There is currently a worldwide shortage of highly qualified and experienced seagoing engineers and this situation is set to continue. This problem has been compounded over recent years by the increasing range and complexity of machinery found on modern vessels such that it has become unreasonable to expect familiarity with every aspect of its operation and maintenance.
In order to compensate for the shortfall in personnel and the high wage costs of those available, operators have increasingly opted to increase on-board automation and computing capability. This policy in itself has led to more complex systems and a change in the skills required of seagoing engineers. It has also introduced the possibility of greater reliability problems associated with sensors and other electronic components, as well as potential problems maintaining safety and environmental protection [1].
3.5 System Healthcare
Marine power and propulsion systems must be continually maintained through their working life so that the designed efficiency and performance is ensured. Increasingly, measures adopted to reduce emissions and improve specific fuel consumption, are only beneficial if operated as designed.
The medium speed diesel industry has experienced changes in its business processes, with power plants supplied and linked to a system healthcare package provided by the manufacturer. This approach passes the responsibility for system performance and its maintenance to the engine supplier and the possibility of financial penalties for failure to meet specified targets. For example, mean time between failures, specific fuel consumption, and availability etc. for a power plant can be used as targets and act as incentives encouraging the production and maintenance of more efficient and reliable systems. For power plant healthcare schemes to operate successfully an integrated condition monitoring, fault diagnosis and maintenance decision-making system must be in place [8].
4. TECHNOLOGICAL DEVELOPMENTS
The demands to improve the environmental sustainability of shipping have resulted in research and development applying a number of new engine designs. Some examples of concepts and designs in various stages of developments are now described.
4.1 Wet NOx reduction in piston engines
The introduction of water to the combustion chamber is a very effective method to reduce NOx formation. The water is vaporised during the combustion process and reduces the maximum flame temperature and cools hotspots within the cylinder. The technologies associated with so called 'wet NOx' reduction techniques fall broadly into three areas discussed below.
4.1.1 Fuel-Water Emulsion
A fuel-water emulsion is produced mechanically prior to the injection pump as shown in the Figure 1. The emulsion is injected into the combustion chamber through the same fuel system used for normal operation with fuel oil. Depending on engine and fuel type, the addition of 20% water to the fuel can yield a reduction in NOx of around 20% with no increase in fuel consumption.
A limiting factor in the fuel-water emulsion system is the maximum delivery capacity of the delivery system and therefore at full load a NOx reduction of about 20% is practical. It is possible to increase the fuel-water ratio above 20%, however this would involve significant design changes and also result in increased fuel consumption, as discussed by Velji et al [9]. In addition to its influence on NOx reduction, the use of fuel-water emulsion improves the opacity of the engine smoke, particularly at low loads and reduces the level of CO emissions, however HC and particulate emissions are slightly increased.
Although this emulsification process has been in operation for a number of years on a few vessels there remain problems that need future development. Increased corrosion and wear of piston rings, cylinder liners and valves, the durability of the fuel injection system and the deterioration of lubricating oil all require further investigation with the possibility of utilisation of alternative designs and materials.
4.1.2 Water Injection
Water may be injected directly into the combustion chamber through separate nozzles, by alternating fuel and water injection through the fuel injection valve, or it may be injected into the air inlet manifold.
(a) Direct water injection through separate nozzles
By using a dual nozzle system, the injection of water and fuel can be timed separately for optimum NOx reduction. Water injection commences before fuel injection so as to cool the charge air prior to fuel injection. By timing the water injection so that it partly overlaps the fuel injection timing, it is possible to achieve significant NOx reductions as outlined by Kohketsu et al [10].
This technology is less complex than fuel-water emulsions but does not have the same improvement in fuel spray atomisation However, compared to emulsified fuels, the capacity of the injection system does not have to be adjusted and emulsion stability problems are avoided so that larger amounts of water can be used [11]. NOx reduction of 50-60% have been reported and trials onboard the ferries Silja Symphony and Aurora of Helsingborg support this [12].
(b) Sequential direct water injection through a single nozzle
This technique involves the design of a fuel injector with the capacity to deliver both water and fuel into the combustion chamber in a single action. Figure 2 depicts such a design. During the time interval between the injections, water is supplied down the centre of the unit and through a capillary into the surrounding fuel chamber. This volume of water displaces an equivalent volume of fuel from the injector chamber, which is returned back to the fuel line.
When the injection pump begins to supply fuel, the pressure in the injector increases, a check valve is closed and the content of the fuel chamber is injected through the nozzle into the combustion chamber. The injection sequence is therefore fuel-water-fuel from the single nozzle. The proportion of water-in-fuel for each given delivery can be varied depending on the position and length of the capillary through which the water is fed to the fuel chamber. Equally, this will have a bearing on the timing of the water injection.
(c) Injection of water into charge air manifold
Injection of water into the charge air manifold is the simplest wet NOx reduction technique and has a similar effect on NOx and specific fuel consumption as the direct water injection method, discussed in depth by Nicholls et al [13]. In this case vaporisation occurs partially in the receiver and partially in the cylinder. This NOx reduction technique is very successful although corrosion and wear of piston rings, cylinder liner and valves as well as deterioration in lubricating oil remain problematic.
