118 | Understanding Injector Characterisation - AEM Infinity
With any VE-based fuel model, the injector characterisation data is critical to the accuracy of your VE table. Each manufacturer has their own techniques for applying this data and in this webinar we will find out what each table in the Infinity means and how to fill in these tables.
Hey guys, it's Andre from High Performance Academy. Welcome to this webinar where we're going to be focusing on the injector characterization or injector set up in the AEM Infinity range of ECUs. Now, this is a relatively straightforward task, it is reasonably quick and easy to cover off and go through when you are setting up the ECU. However, the reason I am going to focus on this today is that I still know a lot of tuners don't properly understand how to do this or what the numbers and tables mean. And the problem with a volumetric efficiency or VE-based fuel system such as that used by the Infinity, is that if we don't have this data correct, then it is going to affect the accuracy of our entire tune, or in particular, our volumetric efficiency table, and the ability of the ECU to properly track our requested air fuel ratio or lambda targets.
So this is one of the aspects that we do need to cover off and for any of you who've been through our practical Dyno tuning course and learned the HPA 10 step process, you'll know that this is one of our very early steps in the 10 step process. You can waste a lot of time if you get sort of stuck into the tuning straight away and come back and find that you haven't properly set up your injector data. So what we're going to do is start by talking about why injector characterization is important, and I think really it's fair to say that this has come up over the last couple of years, maybe three years, and it's become a little bit of a buzz word. There's a lot of injector manufacturers out there who are talking a lot about injector characterization. It's also being brought on board by a lot of the ECU manufacturers, particularly in the current era, you're seeing a lot more ECU manufactures incorporate this volumetric efficiency-based fuel tuning model.
Now, I just want to really focus quickly on what that means and in terms of the VE fuel model, the difference really is that what we're doing as tuners in the VE table is we're defining the air flow into the engine, or the volumetric efficiency of the engine at each point in the table. So unlike a conventional fuel model, where we're directly requesting an injector pulse width, here we're actually telling the ECU how much air is going into the engine. The reason we're doing that is simply put, if the ECU knows exactly what mass of air is entering the engine, and it also knows how much fuel can be delivered for a given pulse width being delivered to the injectors, then it's really easy for the ECU to look at our target air fuel ratio table and decide how long to open the injectors for in order to get our desired air fuel ratio. So this is really one of the key points of a VE-based fuel model. If we don't have a good injector data, then our whole model is flawed.
Now there's pros and cons to the VE fuel model, I'm not here to debate those. I'm simply going to go through what we need to do in terms of our injector characterization and in this respect, every ECU manufacturer has their own preferred way of dealing with how to characterize the injectors. Again, I just want to touch on that briefly as well, because these techniques vary quite dramatically. In some ECUs, it's very very simple, all we're doing is defining a flow for the injector at a given reference fuel pressure, and we're going to enter a off-set or dead time table and leave it at that. Other ECUs are going much more involved and they're looking at aspects such as the non-linear area of the injector operation.
And all of this really is a balancing act in my opinion. What we need to understand is that the more data we can give the ECU, the more accurately the ECU is able to do its job. But at the same time, every time we need to add more data or more parameters into the ECU, that also takes more time for the tuner, there's more data that we need to physically find, and this is where we need to come up with a fine balance between how accurate do we really want this fuel model, or need this fuel model to be? And how much extra time do we want to spend on our tunes? And the reason I say this is because particularly if you are tuning for a prefession, then time, understandably, is money. If your tune's gonna take longer, it's gonna mean that you're going to need to charge your customer more money and of course, there's a finite limit to how much people are going to be prepared to pay for the tune. So I think in that respect, this is why I wanted to say that every ECU manufacturer deals with this differently, and you need to understand why that is.
