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EFI - Does it really work like that ?
This is going to reveal how little I understand, and could well get me
flamed....
I was thrashing round the track in the pouring rain today when I realised I
didn't understand how EFI works.( Pause whilst a Lotus Elise spins off into
the tyre wall) Wheelspin was causing my engine to bounce off the rev
limiter at about 6000 RPM. 6000 RPM = 100 RPSec, or 10mS per rev. It takes
2 revs per cylinder cycle in a 4 stroke engine, so this is 20mS per 4
stroke cycle. With me so far ? . (oops therre goes an Mitsibishi EVO6)
However, the induction phase when air+fuel is being drawn into the piston
via the inlet valves is only 1/4 of this cycle, so it can't last much
longer than 5mS. But I know the injector pulse duration can be longer than
10mS from my ALDL sampled data, so how on earth is it working. (whoa - its
gonna cost a lot to fix that Sierra Cosworth)
A friend at the track said that above a given RPM the ECU gives up trying
to time sequential injecton pulses and basically just squirts fuel
continuosly into the inlet ports, regardless of the phase in the 4 stroke
cycle. Is this really true ?
Someone else suggested that the fuel pressure regulator (FPR) is the
answer. My understanding is that the FPR allows fuel pressure to rise in
the feeder rail in more or less direct proportion to the vacumn in the
inlet manifold, thus forcing more fuel through the injectors at higher
engine speeds. Fair enough for a normally aspirated engine, but my engine
has a turbo, so when its working at full whack, there ain't no vacumn in
the inlet manifold - infact the opposite, its at almost 2 bar.. I know my
car has an FPR, but I just can't work out why.
Then I realised that the same inlet manifold vacumn is used to power the
servo assisted brakes. At this point I felt a bit like a passenger in a
Boeing 747 who starts to wonder why a 400 ton aeroplane can fly when a 10
ton tank can't. Then I realised there is a brake servo vacumn resevoir, so
I calmed down - a bit.
Knowledge is a powerful thing but a little knowledge is a dangerous thing..
Anybody care to put me out of my misery on any of the above points?
Cheers,
Malcolm Robb, LC 0112G
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