PI : DI Ratio Discussion
 

Seems to be plenty of discussion of PI : DI ratio about but I thought this would be a good place to chime in on people's thoughts and if they have tried anything successfully. I think both NA and FI applications are worth a shot as there may be different benefits for each. I think there may be a more optimal map over the OEM for power, but I also get the impression that there are benefits to running PI at various stages of the rev range. I guess this will focus on WOT application as I doubt many will change the lower load regions but any discussion is welcome.


Below is a picture showing the OEM map in WOT load regions between 2-7.4k. Yellow is the % of Direct Injection, blue is Port Injection. As standard it is quite heavily biased toward DI but IMO there is a reason that the OEM decided to re-introduce the PI at higher RPM. There is definitely capacity to run more DI than that. It may be due to DI firing angles and open time requirements that a percentage of PI is required, as I have had previously hinted to me.
https://dl.dropboxusercontent.com/u/...ersClub/PI.jpg


So is a higher % of PI at the very top end beneficial and allows you to run more timing? If you're running boost and aren't running as much timing, is more DI better at controlling knock? Discuss.....

Subscribed.
Injection ratios and cam timing are two areas I've heard very little about on this forum. It will be good to hear some ideas and experiences from those that have played with it.

It's definitely something I'd like to understand more.


On a log I've compared recently, I've had very slight FLKC appear at 6.8k after the PI ratio was decreased despite running the same timing. However, the IATs are also 10C more on the most recent log so I'm not willing to put this down to injection ratio alone.


It may also be worth noting that @Shiv@Openflash uses the OEM ratios for the OTS maps. I've also heard that one tuner relies almost solely on DI for boosted applications and only introduces PI when the DI capacity is near it's limitations. It'd be nice to see some results from actual testing, and I recommend that if you do fancy testing that (a) you know what you're doing and (b) that you do it in similar conditions.


I am running a non-standard ratio map on my car, but as I've not had time to change and test for comparison sake I can't even be sure there's much/any benefit to having changed it.


This is as it currently stands:
https://dl.dropboxusercontent.com/u/...lub/PI%201.jpg


My line of thinking is that the DI will be used to help prevent knock/cool cylinders in the mid range and at low RPM with high load (20% rather than 50% OEM). When the RPMs get higher and the DI capacity is getting near a "safe" limit then I re-introduce PI at a slowly increasing rate, but hit near stock levels at peak RPM as the ignition timing is increasing.

@nelsmar might be the most knowledgeable about leaning heavy on the di until people start tuning the di only fa20det.

The DI on the FA20DIT differs from ours?

It doesn't have pi at all. so tuners will be getting better at utilizing di effectively since well...they have to on the fa20dit.

I believe the DI system on the WRX FA20DIT is subaru design where as ours was toyota design DI/PI system.

Yes. No pi like the pi/di 4ds system we have.

Yeah, we know.
I was wondering if it is even applicable since different DI setup.

An SAE research had shown that DI is much better at E85 than PI.
Wouldn't it be the same for Pump fuel as well?

@jamesm

... ohh wait

some good info here

http://www.ft86club.com/forums/showp...8&postcount=22

whole thread

http://www.ft86club.com/forums/showthread.php?t=3172

We know the DI start of injection, assuming those are reverse engineered correctly (the numbers really represent the physical degrees), but we don't know the PFI end of injection or the DI end of injection timing.

We don't know the combustion parameters at those speeds. We don't know the combustion speed, the combustion phasing, the combustion stability, etc.

We don't know the smoke & particulate emission, or the wall wetting.

I'll speculate anyway. As I've said before, this stuff was mapped on an engine dyno in mostly steady-state conditions. DI is pretty much always better for cooling the intake charge, but the problem is forming the mixture within a smaller crank angle window, especially with side injection like this.

Here's what I mean by that.

http://www.ft86club.com/forums/attac...1&d=1342629544

All you've got is part of the intake and compression stroke to spray the fuel with DI. That's why people are like "omfg duty cycle so low!!1! on DI engines" But if you spray during the expansion or exhaust stroke you are wasting the fuel, because it just leaves the engine unburned. With PFI, you can spray really the entire 720 degree cycle (100% duty). The mixture can vaporize in the intake port and doesn't rely on charge motion in the combustion chamber as much.

http://i597.photobucket.com/albums/t...ps47d69c9b.jpg

Now if you look at that above DI firing angle map, you can see it really ramping earlier and earlier at high speed, just as the PFI is coming on (higher number = earlier, and 360=TDC intake). Normally your SOI gets earlier to give more time for mixture formation to improve combustion stability, but also to limit how late the injection sprays in the cycle.

