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So this block of numbers that I've got here covers the main areas that we're going to be operating the engine in under idle and cruise conditions. So these are the areas where I was just mentioning before, we're more interested now in getting good fuel economy from our engines. If we're an OE manufacturer we're also obviously really interested in minimal emissions from the engine. But for our case we're really looking here at getting good fuel economy. On the other hand though when we got up to wide open throttle our target changes quite dramatically. So you can see particularly in our top row here of our air fuel ratio target table, which is where we're going to be operating at when the engine is at wide open throttle, you can see for the most part I'm targeting lambda 0.

So this is an air fuel ratio that's going to be much more suited to making maximum power from our engine. So this is the key point here, we've got that three dimensional table there that we can move around in and we can choose the air fuel ratio to suit the current operating conditions, whether we want maximum power or we're more interested in fuel economy.

Tuning for Fuel Economy

Now with all of that in mind we do need to talk a little bit about why we need to change those air fuel ratio targets depending on what we want out of the engine. So for instance let's start by talking about the cruise area of the fuel table. So under these conditions we're cruising along the open road, the freeway, the motorway, and we really only have that throttle barely cracked open. We're not requesting very much power or torque from the engine and under these conditions, this is where we want good fuel economy. Now the important factor here is because the throttle body is predominantly closed, we don't have a lot of air and fuel entering the combustion chamber so we're not actually combusting a lot of air and fuel.

Now in turn the key point that we need to understand here is this also is not creating a huge amount of heat inside the combustion chamber.

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So it's quite satisfactory for us to choose an air fuel ratio at cruise and idle around that stoichiometric air fuel ratio lambda 1. Or as we'll see a little bit later, even potentially a little bit leaner than that. Now when we go to full throttle, things change quite dramatically. Under full throttle operation now the driver is demanding maximum power from the engine.

We want every absolute possible newton metre of torque and kilowatt of power out of our engine to make it accelerate as fast as we possibly can. Now at this point we've got that throttle wide open. So what's happening now is we've got a lot more air and a lot more fuel entering the combustion chamber, and under these conditions we're going to need to target a richer air fuel ratio. Now that term richer means that we're adding proportionally more fuel to mix with the air that's entering the cylinder. And there's two reasons why we're going to be targeting that richer air fuel ratio.

First of all because we want maximum power, we want to be very sure that under the turbulent nature of wide open throttle operation, we are injecting sufficient fuel so that all of the available oxygen inside the combustion chamber is being mixed with fuel and combusted, so that's the key. It's not the fuel that really affects the power the engine makes, it's the amount of oxygen making its way into the engine.

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Our job as the tuner is simply to mix the air, the oxygen that's entering the cylinder, with the correct amount of fuel so that we can combust all of it. So that's one reason why we would target a slightly richer air fuel ratio. However there's another subtle aspect here which is probably even more important.

Now under the wide open throttle operation, because we are combusting so much more fuel and air inside the combustion chamber, this tends to generate a lot more heat.

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And this can potentially be dangerous to the reliability of our engine. If we have too much heat going on inside the combustion chamber, this could potentially end up damaging our engine components, could melt pistons, it could melt valves, and the other aspect here is that the higher the combustion temperature goes, the more prone our engine becomes to suffering from detonation so we do need to be very careful with managing the combustion temperature.

So while it might sound a little bit counter-intuitive, by actually injecting a little bit of additional fuel that's essentially passing through the combustion chamber unburned, that additional fuel has the effect of removing temperature out of our combustion chamber, cooling that combustion charged temperature, and helping ensure that our engine remains reliable. Now while I've mentioned that the air fuel ratio, the emissions I should say probably aren't our biggest driver when we are tuning in the aftermarket. Certainly for the OE manufacturers, the emissions is probably their biggest concern.

