885.0h said:
Since your question is broad based and other people will benefit from the answer, here it is...
Joe R.
Air-fuel ratio and how O2 sensors work
An engine is essentially an air pump; combine fuel and air and you get CO2 (carbon dioxide), CO (carbon monoxide), O2 (oxygen), NOx,, (oxides of nitrogen) HCx,, H2O (water) and some other products of combustion. The amount of extra oxygen in the exhaust is an indicator of wither the air/fuel mixture is lean, stoichiometric (most perfect combustion of the fuel) or rich.
Air/fuel mixtures and ranges
Air/fuel mixtures are typically expressed as a ratio between the amount of air combined with 1 part of fuel. In automotive use and using pure pump gasoline (no alcohol); this ranges from 11:1 (rich) to 17:1 (lean). Mixtures at the extreme ends of this range will result in reduced power and efficiency.
Lean mixtures burn slowly, produce lots of heat and give best fuel economy. The range of lean mixture is from 14.8:1 to 16.7:1. Too lean a mixture and the engine runs rough and develops less power. The extra heat generated in the combustion process adds more heat to the unburned air/fuel charge in the cylinder. Excessively lean mixtures are prone to preignition (ping) and detonation both of which can cause engine damage. There is an extra amount of O2 in the exhaust of a lean running engine.
Rich mixture burn quicker than lean mixtures; they produce less heat, lower fuel economy, and produce more engine deposits and air pollution. The range of rich mixture is from 11:1 to 14.6:1. The range of 12.5:1 to 13.5:1 is the best power range for most automotive use. Sometimes a rich fuel mixture is used to drop combustion temperature to keep prevent preignition (ping) and detonation and the resulting engine damage. Rich mixtures are used at high power levels but not because more fuel makes more power. It is because rich mixtures make less heat, therefore allowing more power to be made without overheating the unburned air/fuel charge in the cylinder and the resulting preignition (ping) and detonation.
There is a lower amount of O2 in the exhaust of a rich running engine.
Stoichiometric mixtures are in the middle of the burn speed range. A perfectly tuned engine with a stoichiometric mixture would produce CO2 and water and no other emissions. It would be the most efficient balance of fuel economy and power. In a typical gasoline only engine a stoichiometric ratio is 14.7:1. The amount of O2 in the exhaust of an engine running a stoichiometric mixture is midways between the lean mixture and the rich mixture.
O2 sensors how they work
O2 sensors work as a miniature generator that produces a DC voltage according to the amount of oxygen present in the hot exhaust gases. The output of an O2 sensor is not a straight line. The O2 sensors "switch" between .2 volt lean and .6 volt rich with a curve that looks like the drop off a high cliff. The curve is almost straight up and down, so the voltage shoots from .2 to .6 and back down again 2 or more times a second at cruse. The engine control computer's program knows about the jumping around and uses the output of the O2 sensors to calculate the air/fuel ratio. The program is designed to use it to not only measure the A/F ratio, but also diagnose the quality of the sensors and their wiring.
The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.
Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote
When an O2 sensor quits working properly, the computer goes into Limp Mode with reduced power and economy. The computer uses a set of tables that contain preset values that insure safe operation of the engine without engine damage. This Limp Mode enables you to drive the car until you can get the problem fixed that caused the computer to go into Limp Mode.
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