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Old 04-01-2010, 08:19 PM   #10 (permalink)
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Originally Posted by Daox View Post
It holds the air/fuel ratio to stoichiometric ratios and is in general more efficient than running in open loop.
It's probably worth noting that stoichiometric ratio (lambda 1.0) is the peak thermal reaction, but it is not the best place to run an engine for either fuel economy or power.

Best economy is leaner (around 1.05 lambda), best power is richer (around .86 lambda, but with a lot of variation depending on engine design).

The only reason that vehicles target stoich is emissions. Stoich is the highest EGT, and that gas temp is needed for the CAT (which has a very narrow efficiency band). Also, it is a good spot on the HC, CO2, NOx emissions curve.

As a practical matter, closed loop IS usually more efficient than open loop because the cars always error rich open loop.

Originally Posted by mobythevan View Post
It would not be unusual at all for a car with a wideband O2 sensor to enter closed loop mode quickly. With the wideband the closed loop can control warmup at a richer AFR and then transition to a leaner AFR when warmed up. With a wideband there is almost no reason to stay open loop except start up and maybe after start enrichment because AFR can be controlled across a wide range. With a narrowband O2 sensor it wouldn't make a lot of sense for the car to enter closed loop until the engine is warm because you will only be correcting to 14.7 AFR. My 2 cents
Generally speaking, vehicles with heated narrowband sensors tend to go closed loop a little faster than cars with wideband sensors (which are always heated as well). There was actually a good discussion about this at the recent Consolidated Research Council conference.

Much of the time, the limiting factor in keeping a vehicle from going closed loop right after startup is not rather or not the engine is ready to run at stoich, but waiting on the sensors for reliable measurements.

In an unheated UEGO sensor, the system can't go closed loop until exhaust gas heats the zirconia ceramic to the point that a reaction can occur.

A heated sensor is faster (and generally heated so that the vehicle can get closed loop for emissions purposes faster), but there are still some undesired delays. You can't pre-heat the sensor for startup, because unburnt fuel or condensation would shock cool the ceramic and crack it.

A wideband takes longer to bring up because of the way UEGO sensors work. Stoich is a reversal in chemistry, and effectively correct as long as a reaction is occuring. But the curve off lambda 1.0 with a wideband sensor is complicated, and only 'calibrated' at a specific temp. So instead of just heating above the reaction threshold, a wideband is generally heated more, up to about 750 degrees C, and held there before readings are reliable.

Originally Posted by JeepNmpg2 View Post
Do you know which sensor I should look at, I think mine has two or three O2 sensors (1 or 2 on the manifold and 1 after the cat).
You ALWAYs want to measure lambda pre-CAT. The sensors don't really measure AFR, but equivilency ratio (which is inverted to give lambda). Basically O2 is added or removed to the test chamber until stoich is reached. Since CAT is supposed to be removing hydrocarbons, the post cat sensors are there to see rather or not there is a proper shift from the real (pre-cat) equivelency ratio measurements.

When you get a 'cat efficiency' DTC it generally means that one of the sensors has gone bad, or the shift between pre and post CAT is below expected.

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