Well if I go too early I impinge the piston down causing smoke, and if I go too late I wet the liner causing oil dilution, at least on a side injection engine. At high speed though the pulsewidth is so long that you could be "trapped" and can't move the injection event later or earlier without impinging on something. That's where you dial in the port fuel. We can't see the end of injection timing for the PFI, but I suspect it is closed valve injection. So the port fuel has a long time to mix, helping combustion stability, and the charge cooling is done with the DI. Normally you can get a little charge cooling at high load with open valve PFI injection, but then you're throwing more liquid droplets into the cylinder that don't vaporize as well.

Personally, if you want the most power I recommend you run as much DI fueling as you can get away with, until the high pressure fuel pump can't flow anymore. This is due to the knock benefit. Most people on this subforum don't care about oil change intervals and particulate emissions.

Summary:

DI= more knock relief, potentially poorer mixture formation, more impingement on the piston and cylinder walls, flow limited due to injector flow rate and high pressure fuel pump flow capacity

PFI= more knock prone, more likely to form a more homogenuous mixture with better combustion stability, easier to flow at higher rates (just buy aftermarket PFI injectors and low side pump), not nearly as sensitive to fuel pressure or injection timing

First of all, great post. Thank you for your input. I am trying to get my head around all the info here and the other threads linked earlier. One thing I was curious about is, what exactly does "better combustion stability" mean?

@jamesm (banned, don't wait for a reply) stated he was able to find an extra ~10whp in the most knock prone areas just by increasing the DI %age on 91/94 octane. On e85 it isn't as big of a difference (or a difference at all) because of how knock resistant e85 is.

You'd think an engine makes some amount of torque but that's constantly jumping up and down. It's usually worst at cold start, when pulling spark to reduce knock, and part load where it's being choked down with internal EGR for fuel economy. The ugly zigzags you see on a dynojet sheet are sometimes noise but there really are torque fluctuations. If they're bad enough you'll get hesitations, surging, and misfires.

If one were to manually control an engine on an engine dyno, holding speed with loading, one might see all sorts of fluctuations in torque and combustion pressure. Then one's software could calculate an instability % which corresponds to these fluctuations in torque or combustion pressure. It might look like this, with the bigger changes corresponding to same change in some parameter you are tuning...

http://www.ft86club.com/forums/attac...1&d=1407427337

so is the torque really say 100 lb/ft or is it 100 +/- 5% , or 10%? Is it bad enough that a driver could feel it, or even bad enough that you start getting misfire?

The thing that's relevant to the debate is this: DI can cause poor combustion stability if it's not mixing correctly. However, it relieves knock, and that means more advanced spark and with the right heads probably faster combustion speed.

More advanced combustion due to knock relief improves combustion stability. That's a + for DI.
Poorer mixing due to injection timing and mixing time hurts combustion stability. That's a - for DI.
More retarded combustion due to spark retard for knock hurts combustion stability. That's a - for PFI.
Better mixing due to vaporizing the fuel in the intake port and having more time improves combustion stability. That's a + for PFI.

I can't even keep up with this forum and who's in and or who's out. It's like a middle school clique sometimes.

For E85 it's going to depend on if/how boosted the engine is and how retarded the spark/combustion was on normal E10.

I have grown to LOVE the split DI/PI system on this car. It is more work--yes, but in practice it works incredibly well. Out of all the things we can tweak on this car to make power, the DI/PI ratios and timings are the least touched. The engineers knew what they were doing here, and any tuner who actually spent time trying out the different variations empirically, will be using something very similar to OEM.

The reality here is that DI works BEST by injecting in a very narrow window, just as @arghx7 outlined. Too early and you're blowing out the exhaust, too late and you're still injecting when the combustion event goes off. Another factor is that you don't want to inject too far away from the spark, that's why it isn't just 370 across the board.

We've seen some tuners swear by DI only, and others use 50/50 split or even full on PI in really high HP applications. I don't think either one is a good solution.

Think of it this way: DI fuel delivery will always be the optimal method, UNTIL you make the mixture too unstable or wash the cylinders. Wash is unlikely, but once you approach 7ms injection times (250+whp with e85) you begin to lose some of that benefit. The mixture becomes less stable. Between about 300whp you cross over the threshold where 35% PI has no negative effect. 20% PI on OEM tunes? We have found absolutely no repeatable HP gains, as in 0, like none-what-so-ever. Despite changes in timing, leaner or richer, more or less aggressive cam timing, or different injection timings...I think one car actually made 1hp over 20% PI mix...which is within the margin of error between runs.

