DIY EFI

[Huw Scourfield]

Since I've been prompted to suggest a site for reading, gleaning info on the
DIY EFI I thought I'd at least start a thread on the subject(especially with
an eye on recent ramblings etc, maybe it's time to return to EFI), and
perhaps a bit later will post some URLs for reference. Please feel free to
add/defame as neccassary).

Requirement:
accurate ignition timing.
To do this requires at least a representation of where the egine is, ie cam
sensor. From this can be calculated engine speed, needed for one dimension
of the ignition map. Also required is a representation of engine load, this
can be from a throttle position sensor or manifold pressure sensor. This
gives the second dimension of the
ignition map.
The values programmed into this map may be derived in all sorts of ways, but
they will always have to be corrected for real world/ideal values. Other
corrections would be for coolant temperature, air temperature, maybe
barometric pressure(depending on where you live, halfway up Pikes Peak
etc.).

For an ECU /OEMs to work out what site in the map it needs to be requires,
1, a reading of engine speed,- this merely comes from time difference
between cam sensor events.(OK it can/should be done from crank events, but
I'm trying to keep this simple). One important point to bear in mind is that
the cam event signal must be sufficiently in advance of the required
(maximum advance) spark  event. On reading a cam event the ECU must read the
load sensor, then read the map value for that load and speed from the
ignition map.
Then it must somehow convert the degrees of advance in the map to degrees of
delay from the cam event. The ECU obviously does not work in degrees, but in
microseconds, therefore for each and every ignition event a conversion has
to be done from microseconds to degrees, then back again from degrees to
microseconds. Also bear in mind that the corrections for coolant temp ,
barometric pressure etc. would have to be added the degrees result before
conversion.
It sounds quite complicated but with modern microcontrollers having built in
capture(to measure microseconds between cam events), and compare(to output
the required ign pulse) ports it really is not that difficult to grasp.

For Fuelling, the basic requirement(disregarding sequental injection) is
merely to squirt the right amount of fuel into each, or all inlet ports.
Timing is not of great importance, but quantity is. Fuelling is much simpler
an issue for the ECU and comes down to reading of sensors and corresponding
maps, then adding the necessary corrections. Maybe later.
    Huw

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