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Ethanol & Flex Fuel Tuning: AFR Vs Lambda

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AFR Vs Lambda

06.09

 00:00 - Since the stoichiometric air fuel ratio of ethanol and gasoline are very different this adds a little complexity to tuning for those who like to use units of air fuel ratio to measure the fuel mixture. 00:12 Since I know even experienced tuners who still struggle with this concept, this module is designed to clear up the confusion and give you a solid understanding of how air fuel ratio and lambda change as your ethanol content fluctuates. 00:28 Let's start by defining what lambda is. 00:32 Lambda is simply an alternative way of defining the current fuel mixture, which measures how rich or lean the mixture is relative to the stoichiometric air fuel ratio. 00:44 If we know the current air fuel ratio and the stoichiometric air fuel ratio, then we can swap from AFR to lambda by simply dividing the measured air fuel ratio by the stoichiometric air fuel ratio. 00:58 Let's say for example that we're running pump gas with a stoichiometric air fuel ratio of 14.7:1 and the measured air fuel ratio is currently 12.5:1 To convert to units of lambda, we simply divide 12.5 by 14.7 which gives us 0.85 lambda. 01:19 Likewise we can convert from lambda to AFR units by multiplying by the stoichiometric air fuel ratio. 01:26 For our example with a lambda value of 0.85, if we multiply this by 14.7 we get back to our air fuel ratio reading of 12.5:1 Now that we've covered the conversion between lambda and AFR units, we can discuss how the wide band sensor works. 01:45 First of all we need to understand that the wide band air fuel ratio sensor doesn't have a clue what fuel you're running your car on. 01:53 All it knows is the quantity of unburnt fuel or oxygen in the exhaust system. 01:59 Essentially the wide band sensor is measuring whether we have an excess of oxygen or fuel in the exhaust system. 02:06 When the exhaust gas components are perfectly matched and we don't have an excess of oxygen or fuel, the engine is operating at the stoichiometric air fuel ratio regardless of the fuel. 02:19 Since the wide band sensor is measuring how far away from the stoichiometric air fuel ratio the exhaust gas is, it's actually natively measuring in units of lambda. 02:30 This is a fundamental concept as most tuners incorrectly assume that the wide band sensor is directly measuring in units of air fuel ratio. 02:40 For the wide band metre to be able to display in units of air fuel ratio, we must tell it what the stoichiometric air fuel ratio for the particular fuel we're running on is. 02:52 The wide band meter can then convert from lambda and display AFR. 02:58 For example if the wide band metre is measuring lambda 1.00 then it would multiply this by the stoichiometric air fuel ratio programmed into it in order to display in AFR units. 03:12 So if you now want to use the wide band metre on E85 and you want an accurate measure of the air fuel ratio, you would need to adjust the stoichiometric air fuel ratio to 9.8:1 Now at a measured lambda of 1.00 the metre would display 9.8:1 Most tuners around the world tend to be lazy or lack proper understanding here. 03:37 So instead of constantly adjusting the stoichiometric air fuel ratio, they simply leave it always set to 14.7:1 There's nothing specifically wrong with doing this provided that you understand the implications and particularly if you're dealing with flex fuel where the actual ethanol content could vary substantially, this technique actually makes some sense. 04:00 If you don't have a solid understanding of the implications though you can get yourself into a lot of trouble. 04:06 So let's discuss that in more detail. 04:09 Leaving the stoichiometric AFR set permanently to 14.7:1 as the ethanol content changes, really has the wide band meter working the same as if we were using units of lambda. 04:21 This means that if the engine is tuned to run at 0.85 lambda, the wide band will always display 12.5:1 regardless of the actual fuel composition. 04:33 This means that if we switch to E85 and tune the engine to 0.85 lambda, the meter will still display 12.5:1 because the meter is still multiplying the measured lambda value of 0.85 by the programmed stoichiometric AFR of 14.7:1 In reality though the actual AFR is 0.85 multiplied by 9.8:1 which is the stoichiometric air fuel ratio of E85. 05:02 And this equals 8.3:1 As you can see this is a big difference. 05:08 As I've already mentioned, both gasoline and ethanol usually perform well at very similar target lambda numbers. 05:16 This means that if we leave the wide band meter scaled for a stoichiometric AFR of 14.7:1 we can just aim for the same numbers regardless of the actual ethanol blend. 05:28 This isn't technically the correct way to use a wide band but this is what many tuners do, and to save confusion when talking with other tuners it's important to understand this. 05:40 This is just one more reason why I always recommend learning to work in units of lambda, as then there can be no possible confusion. 05:49 If you want to use units of AFR for your tuning though, decide early on if you'll adjust the stoichiometric AFR to suit the fuel or leave it fixed.