Creating a pulse for injector drivers

[FJB203 at aol.com]

> WE are on a FSAE team and are designing an electronic fuel injection
>  system from scratch.  We are using the Motorola 68hc11 MCU.  We have

First off, good choice on a CPU.  They are cheap, avail in many, many case
styles, and have eight A/D converters built in (you might learn some
limitations as you folks tinker, but that's okay <G>).

>  creating the pulse sent to the injector drivers.  The trouble is with
>  using an output compare to create the pulse.  An output compare can be

Driving a pulse driver can be as simple as an output transister (FET's are
faster than Bipolar of course), however I'm thinking your question is pulse
duration.

The duration can come from a pre-calculated "map" within your eeprom for each
RPM range (500-1k, 1k-1.5k, etc) and your system then adjusts automatically,
following the engine.  Another option is to monitor engine rotation so at
certain points after the injection starts, the injector stops.  However this
introduces mechanical items into the mix.

I'd go with the calculated map approach personally.  BTW, what do you need a
compare for on the output?  Just curious, because I was under the impression
that an injector is basically a coil that when energized, allows a
predetermined amount of fuel into the cylinder based on the internal
bore/valving assembly.

I am not a fuel injection guru by any means...

Frederic Breitwieser
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1993 Lincoln "Boomer" Continental
1989 AG "What color is that" Humvee
1998 "HomeBrew"