Summary

During rapid changes in throttle position, the main fuel delivery map is not sufficient to maintain a consistent air fuel ratio and this can result in hesitation and poor response to fast throttle inputs. 

In this webinar we will discuss why an engine needs additional fuel during transient throttle input and look at the correct approach to configuring and tuning acceleration enrichment using the Link G4+ platform on our Nissan 350Z

00:00 - Thanks for joining us for this webinar.
00:02 Today we're going to be looking at acceleration enrichment tuning and we're going to be looking at that on the Link G4+ or Vi-PEC platform.
00:12 Now of course a lot of what we're going to be looking at is the theory behind acceleration enrichment and this is a universal concept which you can apply to just about any ECU.
00:23 Obviously, there are intricacies or differences between how different ECUs approach and incorporate accel enrichment tuning but the principles behind the theory are always going to be the same.
00:39 So let's actually start by talking about what acceleration enrichment is and why we need it in our engines.
00:48 Acceleration enrichment is unfortunately what we've got into a situation with is the way that it's been dealt with by most mainstream ECU manufacturers is kind of what I'd explain as a work around to why we actually need acceleration enrichment and that's kind of ended up causing some confusion and sort of murkied the waters a little bit for tuners out there as to what's actually going on, what it's doing and why we need it.
01:17 So let's start though with an engine where we have no acceleration enrichment.
01:23 So when we make a sudden change to the throttle position, the ECU is doing nothing to add additional fuel and in that situation if we're driving along at say 3000 RPM and we're just cruising and then we mash the throttle straight to the floor, what we're going to find is that with no additional enrichment being provided by the ECU, we're going to see a momentary lean spike in the air/fuel ratio.
01:51 We're going to see that on the wideband, we're going to see it on the logger and more importantly though we're going to feel it.
01:57 The car is not going to be crisp and responsive to the throttle input, it may hesitate, it may bog and it may be a little bit jerky, OK so that's what happens if we've got no acceleration enrichment and obviously what we're going to be looking at in today's webinar with acceleration enrichment is what the ECU can do to correct that situation.
02:20 But the more important aspect is why we need this.
02:24 Now the mainstream ECUs will deal with acceleration enrichment based off usually rate of change of throttle position.
02:36 So essentially when you make a very sharp quick input to the throttle, that's when the ECU will be measuring that rate of change and go OK the driver's stabbed the throttle, I need to add some additional fuel.
02:48 So all of the acceleration enrichment that we typically deal with is actually based off rate of change of throttle position.
02:55 And that's a little misleading becuase it's not really anything to do with the throttle position that is causing that requirement for additional fuel.
03:05 So let's look at what's happening or discuss what's happening inside the engine when we're driving in steady state, so our foot is consistent on the throttle and we're at say 3000 RPM.
03:16 So we've got our fuel injector mounted somewhere in the intake port and it's pointing traditionally somewhere at least closely approximating the back of the intake valve.
03:26 Now under steady state conditions we'd assume that the fuel is all being sprayed out of the injector, being finely atomised and then drawn into the cylinder.
03:36 However that's not quite the case.
03:38 Regardless of how the injector is positioned, what we're going to find is that some amount of fuel being supplied by an injector is actually going to wet out the port walls, maybe the port walls, the cylinder head and the back of the valve.
03:55 So we end up with a fuel puddle.
03:57 So some portion of the fuel that the injector's being delivered, the injector is delivering actually puddles out onto the port wall and it forms what we call wall wetting or a fuel film.
04:12 Now under steady state conditions the puddle or that wall wetting will tend to evaporate off and be drawn into the cylinder at the same rate that the injector's topping it up because obviously that makes sense.
04:26 If that wasn't the case, either we'd have a puddle on the wall of the intake port that continually grew so at any particular point in the engine's operation, that puddle will be a certain volume of fuel and the injector tops it up at the same rate that the puddle evaporates off and gets drawn into the cylinder.
04:45 So under steady state conditions, that puddle of fuel or that wall wetting has no real effect on the operation of our engine.
04:54 Now before we move on, let's talk about what influences the size of that puddle, the volume of fuel that makes up the puddle.
05:05 So there'll be a variety of aspects there.
05:07 One of the main ones though is manifold pressure.
05:11 So the pressure, air pressure in the intake manifold has a really large influence on the size of that puddle.
05:17 As does airflow which comes with engine RPM.
05:21 Also the engine coolant temperature or more specifically the temperature of the port wall.
05:27 So all of these will influence the size or volume of fuel that incorporates that puddle.
05:32 OK so now we've got to the point where we're in steady state conditions and we have a puddle of a known size.
05:39 What happens and why we need to have acceleration enrichment is what happens, remember I said the size of the puddle to a large extent is defined by manifold pressure.
05:52 When we make a sharp change to the throttle position, we also end up, what we're really seeing in the intake manifold is a sharp change in the manifold air pressure.
06:02 So at the same time we see a large change in the puddle being formed on the port wall.
06:10 So what happens is when we make that sharp change to the throttle we go from part throttle to wide open throttle very quickly and our manifold pressure moves from let's say perhaps 40 or 50 kPa to 100 kPa very very quickly.
06:24 What happens is the size of the puddle on the port wall changes so it grows.
06:28 And what's happening there, as the puddle size increases, that's increasing because it's taking fuel volume from the injector.
06:38 So during that transient throttle change, a large volume of the fuel that the injector's providing actually goes to increasing the volume of the fuel puddle or the wall wetting on the port wall, rather than making it into the cylinder.
06:54 Now remember this is a transient thing, this is only momentary, as soon as the engines regain steady state conditions, the puddle size will have changed but once it's stabilises, remember that the fuel in the puddle will evaporate off and be drawn into the cylinder at the same rate that the injector's topping it up.
