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The Lowly Oxygen Sensor

More power and clean living through science--and Bosch

When we got the invitation to tour Bosch's oxygen sensor plant in Anderson, South Carolina, it had all the allure of eating a sand sandwich. Let's face it, oxygen sensors aren't the most glamorous parts on your car, and nothing about them says "performance." Right? Well hold on there hydrocarbon breath, because a healthy oxygen sensor is imperative for making maximum horsepower on your late-model computer-controlled hot rod. If your O2 sensors aren't working to their full potential, then your full power potential isn't being realized.

The first O2 sensor was installed in 1976 on a Volvo. Yawn. It needed to be replaced every 20,000 miles. That was the inauspicious beginning of the lowly oxygen sensor, so it's no wonder most gearheads either scorn them for their roll in pollution control, or at best, look at them indifferently. At present, the Robert Bosch Corporation has over 218 million of them on the road in the US market; that's over 92 percent of the cars on the road. So we wondered, with such a stranglehold on the market, why does Bosch fly us out and put us up in a fancy hotel just to make a big deal out of O2 sensors?

We'll cut to the chase: the traditional thimble-type sensor (originally pioneered by Bosch) which you've come to know and love is rapidly being replaced by a new kind called the Planar-type. Although they look similar on the outside, the guts are very different--and better in some important ways. But before we dive in, let's review what O2 sensors do.

Oxygen sensors perform a simple duty, they tell the engine control module (ECM) if the exhaust gas is lean (an excess of oxygen corresponding to a low voltage) or rich (a lack of oxygen corresponding to a high voltage). Since O2 sensors aren't terribly accurate when representing air/fuel ratios higher or lower than stoichiometric (a "perfect" air/fuel ratio of 14.7 parts air to 1 part fuel), the ECM likes to see the sensors cross over the stoichiometric mid-point (a voltage around 450mV) as often as possible. That's a pretty good indication the engine is running near its optimal air/fuel ratio. A "lazy" worn-out sensor crosses over the mid-point less frequently, costing power and increasing emissions. In a nutshell, a clean, uncontaminated sensor switches more frequently, and that's good.

Lesson two: a heated sensor is better than an unheated sensor. That's because the ECM can't do its job until the sensor gets up to about 625-650 degrees Fahrenheit--the point at which the sensor's ceramic element becomes electrically conductive. Before 1982, when the heated sensor came into use, it took minutes for an un-heated sensor to get up to temp, or "light off." The heated thimble type chopped that time down to 45 seconds and thus cut emissions commensurately.

In 1998, Bosch introduced the Planar-type sensor. Instead of a thimble shape, it's thin and flat, like a geometric "plane." Without diving into a bunch of arcane engineering jargon, the Planar sensor lights-off faster, in about 10 seconds, as opposed to 45 seconds for the thimble. In an odd twist of logic, a faster-lighting O2 sensor can actually make more power. If you put a Planar sensor on your older car, you're out of luck, but if you're engineering a new vehicle or powertrain for GM, Ford or Chrysler, you'll be able to put a few more oats in the feed bag. That's because the Planar sensor--in conjunction with an increase in ECM computational power--allows engineers to cut cold-start emissions by over 50 percent. When you can do that, you can play a little looser with parameters like camshaft overlap, compression ratio, power enrichment fueling and ignition timing. Now you can see why a car like Dodge's new SRT-4 has Planar sensor technology.

Bosch's planar sensor has a few advantages over the traditional thimble type sensor. For starters, its compact, flat shape provides a more robust seal against the elements and against vibration. It's also more forgiving towards extreme changes in temperature (such as water immersion) which can crack the ceramic structure or breech the seal of traditional sensors. The compact flat profile also requires less power to heat.

Planar sensor technology has more than a toe-hold in the new car market; in the six years since its introduction, Planar sensors account for 53 percent of the O2 sensors in new cars. And it gets better. Bosch has figured out how to use its Planar manufacturing technology to build "wide-band" sensors, which enables some really clean-burning engines to make some really obnoxious power numbers. For instance, you'll find wide-band Bosch Planar sensors in all Cadillac CTS models, including the LS6-powered CTS-v.

