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Pretty simple question....
*disclaimer* I dont believe this is true but cant find docs to back it up
Does all GM ECM/PCMs drop SFI above ~3000 rpm and switch to batch fire?
Im certain its a big NO but again, no docs to back it up.
*edit* did not specify but meant if the ECM/PCM is SFI
Thanks!
Technically, no. Why? Because direct injected engines are SFI and the injection pulse has to occur at the correct time to work with direct injection. Everything I've read regarding injection into the intake indicates that SFI mode stops somewhere around 3000-3500 rpm.
Ok well that settles it. I remember someone stating(RBob?) that sequential, I think the LC2 platform fires the injectors 2 per 2 revolutions no matter what RPM.
Are we gonna win a bet?
LOL i wish!Quote:
Originally Posted by 1project2many
i've never seen it happen, but i don't have a scope either, let alone a dual channel one.
i've also never found any code that would imply it to happen.
http://www.fiero.nl/forum/Forum2/HTML/075502-13.html
Curious myself.
So it does drop SFI and go batchfire or it doesn't?
(not counting direct injection,only port/intake injected.)
I had never heard this till I saw it in above thread i've been following and remembered this thread.
A GM engineer told me the SFI in an LT1 was only done for emissions, did not need it in 94 but 94 would pass 96 emissions tests... and do to time it is batch fire by 3000-3500 RPM. In the code it has to do with Sync and Async fueling as well as time available per revelution, as RPM increase this time decreases and there's not enough time to SFI at that point. Wish I had the proof...
Mark, you have a dual channel scope?
Yes but nothing to test it on... plus I'm in Portland for who knows how long...
We are kidding ourselves if we think that SFI is accurate when engine speeds are high. 6000rpm has 1 cylinder's intake valve opening on every other rotation of the crank or 50 times per second. I'd bet the farm that there is some injector duty cycling overlap independent of intake valve position. I have heard the rumour too. I didn't know if it was an actual change in fueling strategy or simple lack of time to perform sufficient injection in the time allowance. DI is a completely different animal. Its injection occurs during different positions of the piston, no matter where the valve is, very precise.
Those speeds are certainly low enough for modern electronics to work with. Electronically activated diesel injectors have to to stay sequential and both Benz and Isuzu/GM run their diesels in the same RPM range as GM gas jobs. What are the Indy and F1 cars using for injection control? They run to 14k or so.
By switching to batch mode you reduce the dynamic range required of the injectors and greatly reduce complexity and cost of the system. With no significant benefit from SFI at higher rpm, is the added cost to stay SFI worth it??
to me, reverting to batch fire just sounds like more code to do the same thing. I do think the PCM's are more then capable of keeping up though, it's not moving faster then the speed of light or something. I don't know the exact numbers, but an injector firing 100 times a second doesn't seem absurd for an electro-mechanical device.Quote:
Originally Posted by Hog
The PCMs are capable of performing the processing and electrical functions (triggering) of the injectors in an SFI scheme well above the useable RPM range of an engine.
The issue is not the PCM, it's at the other end of the wires, well a little beyond that actually. The higher the RPM gets the less time the valves are open, allowing less time for fuel to enter along with the air. When going to batch fire, some injectors will be injecting fuel when the valves are closed, this is not a bad thing. When they are pointed correctly, that being at the back of the intake valve, the heat from the valve created by the combustion process causes the fuel to vaporize and remain suspended until the next time the valve opens, at which point the vaporized fuel along with any current fuel being injected can be pulled into the cylinder.
This is the main distinction between port injection and direct injection. With DI, there is more time for fuel to be injected into the cylinder, since there is no mechanical restriction or valve to get in the way. DI can and has to stay "SFI" for the entire RPM range in order to operate correctly.
F1 engines are a different animal, and are designed very differently than passenger car engines. They idle at a much higher RPM than a passenger car engine, and have a very different placement for the injectors as well. In an F1 engine the injectors are mounted above the intake runners, in what would be the plenum. This allows there to be an abundance on fuel in the runner and remain a suspended mixture, to aid in high RPM running.
