<snip
Paul, Victor -

Victor is right that with an Inject-Per-Revolution scheme (IPR - I
made it up), a simple MAF sensor/555 timer would need some RPM
feedback and compensation, making it ... not simple.

Paul is wrong that it should be designed/built as "just a rough approx
just keep the engine running".  It should be designed to match the
fuel injected to the incoming air.

Paul is right that VE can change.  However it can change for the worse
over a given RPM range as easily as it can change for the better.  IPR
schemes need crank angle and RPM feedback to work effectively,
otherwise the air/fuel ratio will be all over the map (no pun
intended).

I had always assumed that the 555 timer would inject independent of
RPM or crank angle.  The injectors would sit upstream enough in the
manifold, happily injecting away in fine grained pulses, so the fuel
ingested per intake stroke would average out and only slightly vary.
This would be an Inject-Over-Time scheme (IOT - I made that up, too).

NOTE: That is the beauty of MAF sensors - they tell you the amount of
air going in - you simply inject a matching ratio of fuel, and it'll
burn at 350 RPM, 4000 RPM, or 6500 RPM - hence the simplicity of the
555 timer for controlling fuel injector pulse width.

BUT... If you're going to start varying the injection RATE (tied to
crank angle/RPM), then you need to COMPENSATE the fuel pulse width
with the same variable (RPM), in a feedback to the FORMERLY simple
system.

Paul - just have the 555 Timer system inject in a relatively fast
pulse stream independent of the RPM/crank angle, using the IOT scheme.

In other words, keep it simple.  (Just my 2 cents worth.)

-Vince Orton

I you think it will work that scheme  solves a lot of problems with
high speed
injection. There is not a lot of time  to inject fuel once per rev
at 7500 RPM
anyway. All we need is an oscillator to trigger the 555's.

It does make it  a lot  simpler as we don't need injection triggers
and the associated
circuits. I am getting close  to trying the 555 system as the RX8
engine slide throttle
and intake tubes are near done. The motor mount is done and the prop
dyno is near done.

Paul Lamar

Paul -

The following discussion assumes your 555 Timer design is going to use
the IOT scheme mentioned above.

You don't need to trigger ANYTHING, not even the 555 timers.  They
will put out a train of pulses at a fixed frequency.  The only
variable will be the PULSE DUTY CYCLE which varies according to the
MAF sensor input.  Let me repeat: The 555 timers will run at a fixed
frequency.

Google is your friend for finding simple 555 circuits to do this.  In
1 minute I found: http://home.cogeco.ca/~rpaisley4/LM555.html#17
There are many others.

Remember some fuel injector basics:
- they take about 1ms to open and close, give or take.
- they should be maxed out at 80% duty cycle, or they'll have short
lives.
The Megasquirt manual and FAQs have a wealth of info on this stuff.  I
recommend it for bedtime reading.

Pick a frequency that is fast enough to avoid flooding one chamber and
starving the next.  Let's say 125 Hz.  This translates into an 8ms
pulse width, which coincidentally matches each chamber opening at
7500RPM.

Using that 125Hz injector frequency (8ms pulse width):
Recommended MAX fuel scenario would be:
- 80% on time of 6.4 ms (1ms to open, 5.4ms to inject fuel)
- Off time of 1.6ms
Recommended MIN fuel scenario would be:
- 15% on time of 1.2 ms (1ms to open, 0.2ms to inject)
- Off time of 6.8ms
The MAF sensor output MIN and MAX voltages should correspond with 15%
and 80% duty cycles.  Of course these numbers will vary according to
pulse freq, MAF sensor and injector specifications, but you get the
idea.

The meat of the project is tuning MAF sensor - pulse width -
required fuel.  I still think it'd be easier with a microcontroller,
but hey, let's see what a dirt-simple 555 Timer circuit can do before
knocking it, right?

-Vince Orton


Sounds good I'll try that first.
I was thinking of changing the frequency to change the leaning. Easier
than changing the MAF output.

Paul Lamar