You will find the figures mentioned in this article in the German issue of MTZ 11|2006 beginning on page 830.
Der neue V6-Ottomotor mit Direkteinspritzung von Mercedes-Benz
The New V6 Gasoline Engine with Direct Injection by Mercedes-Benz In spring 2006, Mercedes-Benz presented the new V6 direct-injection gasoline engine – known internally as M 272 DE – in the CLS 350 CGI at the Geneva International Motor Show. This engine represents the first technological further development of the “New generation of V-engines” introduced over the course of the last two years. This is the world introduction of an engine featuring piezoelectric direct injection and spray-guided combustion. The engine runs in stratified mode over extensive part-load ranges and is characterized by very low fuel consumption. The high degree of commonality with the port-injection engine means that it can be produced on the same production line.
1 Introduction
Authors: Peter Lückert, Anton Waltner, Erhard Rau, Guido Vent and Uwe Schaupp
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Gasoline engines with direct injection have a long tradition at Mercedes-Benz. After use in aircraft engines direct injection went into automotive series production in 1954 with the M198 in-line 6-cylinder engine for the vehicle model 300 SL. In 1970, gasoline directinjection was used in a four-rotor Wankel engine in the Daimler-Benz C 111. Direct-injection was used most recently in 2001 in the M 271 4-cylinder engine. The following article describes the new V6 direct-injection gasoline engine – the world’s first gasoline engine with a piezoelectric injector.
The objective of this project was clearly defined: to achieve lower fuel consumption while at the same time improving driving performance. It is known from thermodynamic analyses that spray-guided combustion offers the highest thermodynamic efficiency for gasoline engines. For this reason, a new spray-guided combustion process was developed to production standard based on the engine with port injection described here two years ago (M 272 KE). The key components are a 200 bar hydraulic system with a quantity-regulated high-pressure pump and a directly operated piezoelectric injector, Figure 1. The engine is operated in strati-
V6 Gasoline Engine by Mercedes-Benz
fied mode in the part-load range (λ>1) in order to exploit the full consumption potential of such a combustion process. Emissions are controlled by close-coupled three-way catalytic converters and underfloor NOx storage catalytic converters. Temperature sensors are used upstream of the NOx storage catalytic converters and NOx sensors downstream to control the exhaust gas aftertreatment system in case of excess air. This system meets all current emission standards, of course. This new engine replaces the engine with port injection previously offered in the CLS 350 in Europe.
2 Objectives The demands of our customers with respect to the engine of their vehicle are: – driving enjoyment, characterized by superior and emotional performance – comfort and reliability – segment-specific characteristics of the powertrain. These are of particular importance for Mercedes-Benz as a premium manufacturer. These characteristics are supplemented by demands for maximum utility and economy, whereby fuel consumption will also play a critical part for the latter in the future. Alongside other challenges, the planned CO2-based consumption laws also represent an opportunity for future gasoline engines. Engine developers are aware of the important task to establish a long-term foundation for the combustion engine with respect to consumption and emissions through new technologies. The focus of gasoline engine development is on further reducing fuel consumption while improving driving performance. Thermodynamic analyses show that gasoline engines with stratified direct injection offer the greatest single potential for reducing consumption when compared with all other concepts [1]. In fact, the great potential of spray-guided combustion compared with wall-guided or air-guided systems was recognized early on. The injection systems for wall-guided and air-guided combustion available on the market permitted the thermodynamic advantages of sprayguided combustion to be realized in principle. However, it quickly became clear that series realization with these components was not possible. As a result, Mercedes-Benz initiated component development specifically for the spray-guided combustion process. The new injection valve – the directly-operated, outward-opening piezoelectric injector – was developed together with Robert Bosch GmbH.
3 Base Engine One of the main criteria in the requirement specifications for development of the new generation of V-engines was that this engine should form the modular basis for technological further developments and other drive system variants. Based on this principle of modular technology development, the basic components of the new V6 gasoline engine with port injection [2] were also adopted when designing the spray-guided Mercedes-Benz combustion method. For example, the drive unit, crankcase and thermal management are the same in both variants. The intake system was adapted to the increased rated engine speed. A tumble flap was not needed for the direct-injection engine due to the outstanding mixture formation and optimization of the inlet ducts. The engine management system is also based on the same software structure for both engine variants. It was extended by control functions for the high-pressure components, the modified ignition and the functions for stratified operation and exhaust gas aftertreatment.
4 Design Features The cylinder head concept of the base technology with port fuel injection and the differences compared with the Mercedes-Benz combustion method (direct injection) are described below.
