Pulsed injectors on/off times

[Paul Nicholson]

At 7:29 -0800 2/14/01, <isham-research.freeserve.co.uk at pop.pol.net.uk> wrote:
> > 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.
>
>Does anyone have an equivalent circuit for the injector?

The injector is a simple solenoid. It has inductance and it has resistance, and there is a little whammy thrown in because of the affect of the movement of the valve.

The Nippondenso injectors we were using at Toyota had no suppression built in.

>
>Almost all switched automotive components produced over the last three
>decades have had internal suppression across windings - sometimes a
>simple capacitor, sometimes a diode.  As duty cycle increases, a larger
>percentage of total time will be governed by the actions of any such
>circuit.

For injectors the suppression is put in the drive circuit.

>A coil protection diode would be biased against the applied operating
>voltage.  Any flow through it when that voltage is removed would be in
>the same direction through the coil, acting to open the injector.  The
>problem I have is a lack of quantitative appreciation - is this current
>significant, or it is too small?

It's not really a coil protection diode, it's there to protect the drive circuit. Remember an inductor wants to resist any change in the current flowing through it. When the inductor is switched off, the current continues to flow, and the voltage reverses across the coil. The current is initially the same as the drive current, and it decays as the energy is lost in the protection circuit and solenoid resistance. Without a protection circuit, the coil would place a very high voltage on the drive circuit as it tries to keep the current flowing through the drive transistor that just switched off.

If you want the solenoid to close quickly, then you need to let the coil develop a high voltage across the protection circuit. A simple dioede with a 0.7 volt drop will allow the current to circulate for a long (and imprecise) time, thus holding the injector open. If you put a zener diode in the circulation circuit then the current will decay much faster, and close the injector much sooner.

In a simple injector drive circuit, the injector current is limited by the coil resistance or added resistors. Resistance is bad, it increases the time (and uncertainty) until the injector opens.

What you want is to initially drive the injector with a high voltage to ramp up the current and turn it on as fast as possible, then once the injector is opened, the drive current should be reduced to a lower holding current. When you turn it off, you want to let the injector develop as high a reverse voltage as practical so the injector current decays quickly and the solenoid closes.

One of the reasons for the upcoming move to 42 volts is that it is easier to drive injectors more precisely and efficiently.

Paul