Marine diesel engines of all types are coming under increasing scrutiny as major polluters of the world’s environment and, as a result, considerable effort is being devoted to the reduction of their emissions, but what is the problem?
In general, diesel fuel comprises around 86.5% by weight of carbon and some 13.5% hydrogen. When burnt in an engine, the typical exhaust emissions comprise as much as 99.7% of relatively harmless substances, though this does include carbon dioxide which can result in asphyxiation and is also a prime cause of the so-called greenhouse effect. The remaining 0.3% of the exhaust is particularly harmful.
Emission products
It has been estimated that a typical diesel exhaust will show the constituents detailed in the box below.
This comprises six main pollutants as follows:
Carbon dioxide (CO2) which is formed by the combustion of fossil fuels. The diesel engine has the lowest specific CO2 emission levels of any internal combustion engine when using the same fuel quality due to its superior efficiency.
Carbon monoxide (CO) a colourless and odourless compound that is toxic, results from incomplete combustion but can be minimised by optimal mixture formation and turbocharging. However a concentration of as little as 0.5% can prove lethal.
Sulphur oxides (SOx) are formed by the combustion of the sulphur in the fuel and when dissolved in water form sulphuric acid. Again the diesel produces the lowest specific levels because of its high efficiency and recent legislation is leading to the requirement to burn fuels with ever lower sulphur contents.
Nitrogen oxides (NOx) the high temperature existing in the combustion chamber causes the nitrogen in the combustion air, and in some fuel grades, to react with oxygen in the combustion air to form nitrogen oxides which can cause respiratory illness and lung damage. The amounts produced are determined by the length of time the reaction takes place and its intensity. Unfortunately, a diesel engine's efficiency increases as the pressure and temperature of combustion are raised and to reduce NOx emissions, a lower efficiency and hence higher fuel consumption may be necessary. NOx emissions can be reduced by cooling the charge air and delaying combustion (injecting the fuel later in the cycle).
Hydrocarbons (HC) contained in the exhaust gas are composed of a multitude (as many as 100 or more) of organic compounds as a result of incomplete combustion, which may be caused by short combustion times and low combustion temperatures, as used to reduce NOx emissions (see box)!
Particulates (PM) comprise soot (elemental carbon), ash and the residue of unburnt fuel and lubricating oil. Poor air supply or insufficiently high combustion temperatures result in the free carbon not being totally converted to CO or CO2. The carbon particles combine to form grains of minute size measured in microns, typically around 0.5 to 1 micron. PM10s (ie particles of 10 microns or less) are a major danger to human health, as long-term exposure has serious effects on respiration. They cause lung tissue damage and are thought cause cancer.
