VP, Catalytic Combustion Corp (for Energy Ink)
Because a catalytic substance is not consumed by the chemical reaction in which it participates, a catalyst would never need replacing in a perfect world. In our world, however, ignition failures, backfires, coolant leaks, oil blow-by, load shifts and ricocheting turbo parts happen and they cause damage that afflicts the lifespan of the catalyst. Let’s now turn to how these events work on the catalyst.
Recall that the surface area of the washcoat is key to providing the locations where the precious metals are deposited so anything that reduces that surface area will decay the catalyst’s performance. When a catalyst is overheated, the crystal structure of the aluminum oxide in the washcoat changes from one that has a high surface area to one that is low in surface area.
Under normal operating conditions, the washcoat gradually undergoes this shift in crystal forms. This is called thermal aging, and several years can pass before enough surface area is lost for the catalyst to fall short of its operating targets. However, high temperature excursions can rapidly alter this timeline.
When the temperature on the catalyst’s surface approaches 1,400°F, even the best efforts at stabilizing the high surface area form of aluminum oxide in the washcoat fail. At this temperature, the life of a catalyst is shortened to a few hours, and the higher the excursion, the shorter the excursion time can be. Above 1,500°F, the surface area of the washcoat is ruined in seconds. Every time the temperature exceeds the maximum figure set by the catalyst manufacturer, it damages the catalyst. This damage is cumulative and irreversible.
How could this happen? Imagine what happens when a single cylinder misfires. The unburned air/fuel mixture is pushed into the exhaust and moves toward the catalyst. Once there, some of the fuel is converted by the catalyst and a little extra heat is liberated as a result of the reaction. If this happens once in a while, the slight increase in the temperature of the catalyst does little harm. But, what if it’s a dead hole or if there are more than one cylinder involved? Then you get into a situation where the extra heat drives the temperature of the catalyst beyond its limitations.
“But we have a high temperature shutdown system,” you say. True, but bear in mind that the materials of the converter housing will absorb some of the energy so that the exhaust gas temperature will lag behind the temperature on the catalyst’s surface. Location of the outlet thermocouple can further complicate the situation if it is located more than a scant few inches away from the outlet face of the element or if it is not situated into the exhaust flow.
If the increase is gradual, then the shutdown system has a good chance of catching it. On the other hand, if there is a sudden increase in fuel hitting the catalyst, the damage will be done before the system can respond. A catalyst housing that is glowing cherry red is a clear sign that something is badly wrong.
Preventing this from happening to your catalyst is achieved through minimizing misfires, keeping your shutdown system in good working order, locating the thermocouples as close to the outlet face of the catalyst as practical, and by routine checks to verify that the thermocouples are operating properly.