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EFI, MPFI and diesel injection pump

Short answer

In an MPFI system, an injector per cylinder near the intake valve is controlled by an ECU using sensor inputs to inject precise fuel. It offers better fuel distribution, higher power, lower emissions and good cold starting over a carburettor. The diesel feed pump supplies low-pressure fuel to the jerk-type injection pump, where a cam-driven plunger pressurises fuel and a helical groove controls injected quantity by varying effective stroke.

Explain the working of a multi-point electronic fuel injection (MPFI) system of a petrol engine with its main components. State its advantages over a carburettor. Describe briefly the fuel feed pump and the jerk-type fuel injection pump used in diesel engines.

MPFI system

In MPFI one injector is fitted in the inlet manifold near the inlet valve of each cylinder. An electronic control unit (ECU) decides how much fuel is injected.

 Sensors --> [ ECU ] --> Injectors (one per cyl.)
 MAP/MAF,       |             ^
 throttle,   ignition      fuel rail <- pump <- tank
 coolant T,   timing         + pressure regulator
 O2, rpm

Components

  • Electric fuel pump, filter, fuel rail with pressure regulator (about 2.5-3 bar).
  • Solenoid injectors, opened for a short pulse width set by the ECU.
  • Sensors: air-flow or manifold pressure, throttle position, engine speed and crank position, coolant temperature, intake air temperature and exhaust oxygen (lambda) sensor.
  • ECU with stored maps.

Working: the ECU uses the sensor data to calculate the air mass, and opens each injector for the time needed to give the required air-fuel ratio (about 14.7:1 with a three-way catalyst, using the lambda sensor as feedback). Injection may be sequential, timed with the inlet valve.

Advantages over a carburettor

  • Accurate mixture to every cylinder, better distribution.
  • Higher volumetric efficiency, since there is no venturi restriction; more power.
  • Better fuel economy and lower emissions; works with catalytic converters.
  • Better cold starting and throttle response, no icing.

Fuel feed pump

A low-pressure diaphragm or plunger pump driven by a cam lifts diesel from the tank, passes it through filters, and supplies it to the injection pump at about 1-2 bar.

Jerk-type injection pump

A plunger moves in a barrel with an inlet port and a spill port. The cam lifts the plunger. After the plunger covers the ports it raises the pressure of the trapped fuel to 150-300 bar or more, which opens the delivery valve and injector (the "jerk"). A helical groove on the plunger uncovers the spill port, the pressure collapses and injection stops. Rotating the plunger changes the effective stroke and therefore the quantity of fuel injected for each load.

✓ Answer checked by Claude Opus 5.5 (high effort)

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