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Ethanol & Flex Fuel Tuning: Introduction

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Introduction

03.48

00:00 - VE or volumetric efficiency based fuel models are common with a lot of the current generation of standalone aftermarket ECUs these days.
00:08 And this fuel model actually offers some advantages when it comes to flex fuel tuning.
00:13 At its core, the key to the VE fuel model, is that we're actually telling the ECU how much air is entering the cylinders.
00:21 The principal here is that if the ECU knows what mass of air is entering the engine, how big the injectors fitted to the engine are, and what our desired air fuel ratio is, it becomes quite easy for the ECU to calculate what pulse width to supply to the injectors in order to achieve our target air fuel ratio.
00:41 The advantage of a VE fuel model when it comes to flex fuel tuning is that the mass of air entering the engine will remain relatively consistent regardless whether we're running pure gasoline or pure ethanol.
00:54 Predominantly it's the fuel properties that are changing, rather than the engine's volumetric efficiency.
01:00 If the ECU can account for these fuel properties, then it knows for example that if we move from EO to E85, it will need to add 40% more fuel to maintain a stoichiometric air fuel ratio and this means that the ECU does most of the hard work for us in the background.
01:18 In theory this means that we can disregard the fueling with a flex fuel configuration but of course reality is often a little different.
01:27 As we discussed earlier in the course the actual injector volume flow tends to vary between gasoline and ethanol which alone can result in some error as the ethanol content changes.
01:39 It's also important to understand that the fuel model used in an aftermarket ECU is a generic model that's designed to be effective and work well on 1000s of different engine combinations.
01:50 This is likely to mean that there are some errors that may creep in to affect the fuel model's ability to perfectly account for the ethanol content fluctuating.
02:00 Typically how an aftermarket ECU will deal with the error between gasoline and ethanol is by incorporating a second volumetric efficiency table or alternatively a trim table that can be applied as the ethanol content increases to keep things tracking nicely.
02:18 If everything is working as intended though, we usually find that there's little compensation required, and the difference between the two VE tables is minimal.
02:28 Likewise if the ECU is applying a trim table, we should find that the values required in that trim table will only be in the region of perhaps 0% to 10%.
02:39 Theoretically we can expect to use a linear interpolation between the two VE tables or the trim table as the ethanol content fluctuates, however at ethanol contents between the two limits of the blend we're tuning for, we may see some requirements to bias more towards one table or the other and hence the blend table will still need to be checked and optimised at various ethanol blends to ensure accurate operation.
03:08 The ignition, boost, and cold start compensations are typically taken care of by incorporating a second table for each parameter along with a blend table to define how the ECU will interpolate between them depending on ethanol content.
03:23 An alternative here with the cold start corrections and the boost control is to use ethanol content as one of the axis in these tables instead of providing a completely separate table along with a blend table.