PWM injector driver

[Bernd Felsche]

Brian Michalk tapped away at the keyboard with:

> For regular EFI:
> 
> The Bosch books I have states that the injectors fire once per
> crank revolution, therefore twice per power stroke.

> Does the injector require synchronization with the crank?  It seems
> like it would be much easier to just set up at a certain frequency
> and then modulate the duty cycle.

The timing is to avoid squirting on an open valve as this usually
causes high HC emissions. Not of much importance in an off-road
application, but worth keeping in mind in a road car.

> I understand that at low RPMs, the injector might be to slow to 
> open for a very short duration, and it would cause the engine to run
> rich, so at low RPMs, keep the duty cycle fixed, and vary the 
> frequency.  At higher RPMs, go up to a certain frequency, and then 
> modulate the squirt time with duty cycle.

Some systems only squirt once every two RPM (some Digifants) with
batch firing at low engine loads. That gets around the slow
opening/closing of the injectors (1 to 1.5ms each) and the poor
control around that sort of opening time.

> So my question is: do I really need a crank signal to synchronize
> TDC with my injector firing?

Synchronize on the camshaft so that the injector closes before the
inlet valve opens.

-- 
Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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