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EFI Tuning Fundamentals: Correcting Your AFR

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Correcting Your AFR


00:00 - We finished up the last module with our engine running richer than we wanted it to and we calculated a correction factor based on the measured air/fuel ratio and our desired air/fuel ratio.
00:10 Using this correction factor, we can very easily adjust the fuel delivery to account for the actual volumetric efficiency.
00:17 If we take the 9.24 millisecond injector pulse width that we calculated and multiply this by the correction factor, we get an answer of 8.5 milliseconds.
00:29 So if we go back on the dyno and change the injector pulse width from 9.24 to 8.5 milliseconds then the measured air/ fuel ratio will equal our target of 12.5:1.
00:40 This is by far one of the most important concepts in this course and when you're tuning the fuel delivery of an engine, this single concept can save you hours.
00:51 And most tuners will optimise the fuel table of their engine by trial and error.
00:56 What I mean is that if the engine is lean, then they'll add some fuel to the fuel table and run the engine again to see if it's any better.
01:03 This process is likely to take several iterations before the tuner finally zeros in on their desired target air/fuel ratio.
01:11 This only covers one area of operation and the process goes on and on while the tuner corrects rich or lean areas one at a time.
01:19 More changes will be made and more runs are completed as the tuner slowly chips away at the tune, No doubt they'll be able to get the right result, but this is a slow way of tuning.
01:30 If instead we analyse the measured air/fuel ratio and calculate a correction factor to apply based on the measured air/fuel ratio and our target air/fuel ratio, we can make the required change in one go.
01:43 We can actually tune the entire wide open throttle operating area of the fuel table by making just two ramp runs on the dyno, or acceleration tests on the road or the racetrack.
01:54 All we need to do is look at any areas where the air/fuel ratio is richer or leaner than our target, apply the correction factor at each RPM zone through the rev range and then make the necessary changes to the main fuel map.
02:07 If we do this and then make another run, we'll find that the air fuel ratio now perfectly matches our target and all in just two full power runs.
02:16 Let's stop now and have a quick look at how this concept can be applied on the dyno.
02:21 So far we've learnt how we can calculate a correction factor and how we can use that correction factor to adjust the injection pulse width to correct an error between our measured air/fuel ratio and our target air/fuel ratio.
02:36 This is one of the most important and most valuable concepts in this course.
02:42 Using this particular concept is a really great way of speeding up the fuel tuning on any ECU so I wanted to demonstrate this.
02:53 And I'm going to perform this demonstration using an AEM Infinity Plug & Play ECU on our Nissan 350Z.
03:00 An interesting aspect here is that the AEM Infinity ECU users a volumetric efficiency, or VE based fuel model.
03:10 So the numbers in the efficiency table are actual engine volumetric efficiency values.
03:16 The important point is though, it doesn't matter if your ECU uses a VE based fuel model, whether it's using injection pulse width values in the fuel table, or whether it's using input from a MAF sensor for its load calculations.
03:34 Regardless, this particular correction factor calculation will still work to correct any discrepancy in your air/fuel ratio.
03:43 So before we get started with the demo, let's have a quick look through the Infinity software so you know what you're looking at.
03:50 So first of all here, we have our VE or volumetric efficiency table.
03:56 So as I mentioned, the numbers in this table are actual engine VE values.
04:01 The vertical load axis for this particular table is manifold pressure and of course we have engine RPM on our horizontal axis here.
04:13 To the right we have a graphical representation of the VE table and this just lets us get an idea of the general shape of the engine's VE table.
04:27 To the left here we've got some information that you're going to want to watch during this demonstration.
04:34 First of all here, we have our lambda one value which is the air/fuel ratio or lambda value being measured by a wideband oxygen sensor fitted to the exhaust.
04:45 So this is our measured air/fuel ratio.
04:47 Below that we have our lambda target, so this is our desired air/fuel ratio and this comes from the lambda target table.
04:58 Let's just get the engine running in fourth gear and for the purposes of this demonstration, we're going to run the engine at 2,000 RPM and I'm going to adjust a couple of cells in the fuel table, in the VE table to demonstrate how this correction factor can be applied.
05:19 So what I'm going to do is I'm going to start by running the engine in this particular cell here, which is 50 kPa and 2,000 RPM and when I'm making changes here, when I'm making any changes to the ECU in general, it's always important to make sure that we're as close to central in the particular cell that we're adjusting as possible.
05:42 And the reason for this is, if we're not central in the cell we're adjusting, the ECU will be interpolating from the surrounding cells and that can affect the accuracy of the adjustments we're making.
05:54 So right now you can see that I'm pretty close to 2,000 RPM and 50 kPa and that's where I want to keep the engine running.
06:03 Now at the moment you can see that our measured air/fuel ratio is quite a lot leaner than our target, we're sitting around 1.09 lambda, 1.1 lambda so we're around about nine to 10% leaner than we want to be.
