Next Generation Diesel Engine Potential of Highly Integrated Exhaust Gas Aftertreatment

Transcription

1 DEVELOPMENT Diesel Engines IAV Next Generation Diesel Engine Potential of Highly Integrated Exhaust Gas Aftertreatment IAV has investigated the potential of close-coupled exhaust gas aftertreatment concepts for passenger car diesel engines in conjunction with new options for turbocharging and hybridisation. In the course of simulation studies and engine tests, the potential for reducing CO 2 and pollutant emissions was analysed and evaluated in the WLTC and in a low-load driving cycle. FULL LIFE CYCLE ANALYSIS NECESSARY The comparison of CO 2 emissions associated with different types of powertrains is recommended to consider a full life cycle analysis. Here, consideration should not only be given to tank-to-wheel emissions from fuel combustion, but also those associated with providing and distributing the fuel and from producing and recycling the vehicle. In the case of e-vehicles, CO 2 emissions from producing the drive energy must also be taken into account. This comparison reveals that 42

2 AUTHORS Dipl.-Ing. Matthias Diezemann is Senior Technical Consultant, Business Area Powertrain & Power Engineering, at IAV GmbH in Berlin (Germany). Prof. Dr. Christopher Severin is expert of the research focus alternative drive trains at HTW Berlin, Faculty of Automotive Engineering, and Consultant for IAV GmbH in Berlin (Germany). Dr.-Ing. Maximilian Brauer is Head of Department Advanced Development 2, Business Area Powertrain & Power Engineering, at IAV GmbH in Berlin (Germany). Dipl.-Ing. Frank Bunar is Senior Technical Consultant, Business Area Powertrain & Power Engineering, at IAV GmbH in Berlin (Germany). CO 2 emissions from hybridised diesel powertrains in particular are in the region of those from e-drives, which means they are an important technology bridging the gap on the way to e- mobility or provide a suitable platform for renewable fuels. [1, 2]. DEMANDS The following demands are placed on future diesel-engine propulsion concepts in terms of ecological and economic sustainability [3, 4]: Emission conformity must be achieved under all driving conditions: The Euro 6 limit value of 8 mg/km must not only be met under the legislative framework in the cycle and with regard to the RDE legislation [3], but also in all driving situations commonly occurring in practice. Locally zero-emission driving has to be possible: To reduce the emission level in particularly polluted inner cities, the drive system should have the potential to operate on a zeroemission basis, if locally requested. Ultra-low CO 2 emissions are required: CO 2 emissions in the overall life cycle analysis (LCA) should be below those from an e-motor powertrain. Driving dynamic and power output have to be convincing: High torque over a broad engine speed range is of key importance for driving dynamics. Costs have to be marketable: Despite the higher purchasing costs of a diesel car compared to the gasoline engine, it must still provide an advantage to the consumer in terms of total costs of ownership (TCO). PROBLEM DEFINITION Different driving scenarios produce different challenges in respect of reducing journey-specific pollutant emission, as shown in FIGURE 1 using the example of the temperature level in the exhaust gas aftertreatment system. The following requirements can be derived: NO x conversion at low temperatures in low-load cycles WLTC Low-load cycle City cycle severe BAB cycle Stop and go cycle 4 City mix cycle FIGURE 1 Vehicle speed profiles and exhaust gas temperature curves in the for journey profiles under review ( IAV) MTZ worldwide