So in reality, there's just no reason to overstress the DI system with long IPWs. Take advantage of the dual injection, max out the DI to about 6 ms (this way in sub freezing temperatures you have headroom) and then run port at no more than 80% duty cycle, or about 15ms.

Also, as a comparison the DI equipped FA20 in the 15 WRX...we can't run half the timing on those! DI only we hit the knock threshold at a much lower HP using DI only. Can't explain it and it's still a very new system, but it made me wish for the BRZ setup having tuned it.

Here's an exercise. Go into your DI firing angle map and set the whole map to 260. Take some datalogs and see how it runs.

You can't really see this accurately without drilling the heads for combustion transducers, but keep in mind the difference between spark timing, combustion phasing, and combustion speed.

I'll tell you exactly why you can't put much timing into a FA20DIT besides some situational knock tendency. It burns WAAAAY faster with the high tumble port and combustion chamber shape. Put it into a flow bench and you'll see it flows like crap compares to a BRZ engine.

So I divide my combustion into 4 basic parameters:

1) the time of spark, which you're all familiar with
2) the burn delay in crank angle degrees between 0-10% burn
3) the location of 50% burn - this is the combustion phasing!
4) the bulk burn duration - crank angle between 10-90% burn

with a modern DI turbo engines you'll see high tumble ports. This makes the 10-90% very fast and helps mixing, but they flow pretty bad.

So if I fire the spark at 20 degrees BTDC on a slow burning combustion system it may still burn 50% of the mixture at 8 degrees ATDC (considered to be MBT). If I fire the spark at 5 degrees BTDC on a fast burning combustion chamber it may still burn burn at 8 degree ATDC.

The difference is the combustion speed and the spark, not the phasing.

The reason why an FA20DIT doesn't take a lot of spark is in part because it doesn't need a lot of spark. It burns fast, but the heads don't flow like a BRZ's head. If they didn't have the fast burning head, it would run like crap at part load (poor combustion stability), have high CO and O2 emissions, and probably knock a lot more at full load.

Let me point out that combustion stability perception is kind of relative. When you see a V8 camming, it's having poor combustion stability at idle and low load. It still sounds cool though, even if it's burning poorly.

Likewise when you drive down the road in a modern engine you will have higher instability at part throttle due to dilution with AVCS induced overlap. However it's not unstable enough to notice usually, and you are getting better fuel economy.

Bump for any more tried and tested ratios etc?


I have to say that I prefer a bit of extra control at the top end of the rev range, rather than the same at 5.2k and over. I have tendency to ramp the DI in more gently.


The other thing that has occurred to me is that depending on AFR and induction, that actual percentages will vary as to what's ideal. An example is that a FI car with a lower % PI could still run more PI volume than a NA car with a higher %.

Isn't the shorter length of time offset somewhat by the violence of the expansion of fuel under that much pressure being released into an open space? Wouldn't that help with the mixing?

I haven't seen it put as plainly as this (maybe it was stated in technical terms and I missed it) but if you boost this motor you need to add more fuel. If the DI is effectively maxxed out due to restrictions of the mechanical pump, you can only increase the amount of fuel coming through PI. In that case, a boosted engine has to have the ratio shifted a bit towards PI compared to stock, doesn't it?

As @moto-mike said, its not so much the percentage as the IPW or injection time. On my SC car I run a lower percent PI than stock yet my PI injection time is higher than stock. Think of it like this, 20% of the fuel at 12:1 AFR will be using the injectors less than 20% for 11:1 AFR. Now, let's put aside boost and needing both sets of injectors to fulfill the actual fuelling requirements, if we assume that a certain amount of PI may be required to give a homogeneous mixture, does that sit as a percentage of the mix or as a certain volume of fuel? Again, I understand that this may be over simplifying

Having gone an re-read all the old posts on here, there must be a reason why running a 20:80 split was chosen. I have seen little benefit of leaning heavily on the DI on a NA car. I have taken Mike's advice though on my car, running up to 6ms DI and then having it pegged there until redline seems to.work nicely.

I can see that. Increasing the PI pulse length would increase the amount of fuel per injector cycle. I wish I had a better understanding of how the ECU tries to meet the percentage target. With all those other tables to reference, what exactly is it doing by way of reading what table and then physically activating injectors, cycling pump valves, etc. What does it take to satisfy it that the target percentage was met? If you change a pulse length, does it take that into account? Times like this you really want the programming code.
From what I've read, you want some fuel hitting the back of the intake valves. This not only helps keep them clean, but I bet it has a cooling effect as well. In a high rpm, high compression motor, you need to consider the health of the valves. This could explain the PI in the upper rpms when the DI can handle it.
As far as the benefit of DI in an NA motor, again, high revs, high compression = high heat. The DI has a definite cooling effect, which not only allows for more aggressive tuning, but could also contribute to the long term health of things like valves and piston tops. They don't spray the tops of the pistons just to create a particular combustion pattern. Although that is extremely important, it also has to have a cooling effect on the piston top. It may be small, but every little bit helps.

