Direct injection/port injection explained?

[Shankenstein]

Originally Posted by arghx7

They are describing a very late port injection timing, which is normally terrible for HC emissions... I don't see the cat lightoff strategy on the FA20 surviving the upcoming emission standards. There's too much HC and particulates to overcome. That's one of the reasons why Audi uses DI only.

Do you see Subaru choosing a different playbook of software tricks... or a diesel-like aftertreatment setup?

I'd actually be OK with a particulate filter and NOx trap, but they cost money. Restructuring the combustion chamber and timings could also just target different temperature/pressure ranges. That would prevent the HC from forming in the first place.

Originally Posted by arghx7

If the cat doesn't light off in time, or the engine-out emissions are too high, the engine doesn't get certified and doesn't go to production. So I'm not sure what the original source of that claim is from.

Just alot of complaining from engineers higher up the R&D food chain.

Some day in-cylinder pressure sensors will be cheap enough to obsolete oxygen sensor feedback.

Originally Posted by arghx7

There are a bunch of tricks in the bag. Some involve lean burn and expensive o2 sensors that light off very quickly. Some involve smog pumps (secondary air injection) or HC trap catalysts. Some involve tumble control valves, or expensive DI systems, or valve timing tricks.

As simple as tumble/swirl control is... I'm expecting that the improvements to ANSYS and other fancier FEA programs will yield major gains in this regard. Piston shaping and dual valves are "easy" technology to develop if you can guarantee the resulting performance doesn't yield gains that bite you somewhere else in the operating range.

Originally Posted by arghx7

Sweet! Injection timing tables exist.

That 370 deg is quite interesting. Not sure if they're using fuel to cool certain parts or if they're forcing the maximum value possible.

During my time perusing blue oval code, they did the latter a few times. Generally when they wanted something fully open or closed, you'd see weird values.

[Wepeel]

I've always wondered how the fuel trims are managed for this car - since there are two sets of injectors, are there fuel trims for port and direct? All I know is there are fuel trims for airflow ranges.

[Shankenstein]

Speculation is something I can do quite well.

My understanding of similar fueling systems is that the pedal controls the driver requested torque. The requested torque is compared to the current torque. The delta torque is converted to a delta MAF, which adjusts the throttle angle target.

A weighted rolling average of the last few MAF sensor (or MAP) values gives you a steady-state estimate for air mass. If there has been significant change in the throttle angle (tip-in, tip-out) then some adjustment is made to account for transient changes in air mass. Changes in variable valve timing are also accounted for, along with vacuum sources (like brake boosters). Once they have estimated the air mass entering the cylinder, it's converted to volumetric efficiency (LOAD) by dividing by the displacement (or a similar maximum value).

Once they estimate LOAD, it's used for feedforward fuel mass calculations. The fuel trim is applied here. After some filtering, the oxygen sensor feedback is converted from Lambda (derived from ppm of oxygen that represents "complete combustion") to Fuel/Air ratio (inverse of AFR). The difference between FAR and an adaptive target FAR (slowly adjusts for changing fuel and road conditions).

Alot of stuff plays into this part. Status of the charcoal canister, estimated oxygen stored in the catalyst (this is actually a really hot topic right now), and expected perturbations will all be factored in. Each manufacturer is different here. Delta FAR is multiplied by the LOAD or air mass flow rate. Both yield similar scaling at the end of it all. The % change in fuel mass is calculated here (that's fuel trim).

The feedforward fueling is adjusted by the fuel trim and the total mass (calculated on a per cycle, per radian, or per cylinder basis) is then split up for each injector installed. If the DI system uses split injection, this will get calculated here.

A faster control loop is typically used here since injector pulses need to be pretty stinkin' accurate. The target fuel mass for each injector is converted to a pulsewidth based on fuel rail pressure, battery voltage, and other stuff. This ideally is a linear function, but typically high fuel mass will locally deplete the fuel rail so they increase the open time under heavy load.

As @arghx7 said they may set an "end of injection" time or a "start of injection" time and the injection timing and length are scheduled for the next revolution. The next time it's that injector's turn, it will trigger a short on the injector solenoid as specified. If all goes well, the perfect amount of fuel will be injected, but most likely there will be feedback from the oxygen sensors and changing driver torque demands.

Sorry for not answering your question at all... I enjoy learning about this stuff.

