Is this too simple?

[Jeremy Harris]

Apologies in advance if this has been done/discussed before.  I checked the
digest archives and couldn't specifically find it.

I have a friend who is building a hovercraft.  The fan loads the engine at a
rate proportional to the cube of it's rpm.  The engine (a twin cylinder,
horizontally opposed, 652cc, aircooled ex-Citroen car engine) only needs to
run between fast idle (around 1,500 rpm, limited by torsional vibration in
the drive system) and 6,000 rpm.  Most of the time the engine will be
running at around 4,500 to 5,000 rpm.  Accelleration enrichment isn't really
needed, as the throttle lever can be damped to reduce rate of change.  The
engine will see an almost constant load for any given rpm and low rpm (below
about 3,000rpm) performance isn't at all important (as long as the engine
runs without stalling or damage).  Auto cold start enrichment also isn't
needed, a simple manual enrichment system would be fine.

I have been asked to look at EFI for this engine, and my initial thoughts
are that a very simple system would suffice.  I plan on using a MAP system,
with a single throttle body with one injector mounted to a central plenum
feeding the inlet port runners.  I think that all I need to measure is MAP
and inlet temperature, with the injector timer being triggered by the
existing crank sensor (1 pulse per rev, two pulses per firing cycle).

I've just knocked up a rough breadboard circuit, with a 555 timer modified
so that I could get a better than 10:1 change of pulse width with voltage (1
mS to 10mS range).  I have added a Basic Stamp microcontroller to measure
MAP and temperature and output a voltage to the timer voltage control line.
The timer is triggered by the crank sensor pulse, via a bit of interface
circuitry.

This all seems to work OK on the bench, and I have a baseline set of
calculated pulse widths against MAP and air temp loaded into a look up table
in the Stamp.  The Basic Stamp is a bit slow (it takes around 100mS to run
the loop), but I figure that this doesn't matter too much for this
application.  The idea behind this is to make an EFI system for next to
nothing as a challenge, rather than ultimate performance.

Can anyone see where I have gone wrong, as it all seems just too simple?  I
don't want to build the piping hardware (for me the difficult bit) until I'm
confident that I can get a cheap system to work.

Jeremy Harris
Salisbury, UK
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