Yes, if the rail pressure is high enough, and the spray targeting is appropriate for the injection timing. Raising the rail pressure increases parasitic losses though. Even good spray targeting has a range of injection timing where it won't significantly impinge on the cylinder wall on piston top.

Remember that knock is a bigger problem at low speed and high load, and that's where the most DI percentage is being used. At low speed there is more time for the gases at the end of the combustion chamber to auto-ignite.

Solenoid injectors have a linear and non-linear range of operation. On modern PFI with saturated injectors (voltage controlled) it's simpler because the current and voltage control are simple compared to the old "peak and hold" injectors from the 80s which used a burst of current to open those clunky 80s needle valves and then a lower current to keep it going. The old peak and hold control lives on in solenoid direct injection, but it's more complex (see below).

So basically on modern PFI there's a bunch of stuff in there to translate pulsewidth into mass flow, based on the characteristics of the injector and on the environmental conditions (air temperature, water temperature, etc).

The direct injection is more complicated because they are run with peak-and-hold current control and booster voltage. They also are much trickier to control at low pulsewidths (typically under 1 millisecond). You have to have an injector flow vs pulsewidth model in the code with a bunch of look up tables populated by bench tests and engine dynos for the most part. The basic intellectual property is owned by the supplier and then adapted to the application.

http://www.ft86club.com/forums/attac...1&d=1414421876

With a lower flow DI injector, you flow less and don't need low pulsewidths so much. You spend less time in the "ballistic" non linear range, where the valve opens and closes like you are lobbing a Hail Mary pass at the end of the Super Bowl.

http://www.ft86club.com/forums/attac...1&d=1414421876

So when the DI injector is being driven, you have 4 basic phases:

1) booster voltage phase -- a bunch of current and voltage applied to open the injector quickly

2) high current holding phase - current dithers at a high level as the injector reaches full open

3) lower current holding phase - the majority of the on the time when long pulsewidths are used. the split between high and low current hold has effects on injection noise and behavior of the spray pattern, especially with respect to emissions

4) closing phase - sort of the inverse of the boosting phase

So the ECU signal is the pulsewidth, yes, but there are all these separate sub-operations going on within that pulsewidth that are in separate modules of code with a bunch of lookup tables and settings controlling them. The fuel mass model can calculate flow taking into account all this complexity.

It sounds more and more like unless you have ridiculous research resources at your disposal you should leave the DI portion alone and mess with the PI and the percentage between the two. Unless there are areas that are well known to be optimized for, say, clean burning or economy and the way to change them to be optimized for power is well established.

Just wanted to say...
This has been a refreshing thread to read, since I agree with the technical views of both arghx7 and moto-mike!

We've developed several custom maps for the BRZ and FT86, for both pump gas and ethanol. When I get a gap, I shall share some do's and don'ts.

Okay, this is coming from left field, but this thread plus another thread where a guy talked about running 100% ethanol got me thinking. For the guys running boosted engines and really pushing it, would it be advantageous to have complete control over how much ethanol is mixed and when? If you completely isolated the PI and DI systems; separate tanks, pumps, and lines, you could fill the PI with 100% ethanol, and using these tables have precise control over how much ethanol you are adding to the fuel. This would theoretically give you the ability to really fine tune the system for engines pushing the envelope. Gasoline burns with more energy, but the ethanol raises effective octane. I find myself doubting it would be worth the trouble, but it is a somewhat interesting thought experiment.

^ Ford ran a bunch of experiments with just such a setup. You're basically injecting ethanol as a type of octane booster or racing fuel. If you just want to make power in a simple way (don't care about minimizing E85 consumption) I don't see much advantage to such a complex system.

curious how did you adjust this di ratio if the table only goes to 5200rpm? i cant seem to find out how to do it.

Change the axes ;)

:eyebulge::thumbsup: genius, i always forget you can do that on romraider lol. might you know when do the "warm" and "hot" tables take effect?

It's best just to do them all the same. They're the same from the OEM.

Port injector time
 

I've had a look at port injector duty cycle calculation.

Will post MatLab script later on the MatLab thread.

