Tailpipe Emission Standards

“Tailpipe” emission standards specify the maximum amount of pollutants allowed in exhaust gases discharged from a diesel engine. The tailpipe emission standards were initiated in California in 1959 to control CO and HC emissions from gasoline engines. Today, emissions from internal combustion engines are regulated in tens of countries throughout the world. The regulated diesel emissions include:

  • Diesel particulate matter (PM), measured by gravimetric methods. Sometimes diesel smoke opacity measured by optical methods is also regulated.
  • Nitrogen oxides (NOx), composed of nitric oxide (NO) and nitrogen dioxide (NO2). Other oxides of nitrogen which may be present in exhaust gases, such as N2O, are not regulated.
  • Hydrocarbons (HC), regulated either as total hydrocarbon emissions (THC) or as non-methane hydrocarbons (NMHC). One combined limit for HC + NOx is sometimes used instead of two separate limits.
  • Carbon monoxide (CO).

Emissions are measured over an engine or vehicle test cycle which is an important part of every emission standard. Regulatory test procedures are necessary to verify and ensure compliance with the various standards. These test cycles are supposed to create repeatable emission measurement conditions and, at the same time, simulate a real driving condition of a given application. Analytical methods that are used to measure particular emissions are also regulated by the standard.

Emission cycles are a sequence of speed and load conditions performed on an engine or chassis dynamometer. Emissions measured on vehicle (chassis) dynamometers are usually expressed in grams of pollutant per unit of traveled distance, e.g., g/km or g/mi. Emissions measured according to an engine dynamometer test cycle are expressed in grams of pollutant per unit of mechanical energy delivered by the engine, typically g/kWh or g/bhp-hr. Depending on the character of speed and load changes, cycles can be divided into steady state cycles and transient cycles. Steady state cycles are a sequence of constant engine speed and load modes. Emissions are analyzed for each test mode. Then the overall emission result is calculated as a (weighted) average from all test modes. In a transient cycle the vehicle (engine) follows a prescribed driving pattern which includes accelerations, decelerations, changes of speed and load, etc. The final test results can be obtained either by analysis of exhaust gas samples collected to plastic bags over the duration of the cycle or by electronic integration of a fast response, continuous emission measurement.

Regulatory authorities in different countries have not been unanimous in adopting emission test procedures and many types of cycles are in use. Since exhaust emissions depend on the engine speed and load conditions, specific engine emissions which were measured on different test cycles may not be comparable even if they are expressed or recalculated into the same units of measure. This should be kept in mind whenever comparing emission standards from different countries.

Tailpipe emission standards are usually implemented by government ministries responsible for the protection of environment, such as the EPA (Environmental Protection Agency) in the USA. The duty to comply with these standards is on the equipment (engine) manufacturer. Typically all equipment have to be emission certified before it is released to the market.

Occupational Health and Safety Standards

Applications of diesel engines in confined spaces are regulated through occupational health and safety ambient air quality standards rather than (or in addition to) the tailpipe regulations. The ambient air quality standards specify maximum concentrations of air contaminants called Permissible Exposure Limits (PEL) which are allowed in the workplace.

Gases found in diesel emissions including carbon monoxide, nitric oxide, nitrogen dioxide, sulfur dioxide and many other compounds have their PELs set by occupational health and safety authorities. Diesel particulate matter has also been listed by a growing number of occupational health and safety standards as a toxic air contaminant.

These regulations are set and enforced by occupational health and safety authorities such as OSHA (Occupational Safety and Health Administration) in the USA. The duty to comply is on the end-user (warehouse operator, mine operator, etc.) who has to make sure that the emission control measures which have been employed are adequate to the type and number of polluting equipment. Engine or equipment manufacturers do not have any direct obligations in regard to the occupational health and safety air quality standards.