Let's back up the bus a bit. What is a wide-band sensor and why should horsepower junkies want it? Unlike a standard O2 sensor which switches between rich and lean, a wide-band sensor has an output voltage which is directly proportional to the oxygen content of the exhaust. Put another way, the exact air/fuel ratio is known at other points besides stoichiometric. That's great news to guys who want to tune their engines for max power, since max power usually occurs when the air/fuel ratio is around 12.5:1, not 14.7:1, which is stoichiometric.

The most obvious application for a wide-band sensor is in computer-controlled hot rods. Stand-alone ECUs from FAST, ACCEL and MoTec use wide-band sensors to help control the power enrichment at full throttle. In this case, the ECU chases a preset air/fuel ratio from a load table map, and enriches (or leans-out) the fueling based on a reference signal from a wide-band sensor. In some instances, wide-band controllers have even saved engines that would have ordinarily burned up due to a hurt fuel pump or a clogged fuel filter. Even carbureted engines with no electronic control can take advantage of wide-band technology in the dyno cell. It's common these days to tune jetting and other parts of the set-up using wide-band monitors or data-logging. Nascar engine builders rely on wide-band sensors to help them in real-world track testing by outfitting both sides of an engine. When strong lateral g-forces at high-speed ovals cause the left bank to lean-out and the right bank to run rich, wide-band sensors in conjunction with an accelerometer and data-logging are there to shine light on the problem.

Wide-band oxygen sensors are quickly becoming the tool of choice when it comes to max-effort competition engines. These days, if a serious engine builder is building a high-output nitrous, turbo or supercharged engine, he's using some form of wide-band technology. The quest for power is an obvious reason, but there's another reason: longevity. The higher the output of an engine, the more narrow the tuning window is and the greater the chance of damage. Simply put, top engine builders like delivering engines to their customers in one piece, and wide-band sensors help them do that.

When our whirlwind tour of the Bosch O2 sensor plant was done, we had a new appreciation for this unsung and unloved part. The high manufacturing standards and rigorous testing each sensor is subjected to really got our attention, even if we didn't understand all the chemistry and physics. Planar sensors look to be the odds-on favorite for your next new car, and with wide-band planar sensors quickly gaining ground, it won't take long before they find themselves on more and more new high-performance variants. And the next time we're in the parts store looking for a replacement, we'll be checking the box for the "Bosch" label!

OXYGEN SENSOR QUICK FACTS

* Oxygen sensors (with the exception of wide-band types) are only accurate when they indicate a perfect 14.7:1 air/fuel ratio
* Oxygen sensors are easily contaminated by anti-freeze (such as by a blown head gasket) or the silicon fumes emitted by RTV (such as from a repair or rebuild operation)
* A contaminated oxygen sensor can cause a severe loss of power in a computer-controlled vehicle
* In the first 45 seconds of cold vehicle operation, your engine emits as many pollutants as in 500 miles of highway driving
* Bosch's new generation of Planar oxygen sensors can reach operating temperature in 10 seconds, thus curtailing emissions substantially
* Replacing worn-out oxygen sensors can improve emissions more than all other repairs combined
* Leaded fuel contaminates wide-band sensors after about 200 hours of use. This makes them suitable for short tuning sessions on the dyno and limited use on track, but not for an entire racing season.
* The Bosch Corporation plant in Anderson, South Carolina manufactures an oxygen sensor every two seconds

How An Oxygen Sensor Works

In 1899, Professor Walter Nernst, working in Leipzig, Germany, developed the theory of a "concentration cell" which, much like a battery, uses a gas-tight ceramic electrolyte that becomes electrically conductive above 625-650 degrees Fahrenheit. This "Nernst cell" transfers oxygen ions from "reference air" inside the cell to the outside environment--or from the outside environment to the reference air in the cell. This flow of ions generates measurable voltage reflecting the difference in the oxygen content between the gas outside the sensor and the reference air inside the sensor.--Chuck Ruth, Bosch Corporation

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