My guess is that the fuel trim is applied the total fuel mass. The adjusted amount gets split up as mentioned above.

[arghx7]

Originally Posted by Wepeel

I've always wondered how the fuel trims are managed for this car - since there are two sets of injectors, are there fuel trims for port and direct? All I know is there are fuel trims for airflow ranges.

It could be as simple as adjusting the fuel flow delivery higher or lower, and setting the split according to that mean injection split map. That would eliminate the need for separate trims. Or it could be much more complicated than that, but I suspect Subaru uses a "keep it simple stupid" approach on current vehicles that are in production. See comments below.

Originally Posted by Shankenstein

Speculation is something I can do quite well.

My understanding of similar fueling systems is that the pedal controls the driver requested torque. The requested torque is compared to the current torque. The delta torque is converted to a delta MAF, which adjusts the throttle angle target.

A weighted rolling average of the last few MAF sensor (or MAP) values gives you a steady-state estimate for air mass. If there has been significant change in the throttle angle (tip-in, tip-out) then some adjustment is made to account for transient changes in air mass. Changes in variable valve timing are also accounted for, along with vacuum sources (like brake boosters). Once they have estimated the air mass entering the cylinder, it's converted to volumetric efficiency (LOAD) by dividing by the displacement (or a similar maximum value).

Once they estimate LOAD, it's used for feedforward fuel mass calculations. The fuel trim is applied here. After some filtering, the oxygen sensor feedback is converted from Lambda (derived from ppm of oxygen that represents "complete combustion") to Fuel/Air ratio (inverse of AFR). The difference between FAR and an adaptive target FAR (slowly adjusts for changing fuel and road conditions).

Alot of stuff plays into this part. Status of the charcoal canister, estimated oxygen stored in the catalyst (this is actually a really hot topic right now), and expected perturbations will all be factored in. Each manufacturer is different here. Delta FAR is multiplied by the LOAD or air mass flow rate. Both yield similar scaling at the end of it all. The % change in fuel mass is calculated here (that's fuel trim).

The feedforward fueling is adjusted by the fuel trim and the total mass (calculated on a per cycle, per radian, or per cylinder basis) is then split up for each injector installed. If the DI system uses split injection, this will get calculated here.

A faster control loop is typically used here since injector pulses need to be pretty stinkin' accurate. The target fuel mass for each injector is converted to a pulsewidth based on fuel rail pressure, battery voltage, and other stuff. This ideally is a linear function, but typically high fuel mass will locally deplete the fuel rail so they increase the open time under heavy load.

As @arghx7 said they may set an "end of injection" time or a "start of injection" time and the injection timing and length are scheduled for the next revolution. The next time it's that injector's turn, it will trigger a short on the injector solenoid as specified. If all goes well, the perfect amount of fuel will be injected, but most likely there will be feedback from the oxygen sensors and changing driver torque demands.

Sorry for not answering your question at all... I enjoy learning about this stuff.

My guess is that the fuel trim is applied the total fuel mass. The adjusted amount gets split up as mentioned above.

Based on what the aftermarket has reverse engineered we've seen very little model-based and torque-based code for actual Subaru engine controls. I'm sure it's there to some extent for OBD and failsafe purposes but most of the stuff seems carried over from the days of the Bugeye WRX. If there's a gasflow model, I haven't seen it in production stuff. Nobody's ever shown a fast and slow path torque model or other stuff you find at other automakers.

There's no cylinder pressure model, no MBT spark timing model, no model-based boost control, none of that in production engine controls. Or if there is, nobody's found it, and it's not important for basic engine functions like main fuel and main spark timing. From my experience the Japanese OEM's have been a bit slower to use the more advanced controls. That's not necessarily a bad thing; it's certainly more straightforward for aftermarket tuning.

[mike the snake]

Can someone explain to me about the port injector sizes?

When I ordered my Full Blown Premium stage 1 turbo kit, I ordered the optional 1000cc injectors. I am going for more power than the stock port injectors can handle, especially with methanol injection.

In reading about this, I see injectors of all sizes, 550cc, 750cc, 900cc, 1000cc, and I don't know what to make of it.

1000cc seems pretty big, but my tuner says it's OK to not drive them at or near max, but I don't plan on running E85 so I know I'm not going to come near to maxxing these bigger injectors out.

Any help or suggestions would be greatly appreciated.

[brn12345]

Originally Posted by mike the snake

Can someone explain to me about the port injector sizes?

