PAPER NO.: 139 IMO III Emission Regulation: Impact on the Turbocharging System

CONSEIL INTERNATIONAL DES MACHINES A COMBUSTION INTERNATIONAL COUNCIL ON COMBUSTION ENGINES PAPER NO.: 9 IMO III Emission Regulation: Impact on the Turbocharging System Ennio Codan, ABB Turbo Systems Ltd, Switzerland Simone Bernasconi, ABB Turbo Systems Ltd, Switzerland Hansruedi Born, ABB Turbo Systems Ltd, Switzerland Abstract: Several internal engine measures in combination with advanced turbocharging have been considered for fulfilling the IMO II regulation by the beginning of 0, which requires a NO x reduction of 0% compared to IMO I. In order to fulfill the requirement of the IMO III regulation which will be effective as of 06, a major decrease of the specific NO x emissions (about -80% compared to the IMO I values) needs to be achieved in the ECA s (Emission Control Areas). Sulphur-oxides (SO x ) and particulate matter (PM) emissions are to be controlled by limiting the sulphur content of the fuel used. An alternative measure is the use of SO x abatement equipment such as sea water scrubbers, fresh water scrubbers or a dry exhaust gas cleaning system. For IMO III either external measures (aftertreatment technologies) or a combination of internal engine technologies are required. This paper provides an overview of IMO III solutions with regard to NO x reduction measures and their impact on the engineturbocharger system considering both, -stage and - stage turbocharging. From a selection of possible solutions, two high potential NO x reduction technologies, namely SCR and EGR, have been chosen to be studied in greater detail. Selective Catalytic Reduction (SCR) is a proven technology that basically allows any engine to fulfill IMO III. Nevertheless some configurations require SCR to be installed before the turbine (-stroke engines, -stage turbocharging), which affects the transient operation. The impact on the system and an evaluation of several counter-measures is provided based on transient simulations. Exhaust Gas Recirculation (EGR) is an established NO x -reduction technology in the automotive sector, but is not yet state-of-the art for large engines. An evaluation of several strategies with regards to NO x - reduction, fuel consumption, and other relevant parameters shows the potential and the advantages of recirculating exhaust gases. A further challenge for the turbocharging system is the necessity to provide a variability which allows an engine to fulfill the low emission limits within the ECA s while running elsewhere with the highest fuel economy. c CIMAC Congress 00, Bergen

INTRODUCTION In the coming years marine emission limits for marine engines will be significantly reduced. See Fig. for NOx and Fig. for sulphur. NOx in g/kwh 8.0 6.0 4.0.0 0.0 8.0 6.0 4.0.0 IMOI (..000) IMOII (..0) IMOIII (..06 in ECAs) 0.0 0 50 500 750 000 50 500 750 000 50 Engine speed in rpm Fig. - NOx emission limits for IMO Tier I, II and III, [] S in mass fraction 5.0% 4.0%.0%.0%.0% Global sulphur limit level In Emission-Control-Areas (ECAs) 0.0% 000 005 00 05 00 05 Year Fig. - Equivalent sulphur mass fraction in the fuel, [] IMO Tier II reduces the current IMO Tier I limits on NOx by about 0%, and will be in force from 0. The target NOx values can be achieved with internal measures requiring moderate to slight changes in the engine design. For IMO Tier III the current limits (IMO Tier I) will be reduced by 80%. Either external measures (aftertreatment technologies) or a combination of internal engine technologies are required. This paper first provides an overview of IMO Tier III solutions with regard to NOx and SO x reduction measures. Then, from a selection of possible solutions, two high potential NOx reduction technologies, namely SCR and EGR, have been chosen for study in greater detail. SO x reduction technologies IMO legislation enforces a reduction in emissions of oxides of sulphur. The SO x limits can be fulfilled by reducing the sulphur content in the fuel, or by removing SO x in the exhaust gases (scrubbing). Operating the engine on low sulphur fuel allows compliance with emissions legislation with only slight (low sulphur diesel fuel) or moderate (natural gas) changes to engine design. However, switching to distillate fuels has an impact on the operating costs. Furthermore, oil refineries require large investments and an appropriate timespan to convert residual heavy fuels (HFO) into distillates. See [] for more details. A different approach is to remove the sulphur from the exhaust gases after combustion using scrubbers. There are two main categories of scrubber: dry and wet. Wet scrubbers wash the exhaust gases with alkaline water. Either seawater or fresh water with an alkaline additive can be used. Dry scrubbers remove sulphur without using water, with the advantage that the exhaust gases are not cooled (boilers and SCR may be installed after a dry scrubber but not after a wet scrubber). NOx reduction technologies Basically, NOx reduction strategies may be divided into two categories: external and internal. External measures reduce the oxides of nitrogen in the exhaust gases catalytically while internal measures reduce the formation of NOx at source during the combustion process. Due to the limited NOx reduction potential of some internal measures and the strict limits of the IMO Tier III legislation, some of internal measures may need to be applied in combination. Selective Catalytic Reduction (SCR) employs ammonia (NH ) to reduce NOx catalytically. To avoid handling issues, ammonia is not directly injected but produced from thermolysis of a ureawater solution. SCR is a proven after-treatment technology that basically allows any engine to fulfil IMO Tier III (90% reduction according to []). If the reactor is placed after the turbocharger turbine, the impact on engine design and operation is very low. However, it has major effect on the transient operation of the engine if placed before the turbine. Exhaust Gas Recirculation (EGR) involves mixing a portion of the exhaust gas into the intake air. This decreases the adiabatic flame temperature (see Fig. ) as the combustion gases are diluted with inert gases (lower O concentration). A minor further effect is produced by the higher heat CIMAC Congress 00, Bergen Paper No. 9

