Engine builder looks to future emissions limits
MTU is looking forward to the US, EU and IMO future limitations for NOx and particulate matter emissions, as described by Dag Pike.
Regulations for the reduction of emissions from marine diesels are becoming progressively more severe and users of high speed marine diesel engines will have to conform to new standards in the near future. These regulations, which have been developed by IMO and are endorsed by both the US and the EU regulators, will cover the production of NOx and particle emissions from the exhaust. The regulations set levels for the reduction of these emissions in various stages through to 2014/2017 when engines will be required to have emissions that meet the Tier 4 levels, which equates with IMO’s level 3.
It is a complex scenario that has engine manufacturers working overtime to find solutions that will allow them to achieve the required emission levels and at the same time maintain the levels of reliability and low fuel consumption required for the small craft sector. There are matching regulations for land based engines although here the required final standards are less severe because of the more variable work patterns of these engines.
German engine manufacturer MTU, which supplies engines to a broad spectrum of marine users from recreational craft, through workboats and passenger vessels, to smaller cargo ships, is well advanced with its programme of development of engines and technologies that will meet the required emission levels. Development has now reached the stage where the company is undertaking field trials of engines in vessels with particularly demanding load profiles. To meet the reduced emission levels the company has adopted four technologies that can be adapted to the range of high speed diesels. These are in addition to the existing technology in the form of common rail fuel systems, advanced turbocharging and electronic engine control.
MTU has been using common rail fuel systems on its engines since 1996 when the 4000 series engines adopted such a system, resulting in a significant reduction in emissions. In 2003 when the larger 8000 series was introduced with a higher system pressure there was a reduction in soot particle emissions and for the future, MTU expects to use common rail systems with pressures of over 2000 bar combined with a multiple injection capability and injection rate shaping. Alongside these common rail developments there has been improved turbocharger design.
By using the multiple injection possibilities MTU claim that there can be a 50% to 70% reduction in the pressure gradient which will result in lower noise levels. It will also lead to a 30% to 50% reduction in soot levels.
One of the new technologies being introduced by MTU is the use of the Miller Cycle. This concept has been around for a long time and it uses a pattern of reduced opening and early closing of the inlet valve to achieve a reduction in the NOx emissions. In order to achieve satisfactory combustion with this system, the charge air pressure has to be increased through further improved turbocharger design and this was one of the reasons that it was not previously adopted. Trials have shown that a 30% reduction in NOx emissions can be achieved on engines using the Miller Cycle without any penalty in fuel consumption...
Another emission reduction technology is found in recirculating some of the exhaust gases. A bleed from the exhaust takes a portion of the exhaust gases through a cooler and then passes them into the intake manifold. For this system to work an improved injection system is required alongside increased charge air pressure but it is claimed that NOx emissions can be reduced by up to 50% by adopting this system.
These methods of emission control are incorporated into the engine itself but to meet the future emission standards, after-treatment of the exhaust will be required. In the MTU system this comprises two sections, a particle filter to remove the soot and a catalyst treatment unit. The MTU particle filter under trial is made from uncoated sintered metal which is claimed to remove up to 90% of the particles; this system is currently undergoing long term trials.
For NOx reduction the exhaust gases are passed through a washing system with an ammonia slip catalyst. On bench trials this has shown to reduce NOx emissions by up to 90% across the entire engine performance range except when idling; this system is also undergoing long term trials.
Two 12V 2000 Series MTU diesels have been installed on the ro-ro ferry Meersburg that operates on a route across Lake Constance (Bodensee) on the doorstep of the MTU factory. These engines without the exhaust treatment fitted already meet the stringent emission levels on Lake Constance. With exhaust treatment installed these engines are expected to meet the Tier 4 requirements that come into force in 2014.
The inland waterways vessel Willi Raab has had a 12V 4000 MTU diesel installed and is on a two and a half year trial under a sponsored scheme. The engine is fitted with a particulate filter that achieves a 90% reduction in emissions. Another inland waterways vessel is operating under the ‘Cleanest Ship’ research programme and operating on low sulphur diesel fuel this is showing major reductions in emissions over the whole range.
From these intensive trials using commercial vessels MTU has concluded that meeting IMO Tier 2 limits will be possible by using only internal engine measures but to meet the Tier 3 levels exhaust after treatment will be necessary. The company is confident that its engines will be able to meet the new emission limits, with extended field trials now underway designed to prove the long term viability of the systems. For the leisure market, where MTU has a major presence, these trials will lead to the development of compliant engines but there seems little doubt that the cost of engines will have to rise to meet the new standards. The traditional reliability of MTU diesels is not likely to be affected but the up-coming stringent emission standards will demand a new range of diesels for motor yachts over the next few years.
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