One of my favorite videos:
http://www.youtube.com/watch?v=o_QyUD6V5_I
This one has some very cool elements, like a driver...
http://www.youtube.com/watch?v=5OS_k0V8Bvw
http://www.youtube.com/watch?annotation_id=annotation_84493&feature=i v&sr c_vid=iWSt4CDNvgQ&v=ZOJkl4Agf4c
I was under the impression that SFI didn't necessarily mean the injectors fired when the valve was open, but just that they fired one after the other instead of groups.
Which is tied to when the valve is open, or rather, about to open.Quote:
Originally Posted by ericjon262
i don't see why everyone is going bonkers over injector firing phasing overlapping? it's not a problem at all and is inevitable at higher injector duty cycles.
what he said...Quote:
Originally Posted by RobertISaar
Who's going bonkers?
Maybe the PCM can but injectors are mechaincal. It's not like poof they are open and poof they are closed, it takes time to open and close as well as how much time to stay open. And the time at higher RPM and more cylinders is not seconds but millseconds... or is that microseconds...Quote:
Originally Posted by ericjon262
SFI just means each cylinder injector is timed to the event, so a V8 = 8 events. Batch fire is only 2 events.Quote:
Originally Posted by ericjon262
Even comparing these old injectors for SFI to Direct Injection is like comparing apples to oranges...
Me! 20,000 RPM? I remember that video from earlier and watching the injectors fire with that cover off is awesome. :rockon:Quote:
Originally Posted by Six_Shooter
8 cylinders SFI at 8000RPM(15mS per 2 revolutions), let's say 80% duty cycle, that's 12mS injector on-time. at 80% DC with 8 cylinders, 6 of the 8 injectors are going to be open at any one time(and for a short amount of time, 7 out of 8).
it's not a problem at all on the logic side of things... the interrupt routines for dealing with injector sequencing and timing are lean enough to deal with these kinds of situations. and i don't see why it would be a problem on the mechanical side either, since injectors are "dumb" devices... they just react to signals. they'll deliver the same amount of fuel for the same length of pulse from the injector drivers regardless of being fired in bank, batch, sequential, etc...
i personally don't care for SFI, since it's just more things to go wrong for very little to no benefit. direct injection? different story.
But they're subject to even more stringent limits in terms of achieving maximum economy and power from the fuel system. Even with fuel remaining in suspension it's not as if they're pumping the plenum full of liquid fuel to be kept on tap for later. The question is still valid: Are F1 engines sequentially injected?Quote:
F1 engines are a different animal, and are designed very differently than passenger car engines. They idle at a much higher RPM than a passenger car engine, and have a very different placement for the injectors as well. In an F1 engine the injectors are mounted above the intake runners, in what would be the plenum. This allows there to be an abundance on fuel in the runner and remain a suspended mixture, to aid in high RPM running.
from this article: http://www.formula1.com/news/features/2013/4/14512.html
eight cylinder engine
1 injector per cylinder
100 bar maximum fuel pressure
"At 18,000rpm - before the limiter kicks in - the fuel injector is fired once every 6.6ms for a duration of 2.7ms at full throttle."
At 3.33 ms per revolution those numbers only work for batch fire. And that makes sense. In order to get the amount of range needed to cover 13k rpm difference from idle to WOT with sequential injection you'd need to use a higher maximum fuel pressure. To me that indicates a dynamic range issue.
Haha... so much for squashing this one! :)
... that's kind of odd, 18,000RPM = 6.6667 mS per 4 stroke cycle. so it only fires once per 4 stroke cycle, and for a 2.7mS period? that's a 40.5% duty cycle, unless there is some miscommunication on the site, if it were "fired twice every 6.6ms for a duration of 2.7ms at full throttle" as would be done in double-fire batch(or for some reason, firing twice in SFI), then that's 81% duty cycle, which makes perfect sense.