A narrowband O2 (Oxygen) sensor only reads either rich or lean of Stoichiometric. Stoich is the ration to completely combust the fuel. So a narrowband will only read rich or lean. It does not tell you how far away you are from Stoich.
A wideband O2 is designed to read a board range of air fuel ratios. Some widebands will read from 10:1 up to 20:1. Knowing how far we are away from Stoich we are able to calibrate an engine under all conditions. Especially the areas of full power which are critical to performance and longevity.
The measured ratio is expressed as parts of oxygen to parts of fuel. This is the Air Fuel Ratio. The O2 sensor located in the exhaust the sensor reads the gases as they travel past and the AFR is calculated. Using the information provided by the wideband we can calibrate the AFR to suit our application. We can also test different AFRs to optimise output.
Open and closed loop often get a mention when talking O2 sensors. So what does it mean?
Closed loop operation will only happen when certain conditions are present. Some of these include, engine at operating temperature, light load and idle. In closed loop the feedback from the sensor is used by the ECU to adjust the amount of fuel injected and this adjusts the AFR. When operating in closed loop the AFR is close to Stoich,
In open loop mode the O2 sensor is still operating, however the ECU doesn’t adjust the AFR. So in closed loop the ECU uses the O2 sensor information to adjust the AFR constantly. Open loop is simply the times when this feedback adjustment is not occurring.
From a tuning perspective it is best to disenable closed loop when tuning to prevent the ECU from performing any adjustments during the Dyno tuning process.