more mower

[BigBrother at mail.utexas.edu]

> All four-stroke cycle lawn mower engines and almost all four-stroke motorcycle
> engine fire every revolution.  All the newer distributor-less microprocessor
> controlled car ignitions do as well.  That extra spark just flaps in the
>breeze.

I'm going to end up using a Ford DIS coil to accomplish this.  I don't
know if I'll have a chance to put it on the mower, I'll probably just wait
for the real application.  The Ford coils use the dual fire setup.
Lexus (LS 400) and some Nissans have a single coil for each cylinder.

> My system takes interrupts off the TDC reference and calculates the RPM.
> Based on the RPM, it looks up the advance from a centrigual advance type
>table.
> This value is then used to predict when to fire the next time, in advance of
> actually receiving the TDC reference pulse.  The reference pulse actually is
> set to about 6 degrees BTDC so that it can be used to fire the coil directly.
> This is used before the micro is up to speed, or if it crashes.
> Currently there is no load input to calculations.  The TTL level output of the
> micro drives the ignition coil through a VW Rabbit amplifier module.

That's neat.  Does the VW Rabbit amplifier work well?  Any noise problems?

> By the way, that scheme of one pulse per revolution isn't accurate on any
> engine that can rev very quickly.  I read somewhere that Ford had trouble
> with the controllers for the Taurus SHO because that motor can rev at
> 20000 rpm / second which was too fast for their systems to keep up with.
> I guess that's why all the Motronic systems take pulses off the flywheel
> ring gears as well as a TDC reference pulse.

True, but hey its just a lawn mower!  It wont be pulling off any SHO rev
rates. Although, it would be pretty easy to put more indexing (notch,
magnet, metal) on the flywheel.  But, then it would need to be indexed
again with another setup to reference the actual crank angle.

> I believe the minimum pulse length for the injectors is about 1 to 2 ms.
> There are also considerations on the electrical drive for the injector.  They
> come in different forms.  Some are designed to be used with an external
>current
> limiting resistor, and some are supposed to be connected to more intelligent
> drive electronics that watches the current ramp until the inductance saturates
> and then back the current down to a 'hold' level.  There are some decent
> discussions of this in that blue Bosch injection book. Also, the Motorola
> and National Semiconductor linear databooks describe chips to control
> injectors.  Not that we can get them, but they talk about how they work.

I like the Motorola SEFI ones. The MC33293 or MC33295.
Motorola distributors are supposed to be able to sample these.

Later,
Jeff