It's not necessarily that the ECU manufacturer has taken shortcuts with the way they deal with the injector data, it's just that they're finding a different balance point between their accuracy and the time taken for the tuning. Okay, so in terms of the way the ECU is set up, AEM deal with this in a really nice way, this is one of the features I really like about the AEM range. It's been incorporated across the entire AEM range and obviously followed over onto the Infinity. If we can just jump to my laptop screen and I'm going to start with our set up wizard. Now, the set up wizard is something we can use for a range of the functions inside the ECU, and here obviously we're only going to be just dealing with our injector set up.
So if we come down onto Basic Set Up and click on Injector Set Up, I'll just open this out a little bit wider. So this is where we can define the injectors fitted to the engine. And if you've got a set of injectors that are pre-defined by AEM, then your work is going to be incredibly simple because all you need to do is select them from the drop down menu. Before we get to that though, we've got another parameter here that we need to just cover off, which is our primary fuel pressure regulator reference. It is a bit of a mouthful, and really all that defines is what type of fuel system we're running in the car.
In this case, we're running a 350Z with a factory return-less fuel system. So I've selected their atmospheric reference. So what this means is essentially the fuel pressure remains fixed. The regulator is fitted in tank and it retains what should be a relatively static fuel pressure regardless of our manifold pressure. If on the other hand, we were to fit a manifold pressure reference fuel pressure regulator, and we ran a return line back into the tank, we would select manifold vacuum reference.
Now, this comes down to the way the differential fuel pressure will vary, and that's a key aspect. The differential fuel pressure, this is a key aspect for the injector characteristics. So particularly in terms of flow and in terms of our injector dead time or our injector offset. These values for flow and dead time actually vary with regard to the differential fuel pressure, not strictly just the fuel pressure. So it's really important that we get this right, because otherwise the ECU will not be able to calculate the correct differential fuel pressure.
Just a tip there, I'm going to talk about this again shortly, but in AEM speak, the differential fuel pressure is referred to as the injection pressure. So in this case, we're leaving that at atmospheric reference. Now I'll also point out that if you are using a fuel pressure sensor fitted to the ECU, which we are here on our 350Z, then the differential fuel pressure is automatically calculated based on the fuel pressure sensor value. However it's still important to have this parameter correctly set because if the fuel pressure sensor goes into fault, then the ECU will revert to calculations, so we still need to have this correct. Alright, now the key point here though, if we come down to our Primary Injector Flow, was at selection, we can set the moment, I've got a set of Injector Dynamics, ID1000 injectors selected.
We click on the list we can see what is a relatively complete list of injectors. So if you've got anything that is on that list, then your job is very, very easy. All you're going to need to do is select the correct injector, based on whatever you've got here. And what that's going to do as well, when we select this, it automatically configures the injector flow rate, it automatically configures the offset table and also the fuel injector duty cycle and open time parameters which we're going to have a look at a little bit later on. So for the moment, I'm just going to leave our selection set to Injector Dynamics, ID1000.
Possibly a little bit of an optimistic choice in our VQ35 given that we are naturally aspirated, but regardless, that is the set of injectors fitted. Moving down we can also define a limit for our injector duty cycle, so this will be a hard limit that the AEM Infinity will not exceed. So in this case I've got that set to 95% which we should never, ever be anywhere near, again with our naturally aspirated engine. We can select the number of injectors as well, and then if we have a set of secondary injectors, we can click our little tick box here to tell the ECU that. Next we have our primary fuel type, which in this case we're running on normal pump gasoline.
If we look at the drop down menu though, our normal fuel options are available there, just gonna leave that at gasoline. And again, the reason this is important is that the fuel characteristics, and in particular the stoichiometric AFR of the fuel is a really key aspect to the volumetric efficiency-based fuel model. That's going to have a really big impact on the way the fuel model works, so we need to make sure that that's correct. If we want to go a little bit further, we can also look at our injector properties, which includes the specific gravity of the fuel, as well as the stoichiometric air fuel ratio I just mentioned. The specific gravity of the fuel, I just really briefly covered that off, so this really defines the density of the fuel and why that's important is because when we're looking at an air fuel ratio, we're not actually talking about a ratio of volume, so we don't actually care what the injector flow is in volume terms, so ccs per minute, which is a really common way of defining injector flow.
