Emissions of a gasoline engine:
The exhaust gases of a gasoline engine contain the following substances:
- CO2: Carbon dioxide (Harmful to the environment, humans, and animals at high concentrations)
- CO: Carbon monoxide (incompletely burned gas, also harmful to health)
- CH: Hydrocarbons (unburned gasoline parts)
- O2: Oxygen parts (that have not participated in the combustion)
- NOx: Nitrogen compound (formed only under very high combustion temperatures).
The catalytic converter converts the three harmful components CO, HC, and NOx into three harmless components: CO2, H2O, and N2. The name three-way catalytic converter comes from this.
| Harmful substance: | Adding: | Results in: |
| CO + | O2 = | CO2 |
| HC + | O2 = | CO2 + H2O |
| NOx + | CO = | N2 + CO2 |
During a cold start, enrichment occurs; there is an excess of fuel. Extra CO and HC are emitted during a cold start. Since the catalytic converter has not yet reached its operating temperature during a cold start, these substances cannot yet be converted. With the help of the secondary air pump, extra air is added to the exhaust gases. The additional air increases the temperature of the exhaust gases and warms up the catalytic converter more quickly. The harmful substances can be converted more quickly after a cold start.
Despite manufacturers making utmost efforts to reduce harmful components in exhaust gases, it still occurs even with optimal combustion. The European Union requires manufacturers to make new engines increasingly “cleaner.” The emission standard outlines the limits of the emissions. The table below shows the EU standards for a gasoline engine:
| Stage | Effective | NOx | HC | NMHC | HC+NOx | CO | PM |
| Euro 1 | 1993 | – | – | – | 970 | 2720 | 140 |
| Euro 2 | 1996 | – | – | – | 500 | 2200 | – |
| Euro 3 | 2000 | 150 | 200 | – | – | 2300 | – |
| Euro 4 | 2005 | 80 | 100 | – | – | 1000 | – |
| Euro 5 | 2009 | 60 | 100 | 68 | – | 1000 | 5 |
| Euro 5b | 2011 | 60 | 100 | 68 | – | 1000 | 4.5 |
| Euro 6 | 2014 | 60 | 100 | 68 | – | 1000 | 4.5 |
Emissions of a diesel engine:
- 67% Nitrogen (N2)
- 12% Carbon dioxide (CO2)
- 11% Water (H2O)
- 10% Oxygen
- 0.3% other substances, including soot particles (PM), Hydrocarbons (HC), Nitrogen oxides (NOx), Carbon monoxide (CO)
Nowadays, three-way catalytic converters are increasingly used in diesel engines to convert the minimal amount of CO into CO2 and H2O.