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2011 Engines Review - Catalyst


Time For A Conversion

Unfamiliar with catalytic converters? You shouldn’t be, because you may need one soon.


If you’re like most people, the only time you put serious thought into catalytic converters is when you’re driving down the road and an awful sulfuric scent whacks you straight in the nostrils with a silent but deadly force. All you can do is cross your fingers and hope that it’s someone’s malfunctioning converter and not old, trusty Fido the Garbage Devourer sleeping soundly in the backseat. But the time to delve deeper into the world of catalytic converters has arrived, as the National Marine Manufacturers Association (NMMA) is working closely with the Environmental Protection Agency (EPA) to hammer out regulations that will strongly encourage, if not require, catalytic converters on all gasoline-powered boats within a few years, due to the device’s mitigating effect on engine emissions.

Catalytic converters are useful because internal combustion gasoline engines produce three main gasses that are harmful to the environment: hydrocarbons, oxides of nitrogen, and carbon monoxide. Those gasses normally pass through your engine, out of your exhaust pipe, and into the air, where they act as pollutants. But when a catalytic converter is part of an exhaust system, the volume of these gasses is greatly reduced, creating a cleaner, safer boating environment.


Illustration By Steve Karp

1. A pre-oxygen sensor measures air to fuel ratio in the exhaust before it is catalyzed. 2. The golden cylinder is the converter itself. 3. Another sensor measures exhaust post-conversion; a 14.7:1 ratio means the converter is working efficiently. 4. Clean exhaust exits the system.

The catalytic converter is a deceivingly simple device. It’s about the size and shape of an artillery round and is commonly referred to as a “brick.” Inside it is a finely grated metal honeycomb, which presents the greatest possible surface area to exhaust as it passes through the converter. A specialized wash coat made up of two different components is normally applied to the honeycomb. The first component is a chemical compound that acts as a primer on the surface of the honeycomb by roughening its surface, which allows the second component to stick to it. That component is commonly a mixture of three precious metals: platinum, rhodium, and palladium. Though some wash coats forego platinum because it increases the converters price significantly, it is still fairly common. (An interesting aside, Toyota 4Runners saw a rash of thefts in the 1990s as their high ground clearance made their platinum wash-coated converters easy pickins’ for thieves who could sell them for a tidy sum to morally loose scrap dealers.) When the three toxic gasses in the exhaust come into contact with the mixture of precious metals inside the honeycomb, a chemical reaction occurs that, simply stated, turns them into innocuous gasses and water vapor that pass harmlessly into the environment. The hydrocarbons break down into water and carbon dioxide, the nitrogen oxide splits up into nitrogen and oxygen, and carbon monoxide—that odorless, silent killer—changes into carbon dioxide—the very same stuff you exhale with every breath. It’s somewhat of a miraculous conversion, and one in which the EPA quite rightfully has a fervent interest.


This 8.1 Gi V-8 electronic fuel injection engine from Volvo Penta is about as robust as they come, and it also contains a catalytic converter to ensure that its emissions are as clean as they can be to reduce your carbon footprint.

At Mercury Marine (which consulted on this article) the catalytic converter is an integral part of a three-pronged system known as Emissions Control Technology (ECT). With ECT, the converter works in unison with two separate oxygen sensors—one before the converter and one after it—which test the gas stream to make sure it maintains a stoichiometric level, which is to say the mixture of air and fuel in the exhaust is at an optimal ratio for engine efficiency and cleanliness. Together, the sensors and the converter combine to keep emissions extremely low and your boating environment as pristine as possible.

While some of this chemistry talk about compounds and stoichiometry and precious metals may seem esoteric, it is most definitely worth your while to familiarize yourself with catalytic converters. With the EPA aiming for every gasoline-powered American boat (including ones with outboards) to have a converter installed by as soon as 2016, the time for your own conversion may soon be at hand.

This article originally appeared in the June 2011 issue of Power & Motoryacht magazine.