The fuel injectors are wired in two banks to the ECM. I don't understand how the ECM can fire ONE injector at a time when the injectors are wired in parallel.
Explanation?
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11:25 AM
PFF
System Bot
TONY_C Member
Posts: 2747 From: North Bellmore, NY 11710 Registered: May 2001
They don't fire individually. They "batch fire", this is not a sequentially fired engine. The reason they are on two circuits is just for redundancy, all six fire at the same time. This is the same set up for TPI V8's as well except there are 4 injectors on each circuit.
[This message has been edited by TONY_C (edited 01-07-2011).]
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11:27 AM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
So it's the same process as TBI injection, just happening further "downstream" ... and with six injectors instead of one.
That's essentially correct. The big advantage of port injection vs. a carburetor or TBI is more uniform fuel distribution to each cylinder ... which can yield better performance and lower exhaust emissions, both at the same time. A second potential advantage is better fuel atomization since each injector is spraying near the intake valve.
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How long are these pulses for a motor at idle vs. at WOT?
I don't remember the exact figures (I may never have seen them), but the duty cycle of most injectors tops out at about 80%. At 5000 rpm that would be a maximum injector pulse width of about 19 milliseconds. [(60 / 5000) * 2 * 0.80 = 0.0192 seconds] There is some initial delay in the injector opening, so the pulse width vs. fuel delivery becomes quite nonlinear at low fuel delivery rates ... e.g. at low throttle settings.
[This message has been edited by Marvin McInnis (edited 01-07-2011).]
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01:20 PM
Blacktree Member
Posts: 20770 From: Central Florida Registered: Dec 2001
So it's the same process as TBI injection, just happening further "downstream" in the F.A.S.T. process, and with six injectors instead of one.
Right?
Basically, yeah. If you look at an electrical schematic of a TBI 2.8 vs a MPFI 2.8 V6, they are very similar. The only real difference (electrically) is that there are three injectors on each circuit instead of one. The ECM doesn't even know the difference.
so what your saying is all the injectors fire at the intake cycle and at the exhaust cycle? wow thats wildly inefficent its like your using twice the fuel you have to, thats if im understanding this thread correctly.
...injectors fire at the intake cycle and at the exhaust cycle? ...thats wildly inefficent its like your using twice the fuel...
Keep in mind that the fuel charge doesn't go anywhere until the intake valve opens. It's not twice the fuel, it's half the needed fuel charge at each firing, the exhaust cycle firing just lays on top of the intake valve until the intake cycle opens the valve.
------------------ RickN White 88GT 5spd (<10K miles) White 85GT (SC3800 Series II w/ AOD) White 99 F250SD 7.3PSD 6spd 1956 Ford 860 Tractor w/ Freeman Loader 20Ft H&H Tilt Bed Trailer
[This message has been edited by RickN (edited 01-07-2011).]
You're supposed to choose injectors just big enough that they never exceed 85% duty cycle. But Some guys get carried away with the mods, only to discover they're leaning out at high rpm, because the injectors are at 100% duty cycle. And it is possible to run them like that, so long as you're not leaner than 13:1 at WOT at any rpm. However, they don't last too long that way.
I'm running 30 lbs/hr injectors on a 3.4L engine with stock fuel pressure and the pulse width averages about 2 mSec at idle and 10 mSec at 4k rpm at 10 psi boost.
[This message has been edited by Hudini (edited 01-07-2011).]
so what your saying is all the injectors fire at the intake cycle and at the exhaust cycle? wow thats wildly inefficent its like your using twice the fuel you have to, thats if im understanding this thread correctly.
Nope. the bowl behind the valves is super hot so the fuel instantly atomizes, then when the valve opens it sucks all that vapor+air in to burn. at 2500RPM the injectors fire 80 times a second tiny little pulses of fuel. It's actually very efficient.
so what your saying is all the injectors fire at the intake cycle and at the exhaust cycle? wow thats wildly inefficent its like your using twice the fuel you have to, thats if im understanding this thread correctly.
And it can't use twice the fuel you have to, the exhaust oxygen sensors are there to maintain a 14.7:1 air : fuel mix at part throttle, once the engine is warmed up. 14.7:1 is too rich for best MPG, but going leaner hurts emissions. A modern engine like the LSx could run 17:1 no problem. But the emissions would be bad.
[This message has been edited by Isolde (edited 01-08-2011).]
Injectors don't fire a full fuel load each time, only a partial load. Valves are always closed when the injectors fire. Each fuel load bolls into vapor sitting against the hot valve and in the hot casting. When the valve opens a nicely atomized fuel charge is waiting to enter the cylinder.
Main advantage of port injection over throttle body injection is better cold fuel handling. On a TBI system (and carb'd systems as well) fuel condenses on intake manifold walls when the engine is cold. That fuel is missing from the charge that enters the cylinder, and has to be made up for by enrichening the mixture. Then later when the manifold heats up that extra fuel evaporates into the mixture, richening it and requiring compensating mixture changes. Also, dry air has less mass than air with a fuel load, which I suspect makes it easier to design intake manifold runner geometry.
