Pumps come in two general design categories, Constant volume, and constant pressure.
The constant volume pumps are generally of a constant displacement gear pump type design.
These pumps will typically try to pump 30 gallons per hour at whatever pressure is needed to do the job.
They are capable of producing hundreds to thousands of psi depending on what motor is powering them.
They typically have internal pressure limit valves to prevent them from destroying themselves or rupturing the fuel lines.
They are typically installed with a dedicated pressure regulator that bypasses the excess fuel back to the main tank.
This is the type of fuel pump that is found on the 912is, 914 and 915 engines.
The constant pressure pumps come in two basic flavors, Impeller and Diagram.
They are typically low-pressure pumps.
The diaphragm pump is the typical mechanical pump found on most carburetted engines.
A mechanical cam or pressure from the crankcase pushes the diaphragm out causing fuel to be drawn in.
When released, a spring pushed the diaphragm back in, pushing the fuel out. About as simple as you can get.
Blocking the output of this type of pump results in the diagram and spring holding pressure as the cam stops cycling.
This type of pump is found on all the non-turbo carburetted Rotax engines.
The spring determines the pressure. You can hook a dozen of these in series and the pressure at the output will be determined by the strongest spring the chain.
A variation of the diaphragm pump is the very common aftermarket Solenoid-Driven Piston pump characterized by its clicking sound when operating.
An electric Solenoid pulls back a spring-loaded piston but otherwise, it operates on the same principle as the diaphragm pump.
Blocking the output of this pump results in the piston and spring holding pressure as the solenoid stops cycling.
The Impeller driven pump, often misnamed the Centrifugal Pump, but properly an Inertial Pump, uses a rotating impeller to pump the fuel.
If you were to block the output of this pump you will produce a limited maximum pressure determined by the amount of inertia in the rotating fuel mass.
The pump just harmlessly stalls.
Placing multiple Impeller pumps in series will produce a slight increase in the maximum pressure of a single pump, but far from what you would expect if the pressures of all the pumps in series were added up.
Placing an electrical AUX pump in front of a mechanical pump allows the electric pump to pressurize the fuel lines that were otherwise under a slight vacuum.
This relieves the mechanical pump of some of its workload.
If the Electric pump delivers a higher pressure than the mechanical pump, the diaphragm in the mechanical pump just moves back against its spring and never moves forward.
The electrical mump takes over all of the pumping and just pumps through the mechanical pump as if it were not there.
The pressures do not add. Whichever of the two pumps has the highest pressure is the pressure that will be delivered.
The electrical pumps typically come in two pressure ranges, 4 psi, and 9 psi.
The 9 psi pump should be avoided as it will tend to overpower the Rotax float valves and flood the engine.
