When talking about emissions from engines one often thinks about the smoke intensity. Smoke from ships is often visible and can be different in appearance but can also be invisible.

Exhaust plumes from large engines are more visible than from small engines due to the greater diameter of the plume.

At high loads most modern engines give very little smoke, but during low or transient loads, particularly during start up and manoeuvring the turbochargers deliver less air than is necessary for complete combustion and smoke is created. Smoke is highly undesirable – especially on passenger and cruise ships.

The colour of the smoke can vary depending on its content of different kinds of emissions such as PM, NO2 and water vapour. Black smoke is primarily caused by soot (carbon particles) and there is a clear dependency between the type of fuel used and the smoke formation as heavy fuel oil generates more particles and soot and thus more smoke than distillate fuels. Blue smoke is a sign of the presence of incompletely burned droplets of fuel or lubrication oil. The smoke gets a brownish hue from NO2 and white smoke is simply a sign of condensed water vapour present in the exhaust smoke. The white appearance is more pronounced in cold weather where condensation is greater. An overview of smoke appearances is found in Table 16.

Smoke colour
Soot (carbon particles)
Combustion of lubrication or cylinder oil
Water vapour

Table 16 - Smoke appearance overview.

Smoke that looks clean or invisible is not necessarily clean. Light HC components and very small PM are invisible to the eye and therefore even smoke that appears to be clean can contain relatively high rates of HC and PM.

Furthermore, the unpleasant looking smoke can be “wet” with oil. When the gas cools, in or outside the exhaust system, the oily particles condense on colder surfaces. If this depositing of oily substances happens inside the exhaust system on the ship it can cause a potential fire hazard and increased back pressure. If it happens in the atmospheric air the deposits can damage structures and plants.

In the following the nature of the emissions will be describe along with ways to calculate or estimate the content of emission types in the exhaust gas.