From Cranking to Idle.DIY_EFI Digest V4 #443

[Phil Lamovie]

Hi All,

The problem of Injection timing at cranking is one of engine requirements.

The problem is not as it seems. The engine doesn't run on petrol it runs on
vapor.
When the engine is cold very little vaporization takes place in the cold
air stream.

In fact the only physics available to aid  us is low pressure. Thus the
time to inject is during the exhaust stroke. This allows maximum time for
latent heat absorption and thus vaporization and allows for a vacuum build
up during intake as long a you can convince the customer to keep their foot
of the acc.  pedal.

You will need approx. 300% excess fuel to start the engine. This is due to
the partial vaporization of the fuel. A cranking engine often has HC
components in the exhaust of 50-60% raw fuel. Don't assume that the
compression of the air will supply sufficient heat as the pressure builds
up very late in the piece. And the dwell time at that temp is only a few
ms.  Worse still if you get the A/F wrong you have just chilled your
combustion chamber with a dose of raw fuel. Makes it important to really
over fuel.

Try and imagine the volume of fuel from a Holley double pumper in the two
or three squirts that are applied before cranking.  20 - 40 cc !!! Then
check to see your 5 ms injection volume. Say  grams/ms/injector  GMS
0.03x5x8= 1.2   40cc=22 gms approx.

You're working in grams and Holley in gallons.


You can either multiple fire the injectors by dropping the input divisor or
simply have a table with very big numbers. You may still run out of pulse
width if your injectors are too small. (See Toyota for several hundred
million examples) they still fit cold start injectors in place with very
low winter temps.

The aim of the game is to use the smallest  injector possible that fulfills
the requirements. Often 85% duty cycle is the target. that is of course 85%
of 720 Deg.
or 612 Deg.

Due to the acc of the engine 0 to 600 in 1 to 1.5 seconds the pulses must
be linked to the crank. This will ensure that there are no missed pulses.
This also means that the crank trigger event must result in an injection
event even if the output buffer has not been updated with a sensible rpm
measurement and thus  in fact the ecu may not know how fast the engine is
tuning.

Most manufacturers use a cranking table that cuts out at say 350 rpm. Below
that
each trigger pulse is greeted with the same ms injector output  regardless
of vacuum..

If you hard link the events then it doesn't matter how fast the acc of the
engine really is. If the fueling is  wrong it will simply acc more slowly.

The clue is to use the micro to set up a timer/off event from the
instigation  of injection.

Your electric motor should not be able to unsync a magnetic trigger firing
a gate on a transistor. Though it would confound the heck out of an input
capture rpm reading
as it is using clock events to determine rpm. During cranking it would be
very late in it's determination. By the time it has worked out the rpm it
could be 200% wrong in
its phasing as the engine speed could more than double on a 1/4 revolution.

Hope this cures some headaches and creates many others.


Regards

 Phil Lamovie

 injec at ains.net.au

     cogito ergo zoom

 Injec Racing Developments
 Director of Engineering