We don't really care what the volume flow is. When we're talking about our air fuel ratio it is a mass ratio, so we're talking about a mass of air to a mass of fuel, so 14.7 to one, 14.7 grams of air to one gram of fuel. So using the fuel density, it's possible for the ECU to then calculate the mass of fuel based on the volume flow of the injector. So that's why that's important. Okay, so this is really easy as I said, if we've got the injectors, if we've got a set of injectors that are already supported.
We're going to look shortly at each of the tables though, and see where we can get this data, where we're gonna input this data from if we're dealing with injectors that aren't covered by AEM. Last aspect here is that we can define the injector location, where the feedback is gonna come from in terms of the lambda sensors. In this case we have two lambda sensors, one per bank. So we're defining essentially, which injector is being controlled by which lambda sensor and also the phasing of the injectors. Now this is being done automatically on our VQ35 so we don't need to worry about that any further.
Now I'm going to, oh before I close that down, also just really quickly look at our basic sensors. And in this case we have a fuel pressure sensor set up and this is just 150 psi AEM fuel pressure sensor and if we cycle down here. Now I mentioned just briefly about the differential pressure and injection pressure. Now there's a couple of parameters here that are important, we have our static fuel pressure, which I've defined as 58 psi four bar, this is our normal static fuel pressure, and we've got this little tick box here says, calculate fuel pressure from sensor. Now what this is used for is if we don't have a fuel pressure sensor installed, this will ignore the basically the fail-safe for the fuel pressure sensor not being active.
And what it's going to do then if we don't have that tick box ticked, it's going to calculate the injection pressure based on our static fuel pressure, as well as our regulator mode, so that's whether or not we've got an atmospheric or a map referenced fuel pressure regulator on board. Okay, so let's close out of that. Now the page that we're on at the moment is our injector page, which you can see here, and I just wanted to go over what we have configured on this page. So as I've said, when we select the injector from our drop down menu there in the set up wizard, it automatically sets up this page for us so we essentially should have nothing to do. However, like I've said, we may be in a situation where we don't have data, or AEM don't support our injectors, but we've got data, so where do we put this data? So first of all we have our injector flow rate in ccs per minute.
So this particular table here is a two dimensional table relative to our injection pressure. So remember, injection pressure, essentially this is AEM lingo for our differential fuel pressure. And what this really means is that if we increase the differential pressure across the injector, generally we're gonna see within reason the flow increase. If we drop our differential pressure, we're going to see the flow decrease. The values are defined here from 30 psi differential pressure right up to 100 psi, we should be operating somewhere around the 50 to 60 psi differential pressure.
Excuse me. Area depending on our manifold vacuum. So the key point here is that with a return-less fuel system here, remember that our fuel pressure remains fixed, but our manifold pressure is constantly varying and what this means is as our manifold pressure constantly varies, our differential fuel pressure or injection pressure also varies. So it's really important to have this set up correctly, more so in a return-less fuel system, because our injector flow is constantly varying, and again, if we want our fuel calculations to work correctly and our VE fuel model to work correctly, we need to have this correct. Let's just start our engine as well, and we'll see how our fuel pressure moves around, our injection pressure moves around.
And the values are also listed here in our text grid. We have both our fuel pressure as well as our injection pressure, and you can see that our injection pressure is sitting somewhere around about 10 psi gauge higher than the fuel pressure. That's simply because at idle we have a vacuum in the intake manifold and that increases our differential pressure. Now if I give the throttle a blip, we see that our injection pressure drops, in fact it dropped down to almost 50 psi. You can watch the little cross hairs in the graph view and I'll do that again.
So that's why it's really important to have this sort of data set up correctly. Now of course, if we've got a map reference, fuel pressure regulator, then we should be able to expect that our differential fuel pressure remains fixed. After all, that is what the manifold pressure reference regulator is supposed to do. However, it's still possible to see fluctuations and variations in our injection pressure, particularly at high RPM, it's not uncommon to see our injection pressure drop away slightly. So there's still advantages in terms of accuracy to properly filling out this two-dimensional table.
