EFI - Does it really work like that ?

[Roger Heflin]

On Sat, 15 Apr 2000, Malcolm Robb, LC 0112G wrote:

> 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 ?
>

Yes, this is true.  Otherwise they would need much bigger injectors
which get harder to control at small pulsewidths, and provide worse
mileage and idleing.
 
> 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.
> 
That is not what the FPR is there for.  The FPR reduces pressure to
maintain a constant fuel pressure relative to intake pressure, so the
pressure changes relative to outside air pressure, but not relative to
MAP (intake pressure).   I bet on a blower car it will raise pressure
to maintain constant fuel pressure relitive to MAP, otherwise, if you
get to say 25 psi (+10 over atm) and you had the same fuel pressure as
at 15psi (+0 boost) then you would have less fuel, and probably need
some new pistons.

				ROger

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