|
Contact: 
|
Oxygen sensors Since the introduction of narrow band oxygen (O2) sensors by Volvo in 1976, all cars are factory equipped with them as a part of their engine performance system. They monitor the air/fuel ratio in the exhaust gas. If the air is less in the fuel mixture, than in the perfect ratio, there will be fuel left over after combustion. This is a rich mixture. They are bad because unburned fuel causes pollution and of course, is not economic. When the air is more than in a perfect ratio, there is excess oxygen. This is a lean mixture. Lean mixtures tend to produce more pollutants, and may cause poor performance or even engine damage. The mixture of air and fuel to the engine depends on various things and so when an oxygen sensor fails or degrades, the Ecu-unit cannot control the overall performance of the engine with its various parameters, so it begins to perform poorly with a fixed rich mixture to be on the "safe" side but with increased emissions and a bad fuel economy. The ecu follows the oxygen only in the closed loop. The Ecu-unit that adjusts the so called closed loop follows the output of the sensor in order to maintain a neutral engine performance (lower fuel consumption yet decent engine power and minimal pollution.), to keep it close to the stoichiometric point. As mentioned above, this point is somewhere between the rich and lean levels. Since the output of the oxygen sensor switches between lean and rich very steeply, there isn’t much of difference inbetween the oscillating output, as vieved in a closed loop O2-sensor graph. The output level of these sensors are generally something like 75 - 200mV in the lean area and 750 - 950mV in the rich area. Well just about so... Also depending on the exhaust temperature, which type (zirconia or maybe any other) and the manufacturer. As a standard, the lambda = 1-value, also called the the stoichiometric point, corresponds to 450 mV. Practically, lambda ≈ 1, the stoichiometric area, is within a wider range, between ca 200/250 - 650/700mV, to be somewhat approximate. These values may vary in different articles and reality. And may depend on the condition of the sensor. The curve or the characteristics of narrow band sensors are temperature dependent. If the exhaust gases get warmer, the output in lean area will rise and will be lowered in the rich area. Consequently, a sensor without pre-heating which is not warmed up, has a lower lean-output and a higher rich-output, maybe even exceeding 1 Volt. The temperature influence is smaller in the lean mode than in the rich mode. A "cold" sensor may output a switch voltage between 100 to 850 mV and a warmer sensor may output a switch voltage between 200 to 700mV. Then there is the open loop operation where the ECU disregards oxygen sensor output switches to the open loop mode using default maps to determine the amount of fuel needed for a certain amount of air. Also by other sensors of the engine system, as the air temperature, rpm, throttle position, air flow rate are used. The computer of the ecu while operating in open loop mode is ignoring the oxygen sensor when it suspects the sensor is not working. There are other times also when the oxygen sensor is ignored. When accelerating hard, at a high load, the engine will run rich to provide more power. When slowing down to slower speed the mixture goes lean. There are so many pages on the web as you know but I did take a look and took some that seem ok. If you read these you might learn some more. It is a very good advice that you do since other sources go further in to this subject. Otherwise, google, yahoo or altavista (or anything else) is there for you. External links. This page is very informative and provided with further links. Wikipedia - Oxygen sensors All about lambda sensors http://www.picotech.com/auto/lambda_sensor.html www.auto.howstuffworks.com: How does the oxygen sensor in a car work? Testing an oxygen sensor. How car computers work. What does the computer in a car do? The Oxygen Sensor - A page that provides some theory behind. http://home.flash.net/~lorint/lorin/fuel/lambda.htm More about the closed loop http://www.hondata.com/techclosed.html Oxygen Sensors Q&A http://www.walkerproducts.com/Oxygen%20Sensor%20Questions%20and%20Answers.htm The Oxygen Sensor - Theory and operation. Sensor life and diagnostics. http://www.volvoclub.org.uk A page about oxygen sensors why they are needed and how they work. http://www.forparts.com/o-2.htm A page about an aero-plane, its engine etc. http://home.hiwaay.net/~langford/corvair/first_runs/ Some info about open and closed loop on Zeitronix site. http://www.zeitronix.com/questions/O2questions.htm |