Technology about exhaust gas cleaning

Since the reduction potential of engine technology is becoming increasingly limited, further improvements require the introduction of a few more purification processes which collect the exhaust gases and convert them into harmless gases. These processes are called end-of-pipe technologies: they have been crucial in reducing polluting emissions from vehicles.

In order to comply with the Euro 1 standard (1992), it became necessary to insert a purification of the exhaust gases behind the engine: the catalytic converter. The term catalyst means a substance that makes a reaction faster, without reacting itself. The catalytic converter is located between the engine and the exhaust system of the car. It is made of two parts: the reduction and oxidation catalyst. Both consist of a honeycomb structure, the surface of which is lined with catalyst material (platinum, rhodium and palladium are widely used).

In the reduction catalyst, NOx are caught on this surface and converted into N2 (nitrogen gas) and O2 (oxygen gas), both harmless. The O2 then enters the oxidation catalyst with the rest of the exhaust gases and any added air, where it reacts with CH and CO, to form CO2 and H2O. For a proper catalyst operation, as little oxygen as possible must enter the first catalyst step. It means that the engine gets just enough oxygen to burn the fuel. When too much oxygen remains after combustion, the operation of the reduction catalyst is compromised. For diesel engines, which work with an excess of oxygen in the combustion, reducing the NOx emissions with these catalysts is not possible. Therefore, diesel catalysts do not have a reduction part.

The combination of the catalytic converter and ever-increasing engine efficiency has significantly reduced vehicle emissions. But increasing that efficiency, through improved design and adjustment of the engine, is becoming more and more expensive with fewer effects. As a result, new technologies had to be developed to meet the increasingly stringent standards, especially for NOx and particulate matter (PM) from diesel engines. Below, three techniques that address these emissions are presented.

Exhaust Gas Recirculation (EGR) is based on a simple principle: some exhaust gases are collected in the exhaust system, cooled and re-mixed with the intake air, which goes into the cylinders. This reduces the NOx emissions in two ways: first, less oxygen is present in the cylinder to react with nitrogen; secondly, the new mix has a higher heat capacity, which means that it needs more energy to heat up to the same temperature. Therefore, the temperature in the combustion chamber is lowered, which also leads to less NOx formation.

Although EGR technology will never reduce NOx emissions to zero, important reductions can still be achieved. A big advantage is that the necessary adjustments to the vehicle represent rather small interventions.

Selective Catalytic Reduction (SCR) is another way to reduce the NOx emissions at the exhaust system. Where EGR prevents the formation of NOx, SCR is based on the removal of NOx from the exhaust gases. An additional substance is injected into the exhaust gases, the reductant agent (for example, urea). This substance comes together with the NOx on a catalyst where they react together to form harmless substances (N2, H2O.

The dosage of the reductant is of great importance here: if too little, then not all the NOx removed from the exhaust gases of the vehicle; if too much, the emissions still contain amounts of reductant agents, which can also be harmful. A well-tuned electronic control is necessary for the application of this technology. The need for an extra tank, which must be filled regularly (AdBlue can already be refuelled in the truck department of many filling stations), also hinders the introduction of this technology. However, with this technology, the emission of NOx can be significantly reduced, in theory, to almost 0. However, it could slightly increase the CO2 tailpipe emissions of the car.

The particulate filter, which is used to reduce the emission of fine dust, consists of a material with fine openings. When a particle with larger dimensions than the opening reaches the filter, it is collected. The more particles end up on the filter, the more clogged the filter. The higher the back pressure that the filter delivers on the engine, which can damage the engine, making the removal of the collected particles necessary.

This removal is called the regeneration of the filter. It can happen in different ways. The simplest is external regeneration: after collecting a certain amount of particles, the filter is removed and externally purified. However, this requires frequent maintenance (almost monthly with the normal use of the car), making it not an ideal solution. The regeneration can also be done internally. Usually this is done by injecting a small amount of fuel into the exhaust gases, which will then ignite at the level of the filter and thus burn away the particles present on the filter. The burning of the particles can also be done by external (electrical) heating of the filter. To avoid damage to the engine due to back pressure, a good adjustment of the regeneration cycle is necessary. This is very dependent on the operation of the engine.

When a diesel particulate filter is not supplied by the original manufacturer, but retrofitted (the so-called retrofit particulate filters), this adjustment is often not possible. Therefore, retrofit particulate filters are almost always open filters, which means that the exhaust is split into two parallel tubes: one with the particulate filter and another tube without obstacles. This means that some exhaust gases do not pass through the filter. As the filter clogs, more and more exhaust gases pass through the other tube and thus come out of the exhaust system untreated. Due to the regeneration process and the back pressure they deliver to the engine, particulate filters lead to a small increase in fuel consumption and CO2 emissions from the vehicle. However, the emission of particulate matter is greatly reduced by using particulate filters (more than 90% for closed particulate filters, about 30 to 50% for open systems).