+ Reply to Thread
Results 1 to 4 of 4
  1. #1
    Registered Member
    Location
    Richfield, Ohio, USA
    Member No: 65681 66.cobra is an unknown quantity at this point 66.cobra's Avatar
    Join Date
    Feb 2003
    Posts
    1,809
    Rep Power
    0


    Yes | No

    A/F sensors vs O2

    Has anyone tried or looked into A/F sensors to replace O2s? My RX300 has A/F sensors and I see from a lot of Toyota guys that they put O2's when they fail due to price. A/F sensors are $150, O2s are now down to $40-50. the A/F sensors give a more precise oxygen signal and it seems more cars are using them these days for improved milage. Just wondering if anyone has tried them in their 540 to boost the milage and possibly power?
    "I FEEL THE NEED, THE NEED FOR SPEED"...-Dad
    "BMOC KICKS A$$"........................-Roland

    Click the picture for my Cardomain site.






  2. #2
    Inner Circle Member Inner Circle Member
    Location
    San Jose, CA, United States
    Member No: 21320
    Inner Circle ©156 since: Jul 21, 2003 edjack will become famous soon enough edjack's Avatar
    Join Date
    Dec 2001
    Posts
    8,640
    Rep Power
    23


    Yes | No

    Re: A/F sensors vs O2

    What is the voltage output versus O2 sensors? You can't substitute sensors into a system that is not calibrated for them. Don't play with the system.
    Ed in San Jose. BMW CCA member since 1987 (Nr. 62319). Golden Gate Chapter. '97 540i 6 speed. Build Date 3/97. Aspensilber over Aubergine leather.

  3. #3
    Registered Member
    Location
    Richfield, Ohio, USA
    Member No: 65681 66.cobra is an unknown quantity at this point 66.cobra's Avatar
    Join Date
    Feb 2003
    Posts
    1,809
    Rep Power
    0


    Yes | No

    some reading if you can't sleep....

    same as O2 .1-.9v. I heated an A/F sensor with a torch and it gave me exactly the same voltage output as when I heated an O2 sensor. From what I read it is just supposed to be a more linear and precise signal between that .1-.9v then what a regular O2 will give you. The O2 sensor is more of an on/off switch then sensor. Anything below .5volts is lean, above .5 volts rich. This more linear signal supposedly lets the CPU control the fuel curve better, and get better MPG.


    Zirconia oxygen sensor

    The early introduction of the oxygen sensor came about in the late 1970s. Since then Zirconia has been the material of choice for its construction. The Zirconia O2 sensor produces its own voltage, which makes it a type of generator. The generated varying voltage shows up on the scope as the familiar 1 Hz sine wave, when in closed loop control. The actual voltage that is generated is the difference between the O2 content of the exhaust and that of the surrounding ambient air. The stoichiometric air-fuel ratio or the mixture of air-to-fuel equal to 14.7:1 is theoretically the best mixture ratio for gasoline engines. This is the theoretical air -to- fuel ratio at which all of the fuel will react with all of the available oxygen resulting in complete combustion. At or near this ratio, the combustion process produces the best balance between power and low emissions. At a stoichiometric air-fuel ratio (14.7:1), the generated O2 sensor voltage is about 450 mV. The Engine Control Module (ECM) recognizes a rich condition above the 450 mV level, and a lean condition below it, but does not detect the extent of the richness or leanness. It is for this reason that the Zirconium O2 sensor is called a “narrow-band” O2 sensor.
    [edit] Titanium oxygen sensor

    The Titanium O2 sensor was used throughout the late 1980s and early 1990s on a limited basis. This sensor’s semiconductor construction makes its operation different from that of the Zirconium O2 sensor. Instead of generating its own voltage, the Titanium O2 sensor’s electrical resistance changes according to the exhaust oxygen content. When the air/fuel ratio is rich, the resistance of the sensor is around 950 Ohms and more than 21 Kilohms when the mixture is lean. As with the Zirconium sensor, the Titanium O2 sensor is also considered a narrow-band O2 sensor.
    [edit] Narrow-band sensor

    As mentioned before, the main problem with any narrow-band O2 sensor is that the ECM only detects that the mixture is slightly richer or leaner than 14.7:1. The ECM does not measure the operating air-fuel ratio outside the stoichiometric range. In effect it only detects that the mixture is richer or leaner than stoichiometry. An O2 sensor voltage that goes lower than 450 mV will cause a widening of injector pulse and vice-versa. The resulting changing or cycling fuel control (closed-loop) O2 signal is what the technician sees on the scope when probing at the O2 sensor signal wire.
    [edit] Wide-band sensor

    The newer “wide-band” O2 sensor solves the narrow sensing problem of the previous Zirconium sensors. These sensors are often called by different names such as, continuous lambda sensors (lambda representing air-fuel ratio), AFR (air-fuel ratio sensors), LAF (lean air-fuel sensor) and wide-band O2 sensor. Regardless of the name, the principle is the same, which is to put the ECM in a better position to control the air/fuel mixture. In effect, the wide-band O2 sensor can detect the exhaust’s O2 content way below or above the perfect 14.7:1 air/fuel ratio. Such control is needed on new lean burning engines with extremely low emission output levels. The tighter emission regulations are actually driving this newer fuel control technology and in the process making the systems much more complex and difficult to diagnose.
    [edit] Construction and operation

    The wide-band O2 sensor looks similar in appearance to the regular Zirconium O2 sensor. Its inner construction and operation are totally different, however. The Wide-band O2 sensor is composed of a dual inner layer called “Reference cell” and “Pump cell”. The ECM’s AFR sensor circuitry always tries to keep a perfect air/fuel ratio (14.7:1) inside a special monitoring chamber (Diffusion Chamber or pump-cell circuit) by way of controlling its current. The AFR sensor uses dedicated electronic circuitry to set a pumping current in the sensor’s pump cell. In other words, if the air/fuel mixture is lean, the pump cell circuit voltage momentarily goes low and the ECM immediately regulates the current going through it in order to maintain a set voltage value or stoichiometric ratio inside the diffusion chamber. The pump cell then discharges the excess oxygen through the diffusion gap by means of the current created in the pump-cell circuit. The ECM senses the current and widens injector pulsation accordingly to add fuel.

