Recreation : Potential effects : Engine emissions

The potential effects of engine emissions

Types of marine engines

Engine emissions

Biodegradable lubricants

Accidental discharges

Motorboats use a wide variety of different engines and transmission systems. This makes an assessment of the generic impact of emissions from such craft particularly difficult. The emissions themselves also vary in their importance. Some, such as carbon dioxide, are harmless to the local marine environment, although they may make a very small relative contribution to global environmental effects, such as climate change. Others, such as hydrocarbons, may impact upon the water column and sediment. To compound the difficulty in assessing emissions, there has been little research carried out specifically into the impacts of marine engine emissions on the environment.

This section attempts to identify the different emissions from motorboat engines and the potential impact of these emissions on the marine environment. It begins with a description of the different types of engine and drive systems commonly used.

It should be noted that ‘emissions’ are gases exhausted from engines. They should only be classed as ‘pollutants’ when they have an actual direct or indirect impact on the environment.

Types of Marine Engines

The vast majority of marine craft use one of the following engine types:

  • 2 stroke engine fuelled by a petrol/oil mix or converted to propane
  • 4 stroke engine fuelled by petrol or propane
  • 4 stroke engine fuelled by diesel

Recreational craft engines come in two broad categories - outboard or inboard. These different engine types have markedly different emissions characteristics as outlined below.

Outboard engines are usually mounted at the rear of the vessel and have a self-contained drive unit. The small size, weight and low purchase price make outboard engines popular for smaller craft. Most outboard engines are two stroke petrol units using an oil/petrol mix although four stroke outboards are growing in popularity and the use of propane gas for both 4 and 2 stroke engines is also becoming more popular.

As the name suggests, inboard engines are mounted inside the craft and drive is relayed through various different transmission systems such as shaft drive/propeller, stern drive/propeller, sail drive and water jet drive. The latter are used on personal watercraft.

Inboard engines tend to be used within larger craft and are usually fuelled by petrol or diesel. These engines are generally more powerful and are used where the weight and size of the unit are not the overriding consideration. Many inboard engines are marine derivatives of commercial automotive engines. Although diesel inboard engines are becoming increasingly popular, four stroke petrol engines are usually chosen where installation space is more limited or where higher power to weight ratios are required, as with watersports craft.

Electric engines are becoming increasingly popular on inland waterways, but due to limited power and range, they are not generally used in coastal areas.

Engine Emissions

During the combustion process, internal combustion engines of all types generally produce, in varying quantities, the following substances:

  • Oxides of Nitrogen (NOx), a potential contributor to photochemical smog and to ozone layer damage
  • Carbon Monoxide (CO), a toxic gas
  • Carbon Dioxide (CO2), the most significant cumulative 'greenhouse gas'
  • Hydrocarbons (HC), a constituent of photochemical smog
  • Sulphur Dioxide (SO2), an element in acid deposition
  • Lead, a toxic heavy metal
  • Particulate matter, a potential carcinogen.

4 stroke engines generally produce higher CO, CO2, and NOx, but lower HC than 2 stroke engines. 2 stroke engines emit relatively high levels of HC in both burnt and unburned form, but low levels of NOx. Diesel engines are more fuel efficient than 4 stroke or 2 stroke petrol engines and therefore emit lower overall CO and CO2 However, they produce greater quantities of SO2, Nitrogen Dioxide and particulates. In terms of marine features, it is the effect of hydrocarbons and lead in the water column and sediment that are of key significance.

The impact of marine engine emissions on the environment has been the subject of much discussion over the last twenty years. Despite this, there has been relatively little original scientific research conducted on the subject in recent years, particularly concerning the potential impacts of emissions on specific habitats. There are studies which examine the implications of engine emissions on non-designated marine species and plants (e.g. the Institute of Applied Environmental Research in Sweden reviewed the potential effects on fish). However it should not be assumed that the findings of such reports have significant consequences for the assessment of the designated features within mSAC areas. Features other than the specific Annex I and II features designated within mSAC areas largely lie outside the remit of this report.

Legislation concerning marine engine emissions is to be taken forward in the forthcoming European Directive on marine engines which will specify common European emissions standards. The standards will extend to emissions such as benzene, about which very little is known in the context of impacts on the Annex I and II habitats and species in mSAC areas.