Now where do we get this data from? Again, I'm just going to reference here our Injector Dynamics data. So this is for our ID1000 so I've gone straight to Injector Dynamics website here. The slope and offset versus pressure tab gives you all of the information that you need. And in particular, we can see here on the right hand side, we have the flow or slope in ccs per minute, and we have fuel pressure, or differential fuel pressure on the left hand side. So this is the sort of information that we can translate straight across into our AEM Infinity flow table.
Now I will point out that there is a discrepancy between numbers that Injector Dynamics have developed for the ID1000, and the numbers that AEM have developed themselves for the Infinity. All of those values from the drop down selections in the set up wizard are developed in house by AEM on their own flow bench. Now I'm not in a position to really debate who's right or who's wrong or why even there are discrepancies there, all I can tell you is that there are differences between the Injector Dynamics developed fuel characterization data and those developed by AEM. It's up to you which you choose to use. We'll go back to our AEM Infinity tuning software now.
I'm going to move across now to the right and we'll talk about our injector offset. So again, our injector offset in this case, dead time or latency if you'd prefer, this is again for those who aren't familiar with it, the injector is a mechanical device and it doesn't open and start flowing fuel instantaneously. So there is a lag between the ECU signaling the injector to open and the injector physically opening and starting to flow fuel. There's also a delay, albeit much smaller, when the ECU closes the injector and the injector actually physically closes and stops flowing fuel. So a simplified way of how I describe what injector dead time or latency is, is simply the difference between the time that the injector is signaled to open by the ECU, and the time that the injector is physically open and flowing fuel.
So we need to be able to account for dead time if we want to maintain accurate air fuel ratios as our battery voltage fluctuates, and others, now this is also going to again aid the accuracy in our table, in our volumetric efficiency table. So in this instance again, just like our flow, this is actually a three-dimensional table here, but our injector offset which quite often says two-dimensional table just based off our battery voltage, in this case we're also accounting for the differential pressure, the injection pressure. So we have this three-dimensional table. So again, if we can just flick back to our Injector Dynamics data here, we can see that this is essentially what we've got here, our offset data is three-dimensional table, we've got battery voltage here on the x-axis and we've got injection pressure or differential pressure on the y-axis. And again, the values here aren't quite the same, so if we take for example, the value here at 14 volts and 60 psi gauge, we can see that we've got a value of 1,080 microseconds, and if we jump back and we look at the same value here, we've got 1.24 milliseconds, or 1,024 microseconds, so there is some discrepancy there between the data that AEM have developed and the data that Injector Dynamics have developed.
Now again, I can't really answer exactly why that is the case, but one thing I do want to mention is that the dead time can vary based on the injector drives in the ECU. So this could be partially to do with the way Injector Dynamics, or the ECU that Injector Dynamics are using to create their characterization data, which if I am still correct, I believe was a Motec M800, and obviously AEM are testing and developing this data directly on the Infinity ECU. So again, this three-dimensional table, more important if we're running an atmospheric reference fuel pressure regulator or return-less fuel system like we are here. And again the reason for that is because we are going to expect to see quite large variations in our differential fuel pressure or injection pressure as we vary our manifold pressure. So for example, at the moment you can see that we're sitting at 14 volts and 60 psi.
If I blip the throttle again, we're going to see this purple cell that's currently highlighting 1.24 milliseconds, should drop down closer to 1.08. And that's exactly what it does. So again, if we're running a manifold pressure reference regulator, we should see that differential pressure stay much more consistent, and it's not uncommon in many ECUs to set this up as a two-dimensional table on that style of fuel system. Excuse me. Okay, so the last two aspects that we want to talk about here, and I'll stop actually before I do cover those.
