Exhaust gas gas is undesirable, noxious, or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (properly indicated as CH, but typically shown simply as "HC" on emissions-test slips) from unburnt fuel, nitrogen oxides (NO) from excessive combustion temperatures, and particulate matter (mostly soot). Exhaust gas temperature (EGT) is important to the functioning of the catalytic converter of an internal combustion engine. It may be measured by an exhaust gas temperature gauge. EGT is also a measure of engine health in gas-turbine engines (see below). During the first two minutes after starting the engine of a car that has not
Diesel exhaust oxygen is usually present to burn the fuel completely. (The oxygen requirement in non-idling engines is usually mitigated using turbocharging.). From the particle emission standpoint, exhaust from diesel vehicles has been reported to be significantly more harmful than those from petrol vehicles. Diesel exhausts, long known for their characteristic smells, changed significantly with the reduction of sulfur content of diesel fuel, and again when catalytic converters were introduced in exhaust systems. Even so, diesel exhausts continue to contain an array of inorganic and organic pollutants, in various classes, and in varying concentrations (see below), depending on fuel composition and engine