Electronic Fuel Injection – further operational features
Port Fuel Injection
Port fuel injection (PFI) systems utilize one fuel injector per cylinder. The injectors are mounted in the intake manifold adjacent to the intake valve. With one injector per cylinder, the intake manifold can be a “dry” design to maximize air flow without concern for fuel flow or puddling.
Sequential Fuel Injection
The most sophisticated EFI systems are termed sequential fuel injection (SFI). With an SFI system, the ECU utilizes input from the crankshaft position sensor to time the injector pulse to coincide with the opening of the intake valve. This has no impact on peak horsepower, where the injector must stay open nearly continuously in order to supply adequate fuel. But at idle and under low load conditions, the precise timing of the injection pulse allows for smoother operation, crisper throttle response, and lower emissions.
Oxygen Sensor Control Feedback Operation
An EFI system may be fitted with one or more oxygen sensors. The oxygen sensor is placed in the exhaust stream and responds to the amount of oxygen in the exhaust gasses. The O2 sensor voltage output thus reflects the air:fuel ratio. The ECU can use the input from the O2 sensor to continuously adjust the air:fuel ratio while the vehicle is operating. When a lean condition is detected, more fuel is injected and when a rich condition exists, fuel flow is decreased. This mode of operation is called “closed loop”. Factory systems are fitted with a narrow band (lambda type) O2 sensor. This type of sensor only responds to to changes in air:fuel ratio around the value of 14.7:1. An A:F ratio of 14.7:1 is considered “stoichiometric”. This ratio represents an ideal situation where each fuel molecule has exactly the correct number of oxygen molecules to interact with. In theory, each molecule of fuel is thus completely combusted and no air flow is “wasted”.
Operation at “stoich” is fine for idle of other low load condition. But there is a problem at wide open throttle (WOT) where maximum power is desired. Keeping in mind that engines are airflow limited, to make maximum power, each oxygen molecule must react with a fuel molecule. Even though with an A:F ratio of 14.7:1 there are just enough oxygen molecules for each fuel molecule to burn completely, the combustion process is not perfect. For each oxygen molecule to have time to find a fuel molecule, there must be an excess of fuel over oxygen. In other words, a “rich” condition must exist with an A:F ratio less than 14.7.
Since a lambda type O2 sensor can’t read fuel mixtures much richer than 14.7, the ECU ignores the O2 sensor at WOT and reverts to “open loop” operation. In open loop, fuel requirements are calculated by inference, rather than directly computed from the actual amount of oxygen in the exhaust. This can work well, but is less precise and harder to control than when the closed loop feedback system is operating. The solution is a “wide band” (WB) O2 sensor. A WB O2 can read A:F ratios richer and leaner than 14.7:1 and allows the ECU to actively control the A:F ratio at any desired value. The problem is that WB O2 sensors are relatively expensive and require a more sophisticated ECU. Stock cars are therefore not equipped with WB O2 sensors. The various aftermarket ECU’s we sell and install at Kennedy’s can be equipped with a wide-band option and can function in closed loop nearly all the time.
Please see the page “Aftermarket ECU’s – What’s in it for Me?” to learn more about wide band O2 sensors and the many other advantages these units offer.