jmackro
Well-Known Member
Kasbatts:
I don’t want this to come across as a personal attack, but I have to say that I disagree with pretty much everything you wrote on Dec. 20th.
No, that isn’t correct. The type of electric pumps that go “tick tick tick” will adjust their frequency based on volume. Yes, they deliver a constant pressure (which is what you want), but their volume will vary based on demand.
There is no “extra volume of fuel”. I will admit that if you had a fuel pump that delivered too much pressure, then the set-up described above would serve to bleed off some of that pressure. But a return line with a restrictive orifice is an awfully crude way to decrease fuel pressure – it would not adapt to varying flow rates or fuel viscosity (which varies with temperature). In control systems terms, this is an “open loop” solution. A fuel regulator, which might cost $50, would be far more precise, as it is a “closed loop” solution. Better yet, just use a fuel pump that delivers the correct pressure.
No, a mechanical fuel pump’s output volume is not related to engine speed. Mechanical fuel pumps are sized so that even at idle, they have the capacity to deliver sufficient fuel to meet any volume requirement.
I would encourage you to take a fuel pump apart – either electric or mechanical. You will see that the actual pumping is done by a spring acting against a diaphragm – not by force from the solenoid or engine. The energy from solenoid or engine just serves to load the spring – it doesn’t directly pump the fuel. If the spring exerts a force equal to “F” and the diaphragm has an area of “A” then the pressure is always F/A. Once the carburetor has drawn sufficient fuel from the pump, either the solenoid or the engine-driven cam re-loads the spring, starting another cycle.
I don’t want this to come across as a personal attack, but I have to say that I disagree with pretty much everything you wrote on Dec. 20th.
No, that isn’t correct. The type of electric pumps that go “tick tick tick” will adjust their frequency based on volume. Yes, they deliver a constant pressure (which is what you want), but their volume will vary based on demand.
There is no “extra volume of fuel”. I will admit that if you had a fuel pump that delivered too much pressure, then the set-up described above would serve to bleed off some of that pressure. But a return line with a restrictive orifice is an awfully crude way to decrease fuel pressure – it would not adapt to varying flow rates or fuel viscosity (which varies with temperature). In control systems terms, this is an “open loop” solution. A fuel regulator, which might cost $50, would be far more precise, as it is a “closed loop” solution. Better yet, just use a fuel pump that delivers the correct pressure.
No, a mechanical fuel pump’s output volume is not related to engine speed. Mechanical fuel pumps are sized so that even at idle, they have the capacity to deliver sufficient fuel to meet any volume requirement.
I would encourage you to take a fuel pump apart – either electric or mechanical. You will see that the actual pumping is done by a spring acting against a diaphragm – not by force from the solenoid or engine. The energy from solenoid or engine just serves to load the spring – it doesn’t directly pump the fuel. If the spring exerts a force equal to “F” and the diaphragm has an area of “A” then the pressure is always F/A. Once the carburetor has drawn sufficient fuel from the pump, either the solenoid or the engine-driven cam re-loads the spring, starting another cycle.
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