    If on the other hand the air/fuel mixture goes rich, the pump cell circuit voltage rapidly climbs high and the ECM immediately reverses the current polarity to readjust the pump cell circuit voltage to its set stable value. The pump-cell then pumps oxygen into the monitoring chamber by way of the reversed current in the ECM’s AFR pump-cell circuit. The ECM detects the reversed current and an injector pulsation-reduction command is issued bringing the mixture back to lean. Since the current in the pump cell circuit is also proportional to the oxygen concentration or deficiency in the exhaust, it serves as an index of the air/fuel ratio. The ECM is constantly monitoring the pump cell current circuitry, which it always tries to keep at a set voltage. For this reason, the techniques used to test and diagnose the regular Zirconium O2 sensor can not be used to test the wide-band AFR sensor. These sensors are current devices and do not have a cycling voltage waveform. The testing procedures, which will be discussed later, are quite different from the older O2 sensors.
    [edit] Comparison with mass airflow sensor

    The AFR sensor operation can be thought of as being similar to the hot wire Mass airflow sensor (MAF). But, instead of an MAF hot wire, the ECM tries to keep a perfectly stoichiometric air/fuel ratio inside the monitoring chamber by varying the pump cell circuit current. The sensing part, at the tip of the sensor, is always held at a constant voltage (depending on manufacturer). If the mixture goes rich, the ECM will adjust the current flowing through the sensing tip or pump cell circuit until the constant operating voltage level is achieved again. The voltage change happens very fast. The current through the pump circuit also pushes along the oxygen atoms either into, or out of, the diffusion chamber (monitoring chamber) which restores the monitoring chamber’s air/fuel ratio to stoichiometry. Although the ECM varies the current, it tries to maintain the pump circuit at a constant voltage potential.
    [edit] Testing

    As the ECM monitors the varying current, a special circuit (also inside the ECM) converts the current into a voltage value and passes it on to the serial data stream as a scanner PID. This is why the best way to test an AFR sensor’s signal is by monitoring the voltage conversion circuitry, which the ECM sends out as an AFR-voltage PID. It is possible to monitor the actual AFR sensor varying current, but the changes are very small (in the low milliamp range) and difficult to monitor. A second drawback to a manual AFR current test is that the signal wire has to be cut or broken to connect the ammeter in series with the pump circuit. Today’s average clamp-on ammeter is not accurate enough at such a small scale. For this reason, the easiest (but not the only) way to test an AFR sensor is with the scanner.

    Sometimes the check engine light will come on while you are driving, and after you switch the engine off, you restart the engine and the check engine light will be off. The scanner cannot pick up the fault code unless the check engine light is still on, so it is a good idea to use the scanner while you are driving. On some cars like LEV (Low Emssion Vehicle) 2000 to 2003 JDM Subaru Imprezas with 1.5 litre EJ15 engines, the engine has both a narrow band O2 sensor and an AFR sensor, so it is hard to tell which one needs replacing with a cheap scanner. Since the O2 sensor costs $140US, while the AFR sensor costs $400US, it’s better to let a Subaru specialist determine which one is faulty before you replace it. Also, you can buy a bottle of fuel injector cleaner, which might work for a lot less money than replacing the sensor.
    [edit] Operating temperature

    Another major difference between the wide-band AFR sensor and a Zirconium O2 sensor is that it has an operating temperature above 1200°F (600°C). On these units the temperature is very critical and for this reason a special pulse-width controlled heater circuit is employed to control the heater temperature precisely. The ECM controls the heater circuit.
    [edit] Advantages

    The wide operating range coupled with the inherent fast acting operation of the AFR sensor puts the system always at stoichiometry, which reduces a great deal of emissions. With this type of fuel control, the air/fuel ratio is always hovering close to 14.7:1. If the mixture goes slightly rich the ECM adjusts the pump circuit’s current to maintain the set operating voltage. The current is detected by the ECM’s detection circuit, with the result of a command for a reduction in injector pulsation being issued. As soon as the air-fuel mixture changes back to stoichiometry, because of the reduction in injector pulsation, the ECM will adjust the current respectively. The end result is no current (0.00 amperes) at 14.7:1 air-fuel ratio. In this case a light negative hump is seen on the ammeter with the reading returning to 0.00 almost immediately. The fuel correction happens very quickly.
    "I FEEL THE NEED, THE NEED FOR SPEED"...-Dad
    "BMOC KICKS A$$"........................-Roland

    Click the picture for my Cardomain site.





  4. #4
    Inner Circle Member Inner Circle Member
    Location
    San Jose, CA, United States
    Member No: 21320
    Inner Circle ©156 since: Jul 21, 2003 edjack will become famous soon enough edjack's Avatar
    Join Date
    Dec 2001
    Posts
    8,640
    Rep Power
    23


    Yes | No

    Based on the discussion below>>

    it would appear that the DME input circuitry would have to be designed around the use of an A/F sensor, so a swap would not work.
    Ed in San Jose. BMW CCA member since 1987 (Nr. 62319). Golden Gate Chapter. '97 540i 6 speed. Build Date 3/97. Aspensilber over Aubergine leather.

+ Reply to Thread


Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

     

Bookmarks

Posting Permissions

  • You may post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
1e2 Forum