capacity of the charge air due to increased concentration of CO and H O. The technology is established in the automotive and truck segments, but is not yet state-of-the art for large engines. Exhaust Gas Recirculation 000 T [K] 000 000 0 80-90 0 90 ϕ ϕ [ KW] [ KW] 8 Fig. - Impact of EGR on the combustion flame temperature Two stage turbocharging combined with the Miller Cycle and retarded onset of combustion (late fuelinjection or ignition) reduces process temperature in the cylinder (see Fig. 4). It has the potential to achieve about a 70% NOX reduction at unchanged fuel consumption compared to the IMO Tier I values [4], [5]. According to [6], an 80% reduction in NOx may be achieved if slightly higher fuel consumption can be accepted. This needs, however, to be tested experimentally. Miller Timing 000 T [K] 000 000 0-90 0 90 ϕ - ϕ [ KW] [ KW] 8 Fig. 4 - Impact of Miller timing on the combustion flame temperature Wet methods have a similar effect as EGR. Additional water in the combustion chamber is a valid method of reducing combustion temperature (water vaporization energy, higher heat capacity of the charge air, dilution effect due to lower O concentration). There are at least three methods to achieve higher water concentration in the charge air: () humidification: it consists of water-mist injection in the intake receiver (up to 60% NOx reduction reported in []); () direct water injection: it consists of injecting water directly into the cylinder (up to 50% NOx reduction reported in []); () Fuelwater emulsion: involves emulsifying water to the fuel (up to 50% NOx reduction reported in [7]). SCR SCR reactors installed after the turbocharger have, aside from some pressure loss, minimal or no impact on engine design and system operation. For some application, however, SCR needs to be installed before the turbine stage. This has a major impact on transient engine operation. In this section, the operation and design of an SCR reactor is briefly described. Thereafter the operation of a single stage turbocharged -stroke engine with SCR before the turbine and of a -stage turbocharged 4-stroke engine with SCR between the two turbine stages was investigated on the basis of transient engine simulations. Application limits Conventional SCR reactors operate best in the temperature range from 00 C to 450 C. In this range reactivity is high, urea thermolysis is good, and neither special high nor low-temperature components are required. For temperature above 450 C increasing urea oxidation needs to be taken into account and high temperature components may be required. While surface reactivity is lower for lower temperatures (down to 00 C-0 C), a low temperature reactor may be required and the urea injection system needs to be designed very precisely to guarantee good thermolysis and no deposits in the exhaust manifold. Sulphur has a major impact on SCR operation. The SCR system needs to be designed according to the exhaust temperature and the sulphur content of the fuel, and located differently within the exhaust system. NH reacts with sulphur trioxide (SO ) to form ammonium (bi)sulfate. This is not permanently damaging the catalyst materials and the process is reversible as the temperature increases. However, this glutinous substance masks the reactor surface, hindering NOx reduction and the reactor may clog in a relatively short time. Fig. 5 provides an overview of SCR operating temperature with regard to Medium-Speed (MS) and Low-Speed (LS) engines, depending on the sulphur mass fraction of the fuel. Even with low sulphur fuel, the temperature at the turbine outlet on the MS engine with stage turbocharging and the LS engine is too low to operate SCR after the turbines. Fig. 5 is the outcome of an internal study condensing the information of several sources. CIMAC Congress 00, Bergen Paper No. 9

Exhaust temperature in C 500 400 00 00 ECAs 05 ECAs 00 ECAs 000 Increasing ammonia oxidation T TI,LP MS St Open sea 00 T T TO TO,LP MS St MS St Ammonium (bi)sulphate formation Open sea 0 T TO LS St 00 0.0%.0%.0%.0% 4.0% Sulphur mass fraction in % Open sea 000 Fig. 5 - Allowable operation range of the SCR reactor depending on temperature and sulphur content. Legend: Medium Speed (MS), Low Speed (LS), -stage turbocharging (St), -stage turbocharging (St), turbine inlet temperature (TTI), turbine outlet temperature (TTO) Size of the Reactor A reduction of ceramic volume reduces the investment costs, the space requirement and the thermal mass in cases where SCR is installed upstream of the turbine. Several parameters affect the required design size of a SCR reactor, namely: Targeted NOx reduction level combined with the permissible ammonia slip. Catalytic reactivity, depending on the catalytic material and the exhaust gas temperature. Exhaust gas pressure and temperature. For a given mass flow, the residence time inside the catalyst is directly proportional to gas density. For example, if the SCR is placed before the turbine with pressure of about bar, the required ceramic volume will be about times smaller. SCR Thermal Model To simulate the thermal behaviour of an SCR reactor, a thermal model considering ceramic and metal thermal mass has been set up based on design parameters from a real -stroke marine engine plant and calibrated using measured data. Note: the temperature increase due to the chemical reaction in the SCR has been ignored. The enthalpy peak of the exhaust gases is stored by the thermal mass of the SCR, and will be returned with a time delay, damped and diffused over a longer time scale. The thermal response to a step function (considering constant mass flow) of the SCR used for the stroke investigation is shown in Fig. 6. For a stage turbocharged 4- stroke engine, the thermal model has been scaled, based on design data from a concept design. T [ C] 550 500 450 400 50 T SCR in T SCR out 00 0 50 500 750 000 50 500 750 000 Time [s] Fig. 6 - Step response of the SCR thermal model Single stage turbocharged stroke engine Assuming that stroke engines are being operated with fuel containing 0.% or more sulphur, the SCR needs to be placed where temperatures are above the formation limit of ammonium (bi)sulphate, As shown in Fig. 7 this is the case upstream of the turbine. The size of the cells. This determines the contact surface per catalyst volume, and slightly affects the diffusion process of NOx and NH inside the ceramic channels. For large engines the size of the cell is often limited by dust and particulate deposits. Especially with HFO, larger cells are required due to ash deposition (even if deposits are regularly blown out with compressed air). CIMAC Congress 00, Bergen Paper No. 9 4

