Diesel Exhaust Emissions - Diesel Smoke



Diesel Smoke:

How can we measure the amount of smoke coming from diesel exhaust?

Smoke is usually defined as solid and liquid particles suspended in diesel exhaust gases, which obstruct, reflect or refract light. Smoke from diesel exhaust can be placed into three groups depending on the appearance under direct illumination. The visual appearance of the exhaust gas will depend on the type of illumination and the background against which the exhaust gas is observed. In general, the "colour" of the exhaust gas will appear as white/bluish, or grey/black. Smoke density is usually measured with a Diesel Exhaust Smoke Meter.


What could cause excessive grey/black smoke in diesel engine exhaust?

Grey/black smoke consists mainly of solid particles of carbon, i.e. soot.


What could cause white smoke in the exhaust of a diesel engine?

White smoke is usually caused by the presence of vaporized water and/or unburned diesel fuel in the exhaust gas; for example, a misfiring cylinder or badly damaged engine.


What could cause blue smoke in diesel exhaust?

Blue smoke is usually caused by high concentrations of unburned or partially oxidized fuel or lubricating oil in the exhaust gas. This situation is typical of a diesel engine operating at low temperatures or suffering high oil consumption.


What could cause diesel exhaust to emit an odour?

The diesel exhaust gas odour is a result of a combination of aromatic hydrocarbons and other substances such as aldehydes. Since "odour" cannot be described objectively, conditions affecting the formation of odour-causing compounds are not easily well defined. However, for a given diesel engine, conditions which may affect the concentrations of unburned HC will tend to affect the odour of the diesel exhaust gas.


Of coarse, the diesel exhaust emissions does not contain only soot and particulate matter. The rest of the gases normally emitted by any internal combustion engine are still present. Separating and filtering out those gases from the naturally oily diesel soot however, proves an difficult endeavor.
Diesel soot clogs very quickly the filters of a "normal" exhaust gas analyser (as used for petrol emissions). Those very fine filters are also expensive maintenaince items. Thus an entirely different approach to measuring diesel emissions is used in the form of a Diesel Exhaust Smoke Meter.


Carbon dioxide (C02): (for more read here)

Although this gas is non-poisonous, it still is considered a problem, especially if it is produced in large enough quantities to displace oxygen in the working environment.


Carbon monoxide (CO): (for more read here)

CO is the result of the incomplete combustion of the fuel, caused by localized insufficiencies of oxygen, (rich fuel/air ratio.) Quenching of the reaction by cold combustion chamber walls also increases the CO levels. (Example: cold engine operating temperatures) CO gas is a colorless, odorless, and tasteless gas. Inhalation of as little as 0.3% by volume can cause death within 30 minutes. For this reason, it is important never to allow the engine to run in enclosed spaces such as a closed garage without good ventilation. Increased CO concentrations may be the result of poor mixture formation caused by a defective injection system, injectors with defective spray characteristics, or engine over-fuelling.


Oxides of nitrogen (NOx): (for more read here)

The formation of NOx is dependent on the temperatures during the combustion process, the concentrations of the components nitrogen (N2) and oxygen (02) and the time available for them to react with each other. NO and NO2 are generally lumped together and referred to as oxides of nitrogen (NOx). A rise in the combustion temperature increases the NO concentrations in the exhaust gas. In a diesel engine, the combustion process forms only NO, a small portion of which oxidizes to N02 at lower temperatures and in the presence of 02. The sum of NO and N02 is called NOx. These gases belong to two different classes. Nitrogen monoxide (NO) is a colorless, odorless, and tasteless gas that is rapidly converted into nitrogen dioxide (NO2) in the presence of oxygen - O2 . Advanced injection timing can cause an increase of NO in the exhaust gas. Measures that decrease the NO concentrations, such as low compression ratio or retarded injection timing, also tend to decrease the efficiency of the combustion process. This can result in increased fuel consumption and higher CO and HC concentrations in the exhaust.


Hydrocarbons (HC): (for more read here)

HC in exhaust gases is usually from very small quantities of unburned diesel fuel and engine lubricating oil. Since the measurement of concentrations of different hydrocarbons involves the use of sophisticated instrumentation, only total HC is usually measured and reported. In the presence of nitrogen oxide and sunlight, hydrocarbons form substances, which irritate the mucous membranes. Some hydrocarbons are cancer-causing. Incomplete combustion in a diesel engine produces unburned hydrocarbons. Increased HC levels in the exhaust gas are found when an diesel engine suffers from high

oil consumption, a defective injection system, rich fuel/air ratio, or quenching of the combustion process in the proximity of the cold combustion chamber walls.


Particulate matter (PM): (for more read here)

These include all substances (with the exception of water), which under normal conditions are present as small solid or liquid particles in exhaust gases. PM is usually defined as "any material, other than water, in the exhaust of a diesel engine which can be filtered after dilution with ambient air". These particulates normally consist of a mixture of carbon (soot), hydrocarbons and sulphuric acid. Therefore we can assume that conditions, which affect the formation of soot, hydrocarbons and oxides of sulfur, will affect the particulate emission.


Oxides of Sulphur (SOx): (for more read here)

The SOx formation is caused by the oxidation of the sulphur contained in the fuel with the 02 available in the combustion air. The SOx concentrations depend on the sulphur content of the diesel fuel and the fuel consumption of the engine. SOx reductions in the diesel exhaust gas can only be achieved through the use of low sulphur fuels.



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