Fuel pump is the main structural element of fuel system in petrol engines. It delivers certain amount of fuel under pressure to the injectors (engines with fuel injection) or carburettor (carburettor engines). Depending on the type of drive, there are mechanical and electrical fuel pumps.
Mechanical fuel pump
Mechanical fuel pump (fuel pump) is applied to gasoline engines. It has a mechanical drive from the camshaft (the oil pump drive shaft.) The pump is located directly on the engine.
Mechanical fuel pump is a type of piston pump. Structurally, it comprises:
- building, consisting of two parts and closed at the top lid;
- diaphragm mounted between the upper and lower part of the body;
- rod rigidly connected to the diaphragm;
- return spring, mounted on a rod;
- suction and discharge valves at the top of the pump;
- strainer in the lid of the pump;
Details of a mechanical fuel pump:
Diaphragm is the main working element of mechanical fuel pump. It consists of several (2-3) membranes, between which is located a gasket. Diaphragm (film) is connected to the rod, the other end of which interacts with the elements of the mechanical pump drive. There are different schemes of mechanical pump drive. On domestic cars used construction consisting of a pusher and a balance arm. From foreign manufacturers popular scheme with double-arm lever (rocker).
The pump is driven by the cam on the camshaft. When rotating eccentric pump drive rod moves the diaphragm down against the force of the spring. Volume of the cavity above the diaphragm increases fuel by arises discharge enters the pump through the suction valve from the fuel tank. Thus the discharge valve closed.
Upon further movement of the cam lever pump is released, and the diaphragm moves up under the action of the return spring. Above the diaphragm is pressurized, through which the discharge valve opens and the fuel flows through the discharge pipe to the carburetor. Suction valve closed. Pump cycle is repeated with each revolution of the eccentric.
When the carburetor float chamber is filled, stop the needle cuts off access to the fuel to the carburetor. Diaphragm remains in the down position, and the pump is run empty (nothing happens). Performance of the mechanical fuel pump is automatically adjusted by changing the amplitude of the movement of the diaphragm.
Electric Fuel Pump
The electric fuel pump is used in the fuel system of gasoline engines with fuel injection . In engines with direct fuel injection , and diesel engines, electric pump is used in the low-pressure pre-supply fuel to the high pressure pump . Electric fuel pump pressurizes the fuel in the limit of 0.3-0.4 MPH (for engines with direct injection – up to 0.7 MPH). The use of mechanical pumps in the fuel injection can not be due to low fuel pressure.
Fuel pump with electric drive can be placed in the fuel line or fuel tank. On most modern cars the fuel pump is built into the fuel tank. This provides better cooling pump, reduces the likelihood of loss due to the lack of suction pipe. On the other hand, the system has a maximum length of the discharge fuel, which increases its vulnerability.
Electric fuel pump in details:
Electric fuel pump consists of an electric actuator (motor) and pump (pump itself), placed in a metal case. All elements of such fuel pump are in contact with the fuel. Gasoline has a high electrical resistance which prevents a short circuit. Structurally, the fuel pump is a module, which in addition, includes pump fuel flow, fuel filter, strainer.
The fuel pump is provided with two valves – reverse and relief. Return valve closes the fuel system when the engine is stopped. Reducing valve maintains a certain pressure in the system, bypassing some of the fuel back to the inlet.
By design the following types of electric fuel pumps:
- roller pump;
- gear pump;
- centrifugal pump
In the roller pump fuel suction is created by the rotation of the rotor and movement of its rolls. By increasing the space between the roller and the rotor it creates negative pressure, and thus, fuel fills this space. When the space is full, the fuel supply is cut off. As the rotor moves it reduces space, which in turn opens the outlet and pressurized fuel leaves the pump.
Similarly, the work goes on gear pump, where the fuel is injected and absorbed by the movement of the inner gear (rotor) relative to the eccentric outer gear (stator). The sides of the tooth during the rotation of the rotor in the form of their intervals varying chamber through which fuel is being sucked and pressured.
Due to their design roller and gear pumps are usually installed inside the fuel tank. Modern fuel injection systems are preferred centrifugal (paddle) pumps, which provide uniform (without pulsation) of fuel and produce little noise. However, centrifugal pumps have limited in creation of pressure and performance.
Centrifugal fuel pump
Centrifugal fuel pump usually located in the fuel tank. Impeller (fan) of a centrifugal pump is provided around the perimeter of the many blades. Impeller rotates in the chamber, in which there are two fuel channels of a particular shape, one is suction and another – discharge. Twists of fuel which flows by the effect of blades, provides increased pressure.
The fuel pump starts by the signal from the engine control unit (ECU), which activates fuel pump’s relay. This system provides start of engine by starting electric fuel pump immediately with the ignition on. On some vehicles, the switch-on opening the driver’s door starts electric fuel pump even before ignition on and start of the engine. In addition, each electric fuel pump maintains fuel pressure within narrow limits. The pressure is adjusted by changing the voltage or with a safety valve.
Please check out our auto parts store if you are looking for affordable fuel pump for your car.