IMO TIER 3: Strategies and challengesEMISSIONS The reduction of the permissible sulphur concentrations coming into force in emission protected areas (ECAs) in 2015 will immediately be followed by the introduction of IMO Tier 3 emission stage in 2016. This stage requires a reduction of the NOx limit by 80% compared with the present level (IMO Tier 1). A number of different engine plant approaches are being studied with a view to complying with the future IMO Tier 3 NOx-limits and with the reduced SOx emission targets.

Bert Buchholz, Horst Harndorf, Christian Fink

The IMO Tier 3 limits require far-reaching changes in the technology of large engines. New basic solutions

and coordinated overall strategies are nec-essary to achieve these considerable NOx reductions without causing signifi cant dis-advantages concerning effi ciency and con-sumption.Measures for engine internal emission re-duction can be subdivided into the main areas of air path and fuel path. In addition, exhaust gas after treatment measures may be applied to force the remaining emissions below the limits. A coordinated, load and operation condition depending control of these measures will in future be indispensi-ble to comply with the limits and guarantee safe and effi cient engine operation simulta-neously. This will require complex engine

control systems and control strategies based on maps and physical models. Figure 1 shows the basic elements of an effi -cient and low-emission engines operation.A vital aspect is how to derive meaningful overall concepts from this wide range of individual measures in order to achieve an effi cient and IMO Tier 3 compliant engine operation.

The conventional approach: Use of SCR catalysts The installation of SCR catalysts on board ships has been promoted by the introduc-tion of emission-depending port dues in Sweden and the NOx tax in Norway. Opera-tion even below the limits set for IMO 3 can be achieved by the use of SCR catalysts.In order to stick to the emission limits valid inside the ECAs from 2016, it will be neces-sary to switch to a low-sulphur fuel in addi-tion to operation of the SCR catalyst. Outside the ECA areas, engines can be operated as normal IMO Tier 2 engines using sulphur-ous heavy fuel. As additional NOx reduction is not necessary outside the ECAs, the SCR can be bypassed. Fuel injection systems and air charge groups can stay on the technological level of the IMO Tier 2 strategies, meaning basically a Common-Rail-System with single injection (main injection) and a 1-stage charging with highly effi cient TC. The advantages of SCR-based IMO Tier 3 strategies are the use of established engine and exhaust gas af-ter treatment technologies. Engine internal NOx reduction can be dispensed with due to the high NOx conversion rates of the SCR catalyst.The obvious advantages of an SCR-based concept need to be compared with the in-vestment and operating costs for the SCR catalyst and the urea solution. Using low-sul-phur distillate fuels within the ECAs also has a negative impact on the overall operating costs. The space required for the SCR catalyst and the bunkering of the reduction means

may also present a disadvantage depending on the ship concept.

Exhaust gas recirculation strategies to comply with IMO Tier 3 A strategy for a marked reduction of NOx emissions which is new to the marine die-sel engine industry is the high-load EGR. As has been learned from the Heavy Duty Truck engine industry, using EGR can help to force the NOx raw emissions below the limit of 2 g/kWh. In connection with the operation on low-sulphur fuels inside the ECAs, an introduction of EGR in marine diesel engines can become operational for the fi rst time. An EGR-based IMO Tier 3 strategy within the ECAs will require the installation of an EGR system at the engine. The sulphur ox-ide emissions will be limited by the use of low-sulphur fuels which are also binding for a successful EGR operation.Outside the ECAs, the EGR path can be closed and the engine can be operated on heavy fuel as a standard IMO Tier 2 engine. The advantages of an EGR-based strategy are comparatively low investment costs despite the higher costs for engine and charge air system. The additional space required is not to be neglected, but comparatively low. Ad-ditional operating costs are caused by the use of low-sulphur distillate fuels when sailing in the ECAs. On the other hand, there are a number of challenges. As the amount of recirculated exhaust gas forms a considerable part of the cylinder fresh load, a signifi cant increase of the charging air pressure will be required. This requires the installation of complex 2-stage turbo-charging systems with cor-responding charge air coolers and control measures. The EGR mass fl ow has to be cooled and controlled, too. The high EGR rates required to obtain the IMO Tier 3 lim-its cause a marked increase in particle emis-sions, which will place increased demands on the injection system. Compared to

Figure 1: Basic elements of an effi cient and low-emission engine operation (above), coordination of possible emis-sion reduction measures

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present large diesel engine Common-Rail-Systems, the injection pressures will have to be increased and multiple injection strate-gies will have to be considered (especially combinations of main and post injection). Outside the ECAs, the injection system will have to continue providing a normal IMO Tier 2 operation with heavy fuel while EGR is switched off. The complete system re-quires complex engine control units.There is still a large demand for research and development before these technologies can be put into series production.

