The purpose of the fuel pump in an automobile is to deliver fuel from the gas tank to the fuel injector or carburetor at the necessary pressure. Fuel is drawn from the fuel tank and is propelled into the internal combustion engine through a fuel pump where the combustion of the fuel occurs with air. Regardless of the external conditions, a fuel pump must facilitate the delivery of the right amount of gasoline from the tank to the engine through a fuel line at a specific pressure. In automobile engines, some fuel pumps are mechanically operated with low-pressure fuel distribution, whereas others are electric pumps that push the gasoline to the engines at higher pressure. Your fuel pump choice is based on a number of factors for the system to operate properly.
Mechanical Fuel Pump
Mechanical fuel pumps are low-pressure pumps commonly used in old vehicles that use carburetors in engines. They are commonly called as puller pumps because they use the suction ability to pull the gasoline through fuel pipe and up to the engine. Typically, mechanical pumps handle engines rated under 450 hp.
- Mechanism of operation
- Typically, mechanical fuel pumps are driven by a crankshaft which contains a camshaft having a cam lobe that operates a lever on the pump.
- The other end of the lever is connected to a diaphragm which goes down as needed to refill the pump chamber. When the crankshaft spins, each rotation pushes the lever up which in turn pulls the diaphragm down with it. This builds suction in the pump to drag the fuel out of the tank into the fuel chamber.
- A spring attached to the diaphragm will bring the diaphragm up back to its normal position thereby forcing the fuel from the pump chamber into the carburetor through outlet port.
- When the carburetor is full, no fuel is allowed from the pump chamber into it until the fuel is consumed by the engine and carburetor is ready to accept more gasoline.
The pressure at which the fuel is expelled from the mechanical pump is typically limited between 4 to 10 psi.
Common Issues
The common issues of a mechanical fuel pump include leakage of fuel into the crankcase due to a ruptured or leaky diaphragm, vapor lock in the pump, fuel pressure drop, faulty diaphragm spring etc. Replacing the fuel pump is a DIY project in most cases since it is easily accessible.
Electric fuel pumps are designed for boosted engines such as fuel injection that require higher flow rates and pressure outputs. The electric fuel pumps deliver gasoline under high pressure from the gas tank to the injectors, typically ranging between 30 to 200 psi. Electric pumps are pusher pumps because they generate positive electrical pressure to push the fuel in fuel lines to the engine. The pumps are typically in-tank or externally mounted and come in a variety of pumping mechanisms such as positive displacement pumps, roller vane pumps or turbine fuel systems.
- Mechanism of operation
- Instead of the camshaft, electric pumps use solenoid as an electromagnetic switch to create a pull on the diaphragm and to drive fuel into the chamber.
- When the ignition is on, the Powertrain Control Module (PCM) of engine energizes the relay and supplies voltage to the pump which makes the motor to spin and build pressure in the fuel.
- This makes fuel to run through the inlet and mesh filter where it develops positive pressure and gets pushed to the outlet valve through the fuel line.
- To avoid dirt or other solid contaminants clogging the injector valves, the fuel is passed through a filter before existing and is routed to the individual fuel injectors.
- To regulate the supply of fuel at the proper pressure within the specifications of the engine, electric pumps are equipped with pressure regulators at the end of the fuel rail which is fixed along with the unit inside the fuel tank.
The fuel injectors that are connected to the fuel rail remain closed until the Engine Control Unit (ECU) of the vehicle decides to send fuel into the intake manifold of multi-port fuel injection or directly to the cylinders of direct injection.
Common Issues
Electric fuel pumps fail when there is fuel contamination due to dirt & rust, overheating of the engine due to low fuel level, wiring problems, failed pressure gauge etc. When fuel pump fails the vehicle encounters various issues such as whining noise from the fuel tank, difficulty starting the car, loss of power while accelerating, reduced fuel efficiency etc.
Mechanical vs. Electric Fuel Pump Choice
In simpler times, carburetors relied on mechanical fuel pumps, and EFI engines required high-pressure electric pumps. Fuel pump choice is not so simple anymore because times have changed. Even the most beastly of electric pumps can struggle to keep pace with today’s crop of 3,000-plus horsepower Outlaw and Pro Mod-style forced induction EFI engines.
At this level, belt-driven mechanical pumps are very much the norm. At the opposite end of the spectrum, the rise of late-model engine swaps has increased the popularity of electric fuel pumps at modest horsepower levels more typically associated with mechanical pumps. In between these two extremes, a typical street/strip application can make do with either an electric or a mechanical pump. So, which is right for your car?
Although mechanical and electric fuel pumps each have their share of pros and cons, sometimes it all comes down to practicality. The obvious advantages of a mechanical pump are that they do not make any noise and they do not require any additional plumbing or wiring. However, these days we are starting to see people swap carbureted GM LS motors, Ford mod motors, and even late-model big-block Chevys into older muscle-cars. Since these engines were fuel injected from the factory, they never came with bosses to mount a mechanical pump on the block.
Finally
For those who own a high-performance car, pushing the limits and efficiency of your engine, electric pumps are essential. They are needed in situations where higher fuel pressures are required (like boosted applications, nitrous, and fuel injection — the latter begins at 30,000 PSI). Sometimes, mechanical pumps simply will not fit in your car, forcing you to go electric instead; however, they offer a wide variety of flow rates and pressure outputs, making them incredibly versatile and able to work well in many vehicles. Most electric fuel pumps require a 12-volt circuit with a relay that gives you multiple shut-off options, but they must be properly placed to guarantee efficiency. Although they are more expensive fuel pump choice, they are generally more efficient and safer options.
