NTK Sensor Article

[Brian L Massey]

Late as usual, but here's the outline & comments I promised.

A few remarks first: this about SAE #920234, and the sensor discussed in
this article isn't exactly the same as the Honda sensor; you can see that
from the pictures. But judging from other people's observations, it has
to be very close, so everyone assumes that most material applies to both.

First to David Harris, who asked about the portion where it talks about
temp range of these sensors, here's the important quotation:

"The usable range of the element temperature is from 700C to 950C."

Another:

"It should be noted that the problem of carbon accumulation can occur in
a temperature range less than 700C and there is danger of the element
blackening in a temperature range less than 600C." This is one reason I
guess why everybody says don't operate or leave the sensor in the exhaust
without the heater running.

Here's a rough outline, and then I will mention some juicy bits:

Construction & Mechanism         (how it's built and works)
UEGO Sensor Construction         (goes into the chemistry; some important
things here)
Output Characteristic                 (shows how the pumping current
relates to AFR; also 
                                                discusses mfg. variations
and how output varies with T & P)
Engine Operation                        (discuss results under diff.
engine conditions)
Response Time                        (shows measurements of how quickly
the sensor responds;
                                                 compares to a
conventional HEGO sensor)
Warm-up Properties                  (talks about warm-up and some great
graphs on what
                                                the sensor output does
during warmup, interesting)
Durability                                (about lifetime variations, not
bad for hot gas exposure)
Futher Applications                (discusses other uses like determining
cat deterioration,
                                               misfire detection)

Now for the tidbits: the sensor construction part reveals that with rich
AFR, the sensor is sensitive to...CO (carbon monoxide), HC (unburned
hydrocarbons), and H2 (hydrogen). I didn't realize the significance of
this until I read in the Heywood book about a typical exhaust gas mix
under rich afr. There apparently is a measurable amount of hydrogen.
These three sensor sensitivities have to be taylored to a representative
component mix of exhaust gas. Interesting.

One of the graphs shows changes in pump current (and therefore measured
AFR) based on temp changes: 3.0% per 100C change in the measurement
element, which of course will depend on how well the heater controls that
temperature. Also of interest is a graph regarding same, based on
pressure changes: 3.7% per 0.1atm, but they only give a graph for lean
mixtures, not for rich ones. One thing I noticed in the Bosch LSU
datasheet recently published (thanks Bert & Jorgen!) is that the LSU
looks like it's more than twice as sensitive to temperature changes (6-7%
per 100C), which might explain why Bosch has a more elaborate heater
control mechanism.

On this same subject, another graph suggests that temp is controlled to
within +-0.2 AFR if the heater is simply held at a constant *voltage*!
This has to be due to the sensor's temp coefficient being tuned to
compensate for exhaust gas temps (at a given voltage, the heater heats up
if the gas is cold, and cools off if the gas is hot). They also show the
heater being supplied with a range of fixed voltages from 10 to 14.5V,
and remaining within +-0.2 AFR over the full range of exhaust gas temps
and over the full spread of lean to rich AFRs (well, at least from 12 to
18 AFR). I am fairly impressed with this. As a controls guy, this looks
like a very well-tempered sensor over temperature.

Last, as others have mentioned in the archives, they do show how the
sensor is fast enough to pick up misfires on individual cylinders. Nice
article; for me at least, it was worth the $13.

HTH,
Brian
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