fuel pressure regulator?

[vdubyah73]

Nothing wrong with mine just curious if I have it figured right.
Regulator works off of spring pressure and has vacuum involved.

Vacuum comes from the plenum so vacuum is high when off throttle.

There is almost no vacuum when at full throttle.

Does the vacuum reduce fuel pressure when just idling?

Does the higher vacuum, from lifting off the gas, at highway speeds, reduce fuel pressure even more?

Is the virtual lack of vacuum under full throttle conditions what gives the full throttle enrichment. No vacuum assisting the spring to regulate pressure would give more pressure?

Like I said, just curious?

[Hippie]

I don't know how VW ports vacuum to their regulators, but in general terms, fuel pressure regulators on fuel injected gasoline engines use the vacuum feedback on the regulator diaphragm only to cancel out fuel pressure variations due to changing manifold vacuum.

For example, without this feature, if the fuel pressure being fed into the injectors is 35 psi relative to the outside atmosphere, and the manifold pressure goes into a high vacuum due to throttle closing (or idling) then the outlet side of the injectors (the manifold) goes down while the 35psi inlet pressure would stay the same. This would cause the effective fuel pressure across the injectors to rise and the fuel spray would increase in volume. That would make it impossible for the computer to control lean/rich mixture by changing injector dwell (on) time.
With vacuum feedback, when the manifold vacuum increases for any reason, the fuel pressure to the injectors is decreased.

The net result is that the vacuum diaphram keeps the pressure across the injector relatively constant under changing engine loads so that the computer can supply fuel/air ratio based on the time the injectors are open.



Rob

[Amskeptic]

Does the vacuum reduce fuel pressure when just idling?
Does the higher vacuum, from lifting off the gas, at highway speeds, reduce fuel pressure even more?
Is the virtual lack of vacuum under full throttle conditions what gives the full throttle enrichment?

Yes.
Barely.
No. As Hippie mentioned, the regulator just allows the injection quantity to remain as a constant across different manifold pressure conditions.
Colin

[vdubyah73]

Got it, thanks.

[karl]

Also realize that the fuel pressure regulator acts as an accelerator pump. The ECU sends a time signal to the injectors to open them x amount of milleseconds. It does not know what the fuel pressure is. At 32 psi [as an example] x amount of time injects x amount of fuel. x is an unknown figure. When you step on the throttle, the manifold vacuum drops and fuel presure goes up, usually to 36 psi. The injector still opens x amount of time, but more pressure equals more volume of fuel.

On O2 sensor controlled engines, they have a 60/40 fixed signal when you accelerate to not drive the O2 sensor crazy when you have the enrichment when you accelerate. Otherwise it tries to lean out the rich condition.

[Amskeptic]

Also realize that the fuel pressure regulator acts as an accelerator pump. The ECU sends a time signal to the injectors to open them x amount of milleseconds. It does not know what the fuel pressure is. At 32 psi [as an example] x amount of time injects x amount of fuel. x is an unknown figure. When you step on the throttle, the manifold vacuum drops and fuel presure goes up, usually to 36 psi. The injector still opens x amount of time, but more pressure equals more volume of fuel.

Not more volume of fuel. Same volume of fuel, corrected by the increased total air pressure of wide-open-throttle outside of the injector. It is actually the low absolute pressure of high vacuum at idle that necessitates throttling back the fuel pressure at idle.
"Accelerator pump" enrichment occurs instantaneously with overshoot of the wiper in the AFM, followed by full throttle enrichment via throttle switch on earlier L-Jets and wiper position that triggers full load circuit in later buses.

It took me a while to grasp that "high vacuum" is just a misnomer for "low absolute pressure" and "low vacuum" is actually higher absolute air pressure in the manifold.
Colin