4.1 Cylinder Head Concept The design features of the base technology are shown in Figure 2 left. – cylinder head made of gravity die-cast aluminium – four valves – central spark plug position – valve angle 28.5° – two composite camshafts for each cylinder bank with phase adjusters and roller valve levers as well as vertical hydraulic elements – camshaft mounting in the pressure diecast aluminium cylinder head cover – compression ratio eps = 10.7. The design for the Mercedes-Benz combustion method is shown in Figure 2 right. There are the following differences compared with the base technology: – centrally located piezoelectric injector – spark plug moved towards exhaust valves – compression ratio eps = 12.2 – external twin-pipe exhaust gas recirculation – 200 bar fuel system with volume-controlled high-pressure pump.
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Cross-flow cooling in the cylinder head ensures favourable temperatures at the exhaust-side spark plug and at the centrally located piezoelectric injector. Early consideration of this technological further development in the design phase ensured that both engine concepts can be manufactured on the same transfer line. Production quantities for the respective variants can therefore be adjusted at any time corresponding to the prevailing market situation.
4.2 Hydraulic System Use of the newly developed piezoelectric injector with A-type nozzle resulted in demands on the hydraulic system which necessitated a new development. The principle of high-pressure generation already realized for the M 271 DE in-line engine using the 3plunger radial piston pump was also adopted for the V6. The fuel delivery was adapted to the higher cold-starting requirement of the 3.5 l engine. The high-pressure start already realized for the R4 with start injection during the compression stroke was also adopted. The design of the hydraulic system is shown in Figure 3. The fuel is delivered to the engine from the vehicle’s fuel system, where it mixes directly with the cooled fuel flowing back from the fuel/water heat exchanger. The high-pressure pump is driven by the intake camshaft of the right cylinder bank. This compresses the fuel to a pressure of up to 200 bar and delivers it to the fuel rail of the high-pressure system. The middle of fuel delivery is timed to match the middle of injection and approximately double the injection quantity is delivered. The rail pressure then increases in spite of injection. Since no fuel is delivered during the next injection cycle, the rail pressure falls again. With this control strategy, the average rail pressure during injection is practically the same for all cylinders. A pressure relief valve is located on the fuel rail which also functions as a pressure control valve when the high-pressure pump is operating with constant delivery. The pressure relief valve is closed as soon as delivery switches to quantity control mode; and it then acts only as a relief valve. The fuel control valve directs excess fuel to the low-pressure side, from where it reaches the fuel/water heat exchanger. In this returnless system, the injector’s piezo actuators limit the maximum permissible fuel temperature. Fuel cooling is necessary in order to ensure that this temperature limit is met under all operating conditions. Therefore excess fuel is routed through the fuel/water heat exchanger. MTZ 11|2006 Volume 67
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higher fuel flow of the outward-opening nozzle. These shorter injection times can be realized by the combination with a piezo actuator, and this in turn offers further combustion process benefits. The studies described here show that the outward-opening nozzle in combination with a 200 bar fuel system generates a stable injection spray which is influenced only slightly by the flow and turbulence in the cylinder.
4.4 Exhaust Gas Recirculation System
Figure 1: Comparison of port injection (KE) and direct injection (DE) engines with components
All parts which come into contact with fuel were made of stainless steel or brass in order to ensure compatibility with the fuel qualities expected on the market. The rails are made of mechanically machined forged stainless steel. The housing of the high-pressure pump is manufactured using the same method. In addition to development of the combustion process and hydraulic system, a further important development task involves optimizing the NVH behaviour of the engine. A large part of this work again relates to the injector and its installation conditions in the cylinder head. The main advantage of a piezo actuator compared with a magnetic actuator is its fast response times. This fast opening and closing of the injector leads to extreme stress for the cylinder head structure. Acceleration values of up to 1000 g are reached. In addition to extensive measures in and on the injector, the clamping conditions between the rail and cylinder head were also optimized. The connection was designed in order to minimize transmission of structure-borne sound into the engine structure. In addition, the realized O-ring solution permits especially advantageous packaging with a completely assembled and tested railinjector unit.
