These are of minor importance in commercial shipping and are generally only used as propulsion engines for small craft such as workboats, tugs and the like. They are also used for small generating tasks on larger vessels and for powering lifeboats and rescue boats.
There are more manufacturers in this arena than the other two categories with companies such as Volvo Penta and Scania producing only high speed engines. The German maker MTU straddles the line between medium and high speed with its 8000 series engines that operate at 1,150 rpm but its smaller 4000 series running at 1,800rpm fall definitely in the high speed sector. In the 16 cylinder vee variant, the 4000 engine is a regular choice for high speed ferries with waterjet propulsion systems.
Keeping it clean
Meeting NOx Code requirements is a simple matter for a dual-fuel or gas burning medium speed engine but for operators wishing to run on fuel oil, measures need to be taken to reach the new Tier III and in some cases even the older Tier II regulations.
One of the methods used is adopting the Miller cycle using variable inlet valve closing, so that at full load, the maximum cylinder temperature is reduced inhibiting NOx formation. This is combined with higher compression ratios and slightly later fuel injection timing. For engines operating at part load conditions variable valve timing and increased turbocharger efficiency through concepts such as two-stage turbocharging are required.
Cylinder temperature can also be reduced through using water introduced either with the fuel in a fuel/water emulsion, by direct injection or by adding water to the charge air. Both methods dilute oxygen concentration of the combustion air and use the water vapour for cooling down the combustion temperature peak which leads to a NOx reduction. Emulsions with above 30% water have been seen to reduce the efficiency of the engine and neither method can reduce the NOx emissions sufficiently to meet the regulations. However, in conjunction with other strategies the concept is part of a practical solution that is being pursued. Emulsified fuels have an additional benefit in that they allow for small fuel droplets which give a more complete combustion that can increase engine efficiency and reduce fuel consumption.
Another method is selective catalytic reduction (SCR) in which the exhaust gas is treated with ammonia or urea and fed through a catalytic converter at a temperature of 300ºC to 400ºC. The chemical reactions that take place there act first on nitric oxides before other chemicals such as sulphur oxides. Using SCR a potential reduction of 80% in NOx is possible. With SCR, instead of NOx, only nitrogen and water vapour are emitted.
This only works, however, if the exhaust gas temperature is correct. If it is too high, the ammonia burns rather than forming a compound with nitric oxide. If it is too low, it forms ammonium hydrogen sulphate and gradually blocks the catalytic converter. The same happens if the sulphur content of the exhaust gas is too high. The minimum temperature required depends on the fuel’s sulphur content which can of course vary even for ships operating in SECAs if a scrubber is used to reduce SOx output.
However, SCR is a proven technology both on land and at sea and is operated without problem on most ships where it has been installed. The numbers of ships on which it has been installed runs into thousands and is increasing now that Tier III is in full force. It is also versatile in that the configuration of systems is flexible with components being installed in the engine room or stack as appropriate. Components are also becoming smaller than was typical in the earliest marine systems.
Exhaust Gas Recirculation (EGR)
Another method of NOx reduction is Exhaust Gas Recirculation (EGR) which uses exhaust gas for diluting the oxygen concentration in the charge air which leads to a lower combustion temperature. In combination with Miller cycle, high pressure two-stage turbocharging and common rail systems, NOx reduction rates up to 80% percent have been reached during MAN Diesel in-house tests. Using EGR means that no ammonia or urea is need for SCR and there is also no need for fresh water to be used. A possible disadvantage might be the necessity of costly low sulphur fuels or even distillates to protect the engine against corrosion.
Reducing SOx levels in exhaust emissions can only come about in one of two ways. Either the sulphur level in fuel has to be reduced or abatement technology – commonly referred to as scrubbing – has to be employed.
Unlike with NOx, there are no adjustments that engine manufacturers can make but the use of low sulphur fuel requires additional precautions that need to be taken in the choice of engine lubricants. When it becomes necessary to switch fuels because of the SOx regulations, the process is not without hazards. Lubricants need to be matched to fuels in order to avoid excess corrosion or lacquering which are the extremes of mismatching.
Tribology – the science of interacting surfaces, friction, wear and lubrication – is an important part of engine and lubricant R&D. Attempts have been made by most lubricant makers to develop a universal lubricant for engines that can cope with all fuel types but results have so far been mixed. Some lubricant makers do market universal lubes but others have preferred to match products to different fuels.
For their part, engine makers need to endorse lubricants they consider suitable under a range of conditions and while some may be prepared to permit universal lubricants others may not. The reasons for caution include differences in engineering tolerances and choice of materials for cylinder liners and piston rings. An engine maker’s tests of any lubricant will be done on their test engines which will contain only OEM parts so owners that make use of third party spares may or may not suffer problems. Research in materials and component design is an ongoing process for most engine makers.
The lubrication issue does not normally result in an immediately hazardous situation but other aspects of switching can include fire, explosion or shut down of an engine. For example, low-sulphur fuels may damage existing HFO pumps because of reduced fuel oil viscosity and lubricity leading to overheating and excessive wear. Fuel injection pumps can be similarly affected necessitating their replacement by special equipment such as tungsten-carbide coated pumps. Unless approved by the engine manufacturer, such changes may affect the engine’s compliance with NOx legislation.
When running on HFO many components of the fuel system are either heated directly or will become hot because of the fuel temperature. MGO running through hot piping may vaporise, creating vapour locks that interrupt the fuel supply to the engine. During the changeover, rapid or uneven temperature change could cause thermal shock, creating uncontrolled clearance adaptation, which in turn may lead to sticking/scuffing of the fuel valves, pump plungers, suction valves and, in the worst-case scenario, total seizure of the pump. To maintain an appropriate viscosity if MGO is used in an engine designed to run on HFO, a new cooler may have to be fitted; in some cases it may even be appropriate to install a chiller to remove heat through vapour-compression or an absorption refrigeration cycle.
Ships entering ECAs must have a defined written procedure on board to comply with MARPOL Annex VI Regulation 14. Much advice is available on manual switching processes from classification societies and P&I clubs but arguably the most knowledgeable advisors are the engine makers themselves. The process is not instantaneous and depending on engine and fuel in use can take several hours to be done safely. There are mechanical devices available that can automate the switching process but these are currently not smart enough to determine when to begin the process so as to avoid completing the switch before entering a controlled area.