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EFI Tuning Fundamentals: Choosing The Correct AFR

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Choosing The Correct AFR

18.14

00:00 - One of the biggest decisions we make as engine tuners is deciding what air fuel ratio to use.
00:05 The right air fuel ratio for a particular engine will depend on many variables and it's always a moving target since we're not aiming for a fixed air fuel ratio across the entire range of RPM and load.
00:17 If you've spent any time on internet forums, you'll already know that the seemingly simple question of what air fuel ratio should I run? Is going to result in as many different answers as there are people chipping in with their advice.
00:30 This obviously doesn't help the levels of confusion when you're a novice tuner.
00:36 Part of the reason for these varied answers stems from the fact that there is no single air fuel ratio that we must apply to every engine and every application.
00:45 In fact, as we'll see shortly, an engine will typically make good power across a reasonably wide range of air fuel ratios.
00:53 What we need to understand first is why we're adding fuel in the first place.
00:58 Understandably, it's an essential part of the combustion process, so we need to provide fuel in order to mix with the oxygen molecules and burn so the engine can produce power.
01:08 That should be pretty apparent to everyone, but a little less obvious is that we can use additional fuel, or in other words a richer mixture, to help control the combustion chamber temperature and ensure the reliability of our engine.
01:20 This might sound counter-intuitive at first, but as we add additional fuel and richen the mixture, the combustion temperature is reduced.
01:29 Let's start by having a look a at a graph showing how the air fuel ratio will affect engine power and fuel consumption.
01:36 You can see that as we adjust the air fuel ratio from very lean to very rich, the power tends to increase, plateau, and then drop away again.
01:45 Unsurprisingly, as we richen the air fuel ratio, our fuel consumption is going to continually increase.
01:51 Let's pause now and we can watch this being put to the test in a short dyno demonstration.
01:57 For this particular demonstration, we're going to be looking at how our lambda or air fuel ratio target affects the torque and hence power that an engine produces.
02:07 For this demonstration, we will be using a Nissan 350Z, which is a naturally aspirated 3.5 litre V6 and we're using the Haltech Elite 2500 ECU.
02:19 However the results we're going to see will be equally applicable to any engine and any tuning platform.
02:27 Before we start, let's have a quick tour of the Elite software so you can understand what I'm doing.
02:33 Now on this particular screen here, we have our lambda target table.
02:37 So these are the lambda targets that we want the engine to be running.
02:42 And at the moment you can see I've highlighted this section here which is the wide open throttle area of running between a thousand and two and a half thousand RPM.
02:52 We're going to perform this particular test at wide open throttle and 2,000 RPM.
02:58 To the right you can see that we have our lambda target and then the actual air fuel ratio being shown from the wideband sensor and the exhaust.
03:08 And we're going to be using the torque optimisation function of our main line chassis dyno and let's have a look at what we've got here.
03:17 So at the moment there's nothing to see, on the left hand side of the screen you can see that we have the torque.
03:25 Once we've got the engine running that will show us the torque the engine's producing.
03:29 On the bottom axis you can see that we have our lambda target.
03:35 So this is being transferred from the ECU while the engine's running.
03:40 So we're going to see the dyno produce a plot of torque versus lambda target.
03:46 So let's get running now in fourth gear and we'll go to full throttle and we'll start our test.
03:57 Okay, now that the engine is stable, I can begin our test.
04:03 And what I'm going to do in the Elite software is slowly, over the course of about a second per key press, I'm just reducing our lambda target by 0.01 at a time.
04:18 And at the same time you'll be able to see the dyno will be plotting how that change in lambda is affecting the engine torque.
04:28 And we're down to about 0.93 at the moment, and what you'll be able to see is that the torque, as we go from very lean to very rich, the torque will increase, then plateau, and then as we continue to go rich, it will continue to actually drop, the torque will continue to drop.
04:47 So we're down to 0.82, 0.81, and finally, 0.80.
04:54 So I've completed the test, we'll back off on the throttle and we can have a look at the results.
05:00 So what we've got on the screen right now, you can see our red line here represents how that torque has changed as we've moved the air fuel ratio target from very lean to very rich.
05:15 At the same time the dyno has shown us the point at which we've made maximum torque, that's represented by this point here and it's also being displayed at the top of the dyno.
05:27 So we ended up making a maximum torque of 337 Newton-metres with a lambda target of 0.93.
05:35 Now there's a couple of important points I want to mention here.
05:40 First of all we can see that the graph that we've got does move around a little bit, that's because the lambda reading from the exhaust is never absolutely 100% stable.
05:51 It's always moving slightly up and down, we're not ever going to see a completely rock solid, stable lambda reading or air fuel ratio reading from a wideband sensor.
06:02 So what we really need to do is look at the average torque that we're seeing.
06:06 We need to sort of average that result, that graph that we've got there.
06:11 The other important thing to point out is that through the middle of this graph here we've got a plateau.
06:19 So what I mean by that is from a range of about 0.86, 0.87, through to about lambda 0.98 we really haven't seen very much change in our torque at all.