07:15 So what we've got it this constantly changing puddle size or constantly changing volume of fuel wetting out the port walls and remember that that volume on the port walls will depend on the manifold pressure and also the engine RPM and engine coolant temperature.
07:33 So these are the parameters we're really trying to account for when we're tuning our acceleration enrichment.
07:40 Now just like acceleration enrichment, we've gone from a light throttle opening to a large throttle opening very qucikly.
07:48 The inverse happens when we back off the throttle as well.
07:51 So if we're at a high throttle opening we have a large volume of fuel forming a puddle on the port wall.
07:59 When we back off the throttle and the manifold pressure drops in the intake manifold, all of a sudden that puddle size tends to diminish so we've still got our injector supplying fuel but the puddle size, the volume of fuel in the port wall is being reduced and that's being reduced by being consumed by the engine so when we back off the throttle we have a momentary rich spot where the engine will run richer than normal as the puddle size is reduced until we get to an equilibrium again.
08:29 Now I know that is a little bit of a confusing topic to understand, I know it's a little bit difficult to get your head around.
08:38 Particularly if wall wetting and this concept of a puddle of fuel on the port wall is not something you've ever experienced or heard about before and I know that can be a little bit difficult to get your head around initially.
08:53 Now there is actually a really good explanation of this, it's a well known concept, particularly in OE calibration and it's also referred to, it's referred to as wall wetting, it's also referred to as X-Tau and there's a really good explanation of it that the guys from MegaSquirt have put into their mega manual.
09:15 And I'm going to copy a link into the webinar chat box at the end of this and if you're interested in learning a little bit more, doing a little bit more research, that is a really good resource to have a look through and it will more thoroughly explain exactly what's going on.
09:34 OK so now that we have an idea of what's going on, essentially when we make a change in throttle position, when we're opening the throttle, what we're going to need to do is supply momentarily a little bit of additional fuel to make up for what's happening to the size of that fuel puddle.
09:52 And likewise when we back off the throttle, generally we'll have to remove some additional fuel, remove some of the fuel that's being injected to make up for the fact that that fuel puddle is diminishing in size and being drawn into the cylinder.
10:04 Now while the wall wetting is what's actually going on, it is quite a complex system to tune in an ECU that does properly model what's going on with the puddle size on the wall so most ECU manufacturers take a more simplistic approach and look at an input that's easy for them to measure and an input that's easy for them to use as an axis for acceleration enrichment and of course, the manifold pressure is affected by the throttle position so throttle position is a pretty easy way of adjusting your acceleration enrichment parameters.
10:44 Right hopefully that's been reasonably clear, of course as usual we will have a section for questions and answers so if you've got anything you want me to clarify, please enter that into the chat box and I'll do my best to address that.
11:00 For now, let's have a look however at the laptop software and today's webinar we are using the Link G4 or Vi-PEC software as I explained on our Nissan 350z and what I'm going to do is first of all I'll give you a really quick demonstration on the dyno of exactly what happens when we don't have any acceleration enrichment.
11:22 So what I'm going to do here is I'm just going to bring the engine up to about 2000 RPM.
11:30 And I've got the time graph running here and on this time graph what we've got at the top, we've got our engine RPM.
11:38 The next group down we've got our throttle position.
11:41 We've got our acceleration fuel enrichment percentage being shown here and then we've got our lambda from our left bank.
11:50 So you can see at the moment I'm sitting there at 17.5% throttle, just barely touching the throttle and we're sitting on the dyno at 1950 RPM.
12:00 So what I'm going to do now is I'm going to stab the throttle wide open and we'll have a look at what happens to the lambda as I do that.
12:09 Alright let's do it now.
12:13 OK so I'll just go back.
12:18 Had the time graph running for a little bit of time there.
12:22 Alright, OK so let's have a look now that I've got that area zeroed in.
12:29 We can see exactly what's going on.
12:31 So we've got our lambda, you can see that before I got to wide open throttle we're sitting at around about 0.99 lambda so my target there is lambda one.
12:40 And you can see what happens is as soon as I stab the throttle and we go straight to wide open throttle, we see this lean spike in our lambda so it actually spikes lean to 1.12, 1.13 lambda.
12:52 Reasonably lean and then that's only a momentary spike and then once everything stabilises you see we drop down to about 0.9 lambda which obviously is my wide open throttle lambda target.
13:06 So that's what's happening when we don't have any acceleration enrichment fuel and I've got the acceleration enrichment mode turned to off at the moment in the software.
13:16 Just another explanation, so that lean spike there is being caused by the fuel puddle being topped up by the injector so a momentary lean area where some of the fuel isn't actually making it into the cylinder until we reach an equilibrium again.
13:34 OK so let's see how the Link G4+ software deals with that.
13:40 And if we enter accel enrichment, we can get our accelerator enrichment set up and you can see as I explained, at the moment my mode is set to off.
13:50 If we double click on this we have the option of setting up the acceleration enrichment mode based off manifold pressure or TPS.
13:59 Now while I have just gone through and explained to you that the parameter that's actually affecting the fuel puddle is the manifold pressure and you could expect from that that using MAP would be a better input, there are some downsides to doing so which I'll discuss a little bit later so generally in this sort of ECU, any ECU that is not providing actual fuel film modelling, we're going to want to generally use throttle position as our input and it's going to ask us to make sure that the throttle position sensor is actually installed and it's spanned correctly.
14:43 Once we've got the acceleration enrichment mode set up, we get access to a range of other options that we can set up.
14:52 So first of all we have a load axis input.
14:56 So when we're talking about our acceleration enrichment, there's a few different parameters that are going to affect the amount of acceleration enrichment our engine wants to see.
15:06 One of them is our engine coolant temperature.
