MY RECENT item on fuel cell catalysts got me and several readers thinking about conventional automotive catalytic converters. Are automotive catalysts the same as those in a fuel cell? No. But what does an auto’s cat do anyway?
In fact, a modern car’s catalytic converter performs quite the balancing act. At its best, the device takes byproducts of combustion and transforms them into stuff that’s literally cleaner than the air of a typical urban area.
There are subtleties a’plenty, depending on whether the combustion is spark- or compression-ignited (the latter, aka, diesel). And also whether a gasoline engine is operating in lean-burn mode or not. But it’s a balancing act, nonetheless, as described in my copy of the Bosch Automotive Handbook, 8th Edition, 2011. (There’s also a 9th Edition, 2014, whose details in this regard are unchanged.)
Bosch Automotive Handbook – 9th Edition, Robert Bosch GmbH, Bentley Publishers, 2014.
Modern gasoline engines have three-way catalytic converters, capable of handling all three of the regulated pollutants: unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOX). However, control of NOX is inherently different chemically from minimization of HC and CO.
Chemically, HC and CO control requires oxidation; NOX needs chemical reducing. The three-way cat does both.
Catalysts in the converter, principally platinum, palladium and rhodium, encourage the three chemical reactions described above. Generic hydrocarbons combine with oxygen in the air to produce carbon dioxide and water. Carbon monoxide and oxygen combine to yield carbon dioxide. These are both called oxidation reactions, for the obvious reason that oxygen combines with something to yield something else.
By contrast, the third equation describes the opposite chemical reaction, a reduction: Oxides of nitrogen give up their oxygen to yield pure nitrogen and some more carbon dioxide.
Back before Al Gore, we thought of CO2 as the stuff we exhale and plants just love. Today, it still is, but it’s also dreaded, accounting for about 82 percent of greenhouse gas emitted through human activities.
It’s quite enough to make me nervous about exhaling.
But I diverge from automotive cats. The balancing act is in their performing both oxidation and reduction simultaneously in the same device. Platinum and less costly palladium promote the oxidations; rhodium encourages the reduction.
Optimal balancing depends profoundly on the products of combustion, the engine-out emissions. What’s best is theoretically perfect combustion, with precisely the correct amount of air to burn the fuel. Such an air/fuel mixture is called a stoichiometric one.
The challenge in getting optimal engine-out emissions is solved upstream, in admitting precisely a stoichiometric air/fuel mixture into the combustion chambers and combusting it properly. Back with carburetors and traditional ignition systems, this has been likened to dropping a match down a sewer. Today’s direct fuel injection and computer-optimized ignition do a better job.
However, as noted in other activities, “don’t trust, verify.” And, for this, modern cars have oxygen sensors in their exhaust systems. As their name suggests, these gadgets monitor the oxygen level of the exhaust prior to its entering the three-way cat.
The sensor instructs the engine’s control unit how to dither its intake air/fuel mixture around stoichiometric; this, despite whatever a driver does with the accelerator pedal.
A Bosch oxygen sensor, aka lambda sensor.
Since 1976, Bosch has manufactured 500 million oxygen sensors, also called lambda sensors because engineers designate air/fuel mixtures with the Greek letter λ. Back in 1977, I was an Associate Engineering Editor at SAE, then still known as the Society of Automotive Engineers. Given a technical paper from Bosch in advance of its publication, I wrote an article on the lambda concept and three-way catalysis in SAE’s Automotive Engineering magazine.
Volvo and Saab were among the first production applications of three-way converters. (The Scandinavians were into emissions controls when the rest of European automakers were still claiming it was overkill.)
At a subsequent SAE Annual Meeting, a colleague and I were examining a sparkplug-like device on one of the displays. I asked what it was, and the Bosch engineer said proudly, “This is our new lambda sensor. You can learn about it over there.” He pointed to another story board—containing my article.
I nonchalantly covered my nametag, ambled over and perused it. ds
© Dennis Simanaitis, SimanaitisSays.com, 2015