06:32 The torque has been relatively flat.
06:35 If we go very rich you can see that the torque does drop off and likewise at the very lean end of the scale, again, you can see that the torque drops off.
06:45 But the point here is that the air fuel ratio or lambda target over quite a wide range, in this case about 0.86 through to about 0.98, doesn't actually have a dramatic effect on the amount of torque, and hence the amount of power that our engine's producing.
07:05 When we're choosing the correct air fuel ratio for our engine, it's going to depend to a large degree, on how much load is on our engine, or how much power we're commanding with the throttle.
07:16 For example, at idle and cruise, we aren't using very much throttle, and there isn't a lot of air entering the engine, so there isn't much load on it.
07:24 In this situation, we're most likely interested in achieving good fuel economy.
07:29 And due to the relatively limited amount of air entering the engine and being combusted, controlling the combustion temperature isn't such a consideration.
07:38 If we go to wide open throttle, on the other hand, we now have a lot more air entering the engine, and hence the engine is under at lot more load.
07:46 In this situation, we're demanding maximum power from the engine, so we'll be targeting a richer air fuel ratio.
07:53 There are two reasons why this is the case.
07:56 Firstly, we're now asking for maximum power.
07:59 Remember, it's the air entering the engine that really defines its power potential so we want to ensure that all the available oxygen is being mixed with fuel and combusted.
08:10 For this reason, we're going to be adding a little additional fuel by way of a richer air fuel ratio to make sure that under the turbulent nature of wide open throttle operation, all of the available oxygen is properly mixed with fuel and combusted.
08:24 Secondly though, we're now burning more air and fuel, and hence the combustion temperature will increase.
08:31 This can become dangerous, promoting the onset of detonation as well as potentially exceeding the thermal limit of the engine's components.
08:40 A richer air fuel ratio helps control this combustion temperature and ensures engine reliability.
08:46 We will be discussing detonation in a future module so you don't need to worry about this right now.
08:51 So summing up what we've learnt so far, we need to choose an air fuel ratio that's dependent on the amount of load being placed on the engine, however at the same time, thermal management, or the control of the combustion temperature, also needs to be considered.
09:06 Let's break the engine operation down into a few separate areas as shown ny this graph.
09:12 We can now discuss each of these areas of operation in a little more detail.
09:16 A standard engine will normally idle quite happily at lambda 1.00.
09:21 With a modified engine, and particularly one with a more aggressive cam profile fitted, we may find that the engine will idle best with a slightly richer mixture, perhaps around about 0.90 to 0.95.
09:33 The reason for this is twofold.
09:35 First of all, as we increase camshaft overlap, we end up with unburned fuel in the exhaust system.
09:41 This can result in an inaccurate air fuel ratio reading which doesn't truly represent the actual air fuel ratio of the combustion charge.
09:49 Also as we increase our cam size duration and overlap, our engine efficiency at low RPM suffers and in this instance, often a slightly richer air/fuel ratio target can aid stability in our idle quality.
10:04 There is however a limit to how rich we can go with the air fuel ratio before we run the risk of the spark plugs becoming fouled, which can result in difficulties in getting the engine to start.
10:15 Very rich mixtures can also be responsible for washing the lubricating oil off the bore walls, resulting in excessive wear.
10:22 This excess fuel can also make its way into the sump, diluting the oil and causing long term engine wear.
10:28 To avoid these sorts of issues, I don't recommend tuning richer than 0.90 lambda at idle.
10:35 Now iIn a road car, most of your time's spent in the cruise area of the graph.
10:40 This is the sort of area where you're driving ar freeway speeds at a constant speed and you're only just touching the throttle to maintain your speed.
10:48 There's not a lot of load being applied here and typically engines will be tuned to lambda 1.0 at this point.
10:54 Again, you may find that an engine with a very aggressive camshaft with a lot of overlap may operate more smoothly with a slightly richer air fuel ratio target at cruise, but typically, you shouldn't need to tune richer than perhaps 0.95 lambda.
11:09 The same warnings apply to excessively rich mixtures that we've just discussed.
11:14 Leaning out the engine slightly at cruise can actually result in a small improvement in fuel consumption.
11:20 Normally we find that maximum fuel economy is achieved at approximately 1.05 lambda.
11:25 As the mixture becomes leaner, the torque being produced by the engine is also reduced.
11:30 This means that at some point you need to increase the throttle opening in order to produce enough torque to maintain your cruising speed and this can counteract any fuel savings you may achieve by targeting a leaner air/fuel ratio.
11:44 As we've already discussed, under wide open throttle operation, we need to target a richer air fuel ratio to ensure we make maximum power and control combustion temperature.
11:53 With a naturally aspirated engine, we're limited to atmospheric pressure and we'll be running in the high load zone of the graph.
12:00 We normally find that maximum power for a naturally aspirated engine will be produced at approximately 0.85 through to 0.92 lambda.
12:09 Dyno testing will quickly establish the best air fuel ratio for maximum power so it's important to actually test and find out what a particular engine wants in terms of the air fuel ratio.
