[alfa] RE: Adjustable fuel pressure regualtor

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi Tony,

I saw your question about adjustable fuel pressure regulators.  Before you
decide to fit one you need to consider what you are trying to achieve and the
best way to do that.

The fuel flow rate of an injector is based on a specific differential pressure
between the delivery side and the exit side.  Bosch rate their injectors at
differential pressure of 3bar, which is about 43psig.  At idle the inlet
manifold gets pulled down well below the atmospheric pressure, say to about
40kPa.  This means the fuel pressure needs to lowered by the same ratio, or a
value of 60kPa.  So at idle the Bosch fuel pressure regulator lowers the fuel
rail pressure by about 60kPa to around 2.5bar.  When you floor the accelerator
the inlet manifild pressure rises to just a tiny bit below atmospheric and the
fuel pressure regulator causes the fuel rail pressure to about 3bar.  If you
connect a pressure gauge into the fuel rail and switch on the fuel pump
without cranking the engine the pressure will be the same as you get at WOT.
When the engine is started the vacuum in the inlet manifold pulls the fuel
pressure regulator is subjected to the inlet manifold pressure and the fuel
rail pressure compensate accordingly.

The big factor is that under all of these conditions the fuel rail pressure
remains a constant 3bar relative to the inlet manifold.  This is important
because the variation of the differential pressure changes the injector flow
rate, measured either in cc per minute of lbs of fuel per hour, which ever
measurement you are best happy with.

Now if you start changing the fuel pressure you alter the flow rate.  The
equation is not a linear one.  For example if you double the pressure
difference you do not get twice the fuel flow.  The formula has a square root
function so the fuel only goes up a little bit.  For a 50% increase in
pressure, from 2bar to 3bar, you get an additional 22% or so of fuel.

But the big problem is that when you crank up the fuel pressure the injector
takes longer to open.  If you raise the pressure much above 6bar most
injectors only flow about 35% of the fuel they can flow at 3bar.  At 8bar the
injector will not lift at all!  The thing that keeps the injector closed on
the pintle seat is the fuel pressure, plus there is a small spring to prevent
weeping when the pressure bleeds down.  Increasing the pressure too much is as
bad as running too low a fuel pressure.  If the solenoid cannot bump the
pintle off the seat it will not open properly.

So if the amount you adjust the pressure is small, less than about 50%, then
little changes.  The injector opening time goes up at a linear rate versus
pressure, the flow at a non-linerar rate.  If you double the fuel pressure
theoretically you should get somewhere close to 50% more flow.  That is fine
when the injector is held wide open and is how the flow rate is determined by
the maufacturer.  But in practice we pulse the injectors on & off so the
increased opening time eats into the computed pulse period and we can get less
fuel flowing rather than more.  If the ECU cannot be remapped to cater for the
long opening time with the higher pressure we are screwed and hence limited to
a small external adjustmentment.

One Opel engine I worked on recently had an external fuel pressure regulator
fitted by the owner set to give 5bar constant pressure, there was no Vacuum
adjustment.  The engine ran very lean at the top end and detonated severely.
Relacing the adjustable regulator with the stock one giving a constant 3bar
differential pressure gave plenty of fuel and 65kW more power!  So the money
spent on a fuel pressure regulator might be better spent on something else?

The other factor to bear in mind is that most conventional injectors do not
like being operated at more than about 80% duty cycle.  One test I did on a
random sample of different injectors, when you get to above 85% duty cycle the
pintle sort of floats on the seat and neither fully opens or closes.  The fuel
delivery dropped around 35% from the 80% duty cycle rate.  This was at a
constant 3bar pressure on a test fixture.  Now factor in the increased
pressure, to say 4.5bar, and the fuel drops like a stone to as low as 25% of
the 80% duty cycle rate, in other words the engine now runs dangerously lean
when are demanding a greater fuel flow!

Armed with these facts should enable you to make the right decision.

John
Durban
South Africa
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