15:11 One of them is the engine RPM and we've got access to deal with those.
15:17 Another thing though, if we're at completely closed throttle like we are here idling and we stab the throttle to wide open throttle, we're getting a large change in our manifold pressure, we're going all the way from perhaps 30 kPa, 35 kPa idle all the way to wide open throttle which would be 100 kPa.
15:38 So we're getting around about a 65, 70 kPa swing in manifold pressure so we need a lot of acceleration enrichment.
15:44 If however we are already driving with the throttle down reasonably hard and we maybe have a 70 or 80 kPa manifold pressure and then we stab the throttle the rest of the way, we don't need as much acceleration enrichment because we aren't moving through such a wide range of manifold pressure.
16:04 So this load axis allows us to control that.
16:07 So for a turbocharged engine, or for most naturally aspirated engines that have a large range of their vacuum so something with stock cams for instance, manifold pressure would normally be the input that I would use for that load axis however you can choose throttle position and if you're using a MAF sensor as your load input you can also choose that.
16:33 Then we've got our acceleration sensitivity so there's two aspects with the way the Link G4 Vi-PEC ECUs do their acceleration enrichment.
16:44 We've got acceleration sensitivity and we also have an acceleraiton clamp, the two work hand in hand so essentially this value here defines as its name implies I guess, sensitivity to rate of change of throttle position.
17:00 So the larger this number here, the more fuel will be added for a given rate of change of our throttle position sensor.
17:09 Now one thing I will note with the Link G4 and Vi-PEC ECUs is these values that we're currently looking at here are pretty close to I believe what we've got here is our default settings.
17:23 And I've actually found through, I've tuned literally 100s of Link G4 ECUs over the years, and generally I find that if the main fuel map is correctly tuned, the default values, particularly for sensitivity and clamp are normally very very close and don't require a lot of work.
17:45 OK so that's one clue there for you as well if you do start with the default values in a Link or Vi-PEC file and you find that your car's acceleration enrichment is poor, and you start finding that you're needing to make wholesale changes to the default values, that for me would be an alarm bell, I would be looking at that thinking you know what, something is possibly not right and it would make me start looking a little bit further into what's going on.
18:14 So as I say, the default values should normally be very close to what we want.
18:20 The next one we've got here is an acceleration deadband.
18:24 Now the danger with acceleration enrichment and I have had this in a lot of instances and I've chased my tail and wasted a lot of time with it is the input of additional fuel is based on throttle position.
18:40 So if you're changing the throttle position, it's possible or probable that we're going to get acceleration enrichment being added.
18:47 Now that's fine if we're genuinely requesting a large change in throttle position but often, particularly with an older car or particularly if we've got a faulty TPS or one with poor earthing, it's not uncommon even at steady state conditions to see the throttle position fluctuate very slightly.
19:07 So what that can cause is acceleration enrichment to be constantly pulled in and it makes it very hard to tune the fuel table and get stable air/fuel ratios.
19:19 As you're chasing against a constantly moving amount of acceleration enrichment.
19:24 And that may not be immediately obvious so I'm going to talk about how we can find out if that's happening.
19:30 So first of all, what this deadband means is that the ECU will ignore the throttle position until it's moved outside of a deadband which is plus or minus in this case 5.5% from the current position.
19:44 So what that's allowing is a very small amount of throttle movement before the ECU will even start looking for acceleration enrichment.
19:53 Generally around about 0.2 to 0.5 is a pretty good place for you to have that deadband.
20:02 If you do set it too large, what you're going to find is that the ECU may ignore requests for mild acceleration enrichment.
20:11 So particularly when you're just making a small throttle change, maybe accelerating slightly in a gear, you may find there's a slight lean hesitation there caused by that.
20:20 So that's the danger if you set that too high.
20:24 Now I will see if we can get a situation where we can show you what happens.
20:33 Obviously this car doesn't have any problems with the noise or anything like that on the throttle position but I'm just going to see if I can show you what actually would happen.
20:46 So you can see at the moment I've got the engine running at 2000 RPM on the dyno again, you can see we're sitting at 15.1% throttle.
20:54 Now if I'm ever suspicious that perhaps I have a problem with the acceleration enrichment happening in the background when I don't want it and the clue for me there would be that I can't get consistent air/fuel ratios at cruise and particularly if the numbers in my fuel table are inconsistent, I'm moving up and down a large amount in surrounding rows.
21:21 What I'm going to do is I'm going to go to the run time values display and if we go across to the fuel tab, you can see in the fuel trims here, we've got our acceleration fuel percent.
21:31 Now if you're sitting there at steady state and you're seeing this number jumping up and down, that means that the ECU is requesting acceleration enrichment.
21:40 And that would be obviously an issue, in which case we can change our deadband.
21:46 Now because my throttle position is so stable, we're not going to really see much of this effect on this engine but what I've done is I've just reduced the accel deadband to 0%.
22:02 Now what I'm doing now, if you look at, I'll just move this up so you can see it.
22:06 If you look at the throttle position, what I'm doing now is manually just moving the throttle very very slightly up and down and what you can see is just the smallest hint of acceleration enrichment numbers being pulled in.
22:22 So if we've got a very bad case of noise, I've just changed that back to 0.5%.
22:28 If I do exactly the same thing, now my accelerating enrichment is staying at zero.
22:34 So if you've got a very bad case of noise on the TPS, obviously it also indicates a problem a mechanical or electrical problem that you maybe want to address anyway but it isn't uncommon even if everything's working to see a little bit of noise on that TPS input and that deadband can be used to help iron that out and avoid unnecessary acceleration enrichment.
23:00 OK so the next adjustment we have here is our accel hold.
