Think of the barrel and plunger as the engine’s ultimate metering device. Located inside the rotary distributor-type fuel pump (often called the Bosch VE pump or simply the "injection pump"), this pair is responsible for creating the extremely high pressure needed for injection.

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  The Barrel is a hardened steel cylinder with precise intake and spill ports machined into it.

  
  


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  The Plunger is a perfectly matched piston that slides up and down and rotates within the barrel.

  
  

Their job is a three-step dance:

1. 
   

   Fill: As the plunger moves down, fuel flows into the chamber inside the barrel.

   
   


2. 
   

   Pressurize: The plunger moves up, first closing the ports and then compressing the fuel to pressures exceeding 20,000 PSI.

   
   


3. 
   

   Inject & Spill: At the exact moment determined by engine timing, a helix (a machined groove) on the plunger aligns with the spill port in the barrel. This releases the pressure, ending the injection event. The point at which this happens controls the amount of fuel delivered.

   
   

In short, this assembly is what meters the precise quantity of fuel and determines the exact timing for each power stroke, directly influencing power, efficiency, and emissions.

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