CATEGORIES FOR ARTICLE LINKS BELOW

Sensor Inputs

Most EFI systems measure the same basic 6 inputs;

RPM

Most systems measure rpm off of the ignition coil tachometer pulse or crank triggered magnetic/Hall effect sensors. Rpm is considered a primary input signal on all EFI systems. Most systems generate an injection pulse for every tach pulse so as rpm is increased, the frequency of injection pulses is also increased.

Airflow

On mass airflow type systems, this input is also considered a primary input signal. X amount of air requires Y amount of fuel. As rpm and throttle opening is increased, airflow increases to a point.

Manifold Pressure

On speed density type systems, this input is essential when combined with the rpm signal to calculate airflow. As the throttle is opened, the manifold pressure increases which will require more fuel.

Throttle Position

This input is a secondary input on most systems. It is required mainly for acceleration enrichment when the throttle is rapidly opened. By looking at the rate of change of throttle blade angle, the computer can determine how quickly the throttle is being opened and can supply the extra fuel required momentarily to alleviate the lean condition. Throttle position is measured by a potentiometer attached to the throttle shaft. Think of the TPS input as acting like the accelerator pump on a carburetor.

Throttle Position Sensors (TPS)

Some systems can use blade angle instead of manifold pressure for load information. This method is frequently employed on racing engines with hot camshafts and 1 throttle plate per cylinder where the use of a MAP sensor is difficult.

Water Temperature

Water temperature is a secondary input required mainly to ensure proper starting and warmup of the engine. When the engine is cold, the air to fuel ratio must be very rich to enable enough fuel to vaporize for proper starting. The computer increases the injector pulse width to supply extra fuel when cold and tapers this fuel off as the water temperature increases. Once the water warms past 120 degrees or so, the computer does not need to add any extra fuel.

Water Temperature Sensor

Where a carburetor chokes off air to richen the mixture when cold, EFI squirts in extra fuel to achieve the same effect.

Air Temperature

This is a secondary input required especially on speed density systems. The sensor is usually mounted in the intake manifold or air filter area. As the air temperature drops, its density increases. Denser air requires more fuel. As the temperature of the inducted air increases, the computer reduces the pulse width to compensate for lower density. Mass airflow systems are not critically affected by operation without an air temperature sensor because the airflow meter is already measuring the air mass entering the engine.

Air Temperature Sensor

Oxygen Sensor

This sensor is employed in closed loop systems to modify the basic pulse width after the fact. It is mounted into the exhaust manifold area. By looking at the oxygen content of the exhaust gasses after combustion, the computer can determine if the air/fuel ratio is too rich or too lean for optimum combustion and adjust the next few injections accordingly. This sensor is primarily employed for emission control and to a lesser degree, fuel economy. For the lowest average emissions, the air/fuel ratio must be kept around 14.7 to 1.

Oxygen Sensor

Under full throttle conditions, this sensor input is ignored by the computer so that the engine can produce more power by runner a richer mixture. This is called open loop mode and the computer is supplying the injector pulse width from tables based on all of the other sensor inputs. Once throttle opening and rpm are reduced to cruising conditions, most systems will jump back into the closed loop mode where they will stay for a large portion of the time on most street driven applications.

Page 3/4