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Tackling catalytic converter issues

Thursday, September 28, 2017 - 07:00
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The catalytic converter was introduced nearly 50 years ago because of EPA-ordered cuts in emissions, and in response to those orders, auto industry execs said the EPA’s targets could never be met. But thanks to old-fashioned American ingenuity, they were. What the EPA conceives, our engineers always achieve. Fuel-cell engineer Jonathan Frost once said: "When the U.S. introduced clean air legislation in the 1970s, many engineers said that cleaning up emissions from cars was impossible, but the legislation was passed anyway and new technology was invented in the form of the catalytic converter."

In 1972, Ford President Lee Iacocca said that “if the EPA does not suspend the catalytic converter rule, it will cause Ford to shut down.” He was obviously wrong, but at the time, the prevailing wisdom was that the catalytic converter was a near-impossible concept, overly expensive and inefficient, pegged as an idea that would never, ever work.

These honeycombs are tough enough to do the very precise job required of them, but fragile enough to be rendered ineffective if things go wrong and stay wrong for a while.

The 1970 mandate that auto manufacturers would be required to reduce harmful emissions by 90 percent by the 1975 model year drove Engelhard Industries and Corning Glass to propose the device that would later become the catalytic converter. A “catalyst” foists chemical changes on other elements while resisting any change in itself. The catalytic converter adds the necessary oxygen molecules to CO and HC (the exhaust from rich mixtures), to change those harmful elements into CO2, which is the same thing we breathe out. The exhaust gases flow from the combustion chambers through the catalytic converter’s core, which is a block of ceramic material honeycombed with tiny lengthwise channels, designed to force every cubic millimeter of the gasses into contact with the catalyst material, and that’s where the necessary changes take place.

Lab work finally proved that a catalytic converter would work, but mass-producing them became a new and even more difficult hurdle. Engineers would have to take an abrasive clay mixture and force it through a shaped die at high speed to create the complexities of structure we see in cat-cons today. This process is called “extrusion” and at the time it was very commonly used for creating things like metal pipes and hollow noodles, but nothing anywhere near this complex had ever been attempted. It was a daunting task.

But that wasn’t all. Once that soft block of honeycombed clay emerged from the die, it first had to be cut to the proper length and then heated simultaneously inside and out until it was totally firm. And all this had to be done without distorting those tiny channels or causing the clay to crack, and then they had to find a way to coat all the channels with a layer of very fine platinum particles so that the platinum wouldn’t simply fall off after repeated (and vast) temperature swings. Under normal conditions, a catalytic converter races from ambient temperature to 800° F in 30 seconds, and the temperature of the gases can climb as high as 2,000 degrees. The catalysts on the vehicles I’ve datastreamed lately will normally float between 1000 and 1700°F while driving. In early development, on nearly every prototype, thermal expansion ruined the guts of the converter after just a few drive cycles.

The contrast between a pair of good catalysts (top) and a bad one is easy to spot using the PIDs.  If the rear O2s mirror the front ones this completely (bottom) the cats are no longer cats.

But then it was discovered that samples of clay from one mine in Georgia showed much better heat resistance. This clay turned out to consist of microscopic needle-like units aligned in such a way as to withstand the thermal expansion, and that was the key to making a cat that would last. The cats were off and running, and it has since become our job as technicians to herd them.

Front and rear

Nowadays we’re accustomed to seeing the “light off” cat(s) mounted very near the exhaust manifold(s) to take advantage of the natural heat still present as the exhaust has just made its exit from the combustion event – this front cat is the one sandwiched between the front and rear O2 sensors, and the oxygen that is stored in this converter is extracted from the NOX that is created during the combustion process, leaving N2, which, to quote Bernie Thompson, is the cylinder’s “working fluid” — combustion heats the nitrogen so that it expands against the head of the piston, pushing it down and spinning the crank around. The oxygen extracted from NOX in the front cat is used in the rear cat to handle CO and HC, converting them to harmless CO2, oxygen, and water vapor, which is also created during combustion.

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