06:19 Let's now bring up the calculator, and we'll look at how we can calculate a correction factor to get that air/fuel ratio on to our target of lambda one.
06:31 So remember what we want to do is enter our measured air/fuel ratio, so in this case 1.1 and then we want to hit the divide key and divide that by our target, in this case 1.0.
06:44 That's going to give us our correction factor, in this case the correction factor is 1.1.
06:50 This means that we need to add 10% fuel to that particular zone to get our air/fuel ratio on track.
06:59 Now the number in that particular zone is 81.5 so what I'm going to do is enter that into the calculator so we can multiply that by 81.5 and that's going to give us a new value of 89.7.
07:15 And that's what we need to enter into our VE table.
07:20 89.7 and you'll see that when I press enter, you can see that that's corrected our lambda value and we're now sitting right on our target of lambda 1.0.
07:35 Now important point to note here is, you can see that our air/fuel ratio, our measured lambda value, it does move around a little bit.
07:42 And that's pretty typical.
07:43 We're never looking at a constant 100% steady state value from our lambda and it's our job as tuners to average mentally, I guess you could say, the values that we're seeing from that wideband sensor.
07:57 Okay, so we've made that correction factor there in our 50 kPa zone.
08:02 Just to reinforce the technique, let's just open the throttle a little bit further and we're going to come up to the next site, which is our 60 kPa zone, so again, we're still keeping the RPM constant at 2,000.
08:19 And you can see that at 60 kPa, we're still lean, we've gone to about 1.06, 1.07 lambda.
08:28 So what I'm going to do is make another correction to that particular site and we'll see how that works.
08:38 So let's bring up our calculator again and just looking at the values for our lambda, we're hovering around 1.06 so I'm going to use that as our measured, we're going to divide that again, our target is still one and that gives us a correction factor of 1.06.
08:56 The value in our current cell is 84, so I'm going to multiply 1.06 by 84 to get a value of 89.
09:05 Going to enter that into our cell and again, you can see that our air/fuel ratio is now tracking really nicely on our target.
09:15 And we've done all of that by making one single change to the volumetric efficiency table.
09:21 So you can see how powerful this technique is.
09:24 There's no guesswork, there's no iterative adjustments, we simply look at our measured air/fuel ratio, we look at our target air fuel ratio and we make an adjustment in one go.
09:35 Earlier in the course I mentioned that my preference is to tune using units of lambda.
09:43 And the reason for this is it makes it very easy to calculate these kinds of adjustments to our fuel table very, very quickly.
09:53 And if we bring our throttle now up to 70%, so we'll bring ourselves up to the 70 kPa zone, I should say, not 70%, we're going to make one more adjustment, I'm going to show you how you can do this without needing to use a calculator to get our correction factor.
10:13 So you can see now at 70 kPa we're sitting at around about 1.03, 1.04 lambda.
10:20 So at this point we would normally have brought up our calculator and calculate that correction factor.
10:27 Because I'm using units of lambda though, what I can see is that our target lambda is lambda 1.0, we're running at 1.03 to 1.04.
10:36 This means that we are three to 4% leaner than stoichiometric at this point.
10:43 So what this means is that we can correct that discrepancy by simply adding three to 4% fuel into that table.
10:52 We don't need to go and actually calculate the correction factor.
10:57 So to demonstrate this, let's just bring up our calculator so we know that our correction factor is 1.04, we're adding 4%, the value in our table at 70 kPa is 87.2 so we'll multiply that and we get a value of 90.7.
11:15 We'll enter that in and again, you can see that we are right smack bang on our target.
11:23 Now some ECUs give you the ability to directly make percentage changes to the numbers in the fuel table.
11:30 So this means we don't need the calculator at all.
11:33 All we would do is go to that particular site in the table, in the VE table, and we would multiply that number directly in the table by 1.04, we were adding 4% and that would straight away make the change.
11:47 So we don't need to even use the calculator, we're just basing our decisions, our correction factor on the lambda discrepancy.
11:55 Now, I don't know about you, but personally I find it very difficult to gauge those sort of adjustments when we're looking at air/fuel ratio data, when we're looking at a number such as 13.5 for example, and our target lambda, our target air/fuel ratio is 14.7.
12:14 Personally I find that difficult to mentally make a connection as to the percentage change I need to make in the VE table.
12:23 When we're using lambda, very very simple, very straightforward and it is a very fast way of tuning.
12:29 So hopefully that has given you a demonstration of how powerful that technique is and hopefully now you can see how much time that can save you when you're tuning your fuel table.
12:41 So that's the process that goes on in the background as the ECU calculates what injector pulse width to supply to the injectors.
12:48 Now I did say at the start that you don't need to go through these calculations every time you need to tune an engine, as that's the ECU's job.
12:56 Understanding how the ECU works though, will let you do a better job of tuning the engine.
13:02 The two key points to take away from this section of the course is how to calculate the cycle time directly from engine RPM, and how to calculate the correction factor to apply to your fuel table to achieve your target air/fuel ratio in one change.

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