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Ethanol & Flex Fuel Tuning: Fuel Mixture Aim / Lambda Target

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Fuel Mixture Aim / Lambda Target


00:00 - At this point we've seen how the fuel characteristics of ethanol and ethanol blended fuels vary from those of pump gasoline.
00:08 We've also seen that if the ECU can adequately compensate for these changing characteristics as the ethanol content varies, then it should be able to maintain a consistent lambda regardless of the ethanol content.
00:23 This however may not be what we want to actually achieve.
00:28 It may be that as we move from petrol to ethanol for example, we want to start targeting a different fuel mixture aim or lambda target, particularly under high load or high boost.
00:40 In my own experience I've found the sort of lambda targets that are suitable for gasoline fuel normally also work very well for ethanol fuel.
00:50 In fact with the fuel's ability to cool the combustion charge often you can tune with a leaner lambda target on ethanol than you could on gasoline since we're not requiring a richer fuel mixture in order to cool and control the combustion temperature and prevent knock.
01:08 While we're discussing fuel mixture aims in this module it's important to mention here that I'm talking in units of lambda.
01:16 Since the stoichiometric air fuel ratio varies dramatically between gasoline and ethanol, the air fuel ratio will also change accordingly.
01:26 If we use units of lambda however, lambda one always represents the stoichiometric air fuel ratio, regardless of the fuel blend.
01:35 This means our lambda targets remain consistent even when the actual air fuel ratio values differ.
01:42 This is an area of some confusion, even for experienced tuners.
01:47 And we'll go into it in more depth in the next module.
01:50 So let's look at how we can change the fuel mixture target as we change the ethanol blend.
01:56 This is going to depend on the specifics of the ECU you're tuning, however there are two predominant ways of dealing with this, depending on whether the ECU uses a VE based fuel model or mass air flow metre, or alternatively an injection time based fuel model.
02:14 If the ECU uses a mass air flow sensor or VE fuel model then it will either measure or calculate the current mass air flow passing into the engine.
02:25 Provided it knows the mass air flow, and the injector flow, the ECU can calculate the required injector pulse width in order to achieve a target air fuel ratio, regardless of the ethanol content or fuel properties.
02:39 In this sort of ECU we may have a second mixture aim table that the ECU references when running on ethanol blends.
02:48 And the ECU can move between the two mixture aim tables as the ethanol content varies.
02:55 If the ECU uses a more simplistic injection time based fuel model, where the ECU simply commands a specific injector pulse width, changing the fuel mixture aim can be achieved via a compensation table and manually adding more or less fuel to arrive at our desired target.
03:15 This course isn't intended to provide you with in depth knowledge of what fuel mixture aim values you should be using for a given application, but rather it's meant to give you an understanding of how the changing characteristics of the fuel will affect the air fuel ratio.