23:06 So when acceleration enrichment has been requested, it will hold that acceleration enrichment, it will add that for in this case two injection events and you can double click on that and enter however many you want.
23:20 So the higher the number you enter here the longer the acceleration enrichment will be active for.
23:29 Again, I find that the default values in the Link and Vi-PEC ECUs generally work quite well and I generally don't see the need to change that but if you are seeing a lean spike or more importantly once you've got your acceleration enrichment up and running and it's working reasonably well, if your initial hit of the throttle is reasonably clean and then you see the acceleration, the AFR drift a little bit lean, you could extend the hold time out to 3 or 4 injection events and that may improve it.
24:00 The hold also works however in conjunction with this accel decay.
24:05 So what's essentially happening is when the ECU requests acceleration enrichment, it will add the acceleration enrichment, it will hold it for the set number of injection events and then it will decay it back to 0 and the higher this number, the quicker the acceleration enrichment is decayed.
24:26 Again, typical numbers here generally do a reasonably good job.
24:31 So this is your base setup.
24:33 Now on top of that we also have a few tables available.
24:38 So I'm going to look at first of all our acceleration wakeup load correction table.
24:45 Now this works off the load correction axis that we already looked at, remember we have our accel load axis here and I've got that set up to manifold pressure.
24:56 And this allows us to define how much acceleration enrichment affect we're having at various loads.
25:04 So what you can see here is we've got down at 0, 20 and 40 kPa we've got 1.0 drifting off to 0.8 so essentially a value of 1 in here means that we're having the full effect of our acceleration enrichment.
25:21 You can see though as the manifold pressure increases, we gradually drop that off to the point where at 80 kPa, I'm sitting at 0.3 so only 30% of the requested acceleration will be, acceleration enrichment will be delivered.
25:35 It just comes back to what I was saying before when we're already at a relatively high manifold pressure, we don't need such a dramatic or large increase in our acceleration enrichment.
25:49 OK so that's one of the tables we're going to be needing to tune.
25:53 Again I'm going to sound like a broken record here but usually the default values are reasonably close.
26:00 We also have a cold correction table.
26:03 Remember I sort of talked already about the fact that there are a number of parameters that will affect an engine's acceleration enrichment fuel requirements.
26:13 One of them is our manifold pressure change or our throttle position change.
26:18 One is our airflow or engine RPM and the other is also the engine coolant temperature or more specifically the temperature of the intake manifold and port walls.
26:27 Then you can see here we've got a 2D table set up with, relative to engine coolant temperature and the numbers in here are a multiplier for our acceleration enrichment so you can see up at our operating temperature of 90°, let's just change it at 80 as well, so at 80 and 90°, I've got a value of 1 in there which means that we're using our base acceleration enrichment requests.
26:53 However if we come all the way back down to a cold start of 0 and 10°, you can see that we've got 4.3 and 3.9 in there so we're asking for a multiplication, a lot more acceleration enrichment fuel to be added, based on the cold port walls.
27:12 Now it's important to understand that it's a little bit like tuning cold start enrichment on an engine anyway.
27:20 All of these aspects interact and work together and generally you're not going to be able to tune the acceleration cold load correction, cold correction I should say, until you've got your whole engine tuned, you've got your cold start tuned and you've got your base acceleration enrichment tuned at a normal operating temperature.
27:42 This is where we sort of get into a lot of the parameters in tuning that do take a little bit longer.
27:49 Since I've just mentioned that as well, I should have actually started with this, if we just jump to our fuel table, it's really important to understand that our acceleration enrichment values are being based off our main fuel table values.
28:07 Now this is an area I see a lot of people make the very basic mistake of trying to use the acceleration enrichment to make up for an area of the fuel table that hasn't been correctly tuned.
28:23 So the whole basis is we need to have already tuned the fuel delivery for our engine correctly.
28:30 The base fuel or VE table needs to be correctly tuned, otherwise you're going to have a lot of trouble with your acceleration enrichment, you're going to get yourself into a lot of trouble and waste a lot of time trying to achieve the impossible and using the acceleration enrichment to make up for a lean or a rich area in your main fuel table.
28:54 OK so let's assume that we've got our main fuel table already correctly configured.
29:01 So the two areas that we're going to really be focusing on are our accel sensitivity and then this table here which is our accel clamp.
29:12 So you could think of the accel sensitivity I guess as a coarse adjustment so it's an overall adjustment which defines the amount of additional fuel added for a given rate of change of throttle position.
29:26 But remember we said that the fuel puddle size will depend on our RPM as well.
29:33 So what we have is this clamp table which limits the amount of additional fuel that will be added and this is a 2D table based on engine RPM.
29:43 So you can see here our numbers start at 30% down here at 0 and they decrease to 0% so we're actually not providing any acceleration enrichment at all from 5000 RPM and above and the reason for that is at very high airflow the wall wetting is less of an issue, the air speed through the port is sufficient to really draw that fuel into the cylinder so the requirement for acceleration enrichment is much less.
30:16 OK so now we've got a basic understanding of how all of this works.
30:22 There's another trick that I'm going to just quickly show you that I find quite helpful when setting up the acceleration enrichment and that's if we right click here and we go new view and we set up a time plot and what I'm going to do is add a group with RPM.
30:45 Engine speed.
30:47 Then I'm going to add a group with my lambda.
30:52 In this case I'm just going to look at lambda 1 just for simplicity.
30:55 And then I'm going to add a group and I'm going to add TP main.
31:00 Now despite the fact that we have all of these parameters being displayed in other places on the laptop screen, what I find is when we're tuning the acceleration enrichment, it's handy to be able to see how things are responding in relation to time.
31:19 So the time plot there is a nice easy way of seeing exactly what's going on when we actually made, or across the time when we made a change to the throttle position.
