Fuel Injector Timing

[BOPE]

NCR-PCA, Bob Tucker writes:

Injector- Mechanical valve that "pops" when the fuel pressure reaches approximately 45 psi (3 bar). All injectors in a CI system spray at the same time the same amount, there is no timing as with mechanical injection.

I'm trying to understand how all the injectors can be spraying at the same time.

For instance, if cylinder #1 is on the intake stroke and is being injected with fuel, how can all the other cylinders be sprayed with fuel at the same time?

Instead, I think that the differential pressure across the injector determines when it "pops" and sprays fuel. For example, cylinder #1 on the intake stroke would have a lower pressure downstream of the injector as the piston draws in the air/fuel mixture, whereas the pressure downstream of all the other cylinders' injectors would have a higher pressure, preventing them from opening. Therefore, based on my understanding of differential pressures across the injectors, they do not all spray at the same time.

Well, that's my take on how the system works, but I'm certainly open to all comments.

Jason

[fanaudical]

Regardless of what you think, all injectors on a CIS system are spraying continuously (except for the cold start valve) when the air metering plate is raised high enough and the fuel pump is operating.

Think of it as more of a "fog" than a spray. The fog fills the intake runner ahead of the intake valve until the valve opens on the intake stroke. It then pulls the fuel fog and air into the combustion chamber.

This isn't too different from a carb (which is providing atomized fuel continuously - just further away from the intake valves).

There are tons of books / sites / videos on this. If you have a CIS car, it's easy to pull all the injectors, put them in individual jars, and prove this to yourself...

[BOPE]

Originally Posted by fanaudical

Regardless of what you think, all injectors on a CIS system are spraying continuously (except for the cold start valve) when the air metering plate is raised high enough and the fuel pump is operating.

Think of it as more of a "fog" than a spray. The fog fills the intake runner ahead of the intake valve until the valve opens on the intake stroke. It then pulls the fuel fog and air into the combustion chamber.

This isn't too different from a carb (which is providing atomized fuel continuously - just further away from the intake valves).

There are tons of books / sites / videos on this. If you have a CIS car, it's easy to pull all the injectors, put them in individual jars, and prove this to yourself...

Thank you for that excplanation.

The "fog/spray all the time" doesn't seem efficient to me, hence me trying to make it fog/spray only on the intake stroke via pressure differential.

Jason

[76FJ55]

it actually seems to work pretty well. really not that much different that batch fire EFI. even on sequential injected engines the injector fires prior to the valve opening. With pulsed EFI injectors you duty cycle will surpass the valve open time anyway, commonly approaching duty cycle of around 90%. Only 10% off CIS.

[brighton911]

Think of the speed of which the cycles are occurring and firing the injector at the right movement is not practical - or necessary. Maybe if the engine ran at 500 rpm all the time but in the real world, not necessary. FYI, Motronic systems also fire all injectors at the same same.

[Jeff Alton]

^^^^^^ Motronic in early cars with out cam sensors batch fired. All 3 on one bank at a time, then the other 3 on the opposite bank.

Cheers

[mysocal911]

Originally Posted by Jeff Alton

^^^^^^ Motronic in early cars with out cam sensors batch fired. All 3 on one bank at a time, then the other 3 on the opposite bank.

Cheers

Actually, all six injectors fire at once on the '84-'89 911 3.2s! The change came with the 964 with sequential injection.

[Jeff Alton]

Yes, correct, sorry. Was thinking of systems we have done with bank fire for injectors and wasted spark.

Cheers

[Flat Six]

No different than carbs.