The diaphragm pump transported the fuel, at a reasonable cost and compactly, from the tank to the carburetor. Due to the the mechanical connection it automatically interrupted the fuel transport if the engine was stopped. This safety feature had to be later, additionally integrated into the electric pumps. Generally speaking, diaphragm pumps can also carry liquids which have unfavorable lubrication qualities.
How it works
A plunger is nearly always moved by a cam in one direction, directly or through a lever system, and by a plunger spring in the other direction. The diaphragm follows the plunger only during the downward movement, because the plunger spring is stronger than the diaphragm spring, which can only press the diaphragm upwards, if no disturbances occur in the suction- or pressure area and no valve (e.g., the needle valve of the carburettor) blocks the channel. In this case the diaphragm remains below and only the plunger moves in an idling stroke.The other side of the diaphragm forms the variable pump area which is connected with the supply pipe on the left through a suction valve, and on the right through a pressure valve with the outlet pipe. The pumping chamber can contain a small filter.
The membrane pump is not suitable for higher demands on the consistent fuel-flow requirements. 12/09