4.3 Piezoelectric Injector The piezoelectric injector essentially consists of three main assemblies: the valve group, the piezo module and the compensating element, Figure 4. The valve group also contains the outward-opening nozzle, which produces the conical spray jet with an opening angle of 85°. The needle lift is approx. 35 μm. The piezo module actuates the nozzle needle directly and permits con4
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tinuously variable lift adjustment. The thermal compensating element ensures clearance-free operation and thermal length compensation. Figure 5 top left, shows a Mie image of the spray as it forms under backpressure in stratified operating mode. This spray is produced with only very slight fluctuations between different injection cycles. Various parameters are checked in order to guarantee the quality of the spray. These include the characteristic value Δ, which describes the spray propagation in time and space, Figure 5, top right. Due to the high spray momentum, the outward-opening nozzle produces a stable spray angle under all operating conditions at a fuel pressure of 200 bar [3]. This ensures that the main spray does not strike the spark plug and wet it with liquid fuel in spite of its spatial proximity. The high relative speed with respect to the environment means that a locally stable boundary vortex region is produced by compression injection which is practically independent of the relevant injection parameters. The spray-induced flow, created by injection, also ensures very good air entrainment, Figure 5, bottom left. This creates an ignitable mixture in the boundary vortex. Entrainment takes place in a similar way inside the hollow spray cone. The vaporization rates of two different mixture formation systems are shown in Figure 5 bottom right. It is clear that the injected fuel vaporizes around 4 times as quickly at a system pressure of 200 bar with an outward-opening nozzle, than is the case with a multi-orifice valve and 100 bar system pressure. It is also clear that much shorter injection times are necessary for the same quantity of injected fuel due to the
The exhaust gas recirculation system reduces production of nitrogen oxide in stratified operating mode. The exhaust gas is removed in the left and right exhaust pipes downstream of the close-coupled catalytic converters. Metering and control takes place separately for each exhaust line by means of a poppet valve actuated by means of a servomotor and rotating solenoid. A Hall sensor on each servomotor detects the valve position and this is used for position control in the engine control unit. Both valves are accommodated together in a common watercooled housing. Downstream of the exhaust gas recirculation valves, the collected exhaust gas is routed into the intake manifold after the throttle valve, Figure 6.
5 Combustion Thermodynamic analyses show that a sprayguided combustion process operated with excess air offers the greatest individual potential for reducing fuel consumption in comparison with all other concepts.
5.1 Thermodynamics The thermodynamic advantages of the spray-guided Mercedes-Benz combustion process compared with gasoline engines with port injection or wall-guided direct injection processes are described in [1]. The gasoline engine with direct injection possesses significantly greater potential in stratified charging mode with an adiabatic process cycle or constant-volume combustion than an engine with port injection in λ=1 operation. This is a result of the significantly lower gas temperatures due to charge dilution. The lower temperatures increase efficiency in the case of faster combustion or a reduction in the wall heat losses, and do not primarily lead to an increase in exhaust losses as is the case with operation with quantity control. In addition to the real gas influence and the reduced gas cycle losses, there are additional advantages resulting from improved fuel conversion due to better primary mix-
ture formation, greater variability in the choice of injection strategy and ignition timing, reduced wall and piston wetting as well as fast and near-total combustion. This permits realization of favourable thermodynamic centres, low HC emissions and a large regime in which the engine can be operated in stratified charging mode.
a speed range from idle speed up to n = 4000 rpm and with load demands above 65 %, Figure 7, strategy 1. This double injection results in slightly higher volumetric efficiency and improved combustion stability. Single injection in the suction stroke takes place in the rest of the map area in homogeneous mode.
5.2 Multiple Injection
5.2.3 Starting and Warm-up
The very fast response times of the piezo valve are a precondition for multiple injection with very short intervals during a working cycle. This results in completely new possibilities for realization of different injection strategies, Figure 7.
The cold start phase is of particular importance with respect to compliance with emission limits. For this purpose, the high-pressure start with correspondingly low emissions as already known from the M 271 DE engine was realized. In addition, a particularly effective strategy for fast catalyst heating was developed. This strategy involves dispensing with secondary air injection and utilizing the properties of the piezoelectric injector such as stratified operation capability with reproducible conical spray and multiple injection cycles within a very short time. A catalyst heating strategy which is already familiar for gasoline engines with direct injection is the so-called homogeneous split mode. In this mode, part of the fuel quantity is injected during the suction stroke and the other part during the compression stroke. Significantly improved mixture ignition was achieved with the strategy 3 shown in Figure 7. Here, the fuel quantity is distributed between 3 injection cycles. The first injection takes place in the suction stroke, the second in the compression stroke, and the third delivers only a very small quantity of fuel directly to the spark plug. This strategy permits an extremely late ignition point with further improved combustion stability. It also makes it possible to achieve higher temperatures in the exhaust manifold and catalytic converter with a lower fuel mass, while at the same time reducing untreated HC emissions, Figure 9. A process for catalyst heating on a direct injection gasoline engine was therefore developed which complies with all current emission regulations and also offers development potential for future emission limits.
