Diesel engines on ships
Most ships afloat today are powered by an oil-burning diesel engine of some sort although the number of dual-fuel ships and pure gas burners is slowly increasing. Diesel engines come in various basic types and while direct mechanical propulsion systems are the most common, diesel-electric, combined diesel and gas and other hybrid variants are alternative options that maybe encountered.
Regulation for all engine types
The power and propulsion systems of ships are subject to various regulation but in most cases this is an indirect effect and has more to do with safe operations than the choice or type of engine and machinery.
From an operator’s point of view, engines, propulsion and steering arrangements regardless of type are all considered essential systems and therefore will feature in just about all approved ISM Code safety systems adding another layer to any regulatory requirement for operation and maintenance. On the safety side, the fuel delivery to the engine may be controlled under SOLAS and as a source for potential fire and explosions, the engine room will need fire protection and extinguishing systems.
The engine room is not a comfortable place to work but under the Maritime Labour Convention (MLC) which became effective in August 2013 noise and vibration are now issues that all affected ships must take measures to control both in accommodation and working areas. In July 2014, new SOLAS rules came into effect. The SOLAS changes follow on from MSC91 when the IMO adopted a new SOLAS regulation II-1/3-12 to require new ships to be constructed to reduce on-board noise and to protect personnel from noise.
The new limits are in accordance with the revised Code on Noise Levels Onboard Ships, which sets out mandatory maximum noise level limits for machinery spaces, control rooms, workshops, accommodation and other spaces. The Code supersedes the previous non-mandatory Code, adopted in 1981 by resolution A.468(XII). It will apply to all vessels of 1,600gt or more which:
- are built under a contract signed after 1 July 2014
- with the keel laid after 1 January 2015
- have a delivery date on or after 1 July 2018
As an example of operational regulations, the propulsion engines of ships will produce the exhaust emissions that are controlled by Annex VI of MARPOL and in the case of NOx there is some potential for a direct impact on the engine depending upon the method chosen to prove compliance. Likewise, the Energy Efficiency Design Index may require the main engine’s power output to be restricted in some way in order to meet the CO2 limitations if no other means are available to do so.
The EEDI rules introduced by the IMO in 2013, to reduce shipping’s CO2 output is a non-prescriptive, performance-based mechanism that leaves the choice of technologies to use in a specific ship design to the owner and as long as the required energy efficiency level is attained, ship designers and builders are free to use the most cost-efficient solutions for the ship to comply with the regulations.
The EEDI provides a specific figure for an individual ship design, expressed in grammes of CO2 per ship’s capacity-mile and is calculated by a formula based on the technical design parameters for a given ship.
The effect of the EEDI in reducing CO2 output is designed to become more stringent over time. From an initial Phase 0 (2013-2015) aimed at setting benchmarks, there follow three five-year periods in which the allowed CO2 levels are reduced by 10% each time. The latter stages may be made more stringent in future with the final phase perhaps being brought forward by two years and more phases added.
Unlike most other equipment on board vessels, there are no performance standards or guidelines for the power and propulsion systems beyond vague references to the flag states requirements and a reference in the regulation that calls for interim and special surveys to ensuring the main machinery should be in a satisfactory condition and fit for purpose.
Regulation also comes at a national level under port state rules. Where national regulation is concerned, there are developments such as the Norwegian NOx Levy and Fund, EU regulations or the California Air Resource Board’s (CARB) attempts to limit and measure emissions above and beyond that required under IMO rules. In addition, so long as a ship is registered with a classification society there are the rules and requirements of the class society in question that need to be considered. The demands of MARPOL Annex VI are covered elsewhere in the ShipInsight range of article and publications – most specifically under the Environmental Technology section.
Complying with the NOx Code
However, to recap the requirements of the NOx Code are the only one of the emission regulations where the engine itself rather than the fuel or ancillary systems can be a controlling factor. Under the Code, all vessels built since 2000 must have a Technical File which identifies the engine’s components, settings or operating values which influence exhaust emissions.
