Originally Posted by
Discorde
Quick tech question: what are the relative merits of gear-type fuel pumps over diaphragm pumps? Do most small aero-engines use the latter and, if so, why?
Whether a fuel pump is needed depends on the engine type and the position of the fuel tank(s) relative to the engine.
High wing aiplanes with carburetted engines typically have no fuel pump as the location of the tanks provides adequate pressure at the carb. E.g. Cessna 182.
Low wing aiplanes with carburetted engines typically require a fuel pump and, if one is required, a second is usually provided as a backup. E.g. PA-28-180 which has an engine driven diaphragm pump and and an electric reciprocating pump.
Fuel injected engines typically require a fuel pump regardless of fuel tank position as the required pressure at the injection servo is much higher than can be provided by gravity feed. E.g. FX-3 Carbon Cub which has an engine driven diaphragm pump and an electric gerotor pump. In this aircraft any pressure pulsing with the engine driven diaphragm pump is a non issue.