Senin, 07 Maret 2011

Electronic Fuel Injection (EFI)

If there's one thing that's critical in a high performance engine, then it's fuel control. Think about it: the whole objective of adding a turbocharger, of installing NOS, even of installing a free flow exhaust system, is to improve fuel delivery into the combustion chamber. It is also events in the combustion chamber that can and will destroy a high performance race engine if it's not controlled properly. Here we're talking about controlling the combustion process. Now I've heard many arguments as to why sidedraft carburetors provide better performance than fuel injection and engine management, and vice versa but I always say: it's not about performance, it's about reliability and there's no better system for fuel control than electronic fuel injection. Any endurance race car from INDY Car Racing, to Formula 1, to the World Rally Championship, to the Le Mans Series uses electronic fuel injection (EFI) systems, not just for reliability but because ensuring that the correct amount of fuel is delivered under every condition, will provide the best performance.

EFI is central to engine management. It relies on an engine control unit (ECU) which processes a number of inputs from various sensors on the engine to deliver the correct amount of fuel at a particular RPM and air-flow rate/air density combination. The fuel is delivered through an injector, which is an electronically actuated solenoid valve. The amount of fuel that is delivered is dependent on the fuel pressure, which is usually a constant 30 psi above intake manifold pressure, and the pulse duration of the injector, i.e., the length of time the injector is held open.

Most EFI systems have a standard set of sensors. These include:

  • The Barometric Pressure (BARO) Sensor, which provides the ECU with the atmospheric air pressure reading.
  • The Engine Coolant Temperature (ECT) Sensor, which provides the ECU with the engine's current operating temperature. This is important because fuel vaporization varies for different engine temperatures. A cold engine requires more fuel while a hot engine requires less.
  • The Intake Air Temperature (IAT) Sensor, which the ECU needs to take into account when determining pulse duration.
  • The Mass Air Flow (MAF) Sensor, which is a tube positioned after the air filter in the air intake duct. The MAF sensor has a fine platinum wire that spans across the tube. The wire is heated by electrical current to maintain a constant temperature above ambient. The air flow past the wire cools the wire and more current is required to maintain the constant temperature. Thus, the amount of current required to maintain the constant temperature indicates the air flow rate. The air flow rate is divided by RPM to determine the pulse duration.
  • The Manifold Absolute Pressure (MAP) Sensor, which uses manifold vacuum to measure engine load. An EFI system that uses a MAP sensor does not require a MAF sensor as it can use the input from the MAP sensor to determine the required pulse duration.
  • The Oxygen Sensor (O2S), which is used to measure the amount of oxygen that is not consumed during combustion. This is important for the correct operation of the catalyst converter and is used for emissions control rather than performance or economy. The O2S is located in the exhaust system and is an after-the-fact measure of the air/fuel ratio. Too much unburnt fuel in the exhaust indicates a lean mixture while too little oxygen indicates a rich mixture.
  • The Crankshaft Position (CKP) Sensor, which is important for timing purposes as it tells the ECU which spark plug to fire and which injector to open at any given point in the Otto cycle.
  • The Throttle Position (TP) Sensor, which is another important sensor as the throttle position and the rate of change in the throttle position indicates the what the diver wants the car to do.

The modifications you can perform on an OEM EFI are somewhat limited because the OEM ECU is not reprogrammable. However, there are a number of things you can do to modify the EFI system without having to reprogram the ECU. You can increase the fuel pressure as this is one reading that the ECU does not take into account – it assumes the fuel pressure is a constant 30 psi above intake manifold pressure; you can intercept the pulse signal form the ECU, alter it using input from the manifold pressure and send it to the injector; you can increase the injector nozzle size; or you can increase the number of injectors. However, your best option, performance wise, is to install an aftermarket ECU. In the next few pages we'll discuss each of these options.

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