"...help me understand how the fuel injectors can all fire at the *same time*..."

The FI ecu controls the injectors. The ecu determines the pulse duration and number of pulses needed to deliver the proper amount of fuel for the engine's instantaneous needs.

"...provide the proper fuel amounts..."

The ecu receives inputs from the engine temperature and air flow rate through the AMM (air mass meter). This device measures how much air the engine is "inhaling." Based on that and other data, the ecu can calculate the required fuel. The injectors are "fired," or pulsed, to accomplish this.

"...with the proper timing..."

Timing's not important. The fuel sprays into the intake port above the valve, and does so several times for each power cycle. The fuel mist has an opportunity to vaporize before entering the cylinder. This is actually better (for economy and emissions) than to spray fuel mist into the combustion chamber. Keep in mind that gasoline mist does not burn -- it MUST be a vapor to burn, and the slight pause above the valve, into a hot port, helps vaporization.
--
Don Foster (near Cape Cod, MA)

Thanks for so many excellent answers! I think I understand it much better now!

What a great resource, thanks again!

less confused now...

I will also add...

Strict sequential injection also poses another challenge. Since with sequential injection you only have appx. 1/4 of the time to inject the fuel for a given cylinder, your injector capacity would need to be much larger to deal with the fuel requirements at full load/rpm. Big injectors fix this problem but make it harder to get a clean controlled idle. Since injectors have a minimum open time, there is a lower bound on the amount of fuel that can be injected - this may be more than you need to idle at 750 RPM.

A compromise would be to use a standard injector, run in sequential mode at low load/low RPM mode, and batch fired mode for high RPM/load conditions. (as someone mentioned, at high RPM there isn't much difference between between batch and sequential injection). I don't know if anyone is doing this hybrid approach or not, but I suspect newer cars would have it (as sequential injection does give better results with emissions) Does anyone know?

But when Volvo (read Bosch) was making decisions about injection back in the '80s, I suspect they wanted the simplicity of batch mode, and had no reason for the extra complications of sequentially fired injectors. Keep in mind that batch fired injectors, while not perfect, are light years ahead of carburetors in most respects.

-Steve

Others have answered this question pretty well. The
answer is that some cars far the fuel all the time,
others fire all injectors, others fire the injectos
in groups, and still others fire the injectors one at
a time at the correct time.

Each is a trade-off in terms of complexity versus
efficiency. At high rpms, there is almost no
difference between the continuous and the sequential
fire systems. At low RPMS, the fuel hangs around in
the intake port before the intake valve opens and some
of it will coalesce into bigger drops that don't
burn as well. This only really affects emissions, not
power or fuel efficiency.

The short answer is that, for a 4 cylinder motor,
if the injectors all fire at once, each pulse is timed
to only fire 1/4th the necessary fuel.

A good couple of web sites are:
www.sdsefi.com
www.diy-efi.org
chris

As the engine rotates you simultaneously have 1 piston in the firing stroke (down, both valves closed), 1 piston in the compression stroke (up, both valves closed, 1 piston in the exhaust stroke (up, exhaust valve open) and 1 piston on the intake stroke (down, intake valve open), this happpens on every engine revolution. As you can see, the intake valve opens only on the intake stroke for each cylinder. Any fuel that is injected remains in the head until the valve opens then it is sucked into the cylinder.

That's a very good question. There was a good explanation in the L-Jetronic manual I had for my '75 Opel Ascona... both of which are long gone, and my recollection of the reason is sketchy at best. Remember that for every rev of the crankshaft, only 2 cylinders will fire. I think the actual duration of the injector solenoid opening is about 3 milliseconds. At 6000 rpm, 1 rev takes 10 milliseconds, and at idle 1 rev will be about 100 milliseconds. During one crank rev there will always be two intake valves open for some fraction of the rev - maybe 100 degrees each - so air will nearly always be flowing into some cylinder. Remember also that the amount of fuel per 'squirt' isn't much (roughly 14 pounds of air per pound of fuel), and hopefully with good injectors that tiny amount of fuel is atomized quite well. Even at idle (100 msec per rev) the amount of time that an intake valve is *not* open will be quite small (would need to look a a cam profile to confirm this), only a fraction of that 100 msec. That isn't much time at all for the brief quirt of atomized fuel to settle down anywhere.

I think there would be a practical limit to this fire-all-at-once approach, and it would be at very low engine speeds. Think about the huge stationary diesels used on ships - the ones with con rods 6 feet long - they turn at maybe 100 rpm. Now if they were spark ignition engines, I think the fuel injection pulses would have to be times to the intake valve opening, since each rev would take over one full second.

Just my $0.02 worth.

Actually,
The fuel injection in your car fires two injectors at a time. You only fire two
cylinders per crankshaft revolution so the electronic injection fires the
applicable cylinders. The injection is referencing various factors about the
engine and has a computer map to compare or reference to. The computer references
rpms, air flow, temp, and of course O2 in the exhaust. These factors all 'tell'
the injection how much fuel to inject just before the intake valve opens. Since
the amount of time before the valve opens is such a small amount of time the
injectors that are going to a cylinder that fires that revolution of the crank
receive their injection of fuel. In other words, the firing order is 1342 so
on the first revolution only cylinder 1 and 3 are fired, on the next revolution
4 and 2 are injected fuel. This is LH injection that I'm describing.

I'm sure some of the other electronic injection systems deal with a cylinder at a time basis but the computing power for that is much greater and since cost is also a factor it was economical at that time to have a computer that fires groups of appropriate cylinders with each revolution.

The system in my car is a different injection which does spray fuel all the time.
It is called K-jet and sprays an appropriate amount of fuel that the cylinder will suck in when the valve opens. This type of system is actually more a mechanical system and doesn't time or pulse the injectors. They are only injector valves.

Hope this helps you. Let me know if you would like me to clarify anything I wrote.
Dave 82 242ti 273k mi.

There have been and are a lot of different methods of delivering the appropriate amount of fuel to each cylinder, from throttle bodies (like a central carb with a distributing manifold) to "dry" manifolds that deliver air only, with an injector placed to spray at or near the intake valve. D-Jetronic fires injectors in groups of two, and therefore most of the time against a closed intake valve. Domestic injection systems can be batch fire or semi batch fire where all injectors are triggered on both or one bank of a v type engine. Ford pickup and car 5.0 litre engines are semi batch fire, except for the Mustang HO 5.0 which are sequential. K-Jetronic simply sprays fuel all the time at a particular rate, rather than in a pulse (K is for the German "Kontinuos"). Some deliver fuel in two pulses between intake valve openings.
The problem with injecting fuel on a closed intake valve is that some fuel, especially when the engine is cold, will condense on the intake port, and be moved into the cylinder on a later cycle. Compared to carbs, it is still more accurate, but less so than it can be. So sequential is best, but most complicated from a control point of view.
So it is certainly confusing, except to remember that the best systems today are in fact sequential and timed to intake valve opening points. I think in the future all systems will be, and nearly are now, sequential port type.
My head hurts now.....