Mechanical fuel pumps are devices that use the engine's mechanical motion to draw fuel from the tank and deliver it to the carburetor. They are commonly found in older, carbureted vehicles, utilizing a clever design to ensure a continuous supply of gasoline without requiring electrical power.
Understanding the Core Mechanism
A mechanical fuel pump operates directly from the engine's motion, typically driven by the camshaft or a dedicated eccentric shaft. An eccentric lobe on this rotating shaft moves under a rocker arm or lever. As the shaft rotates, the lobe goes under the pivot level of this rocker arm, effectively pushing one end of the arm upwards. This upward motion then pulls on a diaphragm within the pump's housing, creating a pumping action.
This diaphragm's movement creates two distinct phases:
- Suction (Intake) Phase: As the rocker arm pulls the diaphragm down, it creates a vacuum within the pump chamber. This vacuum draws fuel from the fuel tank, past an inlet check valve, into the pump. The inlet check valve is crucial as it acts as a first check valve, preventing gasoline from flowing back towards the fuel tank.
- Discharge (Pressure) Phase: When the camshaft lobe rotates past the rocker arm, a spring pushes the diaphragm back up. This action pressurizes the fuel within the pump chamber. The pressurized fuel is then forced out through an outlet check valve and into the carburetor. This another valve ensures that fuel drains directly into the carburetor without returning to the pump chamber.
Key Components of a Mechanical Fuel Pump
To better understand its operation, let's look at the main parts:
- Rocker Arm (or Lever): This component translates the rotational motion of the camshaft into the up-and-down movement required to operate the diaphragm.
- Diaphragm: A flexible, rubber-like disc that separates the fuel chamber from the pump's mechanical parts. Its movement is central to creating suction and pressure.
- Inlet Check Valve: A one-way valve that opens to allow fuel into the pump chamber from the fuel tank but closes to prevent it from returning.
- Outlet Check Valve: Another one-way valve that opens to allow fuel out of the pump towards the carburetor but closes to prevent it from flowing back into the pump.
- Spring: Provides the force to return the diaphragm to its original position, aiding in the discharge stroke.
- Camshaft Lobe: An eccentric (off-center) part of the engine's camshaft that directly engages with the rocker arm, providing the mechanical drive.
Step-by-Step Operation
The continuous process of fuel delivery involves a series of coordinated actions:
- Camshaft Rotation: The engine's camshaft rotates, and its eccentric lobe comes into contact with the rocker arm of the fuel pump.
- Diaphragm Actuation: The lobe pushes one end of the rocker arm, which in turn pulls the diaphragm downwards, creating a low-pressure area (vacuum) in the fuel chamber.
- Fuel Intake: The vacuum opens the inlet check valve, drawing fuel from the fuel tank into the pump's chamber. The inlet valve then closes, preventing fuel from returning to the tank.
- Diaphragm Return: As the camshaft lobe moves away, the spring pushes the diaphragm back upwards, increasing the pressure within the fuel chamber.
- Fuel Discharge: The increased pressure opens the outlet check valve, forcing fuel directly into the carburetor. The outlet valve then closes, preventing fuel from flowing back into the pump from the carburetor side.
- Continuous Cycle: This cycle repeats as long as the engine is running, ensuring a constant supply of fuel to the carburetor.
Comparison to Electric Fuel Pumps
While mechanical fuel pumps are reliable and simple, modern vehicles predominantly use electric fuel pumps. Here's a quick comparison:
| Feature | Mechanical Fuel Pump | Electric Fuel Pump |
|---|---|---|
| Power Source | Engine's camshaft (mechanical) | Vehicle's electrical system |
| Location | Typically on the engine block | Inside or near the fuel tank |
| Pressure | Lower (suited for carburetors) | Higher (suited for fuel injection) |
| Noise | Generally quieter, integrated with engine noise | Can be audible, especially when priming |
| Maintenance | Diaphragm wear, lever issues | Electrical issues, motor wear |
| Priming | Requires engine cranking to build pressure | Primes the system instantly when ignition is turned on |
For more detailed information on fuel pump types and their applications, you can explore resources like HowStuffWorks - Fuel Pumps or automotive engineering textbooks.