When I ordered my Full Blown Premium stage 1 turbo kit, I ordered the optional 1000cc injectors. I am going for more power than the stock port injectors can handle, especially with methanol injection.

In reading about this, I see injectors of all sizes, 550cc, 750cc, 900cc, 1000cc, and I don't know what to make of it.

1000cc seems pretty big, but my tuner says it's OK to not drive them at or near max, but I don't plan on running E85 so I know I'm not going to come near to maxxing these bigger injectors out.

Any help or suggestions would be greatly appreciated.

In general you would want the smallest size injectors that would cover your engines tuning needs plus a safety margin. The stock port injectors are in the low 200cc range I believe. Having said that, you can always tune for larger injectors though it would be much more sensitive and in my view more difficult to tune. Now if I was doing this myself, I would go with ID725 injectors just because I have extensively used them and their battery voltage compensation numbers are spot on and they have great low pulse width performance. So theoretically and assuming the oem map is properly scaled, all you have to do is plug is 725 into the injector size table and enter the battery voltage compensation numbers and it "should" be spot on for normal NA driving. Boost is something you have to tune for obviously.

[jamesm]

That doesn't happen in real life. IDs are better than most, but you never just type the advertised flow in and have everything just work perfectly. Well I suppose it depends on your definition of perfect.

Tuning for large injectors is a pain in the ass. Get the smallest ones that suit your needs.

[mike the snake]

Full Blown only has the 1000cc injectors as an option on their website.

Do you think they should have asked whether we were going with E85? Seems almost irresponsible to just sell the kit and larger injectors without at least asking about the application.

I'm going to see if I can't send them back for some more appropriate injectors for my application which is GTX28 on 91 with WMI.

[andrew20195]

Originally Posted by Shankenstein

On a similar note, I heard about Ford and VW using the variable exhaust valve timing to do a controlled backfire to spool up the turbo. Would be interesting if our cold start strategy does this to warm up the catalyst too.

The exhaust cam timing on the FA20 is indeed retarded until the catalyst reaches the target temperature.

[mad_sb]

Originally Posted by mike the snake

Thanks, that helps.

What about something a little more in-depth? Like when does one or the other turn on, or off? When are both on?

I'm getting my car tuned in a week or 2, and was hoping I could have a better understanding of just how my tuner is going to figure out which type of FI is used and when.

For FI (I'm getting a FB kit installed) at WOT, dose the tuner run the DI at say, 90% and then just ramp the PI up as needed?

What about partial throttle? Does the ECU switch between PI and DI or use both together instantly?

Edit: Just watched that little vid and that explains things very well. Thanks very much!

The answers you seek are in the rom file. There is a set of 3 3D tables that control the split between DI and port injection.

Download a rom and romraider or tunerpro and take a look. It's fairly straight forward.

[regnak]

For all you tuners new to the D-4S system: may I remind you to please continue running *some* amount of fuel through the DI injectors during WOT. Running 100% port in high load area is a great way to allow excessive carbon build up on the direct injector's tip and ruin its spray pattern.

[DougW]

On a side note the more fuel you run through the DI is the less air displaced by the fuel in the port and air not fuel is what makes power fuel just supplies the heat to expand the air. I have a friend that races Porsches with the DI, they want a layer of carbon against the DI seal to help protect it from the heat. I get wanting to not clog it up with some spray but why not use DI as much as you can?.
DougW

[mike the snake]

When turbo or supercharged, you'll need to run the port injection to some degree because the DI alone can't handle the requirements for boosting.

Also, I was under the impression that it was the DI that allowed the boosting of our engines with such high compression, and that stock, at highest load/redline that the engine runs mainly off of the DI.

For boosted, I would suspect that the DI is running full blast and the PI is added in as needed at highest loads/rpm.

[andrew20195]

Originally Posted by mike the snake

When turbo or supercharged, you'll need to run the port injection to some degree because the DI alone can't handle the requirements for boosting.

Also, I was under the impression that it was the DI that allowed the boosting of our engines with such high compression, and that stock, at highest load/redline that the engine runs mainly off of the DI.

For boosted, I would suspect that the DI is running full blast and the PI is added in as needed at highest loads/rpm.

Under load, the DI injection timing is during the compression stroke to aid in cylinder cooling. I suspect you're right about that allowing the high cylinder pressures that boosted FA20s are running.