Catalytic converters

By Kevin Ash

Pictures: Kawasaki Press, Yamaha Press

(Click on images for full size versions)
With the new Euro 3 emissions regulations enforced at the end of 2007, many motorcycle engines needed either to be replaced or at least modified to clean up their exhausts, and that meant a lot more were fitted with catalytic converters. The cats themselves are nothing to be especially fearful of as modern ones have little or no effect on power output directly, although a lot of the new engines feel a little flat as the fuelling engineers have less margin to richen the mixture, which they do currently during acceleration for a crisp throttle response. That’s not directly a catalyst problem though.

Kawasaki's 06 ZX-10R cat converter is a long way from the engine and efficiency is reduced. With under-engine systems the cat is closer and heats up more quicklyGenerally the cost of a cat is adding two or three per cent to the price of a bike, although when it needs replacement, which might only be after five years, it could become a dominant factor in the value of a used bike as this won’t pass the MoT without a fully functioning one. Original equipment cats might easily cost £500, and that’s without the rest of the exhaust system.

Most cats are the ‘three way’ type, so called because they deal with three different emissions: NOx (nitrogen oxides), CO (carbon monoxide) and VOCs, (volatile organic compounds, also known as hydrocarbons, from unburnt fuel). Air is almost 80 per cent nitrogen and most of this passes through an engine unaffected, but some combines with oxygen in the heat to form NOx gases, which cause smog and acid rain. Kawasaki's Z1000 has two cat converters in series, located in the exhaust beneath the engineThe first part of a cat converter is the reduction catalyst, a ceramic honeycomb coated with platinum and rhodium – it’s designed this way to present the maximum surface area to the gases, as the chemical reactions take place on its surface (the definition of a catalyst is something which speeds up a reaction without being altered itself by it). This separates the nitrogen and oxygen atoms, which pair up with each other forming N2 and O2, natural constituents of fresh air.

Stage two is the oxidation catalyst, a similar honeycomb to stage one using platinum and palladium. This causes CO and the VOCs to oxidise (burn, in plain English…) forming CO2 and water. CO2 is a greenhouse gas, but it’s not poisonous or considered a harmful pollutant in any other sense – after all, it’s what we breathe out, so banning it would be difficult…

The twin lambda sensors on this R6 are positioned in the downpipes where the four merge into two, just in front of the cat converters. These are much nearer the exhaust ports than the Kawasaki'sThe second stage, like all burning, requires a supply of oxygen, so a lambda sensor is fitted between the cat and the engine which measures the oxygen content of the exhaust. The information is fed back to the engine management computer which constantly adjusts the fuel and air ratio to ensure some oxygen is left unburned to enter the exhaust and get involved in the oxidation process. Which means fuel injection is a prerequisite of a three way cat.

Cats only start to operate at very high temperatures, when they’re said to ‘light up’, so they work best – and soonest after a cold start – when they’re sited close to the engine, another reason we’re seeing fewer long, underseat exhaust systems, and more stubby new R6-style ones, or Buell/ER-6n under-engine types.