New member - Aircraft engine EFI

[Bernd Felsche]

Jeremy Harris tapped away at the keyboard with:

> I've just built a modified BMW R100 air cooled motorcycle engine
> to fit into a light aircraft.  As an electronic engineer I have
> been pondering over whether to go down the EFI route, rather than
> use the big, heavy Bing carbs that came with the engine.  My
> reasoning for EFI has more to do with weight and freedom from
> icing than fuelling accuracy.  Responsiveness and driveability
> aren't big issues for an engine that only needs to idle at around
> 2000rpm or so and spends most of its time running at around 4000
> to 5000rpm (it has a reduction gearbox driving the propeller).

How would the carb. cope with inverted operation?
If you dispose of the carb., does the BMW also have a fuel pump or
does it rely on gravity feed?

You'll need a small electric fuel pump and alternator with battery.

> My thoughts at the moment run along these lines:

> 1) Keep the system as simple as possible.
> 2) Use a single throttle body injection system with one injector

Most of the extra stuff you'll have to add will be way before the
injectors. One more injector is not such a big deal to drive and you
can run injectors in batch mode. It also reduces manifold wetting
and gives more flexibility in the design of the inlet runners tuned
to the required rev range.  A simple, single throttle butterfly is
probably all you'll need.

Besides; if the TBI fails, you're completely without power.

> 3) Use just a throttle pot as the "air flow" sensor (it's for low
> altitude use only)

So you're never going to take it into the mountains for flying from
a ground altitude of 1000m? n-alpha operation is quite simple, but
you might want to consider leaving the system "open" enough to
integrate an EGO sensor (narrow band) which can then be used to
adjust your injection pulse widths according to altitude.

A temperature sensor on at least one of the heads should be
sufficient to trim mixture for hot (re)starts.

Unless you're regulating inlet air temperature to about 25 degrees
C, it's probably a good idea to trim for cold air enrichment as
well. A simple inlet air temperature sensor is sufficient. Doesn't
have to be fast or fancy. You can use the same sensor to tell the
pilot how much to move a pre-heat flap for intake air.

> 4) Seperate out the "measuring" part of the system from the "pulse
> generation" part

> As an outline for a really simple system I am planning to use a
> small, cheap, microcontroller to do the measuring and hold the map
> of throttle angle/rpm to pulse width.  This will load the required
> pulse width into a timer triggered by a crank sensor (I already
> have a Hall effect sensor that gives one pulse per crank rev).

What's the basis for your ignition timing? One signal per rev is
sufficient for batch-fired fuel injection.

> Can any of you experienced people see a major flaw in my thinking here?

I'm not that experienced. What's the glide ratio?

-- 
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