Your automotive fuel injection system has two primary operating conditions. These conditions are called closed loop and open loop. Open loop operation operates without a feedback loop and closed loop operates with feedback.
Open loop operation
Open loop operation of your fuel injection system is an operating condition that occurs when an engine is still cold, or there is not a reliable signal from the Oxygen sensor or sensors. This system uses a database, spreadsheet, or lookup table to determine the exact amount of fuel to inject into the engine for any given set of conditions. This system does not use the signal from the oxygen sensors to determine the air/fuel ratio. This system relies on a pre-determined set of rules that are hard coded into the computer to tell it how much fuel to inject on each rotation of the engine.
What does open loop operation use as inputs?
Open loop operation has to base its fuel metering on something. It needs a few inputs to look up the correct amount of fuel. Different manufacturers use slightly different systems. Most require the following inputs:
RPM – Revolutions Per Minute, This is the speed of the engine crankshaft. This is usually determined by a hall effect sensor and a signal wheel mounted on the harmonic balancer or a cam.
IAT – Intake Air Temperature, This is the temperature of the air as it enters the intake manifold. This is determined by a sensor usually mounted in between the air filter and the intake manifold.
TPS – Throttle Position Sensor, This sensor mounts on the side of the throttle plate and expresses throttle position as a percentage of WOT or Wide Open Throttle. For example, a TPS signal that reads 50% indicates a throttle plate that is exactly half open.
CTS – Coolant Temperature Sensor, This sensor tells the fuel injection computer what the temperature of the engine is based on the temperature of the engine coolant. This sensor is usually mounted near or on the head of the engine in a coolant passage or radiator hose adapter.
MAP – Manifold Air Pressure, This sensor tells your fuel injection system what the vacuum is inside your intake manifold.
A system operating in open loop can use all of these inputs and sometimes more to determine the most appropriate amount of fuel to inject at any given time. Some systems use more sensors than this, and other systems use as few as just the RPM and coolant temp. Generally, the more inputs a system uses, the more accurately it delivers the fuel. Many, if not all, systems use just the CTS and RPM for initial engine startup, and then look at other signals one the engine fires.
What does closed loop use for its input?
Closed loop operation is called closed loop because it uses feedback to tell it what to do as its running. This is often called a feedback loop. The feedback used in a closed loop automotive fuel injection system is the signal from the oxygen sensor or sensors.
What does an oxygen sensor do?
An oxygen sensor monitors your exhaust and determines how much unburned oxygen is leaving your engine. This data is used to determine your air-to-fuel ratio.
The closed loop system continuously monitors the air-to-fuel ratio to determine if it’s injecting the correct amount of fuel. If the oxygen sensor shows that not enough fuel is being injected, the system adds more fuel. If the oxygen sensor shows that shows that too much fuel is entering the engine, the system trims back the fuel. In this closed loop operation, the engine is continuously adjusting the fuel so that the oxygen sensor reading of the exhaust is exactly what it should be. This type of continuous monitoring results in much cleaner emissions, more power, and better fuel economy than an open loop system usually achieves.
How do these systems work together?
When an engine first starts up, it is running in open loop mode. It has to run in this mode until the engine reaches operating temperature and the oxygen sensor heats up to its required temperature. An Oxygen sensor needs to be hot to work correctly. Some Oxygen sensors are electrically heated, while others rely on hot exhaust to heat them. Once the engine and O2 sensor are both in normal temperature range, the system automatically transitions over to closed loop mode. The engine runs in closed loop mode as long as it receives a reliable and accurate signal from the O2 sensor. If the signal from the Oxygen sensor goes out of range, or is interrupted for any reason, the system switches back over to open loop mode so it can keep the engine running. If the O2 signal comes back, and is determined to be reliable, the system goes back to closed loop. Basically, the system tries to run in closed loop all the time if it can, and only runs in open loop at startup or if something is wrong with the engine sensors.