Fuel Pump

A fuel pump is a component in motor vehicles that transfers liquid from the fuel tank to the carburetor of an internal combustion engine .

Carburetted engines often use low-pressure mechanical pumps that are mounted on the outside of the fuel tank, while fuel-injected engines often use electric fuel pumps that are mounted inside the fuel tank (and in some fuel-injected engines). There are two fuel pumps: a low pressure/high volume pump in the supply tank and a high pressure/low volume pump on or near the engine). [1] The fuel pressure must be within certain specifications for the engine to run properly. If the fuel pressure is too high, the engine will run poor and rich, not combusting all the fuel being pumped, making the engine inefficient and polluting. If the pressure is too low, the engine may run lean, misfire or stall.

Some engines, such as older motorcycle engines, do not require a fuel pump because they use a gravity feed from the fuel tank.

Mechanical pump

Fuel Pump

Before the widespread adoption of electronic fuel injection , most carbureted automobile engines used mechanical fuel pumps to transfer fuel from the fuel tank to the fuel bowl of the carburetor. The two most widely used fuel feed pumps are diaphragm and plunger-type mechanical pumps. Diaphragm pump is a kind of positive displacement pump. A diaphragm pump consists of a pump chamber whose volume is increased or decreased by the flexing of a flexible diaphragm , similar in action to a piston pump. A check valve at both the inlet and outlet ports of the pump chamber to allow fuel to flow in only one directionis located. Specific designs vary, but in the most common configurations, these pumps are usually bolted to the engine block or head, and the engine’s camshaft has an additional eccentric lobe that allows the lever on the pump to be driven directly or via a pushrod. Operates by pulling. From the diaphragm down to dead center. In doing so, the volume inside the pump chamber increased, reducing the pressure. This allows fuel to be pushed from the tank into the pump (due to atmospheric pressure acting on the fuel in the tank). The return motion of the diaphragm to top dead center is accomplished by a diaphragm spring, during which fuel in the pump chamber is squeezed through the outlet port and into the carburetor. The pressure at which fuel is removed from the pump is limited (and therefore regulated) by the force applied by the diaphragm spring.

The carburetor usually has a float bowl into which the expelled fuel is pumped. When the fuel level in the float bowl exceeds a certain level, the carburetor’s inlet valve will close, preventing the fuel pump from pumping more fuel into the carburetor. At this point, any remaining fuel is trapped inside the pump chamber, unable to exit the inlet port or outlet port. The diaphragm will continue to allow pressure on the diaphragm, and during subsequent rotation, the eccentric will pull the diaphragm back to bottom dead center, where it will remain until the inlet valve in the carburetor opens again.

Because the pump holds fuel under pressure on one side of the diaphragm and the other side is attached to the crankcase of the engine, if the diaphragm bursts (a common failure), it can leak fuel into the crankcase. Fuel pump capacity, both mechanical and electric, is measured in psi (which stands for pounds per square inch). Typically, this unit is the common measurement for pressure, yet it has a slightly different meaning when talking about fuel pumps.

Plunger fuel pump

A plunger-type pump is a type of positive displacement pump that consists of a pump chamber whose volume is increased and/or driven in and out of a fuel-filled chamber by a plunger with inlet and discharge stop-check valves . It is similar to a piston pump, but the high-pressure seal is stationary while the smooth cylindrical plunger slides through the seal. These pumps generally operate at higher pressures than diaphragm type pumps. Specific designs vary, but in the most common configuration, these pumps are mounted on the side of the injection pump and driven by a camshaft , either directly or via a pushrod . [2]When the camshaft lobe is at top dead center, the plunger has finished pushing fuel through the discharge valve. A spring is used to pull the plunger outward, allowing low-pressure fuel to flow from the inlet valve into the chamber. These pumps can operate between 250 and 1,800 bar (3,625 and 26,000 psi). [3] Because it is connected to the camshaft, the discharge pressure of these pumps is constant, but the rate at which it pumps is directly related to the engine’s revolutions per minute (rpm).

Both pumps create negative pressure to pull fuel through the lines. However, the low pressure between the pump and fuel tank, in combination with heat from the engine and/or hot weather, can cause fuel in the supply line to vaporize. This results in a fuel shortage because the fuel pump, designed to pump liquid, not vapor, is unable to suck more fuel into the engine, causing the engine to stall. This condition differs from vapor lock , where fuel boils in high engine heat lines on the pressurized portion of the pump (between the pump and the carburetor), simultaneously starving the engine of enough fuel to run. Mechanical automotive fuel pumps typically generate no more than 10-15 psi, which is more than enough for most carburetors.