Hydrocarbons - modern outboard engines exhaust below the water surface and, as a result, all hydrocarbon emissions pass through the water. Most remain in the gas phase and are released directly to the atmosphere; the remainder condense and become suspended in the water column or form surface film until they degrade or are released into the atmosphere. Evaporation is the most significant factor in the removal of exhaust products. Hydrocarbons reach the sediment slowly due to the efficiency of the degradation process.

In response to proposed legislation on the Bodensee in the early 1990s, a study on the contribution of outboard engines to marine pollution was carried out by Barlett (FSRC, 1990) in conjunction with the International Council of Marine Industry Associations (ICOMIA). With the exception of one site, the highest concentrations of total hydrocarbons were found to be close to large municipal centres, or near the mouths of major rivers draining industrialised and urban catchment areas, rather than in heavily used boating areas. The report suggested that, in general, residues from outboard motor oils are likely to be negligible in relation to other sources of the hydrocarbon burden in the sediments.

This conclusion is also supported by the TNO Road Vehicles Research Institute (1991), which found that, at present, there is little environmental impact caused by marine engines. Hydrocarbon contamination of sediment is not, to any great extent, caused by the use of 2 stroke marine engines. Butcher (1982) also concluded that outboard engine use was not a major cause of hydrocarbon pollution of the water column, and although hydrocarbons in surface films were more problematic, evaporation was rapid.

However, recent research carried out by the State of California Air Resources Board (1998) found that two-stroke engines can discharge as much as one-third of the oil/petrol mixture unburned into the water. It concluded that there is potential for considerable impact from such emissions. Clearly, such contradictory evidence emphasises the need for further research in this area.

Lead - Butcher (1982) found that lead concentrations were increasing, but that at that time it did not create quality problems in the water column. However, long term build-up of lead in the sediment was considered to be of potential environmental concern. He concluded that sublethal and long term biological effects of leaded fuel cannot be entirely ruled out. The increased use of unleaded fuel has led to a decrease in, although not a total elimination of, the amount of lead in engine emissions.

Biodegradable Lubricants

Two stroke engines use a method of combustion which results in some unburned residual oil and partially burnt oil entering the marine environment through the exhaust. This leads to the familiar sight of small patches of oil forming on the water where 2 stroke marine engines are being run.

Whilst the environmental effects of these emissions are uncertain (see above), legislation has been implemented, or is being considered, in many countries with the aim of severely restricting such emissions. In response, the oil companies are putting increased emphasis on the use of biodegradable lubricants for 2 stroke engines.

Although commonly used mineral oils eventually degrade, the process is very slow and such oils would not normally be defined as biodegradable. Therefore biodegradable lubricants are usually formulated from non-mineral oil fluids, both from vegetable sources such as rapeseed, and from manufactured esters with similar properties.

Biodegradable lubricants are designed to break down into carbon dioxide, water and biomass by the biological activity of micro organisms. The quantity of biomass formation depends on the extent of biodegradation while the speed of the biodegradation process depends on water temperature. In deeper, colder waters biodegradation takes longer than in warmer waters.

Whilst mineral oils remain the dominant lubricant in the marine market, increased use of biodegradable oil in 2 stroke outboard engines could reduce the magnitude of any hydrocarbon impacts on the marine environment. However, the biodegradation process produces a number of toxic by-products which have an unknown impact on the environment. Furthermore, by-products of the 2 stroke combustion process will continue to be emitted with the biodegradable lubricant.

The case for the use of biodegradable lubricants in 4 stroke engines is even less clear. It is also important that consumers are not misled into believing that the lubricant can be disposed of in a more casual manner than common non-biodegradable lubricants. In particular, the used product may contain non-biodegradable components and contaminants which, if disposed of incorrectly, will impact on the environment. There is also uncertainty over the recyclability of the used product.

Accidental Discharges

In many ways, accidental discharges of oil and fuel present a potentially more serious environmental risk than regular discharges from running engines. New European boat safety standards address the issue of fuel equipment in some detail and this should ensure that most boats at least have safe equipment. The handling of the oil/fuel by the boater and fuel dock attendant is another matter. Fuel docks are inspected to ensure safe functioning but incompetent handling of the pumps can cause problems if there is no sealed system in operation.

In most inland waterways the pumping of bilge water into the waterway is prohibited. However, although technically illegal in coastal areas, detection of bilge water pump out is notoriously difficult where discharge does occur and it is uncertain how much oil enters the marine environment in this way.

The discharge of oily bilge water can be minimised by the use of absorbent pads placed underneath the engine installation. These can be regularly cleaned or replaced to ensure the bilge water is not unduly contaminated.


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