Fuel Pump Suction Performance

Question

Electric Fuel pumps and mechanical fuel pumps are well specified when it comes to pressure and flow rate .Reputable manufacturers provide a curve of pressure say PSI Vs flowrate say Gallons per hour. Looking at the curve shows that max flow is available at zero pressure and max pressure is available at no flow .The real operating point is somewhere in between. Low pressure pumps being desirable for carb systems and high for injectors. The level of the fuel pump is close to the level of the tank on cars that I have seen. This means that little or no suction is demanded from the pump. Electric fuel pumps are often mounted close to the tank which is even better.

• What if the tank is very low like my jetboat?
• What if the fuel filter is above the tank ?What if the pump is after the filter which seems sensible from a pump protection viewpoint?
• How much can these pumps suck?
• Does making the pump suck give more chance of airlock problems?

Answer 1

The pump performance is going to depend on the type of mechanism inside the pump.

Gear rotor or vane pumps will give you the best suction because it is the closest to a positive displacement pumps. They are expensive and not often used.

A turbine or jet pump works like a turbo charger. They make little to no suction.

The way a normal fuel system is set up is; the fuel pump lives inside the tank. The pump lives in a cup that is fulled with fuel. The pump can draw fuel from either the cup or the tank through a sock. The sock is a light filter that keeps particulate matter out of the pump.

The pump then pushes the fuel though the filter. You never want to pull fuel through the filer. When this happens a vacuum would drawn on the fuel causing it to evaporate and vapor lock the system. Even without a filter mechanical pumps that are on the engine had problems vapor locking because they were pulling fule from the tank with a suction. This is the reason mechanical pumps have universally disappeared.

Depending on if you have a return or returnless system one of two things happens. In a return system the extra fuel that bypasses the regulator is returned to the tank and usually dumped into the cup. In a returnless system the regulator lives in the cup and extra fuel exits directly in the cup.