fuel rail

[Bernd Felsche]

Nigel Heron tapped away at the keyboard with:

> i'm going to be making a custom fuel rail for my 16v vw out of
> billet aluminum.
> What is the appropriate internal volume of a fuel rail? is there a
> basic guidline based on injector size, fuel pressure, max rpm,
> etc.?

Sorry; no hard and fast formulae...
But it would also depend on injection strategy; batch/sequential
firing including any possible overlapping of duty cycles with
sequential.

Also, coming into the equation is the fuel pump delivery rate and
pressure. The location of the pressure regulator also comes into the
equation(s).

I'm sure that one could sit down and work it out from scratch.
My initial instinct would be to verify that the fuel pump is up to
the job (i.e. capable of feeding all injectors at 100% duty cycle at
the required pressure), and then look at the response of the
pressure regulator.

Given that the amount of fuel injected is very small, any
steady-state analysis of the flow properties of the fuel rail would
be a waste of time.

There are 2 reasons why fuel rails are sized in a particular way:
1. to keep a fairly-constant fuel pressure during injection, and
2. to reduce the noise of pressure pulses.

Both of those can be helped by using a pulse-dampers which is
common in Bosch pulsed-injection systems (though VW tended not to
use them on Digifant at least, perhaps because of high fuel pump
delivery pressure or flexible mounting of the fuel rail). These
dampers are small pressure accumulators, typically mounted on the
fuel rail.

You can always "cheat" and look at the size of the fuel rail(s) on a
higher-performance production vehicle.

-- 
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