12:20 Now let's discuss the situation with a turbocharged or supercharged engine.
12:25 In this sort of engine we'll be operating in the boosted areas of the graph at wide open throttle.
12:30 Remember that what we want to do is choose an air fuel ratio that's appropriate for the amount of load being placed on the engine.
12:38 When we start adding boost pressure, we're simply forcing more air into the engine and the result is a larger combustion event as we burn more fuel and air.
12:47 This results in the combustion temperature increasing dramatically compared to a naturally aspirated engine, and we need to use some additional fuel here to help control this temperature and prevent potential engine damage.
12:59 As we increase the boost, combustion temperature will continue to climb, and that's why as we add more boost, we also tend to target a richer air fuel ratio.
13:10 A forced induction engine will generally make the best power at approximately 0.82 to 0.85 lambda depending on the application, however we may need to end up running a richer air/fuel ratio in order to ensure safety.
13:24 Since we're using the fuel to help control combustion temperature, it stands to reason that the air fuel ratio we choose may depend on the type of use the engine is designed for.
13:34 Let's discuss what I mean with a few examples.
13:38 Tuning a road car is actually relatively straightforward and thermal management usually isn't too much of a concern.
13:45 Now if you value your life and your driver's licence, it's usually difficult to use extended periods of wide open throttle in a road car, unless you're lucky enough to live near an Autobahn.
13:56 Even when full throttle is used, it's usually followed by periods of cruising, allowing combustion chamber temperatures to return to normal.
14:03 In this instance, we don't need to be so concerned with adding additional fuel to control combustion temperature.
14:11 An engine developed for circuit racing or road racing in comparison, is likely to get a much harder life than a road car engine.
14:18 In this type of racing it's quite possible for the driver to be using full throttle for 10 to 20 seconds or more and the engine will constantly be experiencing periods of high load, lap after lap.
14:30 The result is a lot of combustion temperature and we will typically use a slightly richer air fuel ratio target to help cope with this temperature.
14:38 At the extreme end of thermal management is land speed racing.
14:43 This is a very specialised type of racing where engines may be run at full throttle and high RPM for minutes on end.
14:50 Understandably, this creates a huge amount of combustion heat and the number one priority here is to control this temperature.
14:57 In this situation, a richer mixture would be essential even at the expense of losing some power.
15:04 In engines that are very sensitive to knock or detonation we may also find that targeting a richer air fuel ratio can provide some potential power advantage.
15:14 Now I've just introduced two new terms, and I don't want you to worry about them right now.
15:18 Knock and detonation are actually two different terms used to discuss a type of abnormal combustion, that's very damaging to the engine.
15:26 We will be discussing knock in detail in a future module, however for now, all you need to know is that an engine is more likely to suffer from knock as the combustion temperature increases.
15:36 Using a richer air fuel ratio can cool the combustion charge temperature and make knock less likely.
15:43 A side effect is that often this will allow us to advance the ignition timing further and we may see an improvement in power and torque.
15:50 In this graph, you can see that we have a couple of dead zones labelled, which just represent areas in the engine's operation that we simply can't reach.
15:59 Since we can't operate in these areas, the air fuel ratio isn't that important and when tuning, we tend to follow the general shape of the fuel curve and extrapolate this into areas we can't reach.
16:10 Likewise, we have the overrun area which is the area the engine will operate in when the throttle is completely closed, perhaps during a gear change, or as we're coasting down a hill During overrun operation like this it's common to actually shut off the fuel injectors to reduce fuel consumption in which case the air fuel ratio becomes irrelevant.
16:28 If you're not using overrun fuel cut then maintaining a similar air fuel ratio through the overrun area as to what we use in the cruise areas is typical.
16:37 If we look at the tuning strategy used by OEM engineers, we find that in all instances except wide open throttle, the ECU is tuned to target the stoichiometric AFR of 14.7 to one.
16:50 It would be nice to think that this is to provide the best possible fuel consumption, but this actually isn't the case.
16:56 If we look at this graph of tail pipe emissions versus lambda, we can see that at the stoichiometric AFR the engine produces the least combined emissions and it's these tail pipe emissions that are the key considerations for every engine manufacturer, because if the engine isn't emissions compliant, it will never make it into production.
17:16 Cycling the air fuel ratio back and forth across the stoichiometric air fuel ratio is also an essential element of making the catalytic converter function correctly.
17:25 Again, ensuring minimal emissions.
17:28 By now you should have an understanding of what we're trying to achieve when we're tuning the fuel delivery for an engine.
17:35 The important takeaway from this module is that we can't hope to apply the same air fuel ratio to every engine, and every application.
17:43 The key point to consider is managing the combustion temperature of the engine and this will depend on the amount of load placed on the engine, as well as how the engine is being used.
17:54 If you want a more thorough understanding of air fuel ratio and how it affects combustion, as well as how to use the dyno to test and find the optimum air fuel ratio for your particular engine, our Understanding AFR course is an ideal addition.