Fuel pump
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A fuel pump is an essential component on a car or other internal combustion engined device. Fuel has to be pumped from the fuel tank to the engine and delivered under low pressure to the carburetor or under high pressure to the fuel injection system. Some fuel injected engines have two fuel pumps for this purpose: one low pressure/high volume supply pump in the tank and one high pressure/low volume pump on or near the engine.
[edit] Mechanical pump
In earlier cars built before the late 1970s, the pump was mechanically driven by a lobe on the engine's camshaft and therefore bolted to the engine block. It created negative pressure to "suck" the gasoline through the lines. However, the low pressure between the pump and the fuel tank, in combination with heat from the engine and/or hot weather, could cause the gasoline to vaporize in the supply line. This would result in fuel starvation as the fuel pump, designed to pump liquid, not vapor, would be unable to suck more gasoline to the engine, causing the engine to stall. This condition is different from vapor lock, where high engine heat on the pressured side of the pump (between the pump and the carburetor) would boil the fuel in the lines, also starving the engine of enough fuel to run. Mechanical automotive fuel pumps generally do not generate much more than 10-15 psi, which is more than enough for most carburetors.
As engines moved away from carburetors and towards fuel injection, mechanical fuel pumps were replaced with electric fuel pumps, because fuel injection systems operate more efficiently at higher fuel pressures (40-60psi) than mechanical pumps can generate. Electric fuel pumps are generally located in the fuel tank, in order to use the fuel in the tank to cool the pump and to ensure a steady supply of fuel.
Another benefit of a tank-mounted fuel pump is that a suction pump at the engine could suck in air through (difficult to diagnose) faulty hose connections, while a leaking connection in a pressure line will show itself immediately. A potential hazard of a tank-mounted fuel pump is that all of the fuel lines are under high pressure, from the tank to the engine. Any leak will be easily detected, but also extremely hazardous.
Electric fuel pumps will also run whenever they are switched on, which can lead to extremely dangerous situations if there is a leak due to mechanical fault or an accident. Mechanical fuel pumps are much safer, due to their lower operating pressures and because they 'turn off' when the engine stops running.
[edit] Electric pump
Nowadays, the fuel pump is located inside of the fuel tank and is usually electric. The pump creates positive pressure in the fuel lines, pushing the gasoline to the engine. The higher gasoline pressure raises the boiling point. Placing the pump in the tank puts the component least likely to handle gasoline vapor well (the pump itself) farthest from the engine, submersed in cool liquid. Another benefit to placing the pump inside the tank is that it is less likely to start a fire. Though electrical components (such as a fuel pump) can spark and ignite fuel vapors, liquid fuel will not explode (see explosive limit) and therefore submerging the pump in the tank is one of the safest places to put it. In most cars, the fuel pump delivers a constant flow of gasoline to the engine; fuel not used is returned to the tank. This further reduces the chance of the fuel boiling, since it is never kept close to the hot engine for too long.
The electric fuel pump is generally on whenever the car's ignition switch is in the "on" position. The metering of the fuel into the engine is performed by the fuel-injection or carburetor systems.
The ignition switch does not carry the power to the fuel pump, instead it activates a relay which will handle the higher current load. It is common for the fuel pump relay to become oxidized and cease functioning; this is much more common than the actual fuel pump failing. Modern engines utilize solid-state control which allows the fuel pressure to be controlled via pulse-width modulation of the pump voltage.[1] This increases the life of the pump, allows a smaller and lighter device to be used, and reduces electrical load and thereby fuel consumption.
Some cars with an electronic control unit have safety logic that will shut the electric fuel pump off even if the ignition is "on" if there is no oil pressure, either due to engine bearing damage or a non stalled engine, e.g. in a car accident. In case of an accident this will also prevent fuel leaking from any ruptured fuel line. Other cars have an additional roll over valve, that will shut off the fuel pump in case the car rolls over.
The fuel sending unit assembly is the combination of the electric fuel pump, filter, and the electronic device used to measure the amount of fuel in the tank via a float attached to a sensor with sends data to the dash mounted fuel gauge.
[edit] Animation
