In websites I have seen that fuel pumps have a pressure range of say 30-95 psi. Why do they give a range of pressure? I had read that these pressure values refer to the unobstructed flow pressure delivered by pump.
I had read about pressure regulators having boost reference port. These ports are fed with an air line from intake manifold so that under vacuum/boost pressure generated, the boost reference pressure from manifold will control the amount of fuel bypassed by regulator to vary the fuel pressure in the delivery line (To increase the delivery line pressure we should be reducing the amount of bypass/return fuel amount right? Although this site tells that a more obstructed fuel line increase the fuel system pressure ). Thus fuel pressure regulation is done by boost referencing the pressure regulator depending on manifold pressure rather than pump varying its speed. And this is most commonly used system for delivering variable fuel demand.
But in an example calculation they mention that if the injector supplies fuel at an effective pressure of 45 psi when intake manifold pressure is at atmospheric pressure. Then if the engine is boosted to a pressure of 10 psi, without boost reference the effective pressure at which the fuel will be injected at injector will be 45-10=35 psi. When boost referenced, the by pass opening will be controlled such that the delivery line pressure is increased i.e. 45+10=55 psi. So ultimately the effective pressure at the injector will still be 45 psi. But the problem with this calculation is that even when engine is at load (since it is boosted) there is a higher fuel flow and probably higher fuel pressure requirement. So how does an effective injection pressure of 45 psi at injector will be enough under higher load?
I have also read that there are now variable speed pumps which vary the fuel pressure by varying the speed of rotation of the pump based on an electronic pressure regulator. But I think these are not still commonly used.