Motorcycles and mopeds
The emissions regulations for motorcycles and mopeds also cover on-road tricycles and quadricycles. They therefore include vehicles such as the small 3-wheel light trucks used in some parts of Europe. Euro 4 requirements were introduced from 2016 and, subject to an environmental study by the European Commission, a Euro 5 stage is planned in 2020.
2-stroke petrol engines and diesel engines
The emissions limits for this class of vehicle fitted with 2-stroke petrol engines or diesel engines are such that they may not require emission exhaust gas aftertreatment. Nevertheless, it is possible to use oxidation catalysts (or diesel oxidation catalysts) with these engines.
4-stroke petrol engines
Historically, motorcycles for the European market have used either oxidation catalysts or three-way catalysts to meet the emissions regulations. Since the introduction of Euro 3 in 2006, all new 4-stroke motorcycles have been equipped with three-way catalysts.
The principle of operation of these catalysts is the same as for car engines. For the three-way catalyst the exhaust system will include an oxygen sensor or air:fuel ratio sensor that monitors the oxygen content of the exhaust and continuously adjusts the fuelling to match the conditions. This also ensures that the system alternates rapidly between very slightly fuel-lean and very slightly fuel-rich conditions. In this way both the oxidation functions (conversion of CO and HC into CO2 and water) and the chemical reduction function (NOx into nitrogen) can operate simultaneously. For motorcycles, of course, space is a limiting factor. The catalyst generally has to be quite small and built into a short exhaust pipe. Metal substrate catalysts are often used in this type of application.