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EFI Tuning Fundamentals: Air Fuel Ratio

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Air Fuel Ratio


00:00 - An EFI system is primarily responsible for performing two jobs, and the first of these is providing the engine with the correct amount of fuel to match the mass of air currently entering the engine.
00:10 Of course when it comes to talking about the amount of fuel being injected it would be handy if we had a simple way of describing it.
00:17 And the term we use is air fuel ratio or AFR for short.
00:22 This is simply the ratio between the mass of air entering the engine and the mass of fuel being injected.
00:28 As shown here, we can calculate the air fuel ratio by dividing the mass of air by the mass of fuel.
00:34 When we talk about an air fuel ratio such as 12.5 to one what we mean is that we're adding one part of fuel for every 12.5 parts of air.
00:44 The way we measure the air and fuel is by mass, and we could use units of pounds, kilograms or grams, it doesn't really matter.
00:52 Every fuel has a specific ratio known as the Stoichiometric ratio which is the ratio that will result in theoretically perfect combustion.
01:00 In other words this is the amount of fuel required to result in complete combustion of all of the available oxygen.
01:07 The Stoichiometric AFR is defined by the chemical composition of the fuel and hence, it will change depending on the type of fuel we're combusting.
01:17 Attached you will find a PDF document that defines the Stoichiometric air fuel ratio of many of the common fuels you're likely to come across.
01:26 When an engine's running at a fixed RPM and throttle position the mass of air entering the engine won't change.
01:32 So we vary the air fuel ratio by adjusting the amount of fuel being delivered by the injectors.
01:38 When we have a measured air fuel ratio that is larger than the Stoichiometric AFR the mixture contains an excess of oxygen and is said to be lean.
01:47 If, on the other hand, we have an AFR that is less than Stoichiometric, we have an excess of fuel and the mixture is said to be rich.
01:55 You may also hear some tuners use the term fat when they're talking about a rich mixture.
02:00 From the attached document you'll be able to see that the Stoichiometric air/fuel ratio changes quite dramatically between different fuels.
02:08 And if you're going to frequently tune on different fuels I strongly recommend that you begin tuning using the Lambda scale instead of AFR.
02:16 Now let me explain Lambda a little more.
02:19 Lambda references the measured AFR to the Stoichiometric AFR giving a result which describes how rich or lean the engine is running as a percentage.
02:29 This basically takes the Stoichiometric AFR out of the equation making our job a lot easier as we tune on different fuels.
02:37 Using Lambda can make it much faster when we need to make changes to the fueling in order to achieve our desired target too.
02:45 To show you how Lambda works let's look at an example where we have an engine running on pump gas, which we know has a Stoichiometric air/fuel ratio of 14.7 to one.
02:54 If we're measuring an AFR of 12.5 to one we can calculate the Lambda value by dividing the measured air fuel ratio by the Stoichiometric air fuel ratio.
03:05 In this case if we divide 12.5 by 14.7 we'd get a result of Lambda 0.85.
03:12 So we can see that on gasoline an AFR of 12.5 to one gives us a Lambda reading of 0.85.
03:19 On the Lambda scale numbers smaller than one are rich, while numbers greater than one are lean.
03:25 With a Lambda of 0.85 we are 0.15 richer than Stoichiometric which is 15%.
03:33 I'm no mathematician, and you certainly don't need to be, in order to be a great tuner.
03:38 However, when I'm faced with an AFR of 12.5 to one it's hard for me to quickly calculate in my head how much fuel I need to remove in order, for example, to get to 14.7 to one.
03:49 If, on the other hand, I'm using the Lambda scale and I'm running at Lambda 0.85, I know at a glance that I'm 15% richer than Stoichiometric and hence removing 15% fuel will get me to Lambda 1.00.
04:03 For me this makes the Lambda scale a lot easier to work with, because I can calculate percentage changes quickly and easily in my head.
04:12 On top of this, Lambda 1.00 is always Stoichiometric regardless what fuel we're tuning on.
04:18 This is particularly helpful when you're constantly switching between different fuels, as you don't need to worry about what the current Stoichiometric AFR is as it will always be Lambda 1.0.
04:29 Before we finish this module let's quickly recap.
04:33 air fuel ratio, describes the mass of air entering the engine divided by the mass of fuel.
04:39 Every fuel has a Stoichiometric AFR value at which we achieve theoretically perfect combustion and we don't have an excess of hydrocarbons or oxygen remaining in the exhaust.
04:50 The Lambda scale, which I recommend you begin tuning with, means we don't need to know what the Stoichiometric AFR of a particular fuel is.
04:58 Lambda 1.00 is always Stoichiometric.

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