Temperature in C 500 C 400 C 00 C 00 C 00 C TTI TTI, Blw on TTO TTO, Blw on 0% 0% 40% 60% 80% 00% Load in % Fig. 7 - Typical temperature profile at turbine inlet and outlet for a -stroke engine. At lower load with auxiliary blowers. Legend: turbine inlet temperature (TTI), turbine outlet temperature (TTO) Oscillations in the engine-turbocharger system (also known as hunting) due to the thermal mass of the SCR have been observed during operation on stroke marine engine. The phenomenon may be described as two processes that are repeated in a loop: investigate the potential of different control strategies, transient engine simulations have been performed using a Mean Value Engine Model (MVEM) calibrated in Matlab/Simulink. Taking account of the engine topology as shown in Fig. 8 with a catalyst volume of about 0. m /MW, the investigation has been divided into three sections: Investigation at constant load Investigation during load step/rejection Investigation of control strategies Since switching between ECA and non-eca mode does not have a major impact on engine and turbocharger operation, it has not been taken into account for the investigation. Air receiver CMP (a) T SCR,out < T SCR,in : exhaust gases are cooled As the reactor cools the exhaust gases, thus reducing the turbine power, boost pressure and engine scavenging are reduced. This increases the engine s thermal load and hence the temperature at exhaust outlet. Unfortunately, the high enthalpy exhaust gases do not reach the turbine; their energy accumulates in the SCR. A hot wave moves toward the reactor outlet and will be released with a time delay, leading to the phenomenon described in (b). (b) T SCR,out > T SCR,in : exhaust gas are heated As the reactor heats the exhaust gases, so increasing turbine power, boost pressure and engine scavenging increase. The engine s thermal load and the temperature at the engine outlet are consequently decreased. The low-temperature exhaust cools the SCR, a cold wave moves toward the reactor outlet and will be released to the turbine with a time delay leading to the phenomenon described in (a). The reciprocal action of (a) and (b) will cause oscillation of exhaust outlet temperature, turbocharger speed, boost pressure, and will possibly cause an engine shutdown due to insufficient scavenging. Variability in the turbocharging system is nowadays required for reliable operation of marine -stroke engines with SCR before the turbine. For a better understanding of the phenomenon and to Exhaust receiver SCR TUR Fig. 8 - -stroke engine with SCR before turbine Simulation at constant load At constant load, ideally, no oscillations should occur. However, during real operation, slight changes in exhaust temperature may induce small oscillations. Depending on the operating point, they may be either damped or amplified. To investigate this phenomenon, transient engine simulations were performed at different loads. The results for 50% and 70% load are shown. After specifying perfect initial conditions, a disturbance was created artificially by cooling the gas at the turbine inlet by C for a few seconds, see Fig. 9. A stable system response is calculated on the basis of an engine without SCR at 50% load (red curve in Fig. 9), the small oscillations induced are damped by the system itself. The same behaviour can be observed at 70% load for a system with SCR (dark green curve). Unstable system behaviour is observed at 50% load when considering a system with SCR. The slight oscillations induced are amplified until engine thermal load becomes too high due to insufficient scavenging. CIMAC Congress 00, Bergen Paper No. 9 5

T eng,out in C 500 450 400 50 00 no SCR, load 50% SCR, load 50% SCR, load 70% 50 0 00 400 600 800 000 00 400 time [s] Fig. 9 - Transient simulations at constant load. The exhaust gases at turbine inlet have been cooled by C for a few seconds at the time t=00s inducing oscillations in the system. During load step/rejection the gas enthalpies at the engine outlet changes significantly. This induces high amplitude oscillations in the engineturbocharger system. To investigate the stability of the system, transient simulations for a 50%-70% load step were performed assuming a load rate-ofchange of % per second (the 0% load step is consequently achieved within 0 seconds). The impact of the SCR is significant as Fig. 0 indicates. The engine thermal load increases dramatically and large amplitude oscillations are induced. The system is still stable and at 70% load the oscillations are damped. This would not be the case at lower loads where the oscillations are amplified (see 50%-load curve with SCR in Fig. 9). T eng,out and T TI in C 500 450 400 50 SCR, T eng,out SCR, T TI no SCR, T eng,out and T TI 00 0 00 400 600 800 000 time [s] The size of the SCR plays a major role in determining transient behaviour. Fig. shows that a reduction in SCR size decreases both the period and the amplitude of the oscillations. The maximum temperature at exhaust outlet (being an indicator for engine thermal load) is reduced only if the temperature at the SCR outlet starts to increase before the load step is completed (for this specific case Δ Load = 0% time = 0 seconds). Basically, the time constant of the SCR needs to be smaller compared to the time required for the load step. For the case considered, to achieve lower thermal load the SCR should be specified considerably smaller, which is nowadays not realistic. T eng,out [ C] 500 450 400 50 00% SCR mass 50% SCR mass 0% SCR mass no SCR 00 00 00 00 400 500 600 700 800 time [s] Fig. - 50%-70% load step. Impact of SCR size Oscillation Damping Simulations for operation at constant load and with changed load (see section 0), confirmed that a variability is required in the TC system to ensure stable and safe engine operation if SCR is installed before the turbine on a -stroke engine. Two possible control strategies have been investigated (see Fig. ) namely: A) Combination of Air Waste Gate (AWG) with SCR B) Variable Turbine Geometry (VTG) Fig. 0-50%-70% load step. Engine outlet and turbine inlet temperature considering configurations with and without SCR To investigate the sensitivity of the system with respect to the size of the SCR system, load steps considering smaller SCR reactor sizes have been computed. CIMAC Congress 00, Bergen Paper No. 9 6