The Cleaner: Complex exhaust gas after treatmentAs an alternative to the use of low-sulphur fuel in the ECAs, the IMO also allows the use of sulphuric fuels when due to suitable meas-ures the sulphuric oxide emissions have been reduced so that they correspond to an engine operation with low-sulphur fuel (maximal 0.1% sulphur). This has led to a newly in-creased discussion of and research on the possibilities of using scrubbers to eliminate sulphur dioxides from the exhaust stream.A complex exhaust gas after treatment strat-egy for IMO Tier 3 compliance can be based on a combination of SCR catalysts (NOx-reduction) and SOx-scrubbers. When operat-ing ships within the ECAs, this will keep the NOx and the sulphur dioxide emissions be-low the limits, even when using conventional sulphuric heavy fuels. Outside the ECAs, the engine exhaust gases can bypass the exhaust after treatment system under continuous en-gine operation on heavy fuel.Apart from the low fuel costs, another advantage of the strategy will be the pos-sibility to present extremely low emission levels. The engine technology can be car-ried out comparatively simple (IMO Tier 2 level). With an optimal design and opera-tion of the exhaust after treatment plant, the emission levels can be kept below the IMO Tier 3 levels.These very important advantages are, how-ever, confronted by some challenges. So the space required for the exhaust after treatment system is considerable: apart from the SCR catalyst and the urea bunker, a bulky exhaust gas scruber and large stores for new and used scrubbing material (wet or dry) have to be provided. In addition to the considerable investment costs, the op-erating costs have to be taken into account, because the SCR as well as the scrubber require consumables for their operation. Current analyses show that, due to the continuous heavy fuel operation, relatively short amortisation times may be reached. However, this requires a correct adaptation of the complex exhaust after treatment sys-tems to the engine plant, the engine opera-tion profi le and an optimal system control. A sensible integration of all exhaust

Figure 2: Layout principle of a SCR-based IMO Tier 3 strategy

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after treatment elements into the engine room seems to be feasible only with new-building projects.

Alternative solution: Gas operation An interesting and technologically proven alternative is the conversion of the engine plant to gas operation.Using natural gas as fuel, at least in the ECAs, would help to fulfi l all IMO require-ments set for 2016. As natural gas is sul-phur-free the exhaust gas will be so, too. An additional positive effect is the drastic reduction of particle emissions. Apart from these advantages, this strategy will make the exhaust gas after treatment obsolete. Designing the engine for dual fuel opera-tion will permit the conversion to heavy fuel operation outside the ECAs. Diffi culties may be caused by placing the vo-luminous gas tank. Furthermore, infra-struc-tural conditions have to be created allowing the bunkering of LNG in normal port areas.

Latest Common-Rail injection tech-nology and complex engine control: challenges and keys to successIn all the strategies presented above, fuel in-jection and mixture formation play a funda-mental part in complying with IMO Tier 3 limits. The various strategies described show a number of challenges for the development of new injection systems and their control. Thus, CR injection systems will form an es-tablished part of marine diesel engines in the next years.Especially in the EGR-based strategies, high EGR rates will lead to increasing fuel con-sumption and growing particle and soot emissions. Experience gained from large high-speed engines, however, show that a no-ticeable reduction in particle and soot emis-sions can be obtained by increased injection pressures or by multiple injection strategies, consisting of main and attached post injec-tion. In the latter case, the application of injection strategies at common-rail systems

shows various features that have to be taken into account when developing new engines.Looking at the injection rates of a main and a post injection measured at a mod-ern heavy-fuel common-rail system of a research engine, an infl uence of the main injection on the following post injection can be noticed. This is caused by the pres-sure waves generated by the main injection. These form especially between the injector and the high pressure rail. As can be seen in Figure 6, there are sharp differences in the injection rates depending on the posi-tion of the post injection in relation to the main injection. Whereas the main injec-tion is not infl uenced, the rates of the post injection clearly vary depending on dwell times; a fact that has to be taken into ac-count when developing and analysing new injection strategies.