31:34 So now that we've talked about that, let's have a quick look at how we can actually tune this.
31:42 So what I'm going to do, when I do this, it's a bit of a catch 22.
31:48 The dyno is a useful place to set up acceleration enrichment but at the same time a lot of people will make the mistake or at least what I consider to be the mistake of basing their acceleration enrichment tuning solely on the input from the wideband air/fuel ratio meter.
32:11 And my experience is that while the input from the lambda sensor is obviously valuable, that's telling us what the air/fuel ratio is doing in the exhaust, quite often we can see a delay in that input and quite often it can be impossible to get what is truly a completely flat line with no bumps or dips or troughs in our air/fuel ratio.
32:39 And really what's more important than a perfectly flat air/fuel ratio plot across an acceleration event, what's really important, what we're trying to achieve is smooth operation of the engine.
32:52 What we really want to do is make sure that when we go for a sharp throttle change, we're getting very crisp response of the engine, there's no surging, there's no bogging there's no hesitation.
33:03 And on the road that's really easy to feel, all we do is drive the car along at a constant RPM in a constant throttle position, mash the throttle to the floor and we're instantly going to feel if the car bogs or if it accelerates crisply.
33:18 On the dyno we don't get that same sense of feel for how the engine responds so it can dull our senses a little bit so in some ways, while the dyno is useful, I also like to confirm the acceleration enrichment tuning on the road.
33:35 However let's have a look now at what we've got in our little time plot, I'll show you on this time plot and then we'll look at the main logging screen so you can see a little bit larger.
33:45 So what I tend to do is simply run the engine and it's really no different than tuning any of the other cells in either the fuel or ignition tables.
33:57 We want to be central in a particular cell so in this case I've taken the engine up to 2000 RPM and I'm just holding it at a constant throttle position and what I'm going to do is I'm going to start by entering a value of 0 which again will disable our acceleration enrichment, it limits the acceleration enrichment to a percentage of 0.
34:21 I'm going to make a sharp change in throttle and I'll come back to where I am now.
34:30 Let's have a look at that in the logger.
34:35 So we saw exactly what we saw on my original demonstration, actually this time a little bit worse.
34:41 We've gone up to 1.3 lambda.
34:43 Now just to prove the point, on the dyno, I couldn't feel any hesitation or roughness, despite the fact that we're running at 1.3 lambda and I would assume that out on the road I'd almost certainly feel that that's going to cause some hesitation.
35:02 OK so let's start our logger running again.
35:06 Now what I'm going to do is start by adding a value of 20 in there and I'm going to do exactly the same test again and we'll see what our lambda plot looks like.
35:19 So straight away we saw an improvement, let's have a look at it in the larger screen.
35:24 And I'll zoom in a little bit.
35:26 So you can see this time we've got our acceleration fuel request, it's added 12% of additional fuel across that acceleration event.
35:35 Now we've still gone slightly lean.
35:39 But we've only gone to 0.6, sorry 1.06 so around about 6% lean.
35:47 Now this is what I'm talking about with looking at the lambda plot vs what the car's actually doing.
35:54 Now the chances are that based on the fact I've already tuned this engine, that little lean spike that we've got there is actually going to be totally acceptable and it's going to provide really good results out on the road, a nice crisp response from our engine which is what we're trying to achieve.
36:15 However if we were basing our results solely on the lambda data, we'd obviously go well you know what we're still 6% leaner than ideal there and in fact what we'd really want to do is see the air/fuel ratio drop straight down to our 0.90 target so we would go and add additional fuel.
36:35 Let's start the plot running agian.
36:38 And I'll go to 40% so I've actually doubled the enrichment amount.
36:43 I'm going to do another test now and we'll have a look at that.
36:51 Let's have a look at it again on the larger logger.
36:55 So you can see now through the acceleration event, that value of 40% has now, we haven't really gone lean.
37:05 Just prior to me flooring the thorttle, you can see we were running at a 0.97 lambda, so we've gone very slightly lean, it's gone to 0.978 but almost impossible to read.
37:18 However what you have seen now is we've gone a lot richer than we want, 0.83 after the acceleration event so it's added that acceleration enrichment fuel.
37:32 Now this is a case of understanding what you're looking at on the logging screen.
37:41 Now this is the point here where our acceleration enrichment is being added.
37:45 You can see our throttle position, so this is where we've gone to full throttle and the ECU's responded immediately by adding additional fuel.
37:53 You can see our last test we had 12%, this time we've got 19% enrichment being added.
37:58 And you can see that obviously our lambda plot hasn't responded here.
38:02 Sort of a very very brief lag if you like in that acceleration enrichment response and that's why we're seeing that the measured lambda actually falls rich down here.
38:14 Once we're already at full throttle and once our acceleration enrichment event has been cleared.
38:19 So what we're trying to do when we're tuning the acceleration enrichment is we are balancing how rich we, how lean we initially go on the throttle hit, vs what we'll often see is this slight rich hole before everything becomes steady state at equilibrium again which you can see occurs somewhere around about here.
38:42 We've got our 0.89.
38:45 So what we actually see is we see a very slight lean area, almost impossible to read.
38:49 A rich area, then we see it tapers off a little bit lean again and then stabilises.
38:57 So for me what I'm seeing here is the rich area here down at 0.83 is a little bit richer than I'd like and there's two ways we can deal with this.
39:07 One, we could obviously reduce our acceleration clamp value.
39:11 And that will reduce the total amount of fuel being delivered.
39:15 The other thing we can do, remember we talked about our decay rate.
39:20 And we talked about our hold.
39:24 So what we can do is we can adjust these parameters here so if we made the acceleration enrichment decay quicker, that will tend to reduce the effect of this rich area.