AWG Air receiver Exhaust receiver BP SCR CMP TUR VTG Fig. - Variability that have been considered in the TC system The control strategy for A) has been defined so that the valve and the air wastegate can not be operated simultaneously. Case : the SCR cools the exhaust gases. The SCR valve is opened as the boost pressure is too low, reducing the enthalpy differences between engine outlet and turbine inlet. Case : the SCR heats the exhaust gases. The air wastegate is opened as the boost pressure is too high, preventing the exhaust gases from being too cold. Simulation showed that with SCR- and air wastegate the oscillations can be damped effectively and stable engine operation is achieved in a very short time, see Fig.. As some of the exhaust gases es the SCR reactor: NOx cannot be reduced in the SCRed mass flow. This may be critical for operation close to harbour areas, where rapid load changes for manoeuvring and simultaneously low NOx emission are required (NOx not to exceed limit values). The turbine inlet temperature is higher compared to the SCR outlet temperature. This improves scavenging and decreases engine thermal load (compared to a configuration without variability). Control strategy B) uses VTG to control boost pressure. Simulations confirmed that VTG is able to damp the oscillations and guarantee safe engine operation, see Fig.. The exhaust gases flow completely through the SCR, which has the following consequences: It allows nominal NOx reduction even during rapid load changes. T eng,out in C inlet. Higher engine thermal load cannot be avoided. This may be critical at part load, where small changes in the turbine inlet temperature may lead to insufficient engine scavenging, or during large load step. With VTG, additional energy needs to be supplied to the turbocharging system, especially for part load operation. This could be achieved by, for example: () larger and improved auxiliary blowers able to operate up to about 50%-60% load; () a burner installed after the SCR to increase the turbine inlet temperature; () Power Take In (PTI). 500 450 400 50 00 no SCR SCR SCR, BP & AWG SCR, VTG 00 400 600 800 000 time [s] Fig. - Temperature at engine outlet during a 50%-70% load step. Impact of control strategies on the transient response Discussion Simulations confirmed that a single stage turbocharged -stroke engine with SCR before the turbine requires variability to operate safely. Simulations with SCR combined with an air wastegate, or variable turbine geometry showed that stable engine operation can be achieved. SCR and air wastegate are, according to simulations, very effective in controlling oscillations and engine thermal load. However, as the is opened NOx reduction capacity is limited, which may be critical in ECAs. The VTG is, according to simulations, very effective in guaranteeing stable engine operation and allows nominal NOx reduction even during load changes. However, it requires further measures which increase engine scavenging at part load and during load steps. Since SCR cools the exhaust gases it is not possible to increase temperature at turbine CIMAC Congress 00, Bergen Paper No. 9 7

-stage turbocharged 4-stroke engine Considering -stage turbocharged 4-stroke engine, the optimal operating temperature for an SCR reactor is to be found between the two turbine stages, see Fig. 4. Temperature in C 600 C 500 C 400 C 00 C 00 C HP turbine inlet LP turbine inlet LP turbine outlet 0% 0% 40% 60% 80% 00% Load in % Fig. 4 - Temperature profile of a -stage TC engine To investigate the impact on system transient behaviour with SCR between the two turbine stages (see Fig. 5) simulations have been performed with a Mean Value Engine Model (MVEM) calibrated with steady state simulation results using the SiSy software ([8] for references). The SCR reactor model has been reasonably scaled according to a concept design. Air receiver Exhaust receiver CMP SCR CMP Fig. 5 - -stage turbocharging with SCR between HP and LP turbine stage. Simulation cases To investigate transient behaviour with SCR between the two turbine stages, the %-66% load step for an auxiliary engine with G-rating according to [9] has been considered. Three simulation cases have been computed, namely: TUR TUR Results As the exhaust gas temperature at the SCR inlet changes, the temperature at the SCR outlet remains constant for a time because of the heat capacity of the ceramic core. This has two main effects:. The exhaust temperature peak is delivered to the LP-turbine with a time lag and diffused over a longer timescale.. If the steady-state exhaust temperature at LP-turbine outlet is different before and after a load step, the temperature at the SCR-outlet remains constant until the whole ceramic core is heated/cooled to the new temperature. Considering a %-66% load step and a 66%-% load drop, Fig. 6 and Fig. 7, there is a large difference in exhaust gas temperature between HPturbine outlet and LP-turbine inlet. The two effects described above (temperature peak and temperature difference before/after the load step) overlap. Due to the higher LP-turbine inlet temperature, the speed of the LP-compressor increases in the second phase of the load step or decreases much more slowly during a load drop. This has the result that the LP-compressor operates closer to the surge line. T TO, HP & T TI, LP in C 900 700 500 00 No SCR, T TO, HP & T TI, LP SCR, T TO, HP SCR, T TI, LP 00 0 40 60 time in s Fig. 6 - %-66% load step, comparison of temperature at the SCR inlet/outlet for operation with and without SCR %-66% load step, Fig. 6 & Fig. 7 66%-% load drop, Fig. 7 %-66% load step with variation of the SCR size, Fig. 8 CIMAC Congress 00, Bergen Paper No. 9 8

