Hydrocarbons (CH): are formed only by incomplete combustion in cold places and indented combustion chambers. The proportion of hydrocarbons depends on the engine design (unchangeable value), but a mixture that is too rich or too lean increases the proportion of CH in the exhaust gases. Together with nitrogen oxides (NOX), hydrocarbons are responsible for the formation of smog (a difficult to decompose concentration of exhaust gases in the atmosphere).
Nitrogen oxides (NOX): their proportion increases at high combustion temperatures. For example, in engines (engines running on lean mixtures) that are designed to reduce CO and CH emissions (reduced fuel consumption). At high concentrations, NOX can irritate the respiratory system. When combined with water, they form nitric acid (acid rain).
Sulfur dioxide (SO2): is formed mainly during combustion of diesel fuel due to its sulfur content. Under the influence of light, sulfuric or sulfurous acid is formed. Both compounds contribute to the formation of acid rain. The intensification of road traffic currently affects the formation of approximately 3% of sulfurous acids.
Typical poisons in diesel exhaust gases: a diesel engine emits a small amount of CO and CH, which is due to the way it works. Despite higher compression, a diesel engine also emits less nitrogen oxides than a gasoline engine. But diesel engines are guilty of forming other problematic combustion products. For example, soot is a typical component of spent diesel fuel. Soot consists of unburned carbon particles and ash. Sulfur dioxide is also formed during combustion of diesel fuel in a higher concentration than in a gasoline engine (see sulfur dioxide).
Special exhaust gas check (ASU): mandatory in Germany for all passenger cars since December 1993. Cars older than 30 years may be exempt from ASU. On new cars, the ASU sign is valid for 3 years, after which a mandatory check is carried out every 2 years. ASU is carried out by repair shops with the appropriate equipment, as well as by DEKRA and ZOV. When measuring, the exhaust system must be leak-proof and the intake system perfectly functional. If your Focus undergoes regular maintenance, the ASU is carried out there automatically.