Diesel cars and vans
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 catalyst but does not need an oxygen sensor as it does not use 'closed loop' control. PM emissions can be controlled by a wall-flow Diesel Particulate Filter (DPF). These have been fitted to all new cars and vans in Europe since the implementation of the light-duty 'Euro 5' emissions legislation in 2011 which forces their use on all vehicles.
Diesel Particulate Filter showing structure
Control of diesel NOx emissions has been largely handled through engine measures until Euro 5. 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 emissions 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.
However, light-duty diesel vehicles that use specific NOx aftertreatment technology are now available in Euro 6 applications. Vehicles using NOx adsorber and/or urea-based SCR are now available in Europe. These technologies had been previously developed for the US Diesel market where NOx emissions limits have been more stringent.
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 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.
The ‘closed loop’ 3-way catalyst system
Direct-Injection petrol engines
The Euro 6 legislation introduces a particle number limit for Direct-Injection petrol vehicles (both stoichiometric and lean-burn engines). After a temporary relaxed limit, the final Euro 6c/d-TEMP requirement entered into force in September 2017 and aligns with the diesel limit for particle number. It requires the use of Gasoline Particle Filters, especially to ensure particulate emissions are controlled under real-world operation of the vehicles.
In parallel, lean-burn Direct-Injection petrol engines operate in stoichiometric (i.e. λ = 1) 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.