PWM fuel pump

Raymond C Drouillard cosmic.ray at juno.com
Thu Feb 18 05:23:53 GMT 1999


On Wed, 17 Feb 1999 21:05:18 -0700 bearbvd at sni.net (Greg Hermann) writes:
>>Jeffrey T. Birt wrote:
>>
>>> Hi all,
>>>
>>>     I'm new on this list so this is the first post that I have seen 
>on
>>>the subject
>>> of controlling pressure via PWM of the pump.  It seems that trying 
>to
>>>control a
>>> pump this way would cause premature failure of the pump.  Generally
>>>starting a
>>> pump (of any type) is much harder on it than continuos operation.  
>PWM
>>>would stop
>>> the pump and then restart it under load.  It would be much harder 
>on the
>>>pump to
>>> start it with 30 lb or so of pressure
>>> against it.
>>
>>> IMHO, the pump never stops, just runs slower....
>>
>>>    Next, picture this scenario.  Your tooling along @ 20 mph and 
>your pump is
>>> developing (x) lbs. of pressure.  You then mash the accelerator 
>down and
>>>the ECU
>>> determines that you now need (x+y) lbs.  Currently the fuel system 
>is
>>>only @ (x)
>>> lbs., when you accelerate you are effectively lowering the 
>restriction
>>>that your
>>> pump is developing pressure against.  The duty cycle of you 
>injector(s)
>>>go up.
>>> More fuel in the same time period equals less restriction.  The 
>pump has to
>>> compensate for the sudden loss of pressure (x) and try to build up 
>even more
>>> pressure (x+y).  This situation will lead to a lag in fuel pressure 
>on sudden
>>> acceleration.
>>
>>Hopefully, the pump will respond in milliseconds, the controller in
>>nanoseconds - as
>>fast as the injectors.
>>
>>Tom S
>
>Yep--Tom is right---just don't put any kind of an accumulator 
>downstream of
>the pump!!
>
>Regards, Greg
>


Perhaps looking at the TPS will help.  Any quick increase in throttle
opening can be translated into an immediate increase in the duty cycle to
the PWM.  After a brief time (OK, so some experimentation will be
necessary), it can go into "closed loop".

Also, it might be necessary to do this on the reverse side.  A quick
decrease in throttle opening can result in reduced power to the fuel
pump.

Ray Drouillard


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