Pulsed injectors on/off times

[Jorgen Karlsson]

Paul Nicholson wrote:

> Doesn't make sense to me. In order for the injector to close, the
> magnetic field (and hence the current) must be reduced below the
> valve spring force. How fast the current decays depends on the
> load dump circuit. If the injector is turned on before the
> current has fully decayed for a previous actuation, then it will
> simply have a head start based on the current that is already
> flowing in the injector and will turn on sooner.
>
> The only scenario that I can see that would cause a problem would
> be a poorly designed injector driver that uses a higher current
> to open the injector and a lower current to hold the injector
> that might get confused if the injector already had current
> flowing in it, but this scenario can be prevented by common sense
> circuit design. The driver should apply the high current pulse
> for fixed time or until the injector current ramps up to the
> opening current when the injector is turned on. Once the injector
> is open then the injector current can be lowered to a hold current.
>
> In my playing with injectors for TRD back 12 years ago, I had no
> trouble operating them 0 to 100 percent duty cycle on the bench.
> The only caveat is the uncertainty in open/close time which is
> dependent on variables like battery voltage.

There is a problem, the 80-85% rule must be obeyed. But I also suspect that
some injector drivers are more sensitive then other. The back emf issue was
something a friend and I discussed a few weeks ago, we tried to understand
why the injectors floats above a certain duty cycle.

Of course the problem is rpm related, 98% duty cycle at 10Hz isn't a
problem. The EMF as I see it is caused by the injector plunger falling
through the winding. I can't see how that opposing force can help opening
the injector. Either way, if it isn't EMF that causes the problem something
else is. The problem with injector float is very real.

Jorgen Karlsson
Gothenburg, Sweden.