Research work at LKV (Chair for Piston Engines and Internal Combustion Ma-chines) of the University of RostockThe Professorship for Piston Engines and Internal Combustion Machines at the Uni-versity of Rostock (LKV) has traditionally been actively working in the fi eld of ma-rine diesel engines.The subject of the joint projects EMI-MINI I and II was a systematic research into fuel sprays in large, needle-controlled Common-Rail injectors using distillate and heavy fuels. To allow optical measurements of the fuel sprays under engine-relevant conditions, the CR injectors are mounted at an optically accessible high-pressure, high-temperature chamber specifi cally built for this purpose. Figure 7 presents an example of an injection sequence of an evaporating injection spray that is visualised by means of a Combined Schlieren Scatter Light procedure. The re-search carried out yielded important funda-

Figure 3: Basic layout of an EGR-based IMO Tier 3 strategy

Figure 4: Basic layout of a IMO Tier 3 strategy based completely on complex exhaust gas after treatment

Figure 5: Basic layout of a gas-operation based IMO Tier 3 strategy

Figure 6: Injection rates and line pres-sure at injector inlet for main and post injections depending on dwell times

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mental fi ndings for making full use of the potential of modern common-rail systems.Engine internal particle emission reductions obtainable by the use of CR-systems provide additional space for NOx-reduction meas-ures, such as the Miller process or EGR.The heavy-fuel one-cylinder research engine used at LKV is equipped with a modern CR injection system and a freely-programmable engine control unit. In addition, the engine was fi tted with relatively large optical win-dows to allow the analysis of injection, igni-tion and combustion processes at real engine conditions. The object of these analyses to in-crease understanding of the emission forma-tion processes inside the engine, especially against the background of extremely varying fuel properties. Apart from the examination of emission-reducing measures, research at LKV focuses especially on developing new engine control strategies.Furthermore, in cooperation with the start-up enterprise FVTR GmbH, a number of catalyst test stands are being operated. They allow detailed analyses of the catalyt-ic reaction processes in all relevant catalyst and scrubber designs. In addition to a di-rect optimisation of the catalysts analysed, combined physical and chemical model-ling approaches can be developed.

SummaryDepending on the type of ship, the trade route and the strategies of the ship operators, the principle-based advantages and disadvantag-es of the different strategies may be evaluated differently. Therefore, it is quite likely that in the remaining time span until 2016 different strategies may appear on the market. A successful introduction of the EGR-based strategy on the basis of low-sulphur fuels may set free an immense potential for en-

gine-internal NOx reduction. The strategy based on extensive exhaust gas after treat-ment and the use of SCR and SOx-scrub-bers will allow extremely low emissions even when using low-cost heavy fuel.All strategies discussed here have in com-mon that engines and plants have to be op-erated at rather different modes within and outside the ECAs. Combined with this fact, the demands for improved engine control will increase sharply. Most strategies in-clude a frequent change in fuels consumed, which renders this task even more diffi cult. It will make the introduction of new en-gine control strategies indispensible, but it will also require the development of com-pletely new control systems adapted to the operation of marine engines.

The marine diesel engine, this uniquely reliable and effi cient source of propul-sion and onboard energy has by no means reached the end of its development. It has the potential for rigorously reduced emis-sions at continued high effi ciency.

The authors:Dr.-Ing. Bert Buchholz, Forschungszentrum Verbrennungs-motoren und Thermodynamik Rostock GmbH, Rostock, Germany Prof. Dr.-Ing. Horst Harndorf, Dipl.-Ing. Christian Fink, Universität Rostock, Lehrstuhl für Kolbenmaschinen und Verbren-nungs motoren, Rostock, Germany

Figure 7: Analysis of fuel sprays by means of a combined Schlieren/scatter light bypass meth-od, simultaneous recording of liquid and gaseous fuel phases at a common-rail injection spray

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