I used to think a pressure sensor driven EFI system required an air
temperature input and a computer to work.  That ruled out the pressure
sensor in my feeble mind for a simple and cheap 555 system.  I thought I
needed a MAF sensor.  Then I found out that mass air flow sensors are not
linear.  Then I thought well I would use a cheap computer to linearize
the
mass air flow sensor.

I have since changed my mind. All one need do to start the engine is
set the
leaning knob commensurate with the out side air temperature.  Cold day
set
it richer.  That simplified the system greatly and we can use an ordinary
555 system as a manifold pressure sensor puts out a linear signal
unlike a
mass air flow sensor.  TLC555 are linear devices.  PW hence fuel
injected is
directly proportional to sensor voltage.  I now agree with Tracy Crook
almost.

The benefit is a much cheaper system with no need to buy a $80 or $100
mass
air flow sensor and install it on the plenum.  If it is not there it
weighs
nothing and is absolutely reliable.  The other benefit is pressure
sensors
cost less than $10 each so we can use two....  one for each rotor.  A
little
redundancy for back up use.

Since the manifold pressure is linear we don't need the super 555
microcomputer chip and a $1 analog TCL555 will work just fine as is.

I have got to get something working soon. As I will have some engines to
test soon.


Paul Lamar



Hi Paul

Fuel injection that thinks like a "SU"

Electronic Constant Depression Injection "ECDI"

Brilliant!!!!!!

Err....Don't those LM335 temperature sensors give a linear output to
set the
basic enrichment.

Plus the leaning pot for finer adjustment.

Not a pesckiy programying head scratchy brain numbing computa in sight.

Where's the kit???????????????
Where do I GET SOME???????????????

Clive

Temperature compensation requires a computer. One must convert the
output,
linear or otherwise, to a digital quantity.  I have decided not to use a
computer.  Too costly and over kill for an aircraft engine.  The $1
TLC555
chip does 90% of what a computer does in an aircraft EFI system.  It
takes
in an analog voltage from a $10 intake manifold pressure sensor or MAF
and
outputs a pulse width to an injector.  The cost is less than $20 per
injector.

Recently we discovered one of the severe problems tuning Tracy's manifold
pressure driven EFI system.  Those problems soured me initially on
pressure
driven EFI systems.  Apparently the pressure sensor was seeing a lot of
turbulence inside the intake manifold.  The character and intensity of
the
turbulence varied as a function of RPM and throttle opening.  People
had to
set values in the mixture table for every combination of throttle opening
and RPM.  You should not have to do that.  IMHO this was causing all
sorts
of tuning problems.  We discovered this with a high speed (1000
samples per
second) pressure data acquisition system.  We discovered that
measureing the
pressure in the intake chamber of the engine using the apex seal oil
injector hole was ten times more stable than measuring it in the intake
manifold.  See the data plots.

So that is what I am going to use to drive the 555.



Paul Lamar


Paul

The temperature compensation was only an idea.

Not a good one as it turns out.

Ah ha, just, re-read your comment.

Obvious---the pot alters the mixture proportionally to 555 increasing or
decreasing the pulse width across the whole range.

The temp sensor is another input, hence the need for computamebobs,
that we
don't want.

Here's another idea (so don't shoot me if no good)
Could the MAP output be also used to drive a PFM for the fuel pump?
Someone was working on that a while back.

Clive.

Yes I think that could be made to work. However I, like Dave Leonard
prefer a full return system for several reasons. One of which it is
known to work.

Paul Lamar

A PWM fuel pump system needs to precisely match the pump output  volume
to the fuel metered thru the injectors. The injectors have a linear
relationship between applied pulsewidth (less latency) and flow rate,
whereas the pump tends to have a flow rate which is related to the
square of the applied voltage (double the voltage, quadruple the power).
If you wish to use a PWM pump, for aircraft use it would be better to
use a constant-flow injection system with open injectors like
Continental and Lycoming. Use a turbine flowmeter in the pump output
which gives a string of pulses, the number of pulses being a linear
relationship to flow volume.
Average the pulses to give a voltage proportional to flow. (Simple
resistor and capacitor)
Average the output of the e-shaft triggered 555/MAP to give a voltage
proportional to fuel required.
Send both averages to a comparator, whose output switches a transistor
driving the pump.
Use a pot to provide an offset voltage to provide mixture control.
For systems with known VE curve, another simple op amp could provide
enrichment with increasing RPM above a selected threshold, and it could
saturate at the RPM at which the VE curve peaks.

I'm not suggesting a PWM pump is the way to go, just pointing out that
if you control the pump that precisely, you don't need injector
control.  Autos have very stringent requirements which are more
concerned with emissions than with power production.

Victor Roberts