Calculation:

Cycle time ms/cycle
= 1000 ms/s * 60 s/min * 2 rev/cycle / RPM rev/min
= 120000 / RPM ms/cycle

Injector duty cycle %
= 100 % * Port_IPW ms / Cycle_time ms
= Port_IPW * RPM /1200 %

With stock port injectors and stock PI : DI ratio map I am up to about 23% duty cycle at redline and 1.2g/rev load.

Direct injector time
 

This one is interesting and not much info available. I have made quite a few assumptions here in the absence of hard data on Toyota's DI system. Do your own verification before using these to tune your engine...

Since DI only has one rev to work with for fuelling rather than a whole cycle: at an 8000 RPM redline, there are 7.5ms per rev rather than 15ms/cycle that the port injectors have to work with.

I looked at a number of relationships betweek DI_IPW and load in log lines with Port_IPW=0 and found the most linear relationship with the commanded fuel vs DI_IPW*FuelPress:

Commanded Fuel fuel g/rev
= Load air g/rev / Commanded_AFR air/fuel

From what I understand, most DI injectors have up to a 5ms pulse width time. Flow is also proportional to fuel pressure. Luckily for us, the peak DI IPW is 5ms and peak DI fuel pressure on the GDI flow table is 20MPa. Multiplying the peak DI (5ms) and peak fuel pressure (20MPa) essentially gives us a 100 % value that we can use directly.

Scripts will be posted in the MatLab thread.

Figures:
1. Relationship between DI_IPW*FuelPress vs Commanded fuel
2. DI Duty using stock DI system and PI : DI map

Interested on the DI cycle. I run far more DI than the stock setup, my S/C runs about 5.8ms on the ECUtek logs at the higher RPM, compared to ~3.5ms for stock. Is there any way to calculate this at all?


You can log the PI duty on ECUtek although I currently don't....

Ford called this the Bobcat. I recall having a conversation with a coworker that some turboed v8 prototype motor was making 1000 lb ft on a dyno. It was supposed to be an alternative to large diesels, avoiding the costs and packaging of SCR and Urea Injection (IE Scorpion).

http://www.greencarcongress.com/2009...-20090426.html
http://news.pickuptrucks.com/2009/06...el-engine.html

There's some MIT spinout that was working on it, but their webpage is pretty sparse: http://www.ethanolboost.com/

You have to start thinking in terms of crank angle degrees and injection window. On PFI, the fuel can build up on a closed intake valve or spray into an open intake valve. For open valve injection, we can say you've got the full 720 degrees to inject PFI fuel. That's 100% duty cycle. The start and end of injection don't matter at that point.

For DI it's different. In one sense, pulsewidth doesn't matter as much assuming a fixed rail pressure. You're limited by your injection timing window. For max injector flow you can't spray when the exhaust valve is open, or rather you can but it will go right out of the cylinder and be useless. You've got from the start of injection time until a maximum of when the spark plug fires. So if my SOI is 320 BTDC, and my spark is 20 BTDC, I've got 300 degrees window (oversimplifying a bit). The number of corresponding milliseconds varies with the engine speed.

For DI: log your SOI, log your milliseconds duration, calculate your crank angle duration from that, and then calculate your end of injection. The closer your SOI gets to 360 BTDC (top dead center Intake), the more you are hosing down the piston and/or cylinder wall, getting oil dilution, smoke, and other kinds of problems. The same applies to the EOI: the closer your EOI gets to 0 degrees (top dead center Firing), the more you are hitting the piston and/or diluting the oil.

But even if you don't give a crap about hosing down the cylinder and the piston, you still can't throw fuel out the exhaust valve. So your DI will never get anywhere close to 100% duty, because my exhaust valve is open for 200+ crank angle degrees. From what I've seen typical stock tunes don't run more than 200 degrees injection duration on DI due to concerns about smoke and oil dilution, but as I said that can be stretched.

whatever the final technical merit ended up being, basically government subsidies for ethanol and fuel economy credits are being cut so there was no economic incentive to go forward with it.

I haven't looked at SOI parameter, but it does make sense that the injection window would be (SOI - spark timing). We'd have to assume that SOI and spark would be referenced off BTDC in the ROM.

Does anyone have the RAM address for SOI?

Kodename47 has had up to 5.8ms on the DI injector - anyone else gone higher?

I never heard about the outcome. I think the technology is just a little too early, and more dependent on future diesel regulation and diesel after treatment technology. The D35 GTDI was a happy accident from R&D, so who knows... :bonk:

....Then again, we have sub $2 gas, OPEC in shambles, ME in flux, and will have a new administration in 2 years, etc. :cheers: