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A normal zirconia oxygen sensor is only capable of measuring the oxygen content of the exhaust in a narrow air-fuel mixture range, right around stoichiometric (14.7:1). This is fine for normal operating conditions. But, when the car is running in the lean burn mode, the oxygen content of the exhaust is higher than a normal oxygen sensor is capable of measuring.
The Insight uses a LAF (Linear Air-Fuel) sensor for the primary oxygen sensor that is capable of measuring air-fuel ratios as lean as 23:1. This allows the ECM to maintain precise control over the mixture during both normal and lean burn conditions. (Note that lean burn is only used in the 5-speed version of the Insight.)
The engine's LAF Sensor is designed to detect air-fuel ratios as lean as 25:1. The fuel-injection Electronic Control Module uses this data, along with engine rpm, crankshaft angle, throttle angle, car mass, coolant temperature and valve position, to maintain a lean air-fuel ratio below 2500-3200 rpm (depending on throttle position and engine load).
The VTEC-E engine can burn such a lean mixture partly because of a strong air-fuel swirl created in the combustion chamber, created by the mixture's entry through only one of two intake valves during low-rpm operation. Although the overall air-fuel mixture is lean, optimized injection timing, along with the vortex, creates a "stratified" charge - the air-fuel ratio is richer near the spark plug and leaner toward the combustion chamber periphery. The richer mixture ignites more readily and creates a fast-burning, stable flame that promotes more complete combustion.
A standard zirconia oxygen sensor uses a thimble shaped zirconia element that is exposed to the atmosphere on one side, and to the exhaust stream on the other side. If the amount of oxygen on the two sides is different, it generates a voltage. Since the amount of oxygen on the atmospheric side is fixed, the generated voltage represents the amount of oxygen in the exhaust. With the engine not running there is atmospheric pressure in the exhaust, and the voltage is zero since the same amount of oxygen exists on both side of the element. When the engine is running there is less oxygen in the exhaust, so voltage is generated up to a maximum of one volt.
The LAF sensor has two zirconia elements that share a diffusion chamber. There are a total of three chambers:
The zirconia element that is in contact with the exhaust is the sensor element. The diffusion chamber is the space between the two zirconia elements. By applying varying voltage to the control element, the ECM can control the amount of oxygen in the diffusion chamber. Since the diffusion chamber is the reference chamber for the sensor element, this action changes the output of the sensor element.
The ECM monitors the output of the sensor element as the oxygen content of the exhaust changes, and it applies voltage to the element to try to maintain the sensor output at .450 volts. It then monitors the control voltage to determine the actual air-fuel ratio.
Unlike a conventional sensor, the voltage can be positive or negative, and it reads the opposite direction. That is, positive voltage indicates a lean mixture, and negative voltage indicates a rich mixture. The normal operating range is about 1.5 volts.