Diesel Engine Emissions
Exhaust gases emitted from large diesel engines comprise nitrogen, oxygen, carbon
dioxide and water vapor with smaller quantities of carbon monoxide, and oxides of sulfur
and nitrogen, together with even lesser quantities of partially reacted and unburned
hydrocarbons and particulate material, as shown in Figs. 2.4 and 2.5. The composition of
major pollutants in the exhaust gas is a result of the engine process.
The composition of major pollutants in the exhaust gas is a result of the engine
process, its fuels and the means employed to control the emissions. In the following, each
exhaust gas component will be described.
Oxygen (O2): Low speed, two-stroke, crosshead diesels operate with an air excess
ratio of over 3. More than half of the air is available for the combustion process, while the
remaining part is scavenged through the cylinder. Hence, the exhaust gas contains some
13-16% oxygen, and this has to be considered when calculating the concentration of
various compounds in the gas. Some exhaust gas emission regulations refer to 15% oxygen;
so, if the actual content is different, the result will have to be corrected accordingly.
Nitrogen (N2): Given the above-mentioned air excess ratio, it is clear that nitrogen
constitutes the major part of the exhaust gas. As nitrogen is practically inactive, only a
small but, as will be seen later, an important part is involved in the chemical reactions in
Carbon Dioxide (CO2): Basically, the complete combustion of hydrocarbons will
produce carbon dioxide and water vapor, and the relative amounts of these will be a
function of the hydrocarbon composition. Carbon dioxide, although not toxic, has recently
been given much attention because of the so-called ‘greenhouse effect’. The use of
machinery with a high thermal efficiency and of fuels with relatively low carbon content is
the only viable means of reducing carbon dioxide emission.
Carbon Monoxide (CO): Carbon monoxide is a toxic gas. The formation of CO is, in
principle, a function of the air excess ratio and the combustion temperature. The formation
is strongly influenced by the uniformity of the air/fuel mixture in the combustion chamber,
and we expect this to be the decisive factor in our engines. Carbon monoxide is a
compound that is still burnable, so elimination in the engine process is preferable.
Smoke: A traditional measure of the combustion quality, and a traditional way of
qualifying the ‘emission’, is to look at, or to measure, the smoke intensity. The exhaust gas
plume, when it leaves the top of the stack, may be visible for various reasons, e.g. its
content of particulate matter and nitrogen dioxide, NO2 (a yellow/brown gas), or of
condensing water vapor. Although it may be argued that these components are either
subject to separate legislation (NOx, particulate matter) or not harmful (water), it is a fact
that smoke and/or opacity limits are enforced in certain countries, e.g. in the USA.
Unfortunately, methods of measuring smoke and opacity vary (see the section on
measuring methods) and the figures resulting from the different methods are not really
comparable. When considering visible emissions, we should bear in mind that the larger
the engine, the more likely it is that the exhaust gas plume will be visible. This is because,
for a given Bosch Smoke Number (BSN value), the greater the diameter of the plume, the
greater the amount of light it will absorb. For instance, a BSN of 1 will mean almost
invisible exhaust gas from a truck engine, but visible exhaust gas from a large, low-speed
engine. Typical smoke values for the most modern diesel engines are so low that the
exhaust plume will be invisible, unless water vapor condenses in the plume, producing a
gray or white color. However, the NO2 may give the plume a yellowish appearance.
Particulate Materials (PM): Particulate materials in the exhaust gas may originate
from a number of sources:
agglomeration of very small particles of partly burned fuel,
partly burned lube oil,
ash content of fuel oil and cylinder lube oil,
sulphates and water.
Fig:Typical analysis of the exhaust gases from modern two-stroke marine diesel
Fig. : The pollutants in the exhaust gases (% vol) of large diesel engines.
The contribution from the lube oil consists mainly of calcium compounds viz.
sulphates and carbonates, as calcium is the main carrier of alkalinity in lube oil to
neutralize sulfuric acid.