So particularly when you're running your factory car at idle and cruise, the OE manufacturers are absolutely trying their hardest to make sure that the emissions coming out the tail pipe are minimal. If that's not the case they're going to struggle to get that engine into production in the first place, so this is really their key driver. So fuel economy actually takes a back seat to the emissions coming out the tail pipe.

We'll just have a quick look over on my laptop screen at the moment. And what we've got here is a graph of how the emissions at the tail pipe vary with our air fuel ratio.


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So just to confuse matters slightly, this time we are actually using units of air fuel ratio on the horizontal axis of our graph here. So we've got this particular point here, if we can draw down through there, this is our stoichiometric air fuel ratio of So we can see we've got carbon dioxide, we've got carbon monoxide, we've got oxygen, we've got hydrocarbons, and we've also got oxides of nitrogen.

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Now what we can see is that as we move rich which is off to the left, some of these components, some of these emissions components decrease and others increase. Likewise as we move lean which is to the right of our stoichiometric air fuel ratio point, we see exactly the same effect. Some of the components increase and others decrease.

What we actually find, which is the key point, why OE manufacturers are always running their cars at So there's advantages to some of the tail pipe emissions by going richer, advantages to others by going leaner. But at We'll also find that on the same note, we'll see that OE manufacturers are also fitting our engines with catalytic converters in the exhaust system.

And what we'll find is that the factory ECU will actually be commanding the air fuel ratio to constantly dither backwards and forwards across that stoichiometric air fuel ratio, and this is to actually make the catalytic converter work properly and reduce the tail pipe emissions further. So that is why we see that under closed loop conditions in idle and cruise, all of our factory cars will run at So what this means, and we'll see this a little bit later is that we can get a small improvement in fuel economy by targeting a slightly leaner than stoic air fuel ratio during the cruise operation.

Now I will also mention here, there's a caveat to that, this will affect your engine emissions, so if you are tuning or operating in a country or a city where there are emissions rules that you need to meet, this will affect your ability to meet emissions. So just keep that in mind. Also obviously as I've just mentioned, it will affect the ability of your catalytic converter to do its job properly which kinda doubles up on the same thing.

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OK so what we're going to do now is I'm going to get our Nissan z up and running here on our Mainline dyno. And just to demonstrate the effect of the air fuel ratio that we choose to use, we're going to perform two torque optimisation tests. And with these torque optimisation tests, what we're going to do is change the air fuel ratio and we'll see how the air fuel ratio affects the way the engine is operating. So what I'm going to do, let's just jump into our laptop screen for a moment and I'll just talk you through what I'm going to do. We're going to start here by operating the engine at RPM and minus 60kPa.

So what I'm going to do is I'm just gonna fudge the system here, we are going to be running a VE fuel model. And you can see that our target lambda is displayed here, I'll just bring us up to our set point here, RPM. Just get nice and central on that. So you can see that our target lambda is staying there at lambda one. What I'm actually going to do, rather than dealing with this properly, for today's test I'm just going to make some coarse changes here to the value in the cell that we're accessing.

So what I'm going to do here is I'm just gonna start by increasing this quite dramatically, and I'm going to get our actual air fuel ratio or lambda value all the way to 0. So as I'm richening that number, or actually I can only get to 0. Let's jump across to our dyno and I'm just gonna bring up our torque optimisation test here. OK so I'll just talk about what's on this screen. So first of all on the vertical axis here, we have torque in newton metres. So this is being registered by our Mainline dyno.

And on the horizontal axis here we have our lambda which is coming from the ECU. So this is going to be sent across from the ECU. I'll just clear this and what I'm going to do is I'm just going to start this test, and I'm going to lean out the lambda that the engine is running at and I'm going to lean the lambda out all the way, let's just increase this up as well, so we're not getting too much interpolation. I'm going to increase the lambda set point all the way up to lambda 1. So let's do that now, I'll start that test, and we'll just start reducing our value in our VE cell so this is gonna take a little while to go through here, and we'll see that our lambda starts to lean out, we're starting to come through 0.