Π LP [-] 5 4.5 4.5.5.5 66%-% No SCR SCR LP stage V 98,LP [m /s] %-66% No SCR SCR Π HP [-].5.5 HP stage V 98,HP [m /s] According to simulations the system is stable, and all the oscillations are damped for any load and SCR size investigated. The minimum engine air excess ratio (and hence the maximum thermal load) during the load step is not really affected by the SCR. During load step and load drop the LP-compressor operates closer to the surge line, hence it may be necessary to install a compressor (or blowoff valve), depending on: () the size of the SCR; () the minimum surge margin required; () the time response requirement for load drop; (4) the compressor map width. EGR Fig. 7 - %-66% load step and 66%-% load drop, compressor operation To investigate the sensitivity of the system with respect to the size of the SCR (see Section 0), the load step %-66% has been re-computed considering double the size and half the size of the SCR reactor. The smaller the reactor, the more the transient response becomes similar to the reference case, while for a bigger SCR the time required to reach equilibrium is much longer, see Fig. 8. The system is, however, stable at any load and SCR size and no variability is required to damp the oscillations. T TI, LP in C 600 550 500 450 400 50 No SCR, T TI, LP 00% SCR, T TI, LP 00% SCR, T TI, LP 50% SCR, T TI, LP 00 0 40 60 time in s Fig. 8 - %-66% load step, impact of the SCR size Discussion SCR has an impact on the transient response as it acts like a lowpass filter with transport delay for the LP-turbine inlet temperature. However, the impact on the system is not as critical as for a -stroke engine. EGR systems often have a corresponding impact on engine design and the specification of the turbocharger. In this section, some of the most common EGR recirculation strategies are described. Then, considering a single stage turbocharged -stroke engine and a -stage turbocharged 4-stroke engine, a more detailed investigation based on simulations with the SiSy software ([8] for references) will be discussed in detail. Recirculation strategies To recirculate the exhaust gases there are several strategies. In the following section some of the most common are described. Internal versus external EGR Exhaust gas recirculating strategies may be divided into internal EGR and external EGR. Internal EGR is achieved with appropriate valve timing, such that some of the exhaust gases are retained in the cylinder. This is relatively simple to achieve, and no further components are in contact with the EGR gas. However, it is difficult to cool the exhaust gas, hence more EGR is required for the same NOx reduction level. The reduction potential is about 0%. External EGR recirculates the exhaust gases outside of the cylinder. Since the exhaust gases can be cooled, it has higher NOx reduction potential. Special care should be taken for those components on the air side, which are in contact with the recirculated gases. Further, additional equipment may be required to recirculate the gases. The topic will be discussed with more detail in the next section. CIMAC Congress 00, Bergen Paper No. 9 9

EGR cooling requires higher engine water pump capacity. The related power consumption has not been considered for the investigations described in this paper. Low pressure versus high pressure EGR External EGR may be divided into low-pressure and high-pressure EGR. Low pressure EGR recirculates the exhaust gases at ambient pressure (after the turbocharger). It is relatively simple to design and the EGR rate can be readily controlled, but the components (coolers and compressors) need to be designed to operate with exhaust gas. Thermodynamically it is not optimal as the inert gases are recirculated through the whole turbocharging system and the specific compressor work is higher due to an increase in compressor inlet temperature. Further, a degradation of component efficiency in the EGR-line has a direct impact on system efficiency, while for high pressure EGR the impact is marginal (main turbochargers operate with fresh air). The reliability of the system is partially affected, as a failure of a component in the EGR line is also a failure of a component in the main line. High pressure EGR recirculates the exhaust gases at boost pressure (before the turbocharger). In contrast to automotive engines, the pressure difference across the engine is positive (i.e. the exhaust gas pressure is lower than the boost pressure), which increases the degree of system complexity. The system requires a device to pump the exhaust gases into the intake receiver. One big challenge of high pressure EGR systems is the need to provide the variability which allows an engine to fulfil the low emission limits within the ECA s while running at highest fuel economy elsewhere. High pressure EGR has a direct impact on mass flow through the turbocharger and thus on the specification of the flow components. EGR pumps There are several strategies for pumping the exhaust gases from the exhaust receiver to the intake receiver (EGR-intake valve for Miller Cycle engines, EGR-exhaust valve, donor cylinders, ) but in this paper only those configurations with blower and EGR turbocharger have been investigated in greater detail. and due to the electrical power required, the impact on system fuel economy is considerable. An EGR turbocharger is basically an EGR blower powered by a power turbine. The main effects on the system are: () improved system fuel efficiency because no external energy is required; () decreased turbocharging efficiency because the EGR-turbine reduces mass flow through the main turbine. One of the main challenges to be met with an EGR-turbocharger is the mismatch due to the difference of turbine and compressor pressure ratio and mass flow. Single stage turbocharged -stroke engine In this section the operation of a -stoke engine with EGR has been compared with an EGR-blower and an EGR-turbocharger, see Fig. 9 and Fig. 0. Air receiver Exhaust receiver M CMP TUR Fig. 9 - Exhaust gas recirculation with an EGR blower Air receiver Exhaust receiver CMP TUR Fig. 0 - Exhaust gas recirculation with an EGR turbocharger For this specific study the EGR rate has been adjusted to achieve an in-cylinder oxygen (O ) concentration of 7% at start of compression. According to [0], this is a reasonable value, required to achieve about 80% NOx reduction. The boost pressure and the maximum cylinder pressure are the same for all configurations. An electrically powered blower to pump the gases in the intake is a relatively simple solution. For smaller, high-speed engines the solution may be quite valuable. However, for low and medium speed engines the equipment tends to be very expensive, CIMAC Congress 00, Bergen Paper No. 9 0