Degradation of mechanical pumps

As engines moved away from carburetors and toward fuel injection, mechanical fuel pumps were replaced by electric fuel pumps, as fuel injection systems operate at higher fuel pressures (40–60 psi) than mechanical diaphragm pumps. work efficiently. Electric fuel pumps are typically located in the fuel tank, so that the fuel in the tank can be used to cool the pump and ensure a steady supply of fuel. Another advantage of an in-tank mounted fuel pump is that a suction pump in the engine can suck in air through a faulty hose connection (hard to diagnose), whereas a leaking connection in a pressure line will show itself immediately. One potential danger of a tank-mounted fuel pump is that all fuel lines from the tank to the engine are under (high) pressure. Any leak will be easily detected, but it is also dangerous.

Electric pump

The fuel pump in modern cars is usually electric and is located inside the fuel tank . The pump creates a high pressure in the fuel lines, pushing gasoline into the engine. High pressure raises the boiling point of gasoline. Placing the pump in the tank allows the component least likely to handle the gasoline vapor from the well (the pump itself) farthest from the engine, submerged in the cold liquid. Another advantage of placing the pump inside the tank is that it is less likely to catch fire. Although electrical components (such as a fuel pump) can spark and ignite fuel vapor, liquid fuel will not explode ( flammability limitsee ) and therefore submerging the pump in the tank is one of the safest places to put it. In most cars, the fuel pump provides a continuous flow of gasoline to the engine; Unused fuel is returned to the tank. This further reduces the possibility of the fuel boiling, as it is never kept near a hot engine for too long.

Replacing a mechanical fuel pump with an electric one removes an auxiliary load from the engine and reduces engine fuel consumption. In addition, the fuel supply can be monitored more precisely by the Electronic Control Unit (ECU). Since the pump is operating continuously, for example the engine may be turned off at a stop-light to conserve fuel, but still have the required fuel pressure available for a quick start.

The ignition switch does not supply the current required to operate the pump; It supplies a small current which activates a relay which is designed for pump operating current. It is common for the fuel pump relay to become oxidized and stop working; This is much more common than the fuel pump failing. Modern engines use solid-state control which allows fuel pressure to be controlled with pulse-width modulation of the pump voltage. This extends the life of the pump, allows for a smaller and lighter pump, and reduces the power consumed by the pump.

Cars with electronic fuel injection have an electronic control unit (ECU) and can be programmed with safety logic that will turn off the electric fuel pump even when the engine is running. This will prevent fuel leakage from any broken fuel lines in the event of a collision. Additionally, cars may have an inertia switch (usually located under the front passenger seat) that “trips” in the event of an impact, or may have a roll-over valve that turns off when the car rolls over. will turn off the fuel pump.

Some ECUs can also be programmed to shut off the fuel pump if they detect low or zero oil pressure which would indicate an impending mechanical failure.

The fuel sending unit may be an assembly that includes the electric fuel pump, filter, strainer, and electronic equipment used to measure the amount of fuel in the tank. It does this by using a float attached to a sensor that sends a signal to the dash-mounted fuel gauge .

High Pressure Fuel Pump Pump

Pumps for direct-injection diesel and gasoline engines operate at very high pressures up to 30,000 psi [4] and have configurations such as common rail radial piston, common rail to piston radial, inline, port and helix and metering units . Injection pump fuels contain lubricants that prevent the oil from contaminating the fuel. Oil contamination will introduce oil-related emissions and clog injectors . [3] Many diesel engines are common rail , meaning that all injectors are supplied with a common high-pressure fuel pipe supplied by the fuel pump. Fluctuations in common rail damping pressure due to intermittent opening and closing of injectors. Common Rail also makes it easier for fuel pressure to be measured by one transducer instead of one for each cylinder. [3]Port and helix (plunger type) high pressure fuel pumps are most commonly used in marine diesel because of their simplicity, reliability, and their capacity to be increased in proportion to the size of the engine.

port and helix type pump

Port and helix pumps are cam-driven plunger type pumps that operate at one-half the engine RPM for four-stroke engines and at the same RPM for two-stroke . The pump is similar to a radial piston type pump, but instead of a piston it has a machined plunger with no seals. When the plunger is at top dead center , injection into the cylinder is terminated and it is returned to the downward stroke by a compression spring. [2] Because the height of the cam lobe cannot be easily changed, the amount of fuel being pumped to the injector is controlled by a rack and pinion .Controlled by a device that rotates the plunger allowing a variable amount of fuel to be pumped into the area above the plunger. The inlet and outlet ports are located on two sides of the pump cylinder walls, allowing fuel to flow through the compression chamber until the plunger is prompted to close the two ports and begin compression motion. go. The outlet port feeds back into the engine’s fuel tank/settler. The fuel is then forced through a stop check valve, to prevent backflow into the injector nozzle under pressure that can exceed 18,000 psi.

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