Passenger cars & light-duty commercial vehicles
European emissions regulations for light-duty vehicles up to 3.5 tonnes and carrying up to 9 people including the driver - in other words cars, 'people carriers', 4 x 4s and light vans - first reached a level that forced the use of catalysts for petrol engines in 1993. At that time it was still possible to use oxidation catalysts for some vehicles, but the increasingly stringent stages of emissions regulation since then (Euro 2 in 1997, Euro 3 in 2001, Euro 4 in 2006, Euro 5 in 2009 and Euro 6 in 2014) have ensured that three-way catalysts are universally used for petrol engines in Europe.
Typical modern systems for vehicles with petrol or gas engines will include one or more 'close-coupled' catalysts near to the exhaust manifold and often an additional catalyst further down the exhaust, under the floor of the car. 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 slighty fuel-lean and very slightly fuel-rich conditions. In this way both the oxidation functions (conversion of CO and HC to CO2 and water) and the chemical reduction function (NOx to nitrogen) can operate simultaneously.
The 'closed loop' 3-way catalyst system
Diesel engines for European light-duty vehicles normally use a diesel oxidation catalyst to treat the emissions of CO and HC. This is similar to a three-way catalysts but does not need an oxygen sensor as it does not use 'closed loop' control. PM emissions can be treated by the use of a wall-flow Diesel Particulate Filter (DPF). These are fitted to many new cars in Europe and the next generation of light-duty 'Euro 5' emissions legislation is expected to set emission limits which will force their use on all vehicles. Both diesel oxidation catalysts and diesel particulate filters are also available as retrofit solutions.
Diesel Particulate filter showing structure
Control of diesel NOx emissions is largely handled through engine measures at present. The main technique used is to recirculate a small percentage of the exhaust gas into the inlet air so as to reduce the oxygen content and combustion temperature. This in turn lowers the amount of atmospheric nitrogen that is converted to NOx. NOx emission limits for light-duty diesel engines are currently less stringent than those for petrol-engined vehicles as previous and current generations of vehicles would not have been able to meet the tighter petrol-engine limits.
Light-duty diesel vehicles that use specific NOx aftertreatment technology are now starting to be available. A system using Lean De-NOx is available in Europe and vehicles to meet the more stringent emissions limits for the US market are being developed with urea-based SCR and NOx adsorbers.
Lean-burn Direct-Injection petrol engines
Lean-burn Direct-Injection petrol engines operate in stoichiometric conditions (where a three-way catalyst can be used) in some circumstances but in other conditions they operate in lean conditions where the exhaust oxygen content is more like that of diesel and so the three way catalyst cannot reduce the oxides of nitrogen. To control NOx emissions these engines also use a Lean De-NOx catalyst. The adsorber is regenerated from time to time by switching to rich operation for a few seconds so that reductants are produced.
Operating area for lean-burn engines