3 DEVELOPMENT Diesel Engines Basis Euro 6 Variant 1 turbine bypass Variant 2 additional in turbine bypass upstream turbine + upstream turbine FIGURE 2 Topology variants in comparison to the base variant ( IAV) NO x engine-out emission control at high loads [2] large spread of space velocity in the SCR catalyst [3, 4] constantly high NO x conversion rate at rising temperatures in the SCR on filter () [3, 4]. While controlling emissions from highload cycles is more of a dimensioning matter, controlling them in the lowtemperature range is always more complex [3, 4]. At extremely low average driving speeds (down to stop-and-go in congestion situations), exhaust gas aftertreatment temperature can fall below light-off. If this is immediately followed by rapid acceleration, the high levels of engine-out emissions are not treated adequately. To avoid such situations, the exhaust gas aftertreatment system must be kept at a constant temperature. This requires the application of additional exhaust gas temperature management measures like calibration-based measures, strategies for variable valve timing (VVT), hybridisation and exhaust gas aftertreatment (EAT) positioning: With regard to the CO 2 advantages, which must be maintained compared to the gasoline engine, typical calibration-based measures in relevant heating modes (e.g. retarding the start of injection, throttling, post-injection) are detrimental to fuel consumption and therefore not ideal [3, 4]. 44 Regarding VVT strategies discussion focuses in particular on secondary exhaust valve lift and cylinder shutoff as these strategies can be implemented more or less without increases in consumption. Moderate temperature advantages can be achieved and HC and CO emissions reduced during lowload operation [5]. Hybridisation can have an adverse effect on exhaust gas temperature as the average power output in the driving cycle is reduced by the higher efficiency achieved in the powertrain. Measures, such as avoiding trailing throttle and low torque levels as well as active load control, are capable of providing significant temperature and emission-related advantages. Depending on the hybrid powertrain s topology, the engine operating point could be decoupled completely from the driving situation [1]. Avoiding temperature losses as well as minimising the light-off time is a primary criterion in designing the powertrain. Particularly in hybridised concepts, this produces degrees of freedom in positioning the turbocharging (TC) and EAT components. Positioning EAT upstream of the TC provides the optimum in relation to temperature losses and light-off behaviour, but also with regard to the efficiency of any heating and operating strategy measures. IMPROVING REDUCTION OF NITROGEN OXIDE EMISSION DURING LOW-LOAD OPERATION This elaboration demonstrates the potential of different TC and EAT topologies in combination with hybridisation. FIGURE 2 shows schematic diagrams of the topology variants investigated. Compared with the Euro 6 base version, variant 1 has an additional turbine bypass that not only makes it possible to reduce the air/fuel ratio at part load as a heating measure but, in particular, also to bypass the turbine s heating capacity on cold starting. In contrast to variant 1, variant 2 has an additional start in the bypass that permits faster HC/CO light-off and hence faster NO 2 production after cold start. In variant 3, the and AdBlue doser unit are positioned upstream of the turbine. not only positions the upstream of the turbine but also the. In this case, the time until and light-off can be reduced by the maximum extent. BASE VEHICLE The experiment-based studies are conducted for a Euro 6c inline 2.-l fourcylinder engine in combination with a D-segment vehicle with a curb weight

4 of 17 kg. The engine uses single-stage turbocharging and combined high-pressure and low-pressure EGR. The exhaust gas aftertreatment system consists of a close-coupled and. DRIVING SCENARIOS The investigations focused on driving situations with particularly low and therefore critical exhaust gas temperatures so that the selected moderate inertia weight class takes on particular significance as the worst case, FIGURE 1. The reference engine configuration exhibits very good emission behaviour in the WLTC. In the selected low-load cycle with a distance of 6.5 km, a maximum vehicle speed of km/h and an average vehicle speed of just 1 km/h, the vehicle reveals challenges with regard to temperature management and hence emissions. The reference engine configuration described reaches temperatures of just above 15 C upstream of the. NO x conversion is correspondingly low, producing emissions in excess of 2 mg/km and resulting in a poor conformity factor. HYBRID OPERATING STRATEGY For the above-stated reasons, it is likely that diesel engines will at least be electrified in the form of mild hybridisation [1]. This makes it necessary to examine the EAT configurations presented for relevant hybrid topologies in comparison to the all-diesel-engine powertrain. FIGURE 3 compares the test bench results obtained for potential hybrid strategies with conventional operation in the WLTC (left) and the low-load cycle (right). Merely avoiding low levels of torque by using recuperated braking energy is not sufficient to increase temperature to such an extent as to achieve the requisite level of NO x con version. If the described additional hybrid heating strategy is used by applying torque from the point at which the engine is started, temperature can be increased for a short time and then even falls below the base during con ventional operation as a result of the minimal shares from the combustion engine. Nevertheless, it is still possible to produce a positive effect on NO x emission as engine-out NO x emissions are reduced by the shorter engine running time and by the reduced level of consumption achieved via hybrid propulsion. The analysis of conventional EAT positioning shows that despite extensive heating strategy from hybridisation and despite strong hysteresis, it is not possible in the low-load cycle to get the to a tem perature high enough to permit adequate NO x conversion. RESULTS FOR HIGHLY INTEGRATED EXHAUST GAS AFTERTREATMENT In pure low-load cycles, no appreciable advantages can be expected from variant 1 while heating up and in the steady state as only relatively low temperature differences are achieved in the steady state for concept reasons. It is possible to shorten the heating-up time as the turbine s heating capacity is temporarily bypassed. The positive effect of variant 2 on NO x conversion comes from an improvement in the NO 2 /NO x component upstream of the caused by early light-off. This effect is partly offset by the additional heating capacity of the introduced in the bypass. NO x emission [mg/km] 5 WLTC Basis Euro 6c Norm. CO 2 emission [-] NO x emission [mg/km] 5 LLC Norm. CO 2 emission [-] Exhaust temperature downstream Engine out emission Tail pipe emission No hybrid P2-hybrid P2-hybrid with heating Exhaust temperature downstream Vehicle speed [km/h] Vehicle speed [km/h] WLTC time [s] Low-load cycle time [s] FIGURE 3 Hybrid operating strategies in WLTC and load-load cycle with base EAT topology ( IAV) MTZ worldwide

