How Emission Control Systems in Internal Combustion Engines Work

Emissions of Pollutant Gases

The various regulations governing emissions from internal combustion engines set limits on pollutant gases, which vary depending on the engine’s operating conditions and the type of fuel used (diesel, natural gas, hydrogen, etc.).
These emission control systems require high temperatures to function properly and must therefore be located as close as possible to the engine outlet.

For diesel engines, the limits apply to the following pollutants:

• Carbon monoxide (CO)
• Unburned hydrocarbons (HC)
• Nitrogen oxides (NOx: NO, NO₂, N₂O)
• Fine Particulate Matter by Mass (PM) and by Number (PN)

Solutions for Reducing Pollutant Emissions

1. Oxidation catalyst: Located at the front of the exhaust system, as close as possible to the engine. Treats HC, CO, and VOCs through catalytic oxidation.
2. Particulate Filter (FAP/DPF): Significantly reduces emissions of fine particulate matter (PM and PN) by filtering them through porous walls.
3. SCR: The most effective technology for reducing NOx. AdBlue is injected upstream of the SCR catalyst.

In-line exhaust emission control system for a diesel engine

1. The oxidation catalyst

Pollutants such as HC, CO, and other VOCs are oxidized at very high temperatures to form H₂O and CO₂: this is known asthermal oxidation. To enable these reactions to occur at lower temperatures, a catalyst is used to carry out catalytic oxidation.

Its primary function is to treat unburned hydrocarbons (HC) and carbon monoxide (CO), as well as volatile organic compounds (VOCs), aldehydes, and other harmful air pollutants. It is a passive system that does not require the injection of additives or regular regeneration to operate.

2. Particle Treatment in Diesel Engines

Soot particles emitted by diesel engines can be reduced to varying degrees of effectiveness depending on the aftertreatment system chosen.

The Open Filter (POC — Particle Oxidation Catalyst)

The POC is an “open filter” system, unlike the FAP, which is a “closed filter.” It captures and stores particles long enough for catalytic oxidation to convert them.

A saturated POC will still allow exhaust gases to flow through but will lose some of its conversion capacity. Unlike a diesel particulate filter (DPF), there is no risk of the emission control system becoming clogged. Its substrate is typically metallic, consisting of flat and corrugated sheets wound around themselves.

Particle oxidation is achieved through NO₂ generated by the upstream oxidation catalyst (DOC), which reacts with the soot particles to convert them into gaseous products.

Metal substrate constituting a POC

3. SCR — Selective Catalytic Reduction

Nitrogen Oxide (NOx) Abatement

Two technologies are primarily used to reduce NOx emissions:EGR (Exhaust Gas Recirculation) and SCR (Selective Catalytic Reduction).

EGR recirculates a portion of the exhaust gases into the intake, reducing the oxygen supply and limiting NOx production. This can be achieved by cross-connecting valves (internal EGR) or through an external circuit connecting the exhaust to the intake via an EGR valve (external EGR). Although effective, these technologies are not efficient enough to meet today’s NOx reduction targets.

SCR, used in conjunction with EGR, significantly reduces NOx levels and is currently the most effective technology. A liquid urea solution (AdBlue) is injected into the exhaust pipe upstream of the SCR — where it breaks down into ammonia under the effect of heat and undergoes a chemical reaction to convert NOx into nitrogen (N₂), water vapor (H₂O), and carbon dioxide (CO₂).

SCR System

Selective Catalytic Reduction

The SCR catalytic converter is a monolithic type, with a honeycomb structure. The catalytic reaction occurs on the active conversion zones, which are deposited as a thin solid layer on the substrate. AdBlue consists of 67.5% water and 32.5% urea.

The 4 NOx reduction reactions in a urea-injected SCR system

CO(NH₂)₂ → NH₃ + HNCO·Thermal decomposition of urea
HNCO + H₂O → NH₃ + CO₂·Hydrolysis of HNCO
4NH₃ + 4NO + O₂ → 6H₂O·Standard SCR reaction
2NH₃ + NO + NO₂ → 2N₂ + 3H₂O·Rapid SCR reaction
Point of concern: A common issue is the oligomerization and polymerization of HNCO into a solid urea derivative that decomposes slowly even at high temperatures. The consequences are heterogeneous ammonia dispersion and potential catalyst clogging. The injection system must ensure uniform dispersion of NH₃ across the catalyst surface.

Typical Design Data for an SCR

• Maximum gas flow rate (Nm³/h)
• Gas temperature at the SCR inlet (°C)
• NOx concentration at the SCR inlet (mg/Nm³)
• Required NOx reduction (%)
• Maximum NH₃ concentration at the outlet (ppm)
• SCR Pressure Drop (mbar)

Selective Catalytic Reduction (SCR) Reactions