Multipoint - EFI System (Part10. OPEN AND CLOSED LOOP)

The terms open loop and closed loop refer to the operating mode of a computerized fuel system. When an engine is cold, the computer operates open loop (no feedback from sensors). After the engine warms to operating temperature, the system "kicks in" to closed loop (uses feedback from sensors) to control system operation.

Open Loop Operation

As was mentioned earlier, an oxygen sensor must heat up to several hundred degrees before it will functionProperly. This is the main reason computer systems have an open loop mode. The computer has preprogrammed information (injector pulse width, injector timing, air bypass valve position) that will keep the engine running satisfactorily while the oxygen sensor is warming up. When the engine and oxygen sensor are cold, no information flows to the computer. The computer ignores any signals from the sensors. The "loop of information" is open.

Closed Loop Operation

After the sensor and engine are warm, the oxygen sensor, and other sensors, begins to feed data to the com­puter. This forms an "imaginary loop" (closed loop) as electrical data flow from the engine exhaust, to the oxygen sensor, to the computer, to the injector, and back to the oxygen sensor. Normally, the computer system functions closed loop to analyze the fuel mixture provided to the engine. This lets the com­puter "double-check" itself.

Drawings show difference between closed I and open loop mode of computer operation.

Analog and Digital Signals

The signal from the engine sensors can be either a digital or analog type output. The out­put from the computer can also be analog or digital. Digital signals are instant on-off signals. An example of a sensor providing a digital signal is the crankshaft position sensor which shows engine rpm. Voltage out­put or resistance goes from maximum to minimum, like a switch, to report rpm in number form.

An analog signal progressively changes in strength. For example, sensor internal resistance may smoothly. In­crease or decrease with temperature, pressure, or part position. The sensor acts as a variable resistor.

A-Digital signal is ON-OFF signal, like from wall light switch. B-Analog signal steadily increases or decreases voltage signal. It is not an instant ON-OFF signal.