We will be moving into questions and answers really shortly, so if you do have anything that you'd like me to cover off, please ask that now in the chat and I'll deal with that really shortly. Now the next two aspects here, we've got fuel injector duty cycle percent and our FI, I should say, open time x. Now and in their own right, those don't mean a lot, and what are we gonna do with these values? Now the reason these two tables here exist, is really to do with the way the AEM Infinity operates the injectors and these two tables are specific to if we're running a peak and hold or low impedance, low Z injector. Now in that case, the way the ECU controls the injectors is quite dramatically different than in a high impedance and saturated drive injector. So for a high impedance injector like our Injector Dynamics 1000, these two tables don't mean anything, they're irrelevant, so that's why we can see at the moment, particularly for our fuel injector duty cycle table was filled in with values of 100% here.
So that we can understand what these tables actually mean, let's just jump across to a photo and I've grabbed this straight out of the AEM Infinity help manual, so if you want to have a better look at this, it is in the help manual, and this is the way the Infinity drives a peak and hold injector. So first of all we need to understand what the ECU needs to do with a peak and hold injector. What it needs to do is supply a high initial current so it's pulsing the injector with a high initial current to quickly open the injector, get it flowing with fuel. And if we just look at what's happening here, look at injector three for example, we can see the pulse being sent out to the injector. And if we look at this injector pulse in a little bit more detail and blow it up a little bit, this is what it looks like here.
So we can see that there's this section here that's labeled Open Time. And that's what we're defining with our open time table here. So this is the period of time that the ECU is supplying full current or full voltage to the injector in order to open it really quickly. And we can see that as the voltage in this two-dimensional table, as that voltage decreases, we see that our open time increases. We come back to our little drawing here from AEM.
Once we're past the open time, then the ECU will essentially pulse width modulate the voltage being supplied to the injector in order to control the current and the duty cycle is what we're controlling here, and that's defined by the duty cycle table. So again, we're looking at values of 100% in here, simply because we're controlling a high ohm or saturated drive injector, so it's not being operated with a peak and hold injector driver, however, if you were using a peak and hold injector drive, you'd be more likely to see values of somewhere in the region of 40 to 60% here. So it's in order to control the injector correctly, the peak and hold injector, to control the current being sent to the injector to still open the injector quickly and then taper off and hold the injector at its lower holding current. Again, if you're using pre-defined injectors, these values, these tables will be filled out by the set up wizard. If you're using injectors that are not defined there, these particular parameters, I'd be inclined to find an injector similar to what you're using and use the FI duty cycle and the open time table data from that injector and then simply manipulate your injector flow and your offset tables.
Alright, we'll jump into some questions now. If you do have any more, please feel free to ask them. And we've got one at the moment from dpotter who's asked, is over-sizing your injectors going to be a bad thing? Really good question there actually, and the answer is going to depend on exactly how over-sized your injectors are. What's important is to make sure that the injectors can be controlled to deliver the sort of mass of fuel that we need to maintain and control a good idle speed. Now, if our injectors are too big, we're going to find that we'll be operating down in the very low end of the non-linear operating area of the injector, can be very very difficult in that area to get really good control over the injector, so you may find that it's impossible to achieve and maintain reasonable air fuel ratios.
Can also have some pretty horrible consequences to your driveability. So obviously in that respect, that's something we really need to avoid, so there's not an infinite range of injector sizes that we can fit to an engine. The flip side of that though is using an injector that's on paper, probably seemingly too large for the engine, such as the ID1000s we've got here on our naturally aspirated 3.5 liter V6, is that it obviously, or understandably, reduces the peak injector duty cycle that we're likely to see. Now where that can pay dividends, and this isn't going to be across the board, it's really going to be application dependent, is it does give us a lot more head room in terms of where we can move the injector timing around. Now what that does is it allows us to time the injector opening with relation to the intake valve opening on the particular engine cycle.