39:39 Conversely the other thing we're going to see though is this little area here where it starts to stabilise has gone a little bit lean, that's probably going to get a little bit worse.
39:49 The point I'm trying to make here though is, it's important not to try beating yourself up on having an absolute perfectly flat lambda trace that doesn't deviate from your target.
40:04 So it moves from 1 target lambda to the other as soon as you move the throttle.
40:09 It's not very realistic, you don't need to achieve that and that's not what you should expect.
40:15 The more important thing just to reiterate again is what we're trying to achieve is a smooth crisp response to the throttle input and that's really a feel thing rather than looking at what our lambda value is doing.
40:32 So that's really the important point.
40:35 So what I would do, let's just move back to our acceleration clamp table.
40:40 And what I would do when I'm tuning the acceleration enrichment is I would start with the engine sitting down here at idle and what we can do, idle's really easy, because we can just simply have the engine idling and then stab the throttle.
40:57 That's probably one of the easiest areas to tune the acceleration enrichment in because we don't actually have to be driving the car.
41:06 Now once we've tuned that then we would just simply start moving further through the RPM which is obviously going to require us to be driving the car.
41:16 If we look at the plot that we just got with our acceleration enrichment from idle though, you can see that on that first stab of the throttle, we have gone to about 1.3 lambda.
41:33 So that's a little bit leaner than I'd like to see and that would suggest down here that we could benefit from some additional enrichment.
41:43 Now again if you're working off the default values, just again assuming what I have found to be correct, that the default values are generally a reasonably good starting point for most engines, what we can often do is make wholesale changes or overall changes to the entire acceleration enrichment profile.
42:05 So what I mean by that is we keep the same shape as the default value table which we've got here shown on a 2D graph.
42:13 We keep that same general shape and we can make an overall change to the entire table.
42:19 So in that case let's just multiply the entire table by 1.25 so we're going to add 25% to the table, let's have a look at our logger.
42:31 And see how that affects it.
42:35 OK so we'll just stop the logger here.
42:38 And you can see, remember our last test that we did we went to about 1.36 lambda, in this case we've gone to 1.2 so I've made an improvement, at the same time though you can see what's happened here is we've also gone all the way down to 0.69 so we've gone very very rich.
42:57 And this is the balancing act that's important to understand with acceleration enrichment tuning is if I solely based my decisions on this lean spike here, what I'm going to end up with almost inevitably is adding way too much fuel, way more fuel that I actually need to get good crisp response out of the engine and that's going to affect areas like this where you can see we've gone quite rich.
43:24 Now despite the fact that in our first test that we looked at the lambda plot was quite lean, what I actually still had was a really crisp and fast response to the throttle and that's really what I'm basing my main decision on.
43:42 So I'm focusing more on what I can hear from the engine and what I can feel, rather than solely on the lambda data.
43:51 Now there's a couple of reasons for that, one of them is there is a finite response time to the lambda changing or being measured by the lambda sensor and being sent to the ECU.
44:05 So particularly for these very fast transient responses like that, it can end up giving you I guess almost false information to base your decisions on.
44:14 So that's why personally I like to focus more on what I can hear and what I can feel with the engine and on that basis, what I'm trying to do is use generally as little acceleration enrichment as I can get away with or as I need to get that crisp response.
44:31 So let's just start again and we'll see what that looks like with 0% in there and I'l start the logger running.
44:41 So I've just zeroed that entire table and...
44:46 ...OK now that time we saw, now this is just a perfect example of how much value we should be putting on that lambda input.
45:00 This time here with 0 acceleration enrichment you can see that here we've got no movement in the accel enrichment trace.
45:07 We still only registered a lambda of 1.25.
45:11 You probably wouldn't have been able to hear it or notice it in the webinar but I could audibly hear the response of the engine was not crisp and clean.
45:20 Despite that we've actually got exactly the same measured lambda that we saw when we had our last set of data in the acceleration enrichment table and we were getting good results.
45:33 Now if that all does sound a little bit confusing, I would say that one of the worst places to try and use this data from the lambda sensor is at idle.
45:44 Because we've got very low exhaust flow through the engine and this is where the response or latency in that air/fuel ratio reading is probably at its worst.
45:55 So once you're actually up and running and you've got some RPM onboard, you're going to find that everything's going to start reading and making a little bit more sense.
46:03 But at idle it's quite common to get the sort of lag or latency that we're seeing here which almost renders the lambda sensor input just about useless so particularly at idle, we want to be tuning that acceleration enrichment mainly for audible and response that we can feel from the engine.
46:26 OK now one other thing that I will point out that particularly we're going to see when we're tuning that acceleration enrichment from a big stab on the throttle there from idle which we've just done, now remember I said that the fuel table needs to be correctly tuned before we can tune the acceleration enrichment.
46:51 That's really critical.
46:53 When we're doing a large stab on the throttle, from idle, I'll just see if I can capture it, I'll just log a little bit of data here.
47:05 OK now let's have a look at what exactly happened there, when I went to full throttle, we can see the pink trace here has gone to 800 RPM and 0 kPA.
47:19 Now that's an area of the fuel table that almost certainly we're not going to be able to access and we're not going to be able to tune correctly on either the dyno or the road.
47:31 You simply can't hold the engine at 800 RPM and wide open throttle and do a really good job of tuning all of those areas down at sort of 500, 800 RPM.
47:42 So what we should have done there if we've done our job correctly is we would have followed the general trend or shape of the fuel table and followed that down through these areas that we can't really access and tune.
47:55 But even having done that, it doesn't say that we're necessarily correct.
48:00 So this is only an idiosyncrasy just in the very low RPM, the idle areas of the acceleration table.
48:08 If we're really struggling to get good results with the default values in the acceleration clamp table, we may find that we need to actually come back to our main fuel table and address the values that we've got in our main fuel table.
