EFI - Does it really work like that ?

[David Cooley]

At 04:28 PM 4/15/00, you 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)


Actually, Induction is more than 1/4... 1/4 would have an advertised intake 
duration of 180 deg...  Most cams are over 200.
Injection actually starts before the valve opens, and continues until the 
valve is closed


>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 ?


Some do, Some don't.


>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.


To keep a constant differential of Fuel pressure vs. manifold pressure. If 
FP is 35 PSI with 0 vacuum, 0 boost, then at 10 PSI boost you have 45 PSI 
fuel... (35 PSI difference).  at 30" vacuum, you have only 20-25 PSI fuel 
to keep the difference to 35 PSI.  Makes calculating pulse width more of a 
constant, instead of having to create a new injector flow rate for each 
step of pressure in the manifold.

===========================================================
David Cooley N5XMT Internet: N5XMT at bellsouth.net
Packet: N5XMT at KQ4LO.#INT.NC.USA.NA T.A.P.R. Member #7068
We are Borg... Prepare to be assimilated!
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