5.2.1 Stratified Operation The use of 3D flow simulation for development of combustion processes for gasoline engines with direct injection is described in detail in [4]. The vertical sections in Figure 8 left represent droplets of the fuel spray in the spark plug area, the mixture cloud as a lambda distribution and the flame formed after ignition as described by the temperature development. If ignition takes place when there is an optimum mixture composition for ignition at the ignition location, then the flame is propagated faster with multiple injection than with single injection. The positive effects shown by the simulation for multiple injection with respect to ignition and combustion are confirmed in tests. Figure 8, right, shows the influence of the interval between the end of injection and the ignition point on the misfire frequency. The misfire-free range in which the ignition point can be varied with a constant end of injection is significantly greater in the case of triple injection than for double injection. The larger the fuel mass to be injected, the greater the positive effects of triple injection on mixture formation. Triple injection permits the robustness of the combustion process to be significantly improved compared with double injection for operation under high loads in stratified mode. In addition to this improvement in combustion robustness, combustion efficiency can also be increased by triple injection as a result of enhanced homogenization of the mixture cloud and the consequent increase in the rate of combustion. Since the main combustion phase takes place more quickly in the case of triple injection, the end of injection and ignition can be applied later for the same centre of combustion.
5.2.2 Homogeneous Operation In homogeneous operation, double injection is realized during the suction stroke in
6 Exhaust Gas Aftertreatment Concept The good efficiency values achieved mean that exhaust temperatures of below 200 °C are possible with the engine described here in unthrottled stratified operation and at operating points close to idle speed. On the other hand, the coatings still have to meet the demands for high-temperature stability under full-load conditions. In addition, it
must be taken into account that the NOx storage catalytic converters have an active temperature window of approx. 250 to 500 °C.
6.1 Catalytic Converters The exhaust gas aftertreatment concept shown in Figure 10 was defined on the basis of these boundary conditions. Exhaust gas aftertreatment starts with close-coupled catalytic converters in order to ensure that the light-off temperature of the catalysts is reached as quickly as possible. This also minimizes the heat losses in transient operation upstream of the catalytic converters. The NOx storage catalytic converters are installed in the underfloor area in order to avoid exceeding temperature limits. Temperature sensors are installed upstream of the NOx storage catalytic converters and NOx sensors downstream in order to permit precise control of these catalytic converters. The NOx sensors are used for calibration of the models stored in the control unit for adsorption and desorption of the NOx trap. In addition, they ensure compliance with the NOx limit during the service life of the vehicle.
6.2 Control of the NOx Storage Catalytic Converters In stratified charge operating mode, the nitrogen oxides are stored in one NOx storage catalytic converter for each cylinder bank. The stored nitrogen oxides are converted into nitrogen by short phases in homogeneous operation with λ<1. The operating strategy for the NOx catalytic converters is applied in such a way as to obtain the maximum fuel consumption benefit. For this reason, phases with homogeneous operation are used primarily for regeneration of the NOx storage catalytic converter. In order to ensure that this strategy is implemented optimally, it is necessary to know the nitrogen oxide adsorbed in the NOx storage catalytic converter at all times. A model stored in the engine control unit is used for this purpose which calculates a cumulated NOx mass from the untreated NOx emissions at the respective operating points. The decreasing adsorption level of the NOx storage catalytic converters is modelled during regeneration, and regeneration is ended when this reaches zero. The NOx sensor located downstream of the NOx storage catalytic converter also monitors regeneration. Regeneration is also ended if this sensor detects rich exhaust gas, as this is also a sign of completed regeneration. NOx regeneration can be initiated for different reasons: – NOx breakthrough is detected by the NOx sensor: If the NOx sensor detects a previMTZ 11|2006 Volume 67
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V6 Gasoline Engine by Mercedes-Benz
ously defined NOx slip, it is assumed that the NOx storage catalytic converter adsorption is at a level which means that NOx regeneration must be initiated. However, since the NOx sensor can react only when measurable nitrogen oxides are already being emitted, this possibility of initiating NOx regeneration involves slight NOx slip. – Utilization of existing phases with homogeneous operation: If it necessary to change to homogeneous operation due to a torque demand from the driver or a diagnostic or adaptation demand, a check is performed to determine whether regeneration is expedient at this operating point and at the current adsorption level of the NOx storage catalytic converters. Since each regeneration means a slight increase in fuel consumption as a result of brief rich engine operation, regeneration should only be initiated when the NOx storage catalytic converters have reached high adsorption levels. The adsorption threshold at which regeneration is initiated depends on the engine operating point. Realization of these strategies in the NEDC is shown in Figure 11. The diagram shows the modelled temperature of the NOx storage catalytic converter and its NOx adsorption level. A regeneration is requested for the first time after approx. 625 s due to the high NOx adsorption level. In this homogeneous idle phase, the oxygen stored in the catalytic converters and the stored nitrogen oxides are reduced by rich exhaust gas. The NOx adsorption level of the NOx storage catalytic converter is decremented in the model to reflect this. In the idle phases before this, only stored oxygen in the close-coupled catalytic converters is reduced by brief enrichment (λ<1), thereby guaranteeing an optimum λ=1 function. The next NOx regeneration takes place in the acceleration phase to 70 km/h at approx. 830 s. Here, lambda control adaptation forces a change to homogeneous operation. Regeneration is then already initiated at a significantly lower adsorption level of the NOx storage catalytic converter due to the higher load point and the resultant increased amount of untreated NOx emissions. This results in optimum NOx storage capability of the NOx storage catalytic converter during the subsequent constant-speed driving segment. The NOx regenerations are initiated by the NOx sensor in the remaining acceleration phases of the EUDC. The relatively high NOx mass flows in these acceleration phases leads to a slight NOx slip. This is detected by
the NOx sensor, which then initiates regeneration as a result. The additional fuel consumption caused by all NOx regenerations in the NEDC is so small that it is difficult to measure on a dynamometer.
