Exhaust gas aftertreatment: Design, function and diagnostics

The catalytic converter consists of a honeycomb-shaped ceramic or metal body with several thousand channels. An intermediate layer, (wash coat) made of aluminium oxide is applied to the channels, which enlarges the surface approximately 7000 times. In addition, a catalytic layer with the precious metals rhodium, palladium and platinum is vapour-deposited on the intermediate layer. In the stainless steel housing, the catalytic converter is fixed in place by an insulation mat, which compensates for thermal expansions at the same time.

The harmful exhaust gases from the engine are fed to the catalytic converter via the exhaust manifold and flow through the channels of the ceramic or metallic carrier. A chemical reaction is triggered when the exhaust gases make contact with the catalytic layer.

Carbon monoxide and hydrocarbons are oxidised to carbon dioxide on the precious metals platinum and palladium, and nitrogen oxides are reduced to nitrogen (N2) on rhodium. The term three-way catalytic converter means that three chemical conversions take place simultaneously side by side as soon as the operating temperature is reached. Thus, carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) are converted into nitrogen (N2), carbon dioxide (CO2) and water (H2O).

The function of the catalytic converter is monitored by a second lambda sensor, also called an oxygen sensor, behind the catalytic converter. This oxygen sensor measures the oxygen content in the exhaust gas stream and transmits the measured value to the control unit. The control unit compares the values of the upstream (pre) and downstream (post) catalytic converter sensors to evaluate the converter’s functioning. If both values are almost identically high, it can be concluded that the catalytic converter has a low oxygen storage capacity, meaning that the catalytic converter is defective.

The lambda sensor determines the residual oxygen content in the exhaust gas and provides the engine control unit with an electrical signal to regulate the mixture composition. Optimum combustion is required if an ideal conversion rate is to be ensured for catalytic converters. On petrol engines, this is achieved with an air-fuel ratio of 14.7 kg air to 1 kg fuel (stoichiometric mix). This optimum ratio is designated by the Greek letter λ (lambda). Lambda is used to express the air ratio between the theoretical air requirement and the actual air volume supplied:

λ = supplied air volume: theoretical air volume = 14.7 kg : 14.7 kg = 1

In modern vehicle models, exhaust gas temperature sensors are installed at various points in the exhaust gas system in both diesel and petrol vehicles. The exhaust gas temperature sensor detects the temperature, for example upstream of the catalytic converter or diesel particulate filter, and sends this as a voltage signal to the engine control unit. The engine control unit needs this information in order to control the air-fuel ratio or the particulate filter regeneration and thus to effectively reduce emissions. In addition, high temperature sensors help to protect components in the area of ​​the hot exhaust gas flow from critical overheating.

The exhaust gas pressure sensor ─ also referred to as the differential pressure sensor ─ is required on diesel engines in order to monitor the particulate filter system. Increasing accumulations of soot and ash alter the differential pressure in the particulate filter. The differential pressure sensor measures the pressure difference between the exhaust gas on the inlet and outlet side of the particulate filter. This measured value is required by the electronic engine control system as additional information to calculate the regeneration, or purging time, of the particulate filter. This sensor is therefore another element in the systems for regulating pollutant emissions for diesel engines based on European emission regulations.

The NOx sensor comprises a probe and a control unit. These are firmly connected together using a cable harness to form a unit. This sensor unit is installed in the exhaust gas system and is used to recognise nitrogen oxides in the flow of exhaust gas. The NOx sensor is an important component in the aftertreatment system to reduce NOx. It is used in diesel vehicles with carbamide-based SCR systems (selective catalytic reduction) and guarantees that strict Euro 5 emissions values are observed. Consequently the NOx sensor enables the engine system to provide an optimal dosage of AdBlue, effectively reducing nitrogen oxides, which are harmful to the environment. If the SCR system has an upstream and a downstream NOx sensor, the downstream one monitors the effect of the SCR catalytic converter.