Diesel engines for greener Navies
Exhaust gas after-treatment for patrol vessels
In a paper given to the 2010 Defence Technology Asia conference, Christoph Fenske, Senior Manager for Naval Surface Craft at MTU Friedrichshafen, examined the impact of IMO Tier 2 and Tier 3 emissions limits on naval vessels – although navies are not legally obliged to meet these limits, many naval and coastguard authorities intend to follow the rules, with ship procurement programmes demanding compliance.
Following the establishment of Exclusive Economic Zones, i.e. 200 nautical mile zones along national coastlines, the protection of national resources and facilities has become a matter of increasing significance for Navies and Coastguards, promoting a growing number of procurement programmes for offshore patrol vessels.
However, a precise description of the OPV as a class of vessel is conspicuously absent. The scope ranges from straightforward commercial ships armed with small-calibre weaponry for law enforcement, and vessels better described as corvettes or light frigates. Speed requirements are equally diversified. Maximum speeds ranging between 18 and 30 knots are called for depending on requirements.
A typical fast OPV with a speed requirement of around 28 knots, a displacement of 1,200 tonnes and a waterline length of 70 m could have a power demand amounting to 17,500 kW at 100% MCR. Considering the characteristics of this vessel in a diagram in which the Froude number is imposed on the length and speed of the ship, such a vessel would be of semi-planing hull form, with a Froude number of 0.55 whereby Fr = v/(g * L)^(1/2)
Our ship is sensitive to weight owing to the dynamic buoyancy factor at high speeds. Any comparison of possible propulsion solutions soon reveals that only high speed diesel engines, such as the MTU Series 4000 with a power range of between 2000 and 4300kW, Series 1163 with a power range of between 3600 and 7400kW and Series 8000 for the power range between 7200 and 9100kW, are suitable for weight-sensitive craft. The installation of medium speed diesel engines in fast OPVs is not appropriate as the specific weight of the propulsion unit is too high. Any fuel consumption benefits of the medium speed diesel engine cannot compensate for this significant discrepancy.
Gas turbines offer a very high power-to weight ratio, but are viable only in combined propulsion systems of high-specification patrol ships due to their high capital and life cycle costs.
Combined multiple-engine plants, such as a CODAD system comprising typically four Series 4000 engines, have been seen to fulfil the task-specific requirements of fast OPVs
Under the IMO Tier 2 and Tier 3 emissions limits for NOx applicable to diesel engines delivering in excess of 130kW, Tier 2 envisages about 20% lower emission of nitrous gases compared with the previously valid IMO Tier 1 limits and apply to vessels the keel of which is laid as of 2011. Tier 3 limits will apply as of 2016 for vessels operating in Emission Controlled Areas.
Confirmed ECAs are the Baltic and the North Sea as well as the coast of the USA and Canada. ECAs currently under discussion are the coasts of Mexico and Norway, the Mediterranean, South Korea, Tokyo Bay and Australia. Singapore is also considering the establishment of an ECA. In addition, the more stringent requirements of the US Environmental Protection Agency (EPA) in regard of particle and non-combusted hydrocarbon emissions apply to vessels sailing under the US flag in US territorial waters.
Even if official vessels (operated predominantly by Navies and Coastguards) do not necessarily have to observe emission legislation, some Navies require compliance with currently applicable emission requirements, i.e. IMO Tier 2 emission limits at least.
In order to achieve IMO Tier 2 and Tier 3 compliance, MTU is focusingon exhaust gas recirculation (EGR) and the Miller cycle in regard of measures inside the engine. Fuel-water injection has failed to stand the test in Navy vessels in our view.
Exhaust after-treatment systems are also being pursued intensively, as the engine configuration can remain unchanged in this case. This approach offers distinct advantages in maintaining the proven reliability and logistics of the engines, both of which represent decisive factors for Navies and Coastguards when selecting a propulsion system. Exhaust gas after-treatment systems can be used in addition to internal engine technologies in cases where a further reduction in noxious exhaust gas emissions such as NOx or soot cannot be economically realised by means of internal engine technologies.
The key technologies for implementing these approaches are: injection, turbocharging, engine management, mechanical and thermodynamic analysis, and exhaust after-treatment. Only those engine manufacturers which have mastered these technologies will be in a position to achieve an optimum balance between operational behaviour, reliability, life-cycle costs and emissions.