Results with EGR blower and EGR turbocharger Fig. provides a comparison between the fuel consumption of a reference IMO Tier II engine and those for EGR operation with blower and EGRturbocharger. Blower power consumption has been considered, while the power needed to operate gas cleaning devices such as scrubbers is not considered. An electrically driven blower adds energy to the turbocharging systems, with the consequence that turbocharging efficiency increases. Engine scavenging increases, which increases the dilution of the exhaust gases with scavenging gas. For this reason, to achieve the same oxygen concentration at start of compression, higher EGR rates are required (for this specific case slightly above 0%). System fuel consumption is quite high, mainly due to the electrical energy required to power the blower. An EGR-turbocharger reduces mass flow through the main turbine, acting as a wastegate in the system. The consequences are slightly higher engine thermal load and lower turbocharging efficiency. Engine scavenging is decreased, reducing the concentration of oxygen in the exhaust gases. Lower EGR-rates are required to achieve the targeted NOx reduction (for this specific case slightly below 0%). Since no external energy is required, the fuel economy penalty is lower compared to the blower configuration, even considering optimistic blower and electricity generation efficiency. Δbsfc in % 4.0%.0%.0%.0% 0.0% Ref. EGR BLW EGR TC Fig. - System fuel consumption at full load including blower power (assuming 70% blower efficiency and 50% efficiency for onboard electrical power generation) Switching the EGR mode When running on high pressure EGR, flow through the main turbocharger is correspondingly decreased. For this specific case the turbocharger components have been designed for operation with EGR. Fig. shows that turbine area and compressor flow capacity are about 0% lower compared to the reference case. In spite of the lower EGR rate, the main compressor needed for operation with the EGRturbocharger is slightly smaller compared to the configuration with a blower. This is due to lower engine mass flow due to reduced scavenging. The main turbine is also considerably smaller compared to the blower configuration. In fact, because of the exhaust mass flow through the EGR-turbine, the gas throughput of the main turbine is reduced. πc, tot/tot Compressor EGR TC η * sv EGR BLW Ref.. V 98 [m /s] ΔS eff in % 0% -0% -0% -0% -40% Ref. Turbine EGR BLW EGR TC Fig. - Size of the TC components for different configuration. Running in EGR-off mode with a turbocharger having about 0% reduced flow-capacity is a big challenge. Without variability the boost pressure would dramatically increase, the compressor would overspeed and operate in either a very low efficiency area or be subject to surge. An exhaust gas wastegate (WG) is a moderately complex configuration. It allows boost pressure to be reduced and operation of the compressor in a reasonable operation area. However, for this specific case the mass flow through the wastegate is too high and even at part load the wastegate needs to be opened. Engine thermal load would increase dramatically. Variable turbine geometry (VTG) is a state of the art technology which has already proven to be reliable in a number of applications in the field as shown in CIMAC Congress 00, Bergen Paper No. 9

