Groot asked how these sensors differed in another thread, and I was in too much of a hurry to answer him then. Here's a more complete answer.

Both sensors rely on what's called a Nernst cell, invented in the late 19th century. It was found that certain ceramic insulators (zirconium oxide being popular), when heated to 675dF, become electrical conductors of a very useful type. When one side of the cell is exposed to gas A and the other side of the cell is exposed to gas B, oxygen ions will flow across the cell in proportion to the difference in oxygen concentration between sides of the cell. Since ion flow is electric current flow, a small voltage is developed across the cell, again in proportion to the difference in O2 concentration.

Bosch used this to make the first narrowband sensors in the 1970s, using air as the reference gas. There's a fixed curve that defines O2 levels in the exhaust gases with AFR, so this device gave a simple voltage to AFR meter. However, one serious downside is that the cell temperature has a very strong effect on how the cell responds to O2. The first sensors simply used the ambient temp of the exhaust gases to heat up the sensor. EGT varies quite a lot as the engine runs, based on load as well as AFR, so these first unheated sensors weren't all that accurate, and they also took awhile to heat up, so the first few seconds (50-60 seconds) of engine operation was unregulated by the sensor. By the 1980s, powered heaters were used to decrease this warmup time to under 15 seconds. However, the heaters themselves were unregulated, so accuracy wasn't improved, just cold-start response.

In the 1990s, research done over the past several decades on lean-burn engines approached production. However, the O2 sensors then in use couldn't measure these lean mixtures with any accuracy. Two things were done to fix this. One, a regulated heater was used to keep the cell temp in the very narrow range. This substantially improved accuracy, but the cell response curve made measuring mixtures leaner than 15:1 (or richer than 14:1) very difficult. The voltage difference between 15:1 and, say, 17:1 was very, very small, and required an extremely accurate (and therefore expensive) amplifier stage to measure the difference.

Another quirk of the Nernst cell was that if you APPLIED a voltage to the cell, oxygen ions were pumped across it, reversing the normal process. By using two cells, one generating voltage and the other pumping ions, a feedback circuit could be set up to keep the O2 concentration at exactly 0.45mV between the cells. If more O2 appeared on the exhaust side, more ions flowed, so a higher voltage was generated. This was detected, and a higher voltage applied to the pump cell to maintain the balance. This correction voltage was much more linear across a much wider range of O2 concentrations than the single cell approach, which widened the voltage differences between "interesting" AFRs, and thus removed the need for the expensive amplifiers. With current sensors, AFRs of roughly 22:1 to 11:1 can be accurately measured. By varying the pump to measuring voltage feedback curves, you could also calibrate a generic sensor to operate with any fuel (natural gas, ethanol, methanol, which all have different "ideal" AFRs), which fit well with the movement to alternative fuels.

The cost of this new techonology was that the sensor system became a lot more complicated. Whereas before, with the narrowband unit, you simply had a voltage that corresponded with an O2 measurement (so all you needed was the sensor and a cheap voltmeter), you now had to run a tightly regulated heater, and you had to run the pump voltage in a feedback loop with the measuring voltage, then take the difference to get the AFR. There are proven analog hardware circuits to run this feedback loop, but recalibration requires hardware changes. Doing the feedback loop in software allows on-the-fly recalibration (if you switch sensors), and allows for "flex-fuel" operation.

Cool!

More questons:

Why does running a wbO2 sensor unheated harm it?

When that does harm it, what is the failure mode? How long does it take to happen?

Is the harm reversible?

What benefit are you getting by going with a more expensive wbO2 sensor over the cheaper ones? (I'm talking about the sensor itself)

James,

thank you for taking the time to write this out.
very informative.
thank you!

brant

The sensors aren't really more expensive. The cheapest ones (for watercooled VWs) can be had for around $40, which is pretty close to the price of many NB sensors.

The heater/feedback control system is NOT part of the sensor itself. The sensor itself is just two cells (each very like a narrowband sensor) and a simple heater element. The heater control and the feedback circuits are part of an external controller.

If you tried to run a WBO2 sensor w/o a controller, I don't really know what would happen. At best, it would probably function like an unheated narrowband sensor. It may not work at all.

I don't believe running either sensor unheated will damage it, but the WB unit won't read correctly. Since you MUST use a controller with a WB sensor properly, the point is moot. The heater control will be part of the controller.

If you run a narrowband with no heater, you'll not damage it. It will increase its inaccuracy, and it won't read at all for about a minute after startup. You'll also need to ensure an unheated sensor is fairly close to the ports. If it's too far back in the pipe, it will never get up to temperature. A heated sensor, however, will work even well back in the pipe.

The instructions that come with the Innovate LM-1 say to always turn the unit on if the engine is running to prevent damage to the sensor. I also talked to a guy at a dyno shop that said he's killed wbO2 sensors by running them unheated. He said something about them dying "quite quickly". He's also the one that told me that the "really good" wbO2 sensors can cost up to $200.

Being ever curious, I'm trying to find out the details.

I think the innovate literature says never run their sensor without the control unit... implying that it would be damaged.

can't say why though...


james, can you give the part number again for the 40$ units?
are they really just as good?
do you order them through VW, or just a bosche distributor?
(where is a cheap place to get them?)

I'm needing a back up sensor, and maybe I should try one of these cheaper units

Like I said, I really don't know what happens if you run the sensor without the controller, so I'll bow to Innovate's advice on this.

As for "really good", the LSU4 (which Innovate uses) seems to be accurate enough for good tuning. I'm sure something like the Horiba setup (which is costly) is better, but few people need super high accuracy. When you consider how crude using things like spark plug colors is, knowing the AFR to even within 0.5 AFR is pretty damned amazing.

The sensors don't live forever. They get oil-contaminated, and will eventually require replacement. The EPA requires that all new cars not require any emissions related maintenance for the first 50K miles these days (actually, it may be 100K miles now, they've been changing the regs), so one can assume that *on average*, the sensors will last at least that long. $40 every 50K miles isn't just a big deal, IMHO.

According to Bosch's Oxygen sensor FAQ,

An oxygen sensor can fail prematurely if it becomes contaminated with phosphorus from excessive oil consumption, silicone from internal coolant leaks, using silicone sealant in the engine, and some over-the-counter fuel additives. Even a small amount of poorly refined gasoline can kill an oxygen sensor. Environmental factors such as road splash, salt, oil, and dirt can also cause a sensor to fail, as can thermal shock, mechanical stress, or mishandling. However as required by vehicle manufacturers, Bosch sensors are designed and tested with these extremes in mind. Click here to review damaged oxygen sensors.



Nothing mentioned about not heating it up....

Here's something...

"The sensor should never be run without power to the WB unit (a hot sensor burns off carbon residues)"

from http://techedge.com.au/vehicle/wbo2/

I'm not sure I totally buy it, but the idea that it gets clogged if not hot might have some validity.

-Tony

Yes they will get damaged if not turned on.

Some NB sensors are unheated and take 4-5min before they start to read proper.

Leaded fuel will seal over the sensor, If you use leaded race gas you should hook the sensor to your street car, after the race, to burn off the lead deposits.

The Innovate people use the Bosch LSU4 sensor, which is an OEM part for recent 1.8 VWs.

Here is a link to a Megasquirt page with lots of info on where to get the sensor. That page says $30, but the Euro has gone up since, and 1stvwparts now lists the part for $41. The difference between the various subtypes of the LSU4 are basically the lead length and the connector. With NB units, I've seen some parts go for $130+, even though the ONLY difference between that part at the $30 generic parts is the connector. The same scam appears to hold fo the WB units, as well. Same sensor, different connector, huge price difference.