MTU’s Series 4000 was the first off-road diesel engine to offer electronic fuel injection. Injection pressures have since risen from 1400 bar to 1800 bar. Work is currently in progress on systems with significantly over 2000 bar. New injector technologies allow multiple injection and shaping of the injection process. This not only has a positive effect on emissions, including visible soot, but also on the signature of military vessels.
Pre-injection, in particular, influences the pressure rise gradient which is known to have a crucial influence on structure- and air-borne noise levels. Careful adjustment of the injection parameters could lead to a significant reduction of underwater noise.
Post-injection optimises the combustion of non-combusted residues (soot) and thus improves the visible signature and reduces the IR signature.
Such measures must, however, always be brought in line with other requirements such as exhaust emissions or fuel consumption.
High combustion temperatures encourage the formation of nitrous gases. The reduction of top combustion temperatures therefore offers the most leverage for reducing NOx emissions.
The Miller cycle represents a most effective method of reducing combustion temperatures without increasing fuel consumption. The valve control times are changed such that the combustion temperatures and thus the formation of NOx are significantly reduced. This cycle, however, requires a higher charge-air pressure which has to be achieved by means of more efficient turbocharging. MTU is confident that its own in-house developed turbochargers can meet these requirements.
The recycling of combustion gases is already a widespread method used in the automobile industry. Inert gas not used for combustion is returned to the cylinder thus reducing combustion temperatures. The technical challenges presented by long-running diesel engines in transient operation are considerable and can only be mastered by extensive optimization. NOx emissions can be significantly decreased depending on the recirculation rate. Fuel systems featuring high injection pressures and improved charging systems are prerequisite to limiting the simultaneous increase in exhaust opacity (smoke index).
Long-term trials over 5000 operating hours have confirmed the reliability of the exhaust gas recirculation method developed by MTU, mainly on the basis of trials in the rail industry.
Nowadays, NOx emissions of around 3 - 3.5 g/kWh can be achieved by means of exhaust gas recirculation. Investigations are currently underway to determine whether this technology alone, or in conjunction with the other internal engine technologies, will adequately comply with IMO Tier 3 limits. MTU is optimistic of achieving this goal. The outlay involved must still be principally compared with that of exhaust after-treatment systems in order to arrive at the right solution. The overall outlay including aspects of the ship must be afforded due consideration here.
The use of exhaust after-treatment systems is advisable in the case of engines for which an internal engine solution is considered too elaborate. Particulate filters, commonly used in the automobile industry, have yet to prove their worth in marine applications. A burner for active regeneration of the filter is frequently provided to increase the exhaust temperature to the extent that trapped particles of soot are burned. Active regeneration is necessary in cases where the engines operate predominantly under low load conditions as in the case of military vessels. Such a system would presumably be necessary to comply with EPA Tier 4 limits.
A Selective Catalytic Reduction (SCR) reactor may be used as an alternative to internal engine solutions to reduce nitrous gases downstream of the engine to the IMO Tier 3 level. The chemical reaction of urea (ammonia solution) with the nitrous gases in the exhaust produces nitrogen, carbon dioxide and water.
Naval architects will always prefer an internal engine solution as emission compliance is restricted to a fully configured component having known dimensions. Conversely, exhaust after-treatment systems have a significant influence on exhaust routing and thus on the design and arrangement of the superstructure. For example, an SCR reactor for a Series 1163 engine would occupy around 4m³ and weigh about 2,500 kg. A mixing pipe facilitating uniform mixing of urea solution and exhaust gas is also needed. The on-board supply system for injecting the urea solution requires an additional tank with a capacity of around 3-5% of the fuel consumption, the associated pumps, valves and a control system. A catalytic reactor has a service life of some five years or up to 12,000 operating hours. The catalytic reactor must then be removed and overhauled.
Existing vessel designs are potentially in need of adaptation in order to integrate SCR reactors. The weight distribution is altered and thus the stability of the vessel. It will be advisable to take SCR reactors into consideration for new vessel designs requiring Tier 3 compliance.
MTU believes that it is equipped to meet these challenges which affect not only the engine manufacturer but also the ship designer in achieving the balance between operational and emission behaviour of the propulsion system.
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