The file is prepared by the engine maker and approved by the flag state. It must be retained onboard for the whole life of the engine and will be used to ensure compliance. The engine to which the Technical File refers is to be installed in accordance with the rating (kW and speed) and duty cycle as approved together with any limitation imposed by the Technical File. The Technical File must, at a minimum, contain the following information:
- Identification of components, settings and operating values of the engine which influence its NOx emissions
- Identification of the full range of allowable adjustments or alternatives for the components of the engine
- A full record of the engine’s performance, including its rated speed and rated power
- A system of onboard NOx verification procedures to verify compliance with the NOx emission limits during onboard verification surveys
- A copy of the test report for an engine tested for pre-certification or a test report for an engine installed onboard ship without pre-certification
- If applicable, the designation and restrictions for an engine which is a member of an engine group or engine family
- Specifications of those spare parts and components which, when used in the engine, according to those specifications, will result in continued compliance of the engine with the NOx emission limits
- The Engine International Air Pollution Prevention Certificate (EIAPP)
Compliance with the code can be achieved using one of three options alone or a combination. The first option is to run the engine always within the parameters as laid down in the technical file and to use only OEM spare parts when any component identified in the technical file requires replacement.
The second is to install a continuous monitoring system of the type offered by manufacturers such as Kittiwake, Martek Marine, Green Instruments or Norsk Analyse, among others. Some of these systems can measure other exhaust gases and might be able to provide evidence of compliance with other regulations such as SOx emissions limits in SECAs or in ports where low-sulphur fuel is mandated. The third option requires the engine to be tested at regular intervals by approved service providers.
All engine types and power configurations are affected by the regulations above although engines running on LNG and some of the latest alternatives such as methanol or ethane do not produce SOx.
The regulation of engines using LNG and similar fuels has only recently come under the auspices of the IMO having previously been down to flag states to set rules on a case-by-case basis. More details of the IMO rules are included in the section on dual-fuel and gas burning engines.
Internal combustion engines were first used for ships in 1912 but took more than 50 years to fully replace steam engines. Early motors were not necessarily more efficient than steam but did allow owners to employ far fewer crew as stokers to carry coal from the bunkers and to shovel it into boilers were replaced by simple pumps to move oil from tanks to the engine. Thus there was an immediate saving in crew numbers that will not be repeated if oil is replaced by LNG as a fuel.
Diesel engines can be either two-stroke or four-stroke with the former being low-speed engines and the latter either medium- or high-speed. Two-strokes are used for direct mechanical propulsion only, but four-strokes can be used either in mechanical propulsion systems using a gearbox to reduce the engine speed to one more suited to driving a propeller, or in a diesel-electric configuration where the engine drives a generator to produce electricity which is then directed through cables to electric motors to drive the propeller or other propulsor type.
Until the mid-1930s marine diesels were invariably four-stroke and ran on distillate rather than residual fuels. Today the giant two-strokes with their better power to weight ratio are the engine of choice for most large cargo vessels. High-speed diesels are rarely encountered on commercial ships except as generators but are regularly used for propulsion in small tugs, work boats and ferries.
Diesel engines can run on many types of oil fuel from heavy residuals through to light distillates but although it is possible (and frequently necessary) to switch between fuel types, ideally an engine performs best when all parameters are matched to a single fuel type. Because of the polluting effect of engine exhaust when burning oil fuels, there has been a move to persuade owners to switch to burning LNG. This can be done either using a pure gas engine or a dual-fuel engine that is capable of running on either oil or gas fuels.
The imposition of a 0.5% m/m sulphur limit on marine fuels used outside of ECAs in 2020 may change the driving factors in the choice of fuel type although most analysts expect that residual fuels will still be the preferred choice of most operators. This infers that either sufficient compliant residual fuels will be available or sales of exhaust gas cleaning systems will accelerate.
The development of dual-fuel engines is relatively recent and although most major engine manufacturers now have models in their portfolios, they are still not fully accepted by the majority of shipowners and operators.
Back in the 19th century when internal combustion engines were in the very early stages of development, two men Nikolaus Otto and Rudolph Diesel devised different means of initiating combustion of the fuel. Otto’s method was to compress the fuel to a particular volume and to then apply a source of ignition in the form of a spark. Diesel’s method was to continue to compress the fuel until it ignited spontaneously due to the heat produced by the higher compression used.
At similar pressures, the Otto engines are considered more efficient but because Diesel engines make use of much higher pressures, in practice they are more efficient and consume less fuel. Modern oil burning engines mainly rely on the Diesel cycle but dual-fuel engines need an alternative ignition source when operating in gas mode.
Wärtsilä dual-fuel engines make use of the lean-burn Otto process in which gas is admitted into the air inlet channels of the individual cylinders during the intake stroke to give a lean, premixed air-gas mixture in the engine combustion chambers and ignition is obtained by injecting a small quantity of diesel oil directly into the combustion chambers as pilot fuel which ignites by compression ignition as in a conventional diesel engine. By way of contrast, in MAN Diesel’s high-pressure MEGI DF engines the gas is injected only after the combustion air is compressed, after which it is ignited by the pilot oil injection.