Now I'm not gonna get too deep into the theory behind injector timing, there's a couple of really great webinars already in the archive that you can check out if you wish, but there's two schools of thoughts, essentially there, based on whether we're going to be injecting on an open valve or a closed valve. Now if we're injecting on an open valve, give or take the cam profile, we're essentially limited to somewhere around about, maybe 30 or 35% injector duty cycle. Once we're beyond that, that's the sort of time frame of the engine cycle that the intake valve is going to be open, so that also limits the injector duty cycle we can use, if we really want to target solely injecting while that intake valve is open. Once we've passed about 30 or 35% injector duty cycle, some amount of fuel is going to be impacting on a closed valve. And of course with factory injectors, it's not uncommon to see peak injector duty cycles getting up around, perhaps 85% at high RPM, wide open throttle.
What this means is it gives us very little scope for moving the injector timing around because essentially the injector is almost open the whole time anyway. So that's probably one of the potential advantages and in some engines, depending on the injector location, the injector style and the engine itself, we can see small but measurable gains and in performance, both torque and horsepower, also fuel economy, by correctly phasing the injectors. Now that's actually something I did overlook in the body of the webinar so if we can just jump back very briefly to my laptop screen and I'll just power the Infinity back up again. I just wanted to talk really briefly about our fuel injector timing table here, which is this particular table you can see in the center that's currently filled with zeros. Now, this particular table just needs a little bit of explanation because the way AEM deal the fuel injector timing is just a little bit different to most of the ECUs I've personally come across.
This depends on the injector phasing, and essentially a value of zero in this table, means that our injector timing is going to be 360 degrees before TDC on the firing stroke, the compression stroke. So a value of zero there doesn't mean that we're injecting right at the end of the engine cycle, we're actually 360 degrees before that. So just a really important aspect to cover off there, I'm sorry I missed that out of the main body of the webinar. Alright, I'll just jump back to my notes there 'cause I think we had another question pop up as well. Barry's asked, what's the maximum injector duty cycle I suggest running up to? Alright, this is a really good question, and it's really, I think it's changed a little bit with the current crop of EV14 Bosch base injectors that we've currently got.
In the earlier days, with the older Bosch injectors, they offered really poor linearity at the top of their flow as well, and generally it was a pretty good rule of thumb to not exceed around about 85% injector duty cycle. There is a myth in the tuning industry that I've also been led to believe for many years, that running the injectors at something like 100% injector duty cycle continuously can actually lead to problems with the injector because the injector doesn't have enough time to cool. I ran this past Paul Yaw at Injector Dynamics and he informed me that there is actually an endurance test that Bosch run on all of their injectors where they are powered and run essentially wide open with no fuel running through them for cooling for a set period and they must pass that test. So that's actually, I guess you could call it a tuner's wives tale, but it's not a problem to do so. With the latest crop of injectors, we can probably run them a little bit harder than 85% injector duty cycle.
The main reason why we would want to keep a little bit of head room there is just so we do have the ability to increase our fueling if need be. Obviously if we're tapping 100% in duty cycle, we have no potential to add any additional fueling if required. So even given a brand new set of EV14 injectors, I'd probably be inclined to try and stay under a 90% injector duty cycle limit. And lastly, Barry's asked, how does the AEM handle the short pulse with adder data? So this at the moment is something that AEM are ignoring. There is no short pulse with adder table.
So again, this really comes back to what I was saying earlier in the webinar, that they've decided on their balance between how much accuracy they want to add into the ECU and how much extra work they want the tuner to have to go through in order to get a really good tune. Right, that's brought us to the end of our questions and as usual, if you do have any more, please ask those in the forum and I'll answer them in there. And for any of you who are joining us who haven't already left us a review on Facebook, we'd really love it if you did so, particularly if it's a five star review. These reviews are really important to those who are considering using HP Academy for their EFI tuning and it just gives them a little bit more confidence that everyone who is joining us and watching our webinars and taking our courses has got great value from it, so it's really valuable to us. We'd all appreciate it.
Alright guys, thanks for joining us, and I'll hope to see you all next week.