question: Why do some fuel pumps pump very quickly while others take forever?
comment: a pump is a mechanical machine, which consumes power and degrades over time so 1) a gas station could save money putting in lower quality pumps or putting pumps which are slower but consume less electricity to run 2) another option is that gas is volatile, it means that its a liquid that evaporates quite easly, so using a slower pump can reduce problems caused by gas vapor and pressure buildup in the gas tank.
passages: A single-piston pump delivers liquid to the outlet only during the discharge stroke. If the piston's suction and discharge strokes occur at the same speed and liquid is metered out half the time the pump is working, then the overall metering rate averaged over time equals half the average flow rate during the discharge stroke. Some single-piston pumps may have a constant slow piston motion for discharge and a quick retract motion for refilling the pump head. In such cases, the overall metering rate is practically equal to the pumping rate during the discharge stroke.
  
  Section::::Pumps used in high-pressure chromatography.
   Pressurized refuel systems operate via sealing the fuel tank at the appropriate fill point thus creating internal pressure. The automatic shut off nozzle shuts off when pressure in the fuel system reaches 9 psi (62 kPa) or greater. Due to the pressure introduced into the fuel tank, most tanks require additional structure to prevent bursting or damaged weld joints.
  
  Section::::Non-pressurized refuel system.
   BULLET::::- Turbopumps to pump liquid propellants are complex to design, and can suffer serious failure modes, such as overspeeding if they run dry or shedding fragments at high speed if metal particles from the manufacturing process enter the pump.
   Combat and some speed models use rubber tubing ("bladder" tank), baby pacifiers, or fountain pen ink bladders, inflated with fuel from a large syringe, to hold the fuel under fairly high pressure. The fuel line is pinched off to prevent fuel loss until the engine is started. The high pressure of fuel delivery permits the use of a larger intake on the engine, allowing more air flow than would otherwise be possible, and thus more power. This type of fuel delivery is by far the most steady until the fuel finally runs out.
   A fuel pump is a frequently (but not always) essential component on a car or other internal combustion engined device. Many engines (older motorcycle engines in particular) do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank or under high pressure to the fuel injection system. Often, carbureted engines use low pressure mechanical pumps that are mounted outside the fuel tank, whereas fuel injected engines often use electric fuel pumps that are mounted inside the fuel tank (and some fuel injected engines have two fuel pumps: one low pressure/high volume supply pump in the tank and one high pressure/low volume pump on or near the engine). Fuel pressure needs to be within certain specifications for the engine to run correctly. If the fuel pressure is too high, the engine will run rough and rich, not combusting all of the fuel being pumped making the engine inefficient and a pollutant. If the pressure is too low, the engine may run lean, misfire, or stall.
   Metering pump
  
  A metering pump moves a precise volume of liquid in a specified time period providing an accurate volumetric flow rate. 
  
  Delivery of fluids in precise adjustable flow rates is sometimes called metering. The term "metering pump" is based on the application or use rather than the exact kind of pump used, although a couple types of pumps are far more suitable than most other types of pumps. 
   Section::::Working cycle.
  
  The cycle is as follows:
  
  BULLET::::- The fluid enters and fills the chamber from the inlet valve. The outlet and pressurant valves are closed.
  
  BULLET::::- The inlet valve closes, the outlet and pressurant valves open. The presurant forces the fluid through the outlet valve.
  
  BULLET::::- As the chamber empties, the presurant valve closes and the inlet valve opens, followed by the outlet valve closing.
  
  BULLET::::- The cycle is repeated.
  
  Section::::Pumping rate.
  
  Rocket engines requires a tremendous amount of fuel
  
  at high pressure. Often the pump costs more than the thrust 
   Depending on the size of the pump, it generally produces of liquid at each stroke. Often this is an emulsion of crude oil and water. Pump size is also determined by the depth and weight of the oil to remove, with deeper extraction requiring more power to move the increased weight of the discharge column (discharge head).
   turbopumps in that the weight is about the same, the unit, engineering and test 
  
  costs are less and the chance for catastrophic failure is less. This 
  
  pump has the advantage over pressure fed designs in that the weight of 
  
  the complete rocket is much less, and the rocket is much safer 
  
  because the tanks of rocket fuel do not need to be at high pressure. The 
  
  pump could be started after being stored for an extended period with 
  
  high reliability. It can be used to replace turbopumps for rocket 
   In addition to the propellant turbopumps, staged combustion engines often require smaller boost pumps to prevent both preburner backflow and turbopump cavitation. For example, the RD-180 and RS-25 use boost pumps driven by tap-off and expander cycles, as well as pressurized tanks, to incrementally increase propellant pressure prior to entering the preburner.
  
  Section::::Variants.:Full-flow staged combustion cycle.
   For reliability, it is rare to use an external drive mechanism, either a separate belt drive or external gears, although camshaft-driven pumps often rely on the same timing belt. Additional separate belts are sometimes used where dry sump pumps have been added to engines during tuning.
   Because one side of the pump diaphragm contains fuel under pressure and the other side is connected to the crankcase of the engine, if the diaphragm splits (a common failure), it can leak fuel into the crankcase. The capacity of both mechanical and electric fuel pump is measured in psi (which stands for pounds per square inch). Usually, this unit is the general measurement for pressure, yet it has slightly different meaning, when talking about fuel pumps.
  
