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Fuel Injection: Fuel Control

Last time we covered how the fuel system is configured; This time lets consider how the flow of fuel into the engine is controlled by electronic fuel injection.

A carburetor works on Bernoulli's principle – the faster air moves, the lower its pressure. The throttle does not directly control the flow of fuel but actuates carburetor mechanisms which meter the flow of air being sucked into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the air stream. A carburetor must provide the proper air/fuel mixture across a wide range of ambient temperatures, atmospheric pressures, engine speeds, engine loads, and centrifugal forces not to mention dealing with cold starting, hot starting, idling, acceleration, full throttle / high load, part throttle / light load. Various circuits within the carburetor allow it to deal with these differing requirements.

Fuel injection atomizes fuel by forcibly pumping it through a small nozzle under high pressure. The electronic fuel injector is normally closed, and opens to inject pressurized fuel as long as electricity is applied to the injector's solenoid coil. The duration of this operation, called the pulse width, should be proportional to the amount of fuel required.

Whilst a carburetor is a self-contained fuel metering system, an EFI system requires several peripheral components in addition to the injector(s), in order to duplicate its functions. Central to this is the Engine Control Unit (ECU), a computer dedicated to generating injection pulses based on calculations made using information from various engine sensors.

To know how much fuel to add, or rather the pulse width to generate, it is necessary to know the mass of air the engine "breathes" during each induction event (the “air-charge”). This is proportional to the intake manifold's air pressure/temperature, which is proportional to throttle position. A Mass Air Flow (MAF) sensor can directly measure air mass but an alternate method (known as speed-density and used by my choice of ECU) uses a Manifold Air Pressure (MAP) sensor, engine RPM, and an air temperature sensor to calculate air mass.

Complete combustion of petrol can only occur if the air and fuel is present in the exact stoichiometric ratio, which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with no undesirable polluting leftovers. To achieve stoichiometry, the air mass flow into the engine is measured and multiplied by the stoichiometric air/fuel ratio 14.7:1 (by weight) for gasoline. The required fuel mass that must be injected into the engine is then translated to the required pulse width for the fuel injector for that induction event.

Cold operation and heavy loads require richer mixtures than stoichiometric (mixtures can range from 10:1 to 18:1).

An Exhaust Gas Oxygen (EGO) sensor can be used in the exhaust to read the actual air/fuel ratio (AFR) achieved to compare with the AFR predicted for the induction event, giving the ECU feedback on the accuracy of calculations. In steady-state conditions this can be used for “closed-loop” control to ensure precise AFR ratios.

A Throttle Position Sensor (TPS) is used to monitor the position of the throttle. The ECU can monitor the TPS value for rapid opening of the throttle to allow it to add extra fuel - mimicking the accelerator pump functionality within a carburetor.

Finally to allow trouble-free cold starting an Idle Air Control (IAC) valve is controlled by the ECU to allow extra air (and therefore fuel) into the engine when cold (similarly to holding the throttle slightly open). The Coolant Temperature Sensor provides the information to the ECU as to how warm the engine is – On an aircooled engine it is typically placed close to a cylinder head out of the air flow (physically the sensor is often identical to the air temperature sensor).

So, in all, to replace a carburetor you will require:-

  • An ECU
  • Fuel Injectors
  • Throttle Bodies to replace the carbs
  • Air Temperature Sensor
  • Coolant Temperature Sensor
  • TPS sensor
  • MAP sensor
  • RPM hookup
  • EGO sensor
  • IAC valve for easier cold starting,

and a ton of wiring!

Continue to part 3