Hi Randall
The shuttle is powered by the fuel pressure, 80psi (ish) before the shuttle
will even move, 100 - 115psi for normal running, the fuel entering one side
moves the shuttle across pushing the fuel from the other side to the
relevant injector, the shuttle makes six strokes (three round trips) per
revolution of the metering unit 1 full firing cycle (2 turns of the crank).
There are 6 outlets set as 3 pairs spaced at 120 degrees there are another 3
pairs of holes (inlets) spaced at 120 degree intervals (opposite the
outlets). There is a central rotor with 1 hole at one end and another hole
180 degrees out of phase at the other end, these are positioned such that
they line up with the inlets and outlets alternately, the shuttle and stops
sit in a hole on the central axis of this rotor, such that the shuttle is
reciprocating along the axis of rotation.As one hole in the rotor lines up
with an inlet, fuel flows in pushing the shuttle to a fixed stop, then after
60 degrees of rotation, that hole lines up with an outlet, the hole on the
other end lines up with an inlet and the fuel pressure pushes the shuttle
back the other way, to an adjustable stop, forcing the fuel previously taken
in, to exit to the injector, another 60 degrees rotation and the shuttle
returns and the cycle continues.
The position of the adjustable stop is what governs the fuel flow, by
varying the stroke of the shuttle. The position of the adjustable stop is
controlled by a linkage connected to a diaphragm which closes off the bottom
of the vacuum chamber, a link from this diaphragm lifts a roller up when
exposed to the manifold vacuum, the roller runs on a track set on an angle,
nearer the adjustable stop at the top and farther away at the bottom, thus
when the roller is lifted by a vacuum the fuel flow is reduced. Conversely
when the vacuum reduces the roller drops (spring assistance) and the travel
increases thereby increasing the fuel flow.
There is no advance / retard for the fuel delivery either!
Graham.
----- Original Message -----
From: "Randall Young"
Subject: RE: PI
> But isn't the motion of the shuttle itself derived from engine rpm ? One
> stroke per cylinder firing ? (Or perhaps it's 3 strokes per revolution,
I'm
> not clear on whether there is 1 shuttle or 6.)
>
> I'm guessing those are idle figures ... lower than the carb version
because
> of the more radical cam used in the PI engines. But the exact vacuum
makes
> no difference to my argument ... the same thing applies even at 1" of
> vacuum, the engine requires nearly twice as much fuel at 4000 rpm as it
does
> at 2000 rpm (for the same manifold pressure) *because* it's also drawing
> nearly twice as much air and the fuel/air ratio must stay within a very
> narrow band for the engine to run.
>
> Randall
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