[]. However, like the waste gate configuration, it is not able to guarantee satisfactory operation in this specific case, with an EGR rate of about 0%. To run in non-eca mode there are at least two feasible configurations: Running in a non-eca with moderate EGRrates and WG or VTG. This allows engine operation with an IMO Tier I injection profile (higher NOx, lower fuel consumption). However, high turbo-charging efficiency cannot be guaranteed in both modes, see []. Sequential turbocharging, consisting of additional turbocharger which compensates the missing 0% capacity when running in EGR-off mode allows the engine to run as a conventional IMO Tier II design, see the topology on Fig.. high efficiency cannot be guaranteed for both modes, safe engine and turbocharger operation can be achieved with VTG or a wastegate, see []. However, higher EGR rates will very probably be required for IMO Tier III. For a non-eca mode there may be at least two possible configurations: () operation without EGR, in this case sequential turbocharging is required; () operating with a moderate EGR rate, in this case VTG or a WG might be sufficient to guarantee safe engine and turbocharger operation. -stage turbocharged 4-stroke engine In this section the operation with EGR-turbocharger has been investigated considering a -stage turbocharged 4-stoke engine, see Fig. 4. HP C LP C Air receiver CMP EGR C Air receiver engine CMP TUR EGR T Exhaust receiver HP-turbine HP T LP T Exhaust receiver Sequential TC Main TC Fig. - EGR off operation with sequential turbocharger Discussion Simulations comparing the high pressure EGR solutions EGR-blower with EGR-turbocharger showed that: TUR Higher fuel consumption is expected for the blower configuration. Lower turbocharging efficiency, hence, higher thermal load is expected for the EGR-turbocharger configuration. Switching between ECA and non-eca modes is a major challenge. Depending on the EGR rate required for IMO Tier III and the strategy for a non- ECA mode, the solution can be reached in different ways. In the case of a moderate EGR rate (up to about 0%-5%), operation in EGR-off mode is possible without sequential turbocharging, considerably reducing the complexity of the system. Even though Fig. 4 - EGR turbocharger with stage turbocharging Compared to low speed engines (-stroke), medium and high speed engines require a lower EGR rate to fulfil IMO Tier III. For this investigation an EGR-rate of 0% has been assumed. Variable injection timing is required to control the maximum cylinder pressure in both EGR modes. To compare different recirculation strategies with the same combustion air excess ratio, variability of intake valve timing has been applied. Analogously to the -stroke case, the turbocharging components are too small for operation in EGR-off mode. To partially compensate for this effect, variability intake valve timing has been considered. Namely, a greater Miller effect for EGR-off operation which moves the operating points toward a higher pressure ratio where higher volume flow rate can be achieved. However, intake valve variability is not considered sufficient to guarantee good and safe compressor operation. The LP compressor and, in part also the HP compressor operate very close to the choke limit. Turbocharging efficiency is very low, thermal load increases, and engine fuel consumption is dramatically affected. CIMAC Congress 00, Bergen Paper No. 9

Additional control elements required Two possible configurations have been investigated, see Fig. 4, namely: A. Turbocharger specification for EGR-off operation, engine opened for EGRon operation. B. Turbocharger specification for EGR-on operation (smaller TC components required), high pressure turbine for EGR-off operation. LP stage EGR off EGR on Miller HP EGR on EGR off EGR off EGR on HP stage Miller HP If the turbochargers have been specified for EGRoff operation (case A) they will be too big when running in EGR-on mode. With an engine opened for EGR-on mode, the turbocharger mass flow is increased. The operating points in the compressor maps move toward safe and high efficiency areas, as shown in Fig. 5. bsfc [kg/kwh] 5% 4% % % % 0% EGR off EGR on Miller LP stage Engine engine EGR on EGR off EGR on EGR off HP stage EGR off EGR on Miller 0% 0% 0% 0% eng-bp [%] engine Fig. 5 - Compressor operation with an engine for EGR-on operation If the turbochargers have been specified for EGRon operation (case B) they will be too small when running in EGR-off mode. An HP turbine is opened for EGR-off operation. This increases the pressure ratio of the LP compressor and moves the operating points toward safe and good efficiency areas in the compressor map as shown in Fig.6. bsfc [kg/kwh] 5% 4% % % % HP-turbine EGR on EGR off 0% 0% 0% 0% Tur-BP [%] Fig. 6 - Compressor operation with HP turbine for EGR-off operation Results A variation of mass flow through the engine- and HP-turbine has been performed. The operating points in Fig. 5 and Fig. 6 correspond to the maximum values in the respective configurations. According to simulations, fuel consumption at the respective design point (EGR-off for A. and EGR-on for B.) is basically the same. However, as the EGRmodes are switched, fuel consumption for the solution with engine (A.) is considerably higher. There are at least two reasons for this:. The ed mass flow cools the exhaust gases at the turbine inlet reducing the specific exhaust gas enthalpy at the turbine inlet.. Pressure is lost in the gases ing the high pressure turbine, but enthalpy is conserved and is partially used by the LP stage. CIMAC Congress 00, Bergen Paper No. 9

EGR without EGR-pump For the configuration with HP turbine for EGR-off operation (B.), a different recirculation strategy has been investigated (C.), namely, a simpler solution recirculating exhaust gas by connecting the HP exhaust gas receiver with the LP air receiver. Given the relevant pressure difference between the two volumes, exhaust gas can be recirculated without the need for an EGR pump, as shown in Fig. 7. Simulations, see Fig. 8, showed that operation in EGR-off mode is basically the same as for the configuration with EGR-turbocharger and HPturbine (B.). The slight lower fuel consumption is mainly due to the increased flow capacity of the HP components. However, operation in EGR-on mode requires about.5% more fuel. This is mainly due to the pressure loss in the EGR-line due to the pressure difference between the connected volumes. The HP compressor, as well as the two coolers, need to be designed for operation with exhaust gases and an increase in system-sensitivity due to an efficiency drop or failure at the HP compressor need to be considered, see Section 0. Air receiver Exhaust receiver HP-turbine HP C HP T DPF / Oxicat Fig. 7 - EGR line without EGR-pump LP C LP T bsfc [kg/kwh] 5% 4% % % Δbsfc EGR-on Case B, EGR-on Case C, EGR-on Case B, EGR-off Case C, EGR-off % Δbsfc EGR-off 0% 0% 0% 0% Tur-BP [%] Fig. 8 - Fuel consumption, comparison of configuration with EGR-turbocharger (case B) and without EGR-pump (case C) CONCLUSION Although the challenges are different for the SCR and EGR configurations considered in this paper (transient behaviour for the SCR, and switching of ECA modes for EGR), the requirements on the turbocharging system are similar. Namely, wide compressor maps and additional variability in the turbocharging system are needed. Additional variability on the engine side also needs to be considered for configurations operating with EGR. To control maximum cylinder pressure as the EGR mode is changed, variable injection timing is required, while to control the combustion air excess ratio in both EGR modes, variability of valve timing is required. SCR If the SCR reactor needs to be installed before the turbine to prevent the formation of ammonium (bi)sulphate, engine transient behaviour is affected. For single-stage turbocharged stroke engines the SCR needs to be placed before the turbine, which dramatically affects transient behaviour. According to simulations, the engine can be safely operated only with additional variability in the turbocharging system. An SCR combined with air wastegate, or variable turbine geometry can guarantee stable engine operation. However, NOx reduction potential is reduced as the SCR- is opened while VTG requires an additional device to increase engine scavenging during transient phases when the exhaust mass flow is cooled by the SCR. For -stage turbocharged 4-stroke engines with the SCR reactor between the HP and the LP turbines, CIMAC Congress 00, Bergen Paper No. 9 4