48:23 And remember this really comes back to what I was saying, if you start needing to make really large wholesale changes to these default values, particularly in the Link and Vi-PEC product where my experience has been that the default values are really quite close in almost all instances, if you need to make these wholesale changes, generally that suggests that something else isn't right and we need to look a little bit deeper.
48:48 Obviously once we're past idle and we're into the areas of the main fuel table that we have been able to properly tune in steady state on the dyno or out on the road, then we shouldn't have those issues.
49:01 OK so the, back to our accel clamp table.
49:06 Now remember the values that we had in this table ended up being at 0 by 5000 RPM.
49:15 So the technique that we use to tune it, if we're on the dyno it's really easy, we just run the car at each of the RPM zones, go from a very light throttle, steady state condition, smash the throttle straight to the floor and look at the response, feel the response and make changes accordingly, then what you're going to find is you'll get a general trend with this table, is as the RPM increases, you're going to find that you need less and less fuel.
49:44 And again in my experience, generally somewhere around about this 5000 RPM point you will find that you don't need any acceleration enrichment and above that point just due to the high air speed and the turbulent nature of the airflow at high RPM, the values in the table can be set to 0.
50:04 OK we'll get into some questions now so if you do have anything you want to know, this will be the time to ask them.
50:13 Hopefully that has cleared up some of the issues around acceleration enrichment tuning.
50:19 I know that it is an area that a lot of people do struggle with and I think that there's 2 areas mainly that cause, let's say 3 areas mainly that cause these problems.
50:33 The first is, and this will be almost certainly the biggest problem is people trying to use acceleration enrichment parameters to make up for a poorly tuned main fuel table.
50:46 So that's your very first place to start is to make sure that the rest of the engine is correctly mapped before we start trying to set up our acceleration enrichment.
50:55 The second problem people have is a lack of understanding of why acceleration enrichment is needed and what they're actually trying to achieve by setting the values in the table.
51:08 Hopefully this webinar has answered some of those questions and given you a better understanding of what you're trying to achieve.
51:15 My whole point with tuning in general is if we understand the principles behind why we're doing something, generally that's going to allow us a better understanding of how that particular aspect of tuning works and if we understand it we're always going to be able to make more educated decisions on the changes we need to make..
51:35 And my experience is that's going to allow us to do a better job of that particular tuning situation.
51:41 So that's the second problem and then the third issue which I've discussed as well is people trying to base their acceleration enrichment tuning solely on the data from the lambda sensor.
51:54 And while obviously there is some value there, we do want to watch what's happening with the air/fuel ratio, much more important in my opinion is to tune until the engine's response feels and sounds crisp, that's much more important than a perfect number on a lambda meter.
52:15 If someone brought me a car and said look how good a job I've done of the acceleration enrichment tuning, and I drove the car and the lambda tracked perfectly between one target and the next but the car hesitated or jerked or wasn't responsive to my foot, that's what I care about, I don't really care about a number on a screen.
52:34 So that's really important to understand, only use the lambda as a guide and understand that particularly with these transient response tuning situations, there is the possibility that the response you're getting from the lambda sensor may not be entirely useful or it may be a little bit misleading.
52:56 OK we'll have a look at we'll see if we've got any questions.
53:00 Jono P's asked, I have a question, should the acceleration enrichment be turned off when initially tuning the main fuel map? OK well it really depends on whether or not you've got problems with noise on the accelerator, the TPS input.
53:20 Now as I discussed, that is an issue, now I don't see it very often, probably might see that raise its head maybe once every 6-12 months, it's quite rare and I do tune a lot of cars so the fact I'm only seeing it that infrequently, it is very rare.
53:39 The problem is when it does come up, it can be quite frustrating and quite time consuming to figure out what's going on, that's why I've added that in as a tip if you have problems getting good control of air/fuel ratio.
53:52 It is something that is always worth having a look at and making sure that your accel enrichment isn't coming in.
53:57 So when you're tuning in steady state the very term steady state's a bit of a giveaway there, we should be holding the engine steady in a particular cell before we make changes to the air/fuel ratio, to the fuel delivery.
54:11 So hence the acceleration enrichment should be having 0 affect.
54:15 So in that instance, no there should be no need to turn the acceleration enrichment off.
54:21 It is however, if you're tuning out on the road, this is one aspect to keep in mind as well, if you're tuning out on the road, or you're using an auto tune function, or you're logging or creating a histogram of air/fuel ratio based on three dimensions, air/fuel ratio, vs RPM and load, it's important when you're trying to create that data or datalog that to make very smooth changes to your throttle position and move smoothly through the map because if you do use sharp changes in throttle position, you're going to get acceleration enrichment coming in which will affect momentarily the air/fuel ratio and that can really mess up your logs.
55:02 Again it's just important to understanding how that is working, what it's doing and how it's actually going to affect the results you get.
55:11 We've got a question here that Ben's added, when would you use manifold pressure as load input.
55:17 OK generally I very seldom use manifold pressure as a load input for the acceleration enrichment strategy.
55:29 Now the reason for that is twofold, first of all, the throttle position is generally a mosredirect measure of the driver's request and I know that right at the start I talked about the wall wetting and how it's actually the manifold pressure that affects the size of the fuel puddle.
55:52 However what we also find is that generally in the manifold pressure signal that the ECUs receiving will be filtered and it can be filtered quite heavily which means that there is a very minute response time to that manifold pressure changing.
56:08 So that can affect the ability to use that parameter, that input on an ECU that isn't designed specifically around a wall wetting strategy.
56:19 It can affect the ability to use manifold pressure as a really effective parameter and generally throttle position is the one thta I would use.
