Emission Project Leads MaK Toward IMO III
Shared research provides clear mission and specific focus
According to a report from Caterpillar’s Marine Power Systems Group, the second phase of the Emission Minimization (EMI MINI), a joint research project to reduce marine engine emissions, is nearing completion. The first phase of EMI MINI I ran from 2002 to 2005 and was funded by the German government and involving a number of diesel engine industry companies including Caterpillar Motoren, AVL Deutschland, L’Orange, WTZ Rosslau and the University of Rostock.
Caterpillar said the first phase produced major building blocks for its MaK low emission engine (LEE) technology. The second phase, EMI MINI II, which runs from 2006 to 2009, has now almost been finished. This phase has placed special emphasis on optimizing the Caterpillar common rail (CCR) system to allow for MaK emission levels 50% below current IMO I regulations.
“The combined efforts of five experts in marine engine technology have borne fruit,” said Dr. Udo Schlemmer-Kelling, manager of research at Caterpillar Motoren in Kiel. “Beginning with comprehensive spray chamber investigations, we have not only improved the software tools to better understand the combustion process, but also set precise development targets for the injection system manufacturer, carefully verified results in a single-cylinder research engine and then combined it all in an advanced MaK LEE production engine. EMI MINI has proven that research and serial production can be linked in a highly efficient way,” he said.
In 2000, Caterpillar Motoren identified three emission levels for the MaK marine product to meet short- to medium-term emission regulations. These were a baseline IMO I engine, an IMO-compliant engine with invisible smoke emissions and a low-emission engine that meets the expected NOx emission range of IMO II and is also smoke invisible.
In addition, the strategy favored the enhancement of proven technology inside the engine, which Caterpillar noted has advantages in terms of cost, complexity and maintenance. Since then, IMO I-compliant engines have become the norm, more than 80 MaK marine engines rely on flexible camshaft technology (FCT) for invisible smoke, and the first MaK IMO IIcompliant engines are already in service at sea. Thanks to EMI MINI I, the NOx emissions of these LEE engines have been reduced by 30%, Cat said. To achieve emission levels as low as 50% below IMO I, however, additional research had to be carried out within the framework of EMI MINI II.
The University of Rostock contributed a detailed investigation of injection spray parameters on a special test bench, yielding important data on how to optimize injection components for different fuel qualities. The knowledge gained from this has also been used by AVL to develop, improve and verify calculation models in their CFD-FIRE software, enabling Caterpillar Motoren to calculateand predict the combustion processes of MaK marine engines. In this simulation, the influence of the nozzle flow on spray propagation and droplet breakup and thus on the mixture formation, combustion and emission formation was demonstrated.
This not only helps to reduce development time and costs, Cat said, but also allows it to view the chemical and physical processes inside the engine that cannot be achieved with current measurement techniques. The results of spray investigation and simulation led to improvements in the single-circuit common rail system used on MaK marine engines. L’Orange optimized the design of the electronically controlled injectors composed of the nozzle element, pilot valve and oilcooled solenoid. Special emphasis has been placed on multiple injection capabilities as a prerequisite for future emission reduction.
Further tests with different rail pressures, injection parameters, compression ratios and charge-air temperatures took place at WTZ Rosslau using a singlecylinder research engine. These investigations have shown that by combining certain NOx reduction measures with common rail capabilities, i.e., variable injection pressure, variable injection timing and multiple injections, a NOx reduction of 50% compared with IMO I can be achieved with only a slight increase in fuel consumption and filter smoke number, Cat said.
Parallel to the research, simulation and development efforts taking place at different locations, Caterpillar Motoren Kiel also carried out real engine tests on a MaK 6 M 32 C and a 6 M 20 C engine, both equipped with a Caterpillar common rail fuel system. On these engines, different combustion chamber designs and compression ratios were verified. The Miller Cycle strength was gradually modified and combined with various common rail trims.
For detailed investigations regarding functional safety, lifetime and wear of components, an additional test rig capable of marine diesel and heavy fuel oil operations was used. For transient operation, Cat considered it very important to avoid visible smoke emissions. The MaK engine concept for improved emission reduction combines the advantages of flexible camshaft technology and the Cat common rail fuel system to allow independent adjustment of the air and fuel systems during operation. The flexible camshaft technology system can switch the Miller cycle on and off, which influences the amount of trapped air in the cylinder. The common rail system can change the injection parameters during the acceleration process. As a result, smoke emissions are significantly reduced and remain below visibility at any given load.
The base line for all emission reduction measures was an IMO I serial production engine with a NOx cycle value of 11.7 g/kWh. Test bed trials with modified MaK 6 M 32 C and 6 M 20 C engines showed that the Miller Cycle is a suitable tool for even further reduction of the NOx. This causes the loss of peak cylinder pressure, which can be compensated for by an increased compression ratio, thus keeping efficiency con-stant. And when equipped with CCR and FCT, MaK marine engines can meet the requirements of IMO II without visible smoke, Caterpillar said. Even though IMO II with 30% less NOx has been achieved, it became apparent that NOx values of 50% below IMO I were not achievable without penalties in terms of efficiency. Injection system trims such as adapted rail pressures, timings and multishots, were able to reduce the NOx cycle value by only about 10% below IMO II (40% below IMO I). Trimming the engine to 6 g/kWh NOx (50% below IMO I), resulted in an efficiency drop of 2 to 3%. An increase in smoke emissions could be overcome by appropriate injection trims. However, to achieve the given target of EMI MINI II, it was found that additional research was required.
Caterpillar Motoren carried out additional development and simulations, and found higher compression ratios and a stronger Miller Cycle were the right tools for the NOx target of 6 g/kWh. But a stronger Miller Cycle requires a higher boost that can only be delivered by a twostage charging system. Also, for higher compression ratios, a redesign of the cylinder head would be needed. Smaller and, consequently, more valves provide the opportunity to reduce the valve lift without sacrificing engine breathing. A reduced valve lift permits an increase in the compression ratio without compromising the piston crown design.
Combined, these measures not only keep efficiency constant even at 6 g/kWh NOx, but also provide the basis for a further reduction to 2 g/kWh NOx, and thus to IMO III requirements, Cat said. The EMI MINI project underscored the potentil of the current MaK medium-speed marine engine design. Introducedin 1992, the MaK long-stroke engine family comprises the M 20 C, M25 C, M 32 C and M 43 C models. Caterpillarsaid with these latest findings, it has become clear that this platform isnow capable of meeting the IMO III emission limits not expected until 2016.