the impact on the system is moderate compared to -stroke engines. The main effect is a smaller surge margin during load steps, which may require a compressor or blow off valve. EGR To run with the highest fuel economy on the open sea, one of the main challenges of high pressure EGR systems is to provide the capability to switch safely and efficiently between the ECA modes. Studies with different exhaust recirculation strategies have been performed for -stroke and 4- stroke engines. Depending on the strategy for achieving the IMO Tier II mode (non-eca), the variability required for -stroke engines may be different. If IMO Tier II is achieved with moderate EGR-rates, VTG or a WG might be sufficient. If IMO Tier II is to be achieved during EGR-off operation, sequential turbo-charging is required. According to the simulations performed, exhaust gas recirculated by means of an EGR turbocharger reduces overall fuel consumption but leads to higher engine thermal load compared to a configuration using an EGR blower. For -stage turbocharged 4-stroke engines, the variability of the intake valve considered is not sufficient to guarantee safe and efficient engine operation in both ECA modes. According to the simulations, an HP turbine opened for EGRoff operation and variability of injection timing allows efficient engine and turbocharger operation in both ECA modes. Simulations have been performed on the basis of MDO fuel. With a gas cleaning device in the EGR line (i.e. a scrubber) it should be possible to run the engine on HFO. From the thermodynamic point of view no relevant changes are expected. ACKNOWLEDGEMENT The authors would like to thank Mr. P. Schuermann and Mr. T. Huber for the transient simulations of the configurations with SCR before the turbine. Many thanks to Dr. D. Brand for his valuable support during the investigations. REFERENCES [] VDMA, 008, Exhaust Emission legislation, Diesel- and Gas Engines, VDMA Engines and Systems, Frankfurt am Mein (D) [] HAMMETT, J.: Abatment technology, RFO vs DISTILLATE impact on costs & emissions, CIMAC Circle 009, Marintec Shangai, December 009 [] CIMAC, #8 November 008: guide to Diesel Emissions Control of NOx, SOx, Particulates, Smoke and CO [4] CODAN, E., MATHEY, C.: Emissions A new Challenge for Turbocharging, Paper NO. 45, CIMAC, Vienna, 007 [5] CODAN, E., Vlaskos, J.: Die Aufladung von Mittelschnellläufender Diesel-Motoren mit extreme frühen Miller-Steuerzeiten, 9th Supercharging Conference, Dresden, 004 [6] CODAN, E., MATHEY, C., VOEGELI, S.: Einsatzmöglichkeiten und Potentiale der - stufigen Aufladung, 4th Supercharging Conference, Dresden, 009 [7] Exhaust Gas Emission Control Today and Tomorrow Application on MAN B&W Two- Stroke Marine Diesel Engines, paper published by MAN B&W Diesel, Copenhagen, August 008 [8] CODAN, E. et al: Das Programmpaket von ABB Turbo Systems AG für das Design und die Optimierung von Aufladesystemen, Proc. st Conference on engine process simulation and supercharging, Berlin, 005, pp. 60-76 [9] International Standard, ISO 858-5, Second edition, 005-07-5 [0] MATTES, P, et al.: Untersuchungen zur Abgasrückführung am Hochleistungsdieselmotor, MTZ Motortechnische Zeitschrift 60, 999 [] BERNASCONI, S. et al.: Field experience with variable turbine geometry on ABB turbochargers and potential new application areas due to the introduction of IMO Tier III regulations, The Worldwide Turbocharger Conference, Hamburg, 009 Authors: Ennio Codan ABB Turbo Systems Ltd Department ZTA Bruggerstrasse 7a CH-540 Baden Tel.: +4 58 585 40 64 Fax: +4 58 585 4 E-Mail: ennio.codan@ch.abb.com CIMAC Congress 00, Bergen Paper No. 9 5

Simone Bernasconi ABB Turbo Systems Ltd Department ZTA- Bruggerstrasse 7a CH-540 Baden Tel.: +4 58 585 6 4 Fax: +4 58 585 4 E-Mail: simone.bernasconi@ch.abb.com Hansruedi Born ABB Turbo Systems Ltd Department ZTA Bruggerstrasse 7a CH-540 Baden Tel.: +4 58 585 95 Fax: +4 58 585 4 E-Mail: hansruedi.born@ch.abb.com CIMAC Congress 00, Bergen Paper No. 9 6