Pulsed injectors on/off times

[Paul Nicholson]

At 0:57 +0100 2/13/01, Jorgen Karlsson wrote:
>
>The thing that happens is that when the injector don't have time to close
>before it is commanded to open it will cause back EMF (electro magnetic
>force). The impendance of the injector gets soo high that the current over
>the winding builds slowly. This in turn makes the injector take a very long
>time to open, then when it has opened it all happens again as it is
>commanded to close. This is how the injector can stay half open even if it
>is commanded to be open 95% of the time.

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.

Paul