56:29 So when would you use it? The actual question, in some instances, and it's fairly rare, very rare in my experience, we may find that we have an engine that we're tuning that doesn't have a throttle position sensor.
56:42 Really here the correct fix would be to fit a throttle position sensor but if we have no option, obviously we can use manifold pressure as that input.
56:51 The other one would be occasionally I have had an instance where the throttle position sensor was so noisy it was almost useless for acceleration enrichment and in that case again really the fix is to correct whatever's wrong with the TPS either replace the TPS or check the wiring.
57:12 However if you needed to, you can get around it using manifold pressure.
57:18 Ben's also asked, does it matter for response time, where abouts in the exhaust the sensor is? Yeah I've talked a little bit here about latency or lag in that air/fuell ratio sensor's response and it is quite critical and again this really comes back to how much value you place on what you're seeing from the air/fuel ratio sensor.
57:42 Particularly remember at very low engine speeds, the exhaust gas, the mass flow through the exhaust system is obviously also low, we just simply don't have much air and fuel going through the engine and hence it will take a little bit longer, there'll be more of a delay in the changed air/fuel ratio actually making it to the sensor, being registered and read.
58:07 That is obviously made worse if you're measuring the air/fuel ratio a long way back through the exhaust system.
58:14 My preference is always to use a sensor mounted as close to a collector or turbo outlet as we can to ensure that we don't have any sort of transport delays in the air/fuel ratio being read but sometimes that's not possible and again it just really comes down to understanding what's going on there and how that will affect your tuning.
58:39 Now one other thing that I did want to mention that Ben has brought up there just regarding manifold pressure is I have struck problems in the past with getting really poor acceleration enrichment response from an engine and now let's just go back to our fuel table in the laptop software, now while our acceleration enrichment request is based off throttle position in this instance, our main fuel table, our main load axis is still manifold pressure, in this case manifold gauge pressure.
59:20 Coming from the internal MAP sensor inside the ECU.
59:23 Now remembering that the acceleration enrichment is applied based on the value from the main fuel table.
59:32 It's still important that we have crisp and very very quick response from our manifold pressure sensor.
59:39 Now this is a problem I've only struck a handful of times over 14 odd years so it's very rare but it is worth discussing in this webinar in that if we have a very very long vacuum hose to our manifold pressure sensor, or there's a partial blockage in that vacuum hose, what that can to is it can cause a delay in the vacuum signal changing.
01:00:06 So what that means is that we go from idle to wide open throttle and instead of our manifold pressure instantly tracking to 100 kPa, to atmospheric pressure, as soon as the throttle is open, we might see a 10th of a second or even less of a delay in that vacuum signal moving, it moves slowly so what happens is in our fuel table, instead of jumping straight down to this area here, what we'll see is that the throttle position goes open but we actually see a slow drop in the manifold pressure down to atmospheric.
01:00:42 What that means is that for at least the first portion, the ECU will be accessing these values down here even though the actual manifold pressure would be up here.
01:00:53 And obviously that means that its fuel delivery is incorrect.
01:00:57 Now again this comes down to that situation where if we don't realise that's what's happening, we end up in this strange situation where we're trying to account for an actual problem with the ECU installation using acceleration enrichment and I guess the one takeaway, and this can at least be applied to the Link and Vi-PEC acceleration enrichment, the one takeaway or most important takeaway from this webinar is if you are finding that you're needing to make these wholesale changes to those parameters, then there's a chance that you need to look elsewhere because there's a chance that you've got another problem that you're trying to tune around.
01:01:37 It's all about being aware of these potential issues and understanding what to look for.
01:01:41 So if I think I've got, first of all, this sort of a problem with a slow MAP sensor response won't actually show itself up in steady state when we're tuning the main fuel table because we're moving so slowly between zones.
01:01:56 And that delay that I'm talking about in the MAP signal is generally a 10th of a second or perhaps less so it's very small and we won't see it in steady state.
01:02:07 What we're going to find is when we're trying to do the acceleration enrichment, that's where it becomes most obvious.
01:02:13 We're also going to get some really funny transient problems, hesitations, poor drivability when we've actually got the car on the road or the track and the way to look for that is simply, I'll actually start the engine and we'll show you.
01:02:27 So you can see obviously our little yellow box tracks our current site that the ECU's accessing and if I just mash the throttle straight to the floor, you can see that there's an instant jump, there was an absolutely instant jump to 0 kPa here, to our stmospheric pressure, there's no hesitation.
01:02:53 So if I think I've got that sort of a problem, what I'd be doing is I'd be looking at this and I'd be logging it and if we look at the logging, we'll show you exactly what we're seeing here.
01:03:03 So we can add in another group and another parameter and we'll add manifold pressure.
01:03:12 And let's move that up so we've got it right beside our throttle position.
01:03:18 And you can see that this is our throttle position here, unfortunately they're both blue traces but you can see that the manifold pressure tracks to 0 kPa gauge immediately as soon as we go to full throttle.
01:03:35 Otherwise what we'd see is a slower rise and we would have been at full throttle for a 10th of a second or so, maybe less and our manifold pressure would still be gradually rising.
01:03:47 So that's something there to look for.
01:03:49 Alright I'll see if we've got any other questions.
01:03:52 And we haven't so we'll leave it at that for today.
01:03:55 Hopefully that's given you a little bit more insight into what is actually going on behind the scenes with acceleration enrichment.
01:04:03 As I mentioned I'll add into the chat box the link to the Mega Manual section from MegaSquirt on fuel film modelling so you've got a little bit more of a resource there if you want to do some further reading and for those of you who are watching this in the archives at a later point, we'll put that into the forum as well If you do have any further questions on accel enrichment, please ask the in the forum and I'll be happy to help you out there.
01:04:31 Thanks for joining us.