While most of the media attention these days focusses on “carbon” or "greenhouse gasses", there are potentially harmful emissions that the marine engine sector is working to reduce.
Undesirable emissions from non-compliant small engines include:
Hydrocarbons: (HC) – basically unburnt or part burnt fuel and oil - from incomplete combustion or scavenging losses of the engine. Uncontained evaporation and vapour leaks from fuel tanks is a second source of Hydrocarbon emissions.
Traditional two strokes (carburetor or EFI, but not direct injection DI) are much higher emitters of HC. This is in part because unburnt fuel and oil are pushed out the exhaust during ( which also explains the higher fuel consumption) . two stroke fuel is a mixture of petrol and oil – and this also adds to the HC load.
Hydrocarbons contribute to the formation of ground level ozone via photochemical reactions and can contribute to a range of related respiratory diseases. Commonly visible as blue exhaust smoke, or an oil slick on the water.
Oxides of nitrogen: (NOx) derive from high temperatures during combustion, and contributing to ground level ozone, may also cause a range or respiratory diseases. In the presence of water forms Nitric Acid, which contributes to acid rain, a cause of environmental degradation.
Carbon Monoxide: less an Air Shed Pollution but a deadly poison, especially in high concentrations that have the potential to build up in closed rooms and boat cabins.