[Diy_efi] CIS EFI via Fuel Pump PWM

[Mike]

If you are considering controlling the high pressure fuel pump
then thse factors are an issue:-

a.	Rotational inertia of rotor in pump
b.	Potential for instantaneous cavitation - especially
	for idle to WOT step change in load - depends of low p pump.
c.	Characteristics of fuel pressure regulator at the injector
	rail - propensity to oscillate during step change in pressure

Combinatorially (you'll hear that more and more) is the issue of
the pipe's expansion or equivalent 'spring constant' in relation
to items a and b and c above and the potential this has for under-damped
response to transients !

If it were me and I was hell bent on doing pump PWM to control flow,
I'd be inclined to put the high pressure pump as close to the
pressure regulator and fuel rail as possible *and* use switch mode
driver for the pump controlled by a PID algorithm, the switch mode
driver output would be capable of ramping up the pumps rotor as
fast as possible when WOT is called for and using a pressure drop
calibrate dump valve to return line pressure back to idle when WOT
is released - and for that of course you'd need a pressure sensor,
I'd guess the switchmode driver output would be from 9 to 20v
and handle 100-120W.

Certainly do-able and different, but given the great ease in controlling
injectors via sync'd PWM, I'd look at this as the primary control
mechanism with a PWM solenoid operated operated pressure regulator
for a trim - especially if the injector PWM is becoming maxed out
due to factory injectors and unwillingness to change injectors and
remap and use a switch mode power supply to ensure the high
and low pressure fuel pumps are driven with a constant 13v regardless
of battery, alternator voltages or line drops etc etc

rgds

mike




At 10:24 PM 5/8/2002 -0500, you wrote:
>
>
>Bernd Felsche wrote:
>> If fluid isn't compressible, how does it transmit sound? :-)
>> Yes, fuel *is* compressible.
>> 
>> The longer the fuel line, the longer the propagation delay.
>
>The propagation delay is the speed of sound.  I doubt that's worth
>considering.  The delay in building pressure is due to the
>compressibility of the fuel and the stretchy-ness of the fuel lines.  I
>get to use words like that because I'm not an ME.  Could someone who is
>turn on their calculator and calculate a worst case time constant for
>the pressure buildup?  
>
>I'll bet a virtual beer that neither makes a significant difference.
>
>--steve
>
>-- 
>Steve Ravet
>steve.ravet at arm.com
>ARM,Inc.
>www.arm.com
>
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>

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