[Diy_efi] pwm explanation

steve ravet sravet at arm.com
Fri Jan 31 19:00:44 GMT 2003


The open/close signal to the injector is not usually referred to as PWM,
it's just the injector on time.  PWM comes in to play if you have a low
impedance injector that needs current limiting.

Are you trying to use the PWM hardware to open/close the injectors? 
injector timing is usually synchronized to engine position, but the your
PWM hardware probably won't do that, it's synchronized to the clock that
runs the chip.  There have been discussions in the past about fueling
that is totally asynchronous to engine position, but I don't remember
the pros/cons.  Some archive searching should turn it up and you can
decide for yourself.

If you use the PWM hardware, then your software needs to decide on fuel
delivery between 0 and 100% of the injector's capacity depending on rpm,
map, or whatever else your inputs are.  Then it updates the PWM hardware
accordingly.

If you want synchronous fueling then you'll have an interrupt that comes
from a cam or crank sensor.  The interrupt routine will load a one shot
timer with a value according to the fuel requirements calculated, the
timer output will drive the injector.  That's an example, there are
other ways to do it.

You have saturated injectors, they should measure something like 12 ohms
or so.  If so then only an output stage to drive the injector is
needed.  If you had PnH injectors (~2 ohms) then you'd need some kind of
current limiting circuitry, either an additional PWM stage or a circuit
that reduces the applied voltage based on the amount of current drawn.

--steve

Bino Lardo wrote:
> 
> Hi
> 
> I would like to know if can help me whit the pwm to drive the injectors.
> I use a microcontroller to make something like the megasquirt. I understand
> how to create one voltage but where im lost is how do I determine the amout
> of time the injector is on ( a other timer?) and if the voltage has to be
> steady or if it varies during the operation of the injector. the injector
> are saturated. the microcontroller is a atmega 128.
> 
> thanks!
> 
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-- 
Steve Ravet
steve.ravet at arm.com
ARM,Inc.
www.arm.com

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