so the pulse width delivered to the injectors would become 16 milliseconds.
03:04 The aim behind this is to achieve a consistent air fuel ratio as manifold pressure changes.
03:10 Of course, we might not want to run the same air fuel ratio at 200 kPa as we ran at 100 kPa and as the manifold pressure changes, we're also likely to see a change in volumetric efficiency but this background calculation will mean that we don't have to make such dramatic changes to the fuel table values.
03:28 The advantage in my opinion over an ECU that directly defines the injector opening time is that we tend to see a flatter fuel table and the shape of the fuel table becomes a closer approximation to the engine's actual volumetric efficiency.
03:41 As a tuner, I always like to be able to visualise the shape of the fuel table as this provides me with a lot of clues about how the tune is progressing and if there are any potential problems.
03:52 One important aspect to point out here, is that with both of these types of fuel model, we need to adjust the numbers in the table to achieve a richer or leaner air fuel ratios.
04:02 Likewise, if we change the size of the fuel injector fitted to the engine, we'll also need to adjust the fuel table to achieve the correct air fuel ratio again.
04:11 Let's now move on and discuss the VE, or volumetric efficiency fuel model.
04:16 In this type of ECU, we don't actually have a fuel table at all, but more correctly we have an efficiency table, like this example shown here.
04:25 In this table, we're defining at each point what the engine's volumetric efficiency is, or in other words how completely it's filling its cylinders with air.
04:33 This is an important difference to the injection time model and can offer some subtle advantages.
04:39 With the VE fuel model, we need to provide some additional information to the ECU, such as the engine displacement and the number of cylinders, the stoichiometric air fuel ratio of the fuel we're running and the flow data for the injectors fitted.
04:53 Finally, there will be a separate table in the ECU that defines the target air fuel ratio, as you can see here.
05:00 This table defines the air fuel ratio we want the engine to run at each point in the load and RPM.
05:07 That might sound like a lot of information, but there's a good reason why.
05:10 If the ECU knows the mass of air entering the engine and it knows how big the injectors are and it also knows what our desired air fuel ratio is, it becomes very simple for the ECU to calculate the required injector pulse width in order to achieve our aim.
05:26 The advantage of the VE fuel model over an injection time-based fuel model is that if we correctly tune the efficiency table, then we can then make changes to the air fuel ratio through the target air fuel ratio table without the need to retune the engine.
05:41 Likewise, if we swap to a larger set of injectors and correctly enter the data for these injectors in the ECU, the engine should still be able to meet our original air fuel ratio targets.
05:52 In this module, we've learned about the two most common fuel calculations in aftermarket ECUs.
05:58 Injection time, where the fuel table defines the injector opening time directly and volumetric efficiency where the airflow into the engine is defined by the table itself.