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7 Technical Data The engine described here allows speeds of over 120 km/h to be achieved in stratified operating mode in the vehicle CLS 350 CGI, Figure 12. The system switches to homogeneous operation with λ=1 control outside the active temperature window of the NOx storage catalytic converter. This means that the entire map area is covered with active exhaust gas aftertreatment. In addition to the potential of very low consumption, the engine also offers the possibility of achieving speeds of up to 250 km/h (governed) and fast acceleration. The consumption map of the CGI engine is shown in Figure 13. The absolute minimum is 235 g/kWh, which corresponds to a very good value. A consumption of approx. 290 g/kWh is obtained at the comparison point n=2000 rpm and pme=2 bar. Even the best car diesel engines do not achieve this value. The engine parameters of the base engine with port injection and the engine with direct injection are summarized in the Table. The 3.5 l V6 direct injection engine has a 15 Nm higher torque and a 15 kW higher rated output compared with the base engine with port injection, Figure 14. This increase is a result of the known advantages typical for direct injection and an intake tract designed for the higher rated engine speed. This is achieved in spite of the slightly increased exhaust gas backpressure resulting from the additional NOx storage catalytic converters.
8 Summary Realization of direct injection in the Mercedes-Benz combustion process means that it has been possible to successfully achieve the theoretically forecast potential of this new technology under series conditions. The step from the first to the second generation, from wall-guided combustion to sprayguided combustion, required in particular a newly developed piezoelectric injector, a 200 bar fuel system with quantity-controlled high-pressure pump and an external exhaust gas recirculation. Development was focused on ensuring spray stability at all operating points and for the entire service life. In addition, it was and still is necessary to master the geomet-
ric tolerances and precision of electronic control. This is achieved, among other things, by using an outward-opening nozzle. In addition, the piezo actuator permits multiple injection even in stratified operating mode thanks to its fast valve dynamics and partial needle lift. As a result, it is possible to significantly improve combustion robustness under high loads in stratified mode by means of triple injection, for example. Seen from an emission point of view, this injection method also offers the possibility with respect to catalyst heating, for example, of optimizing the exhaust gas composition, increasing exothermics and therefore reaching the light-off temperature of the catalysts more quickly. The new engine, which makes this technology available for the first time, will replace the previously offered M 272 KE with port injection in the CLS 350 in Europe. The described engine modifications on their own result in a reduction in consumption of 10 % compared with the base type with port injection in the NEDC. With 15 kW more power and 15 Nm more torque than the base type, this new engine underlines the sporting character of this dynamic vehicle series.
References [1] Kemmler, R.; Schaupp, U. et al.: Thermodynamischer Vergleich ottomotorischer Brennverfahren unter dem Fokus minimalen Kraftstoffverbrauchs. 11. Aachener Kolloquium 2002 [2] Lückert, P.; Waltner, A. et al.: Der neue V6-Ottomotor M 272 von Mercedes-Benz. In: MTZ 65 (2004), Nr. 6 [3] Waltner, A.; Schaupp, U. et al.: Anforderungen und Entwicklungsschwerpunkte für Ottomotoren mit Direkteinspritzung bei Mercedes-Benz. 6. Internationales Stuttgarter Symposium 2005 [4] Bezner, M.; Enderle, C. et al.: Einsatz der 3D-Strömungssimulation bei der Entwicklung ottomotorischer Brennverfahren mit Direkteinspritzung. 6. Tagung Direkteinspritzung im Ottomotor, Essen 2005