  Section::::Mechanical pump.:Plunger-Type Fuel Pump.
   Some ECUs may also be programmed to shut off the fuel pump if they detect low or zero oil pressure, for instance if the engine has suffered a terminal failure (with the subsequent risk of fire in the engine compartment).
   BULLET::::- Bit speeds can be very high as such bit selection is important. The high speeds may limit the use of certain types of bits.
  
  BULLET::::- Special pump requirements may be needed as particular pressures and flow rates may be needed to maintain proper and efficient motor operation.
  
  BULLET::::- If used for directional control the down-hole assembly may be long and this may take time to assemble on the rig floor.
   Section::::Mechanical pump.
   Light passenger vehicle pump flow rate ranges up to about per minute (the United States limits this to per minute); pumps serving trucks and other large vehicles have a higher flow rate, up to per minute in the UK, and airline refueling can reach per minute. Higher flow rates may overload the vapor recovery system in vehicles equipped with enhanced evaporative emissions controls (required since 1996 in the US), causing excess vapor emissions, and may present a safety hazard.
   BULLET::::2. The very small float bowl/fuel chamber can result in fuel starvation in extreme high-performance situations, but can usually be traced to a fuel delivery problem to the carburetor, such as a weak fuel pump or a worn/rounded camshaft eccentric that drives the fuel pump lever. The latter situation can be resolved with an external (electric) fuel pump.
   Delivery rate starts at very close to swept volume when there is no pressure difference, and drops steadily until there is no effective transfer when the pressure ratio reaches the maximum boost ratio.
   Some carbureted engines have low-pressure fuel pumps: they are typically designed only to overcome a loss of suction in the fuel line near the engine due to fuel evaporation in hot weather, to supply sufficient fuel to maintain stoichiometric combustion under heavy load with wide-open throttle, or a combination of the two. Fuel demand is low at idle and there is more than enough manifold vacuum to draw sufficient fuel for combustion, even if the fuel pump is switched off.
   Non-pressurized refuel systems reduce fuel spills and provided more accurate filling when used with automatic refuelling nozzles. The system is also designed to prevent overfilling or manual override of the fueling system. David Juleff of Shaw Development, LLC was the design engineer that tailored an aerospace non-pressurized refuel system to be first introduced on the Caterpillar D11 track type tractor in 1998. A number of non pressure fueling systems are available in OEM and retrofit applications from FloMeg, Shaw Development, Banlaw , Hydrau-Flo®, FastFillSystems  and others
   BULLET::::- Lobe pump
  
  The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa (when new) and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa.
  
  A positive displacement vacuum pump moves the same volume of gas with each cycle, so its pumping speed is constant unless it is overcome by backstreaming.
  
  Section::::Types.:Momentum transfer pump.
   Plunger-type pumps are a type of positive displacement pump that contain a pump chamber whose volume is increased and/or decreased by a plunger moving in and out of a chamber full of fuel with inlet and discharge stop-check valves. It is similar to that of a piston pump, but the high-pressure seal is stationary while the smooth cylindrical plunger slides through the seal. These pumps typically run at a higher pressure 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 the camshaft, either directly or via a pushrod. When the camshaft lobe is at top dead center, the plunger has just finished pushing the fuel through the discharge valve. A spring is used to pull the plunger outward creating a lower pressure pulling fuel into the chamber from the inlet valve. These pumps can run between 250 and 1,800 bar (3,625 and 26,000 psi). Because it is connected to the camshaft, the discharge pressure of these pumps is constant, but the rate at which it pumps is directly correlated to the revolutions per minute (rpm) of the engine.
   In some cases the actual pump may be sealed and immersed inside the fuel tanks on a site, in which case it is known as a submersible pump. In general, submersible solutions in Europe are installed in hotter countries, where suction pumps may have problems overcoming cavitation with warm fuels or when the distance from tank to pump is longer than a suction pump can manage.
   In a continuous injection system, fuel flows at all times from the fuel injectors, but at a variable flow rate. This is in contrast to most fuel injection systems, which provide fuel during short pulses of varying duration, with a constant rate of flow during each pulse. Continuous injection systems can be multi-point or single-point, but not direct.
   One of the most important functions for the pump is to accurately measure the amount of fuel pumped. Flow measurement is almost always done by a 4 stroke piston meter connected to an electronic encoder. In older gas pumps, the meter is physically coupled to reeled numerical displays (moving wheels or cylinders with numbers on the side), while newer pumps turn the meter's movement into electrical pulses using a rotary encoder. In the U.S. flow speed is limited to 10 gallons per minute for cars and 40 gallons per minute for trucks. This flow rate is based on the diameter of the vehicle's fuel filling pipe, which limits flow to these amounts.
  