Euro 4 engine and aftertreatment technology

NOx removal technologies

Exhaust gas recirculation (EGR) 

EGR is a technique that directs the exhaust gas back into the air intake. Because these gases already have been used by the engine, they have a lower oxygen level. By reducing the oxygen level in the air intake there is less oxygen available to allow nitrogen oxides to form. The exhaust gas in the air intake also absorbs more energy during the combustion process, which lowers the peak in cylinder gas temperature and also helps to lower the level of NOx (high temperatures are needed for NOx formation).

Lean NOx traps (LNT), NOx absorbers and NOx storage reduction catalyst (NSRC) 

An LNT catalytically assists the oxidation of nitrogen oxide (NO) to nitrogen dioxide (NO2) and stores the NO2 in an adjacent trapping site as a nitrate (NO3). The stored NO2 is removed in a two-stage reduction process by temporarily inducing a rich exhaust condition using extra fuel. Consequently, there is a fuel consumption penalty. Lean NOx traps use precious metal catalysts to carry out the conversion of NO to NO2.

NOx removal technologies

Sulphur from the fuel or lubricant can react catalytically with oxygen to form sulphur dioxide (SO2), which can be trapped and form stable sulphates. These sulphates are more stable than the nitrates and render the NO2 absorbing capabilities of the system ineffective. The excessive temperatures needed to de-sulphate the system tend to degrade the lean NOx trap, reducing its performance over time. SO2 also can be catalytically converted to sulphate in the system, which contributes to particulate emissions.

Lean NOx catalysts (LNC) and DeNOx catalysts 

LNCs are based on the catalytic reduction of NOx to nitrogen, using hydrocarbons as the reductant. A passive LNC uses hydrocarbon from the exhaust stream. Active LNC enriches the exhaust stream with additional fuel, which obviously has a fuel consumption penalty. The LNC uses platinum or zeolites to reduce the temperature at which the NOx is reduced to nitrogen in the presence of hydrocarbons.

Selective catalytic reduction (SCR)

SCR is an effective way to remove NOx emissions using a solution of urea (source of ammonia) to reduce the nitrogen oxides to nitrogen. The process starts with a DOC, which converts much of the NO to NO2 and removes hydrocarbons. Converting much of the NO to NO2 lowers the temperature required for the next stage of the reaction, where the ammonia in the urea is used to reduce the nitrogen oxides to nitrogen and water. Titanium, vanadium or zeolite catalysts often are used for SCR systems.

Particulate removal technologies

Diesel oxidation catalyst (DOC)

The soluble oil fraction (SOF) is emitted as a particulate when it condenses. DOCs promote the oxidation of hydrocarbons, SOF and carbon monoxide (CO) to harmless CO2 and water. The DOCs generally use precious metal such as platinum or palladium to catalyse the reactions.

Diesel particulate filters (DPF) 

DPFs actually capture the particulates and prevent their discharge from the exhaust pipe. Collected particulates and soot are removed from the filter by burning them off at high temperature. A catalysed DPF, as its name suggests, uses a catalyst (often precious metals such as platinum, palladium and rhodium) to help reduce the temperature required for this process. Fuel additive catalysed filters lower the temperature for soot burning. However, the ash from the additive remains in the filter after the particulates have burnt off, thereby contributing to the total ash in the filter.

Particulates are more easily oxidised/burned in the presence of NO2 than oxygen, by lowering the temperature at which oxidation takes place. This discovery enabled the continuously regenerating trap (CRT) to be developed. The CRT contains a DOC upstream of a diesel particulate filter, which converts more of the nitrogen oxides to NO2. The NO2 is then used to lower the temperature to oxidise/burn off the particulates from the trap.