A mechanic told me the air pumped to the catalytic converter only dilutes the exhaust to make it "look" cleaner. Is this true? Or does the air actually react with the catalyst and unburnt fuel?

 

One of the jobs of the Cat is to burn off fuel that wasn't burnt in the combustion cycle. Since rotaries are tuned to run rich there is a significant amount of unburned fuel heading straight for the cat. Fuel needs Oxygen to burn. Our oxygen is provided with an air pump. Most fuel injected water-cooled piston cars are tuned lean enough to not need to worry about additional air, so no air pump.

 

Better get a new mechanic - the above post is correct.

 

A bit more: the catalyst on the cat converter is platinum plating on a metal substrate. When Pt gets hot, it can cause hydrocarbons to undergo reactions that otherwise might not occur (a catalyst), in this case, the breaking down of the carbon-hydrogen chains and circles (aromatics) into carbon and hydrogen radicals, which, when there is oxygen around, can cause the carbon to combine with the oxygen (CO and CO2) and allow the hydrogens to bind oxygen to form H2O. Note that the atmosphere is ~80% Nitrogen, so the oxygen CAN and WILL form bonds with the nitrogen, causing the dreaded oxides of nitrogen (NOX), a known green house gas. If I recall correctly, Pt results in fewer NOX products than other reactive metals in the presence of hydrocarbons, and most NOX control occurs in the combustion chamber via lower temp combustion. Air pump directs fresh air to the cat converter to reduce unburned hydrocarbons, an especial problem from our rich running beasts. It also directs air right at the exhaust port (port air injection) to provide more complete combustion at this point. It really doesn't hurt performance much, shuts off at higher rpms anyway, so why not go with it?