5 DEVELOPMENT Diesel Engines Time to torque ( % of max.) [ms] Time to torque at 2 rpm at cold exhaust system Electric energy [kwh] Basis with EM 2 kw with e-boost with EM 2 kw with EM 8 kw with e-boost FIGURE 4 Time-to-torque behaviour ( IAV) and have a high thermal mass. If they are positioned upstream of the turbine, the temperature at the turbine inlet responds with a correspondingly long delay. Isentropic turbine work is directly proportional to the inlet temperature. As a result, power output approximately proportional to inlet temperature can only be applied to the charger shaft and converted into boost pressure. This means the potential boost pressure buildup in variants 3 and 4 is more or less proportional to the outlet temperature in the exhaust gas aftertreatment system [2]. By way of example, FIGURE 4 shows the response behaviour (time-to-torque) for an engine speed of n = 2 rpm. It is apparent for variant 3 that the drawbacks in boost pressure buildup can only be compensated for with an e-booster. also requires an e-booster or high-volt hybridisation. FIGURE 5 provides an overview of the benefits that can be achieved with each variant for a conventional vehicle as well as for a hybridised system with heating strategy based on temperature thresholds. It becomes clear that successively moving EAT components upstream of the turbine can do much to gain robustness in NO x aftertreatment. This effect is further reinforced in hybridised systems as possible load point shifts and associated higher temperatures have a direct effect on exhaust gas aftertreatment. In combination with hybridised heating, variant 4 exhibits the best results in terms of NO x emissions and fuel consumption. SUMMARY AND OUTLOOK In addition to continuous optimisation, non-hybridised systems with conventional exhaust gas aftertreatment layout require additional measures to exercise a positive influence on engine-out emis- NO x emission [mg/km] Basis Variant 1 Variant 2 No hybrid / e-booster P2-Hybrid P2-Hybrid with heating Norm. CO 2 emission [-] 46,2,4,6,8 1, Bypass Pre turbo FIGURE 5 NO x emissions of individual variants in the low-load cycle ( IAV)

6 Variant CF < 1 Drivability Fuel consumption Cost Local ZEV Basis Bypass (variant 1/variant 2 ) with e-booster MHEV + with e-booster + HEV FIGURE 6 Summary and evaluation of concepts under review ( IAV) sions and exhaust gas temperature in low-load cycles. Using a turbocharger bypass to enhance cold start performance (variant 1 and variant 2) permits moderately better robustness in relation to controlling emissions at low additional costs, FIGURE 6. Positioning the upstream of the turbine (variant 3) in combination with an e-booster provides far greater reliability in controlling emissions from low-load cycles particularly if a solution can be found for positioning AdBlue dosing upstream of the turbine. Moving the upstream of the turbine as well (variant 4) provides huge improvements in terms of robustness to low-load cycles. Given the tense situation regarding maximum temperature in the, a system of this type is limited in terms of output (in the order of P spec. = 45 kw/l). The e-booster or high-volt hybridisation remain obligatory for non-hybrid concepts. In combination, the effects of both aspects hybridisation and maximum reduction in heating capacity are particularly efficient. REFERENCES [1] Blumenröder, K.; Bunar, F.; Buschmann, G.; Nietschke, W.; Predelli, O.: Dieselmotor und Hybrid: Widerspruch oder sinnvolle Alternative? International Vienna Motor Symposium, 27 [2] Severin, C.; Bunar, F.; Brauer, M.; Diezemann, M.; Schultalbers, W.; Buschmann, G.; Kratzsch, M.; Blumenröder, K.: Potential of Highly Integrated Exhaust Gas Aftertreatment for Future Passenger Car Diesel Engines. International Vienna Motor Symposium, 217 [3] Bunar, F.; Schirmer, S.; Friedrichs, O.; Grünig, C.; Hansen, K.; Hartland, J.: High Performance Diesel NO x -Aftertreatment Concept for Future Worldwide Requirements. 23 th Aachen Colloquium Automobile and Engine Technology, 214 [4] Krämer, L.; Buschmann, G.; Stiegler, L.; Bunar, F.; Richardson, S.; Hansen, K.: Mit der Diesel-ANB auf dem Weg zum Super-Ultra-Niedrig-Emissionsfahrzeug (SULEV). International Vienna Motor Symposium, 213 [5] Brauer, M.; Diezemann, M.; Pohlke, R.; Rohr, J.; Severin, C.; Werler, A.: Variabler Ventiltrieb aktives Abgastemperaturmanagement am Dieselmotor. ATZlive conference Der Ladungswechsel im Verbrennungsmotor, 212 MTZ worldwide