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R. Sauersteina R. Dabrowskib M. Beckerc H.-P. Schmalzld R. Christmanna The Dual-Volute-VTG from BorgWarner – A New Boosting Concept for DI-SI engines (2009) a BorgWarner Turbo Systems b Hochschule Mannheim, TurboAcademy c BorgWarner Inc. Engine Systems Group d BorgWarner Inc. 1 Introduction A concept strategy consisting of exhaust gas turbocharging, direct injection and variable valve timing gear has become established as a compromise between CO2 savings potential and effort in the last few years in the case of SI engines. The increasing demand for optimizing the overall system requires a continual improvement in the components and how they interact with each other. The required increase in the boost ratio in the case of downsizing concepts means that this is becoming more and more important. This results in new requirements in respect of the characteristics and performance capability of the components: ¾ Coping with combustion • Reliable ignition at high boost pressures • High-performance ignition systems for lean mixtures / mixtures diluted by exhaust gas • Avoidance of harmful work cycles • High conversion rates ¾ Flexibility of the charge cycle • Adaptation of timing optimum for operating point o Increase in cylinder recharge o Reduction in cylinder residual gas mass o Reduction in charge cycle losses o Reduction in cylinder charge temperature • Adaptation of the turbocharging process optimum for operating point by variable pressure build-up behavior ¾ Extending the operating temperatures by high-temperature materials ¾ High / low-pressure EGR to improve emissions and limit the exhaust temperature BorgWarner, being a systems supplier, pursues a policy of ongoing optimization of the relevant components exhaust gas turbocharger, variable valve timing gear, EGR modules and ignition technologies in order to comply with future emission requirements. 2 A new boosting concept for 4-cyclinder DI-SI engines that attempts to unite the advantages of variable turbine geometry (VTG) with those of consistent separation of exhaust routing is presented below. Requirements and degrees of freedom for exhaust gas turbocharging on an SI engine Owing to the differing throughput characteristics of turbo-machine and piston machine, thermodynamic coupling of turbocharger and internal-combustion engine leads to known problems in respect of the engine-related stationary and transient behavior. When designing turbocharging systems for SI engines, these problems occur to a greater extent owing to the higher throughput spread required. A pressure ratio p3/p4 occurs which can be converted to a certain boost pressure p2 in accordance with the first turbocharger main equation (equation 1) depending on the exhaust flow throughput by the engine. L A L A 1 1 p,A32 T 4 V T V mT mV 1 V 1 p,L 3 cTp m p1 1 p m T c p κ κ − κ − κ ⎡ ⎤⎛ ⎞⎛ ⎞⎢ ⎥⎜ ⎟π = = + ⋅ ⋅ ⋅ η ⋅ η ⋅ η ⋅ η ⋅ − ⎜ ⎟⎢ ⎥⎜ ⎟⎝ ⎠⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦ & & (equation 1) Smaller fixed-geometry turbines with wastegate control which allow high turbine pressure ratios and, consequently, high boost pressures even at low exhaust gas flow rates owing to their pressure build-up behavior are used today on turbocharged passenger car SI engines to implement an effective torque build-up and a good dynamic driving behavior. However, such a design with increasing full-load engine speed and, in particular, in the nominal output range, leads to an undesirably high pressure level p3 upstream of the turbine despite the control facility at the exhaust end. The negative flushing gradient (p2S – p3) which occurs in this case results in, besides the increased charge cycle work pmi, LW, a high residual gas content xRG which significantly increases the knocking tendency and results in the upward fuel consumption spiral with retarded ignition point, high exhaust gas temperature T3 and the corresponding need to enrich the fuel in order to protect the components against heat. In the reverse, turbines with high absorption capacity permit, admittedly, good effective engine efficiencies at high full-load speeds but are not able to provide satisfactory starting torques above all for SI engine applications. 3 The conflicts discussed between attractive low-end torque and good specific fuel consumption at nominal power cannot be solved with fixed-geometry turbochargers. Despite the more difficult temperature boundary conditions by comparison with the diesel engine, this fact led to the series introduction, for the first time, of a variable turbine geometry (VTG) by BorgWarner [1] in the SI engine passenger car segment as well. The blades in the guide frames (Figure 2.1) vary the build-up behavior by changing the angle of incidence of the absolute flow into the turbine rotor so as to make available an adequate turbine size in a broadly spread area of the mass flow throughput by the engine. This means that the VTG acts directly on the actual core problem of coupling piston machine and turbo-machine. Moreover, it has the energy-related advantage that the complete exhaust gas mass flow is routed via the turbine rotor. This raises the mass flow ratio in the first turbocharger main equation (see Figure 2.2) and has a positive impact on the turbine efficiency. Figure 2.1: Turbocharger with variable turbine geometry [2] One other focus in optimizing the interplay between SI engine and turbocharger is to minimize the feedback of the charging device on the engine work process. Central importance is attached in this case to the configuration and the geometrical design of the combination of exhaust gas manifold and turbine housing. In order to reduce the impairment of the charge cycle by the pressure surge of the relevant cylinder neighboring in the ignition sequence shortly after “Outlet opens”, an ignition sequence manifold and dual-flow turbine housing [3] are used, amongst other things, on turbocharged 4-cylinder SI engines. The associated decoupling of the exhaust gas pulses achieves a positive dynamic flushing pressure gradient over the individual cylinder during 4 the valve overlap phase so that, in the case of DI-SI engines, in principle, a “scavenging” charge cycle is possible. In addition, in the case of dual-flow housings, the kinetic energy contained in the exhaust gas is better used to increase the useful enthalpy gradient at the turbine (surge boosting) owing to the smaller line cross-sections in the case of scroll separation. In order to further-increase the influence of surge boosting and, thus, the available energy at the turbine, it is possible, with the aid of the dual-flow housing (see Figure 3.1.1), to move scroll separation and, thus, the pressure pulsations contained in the exhaust gas as far as directly against the turbine rotor. 5 Figure 2.2: Mechanisms of action in the case of SI engine turbocharging 6 The advantages for the SI engine working process, resulting from variable pressure build-up behavior and consistent exhaust routing, lead to a logical further development in the form of uniting the functions of the two design principles (dual-volute VTG). Moreover a combination of multi-scroll turbine housing, VTG and variable valve timing gear (VVT) would appear promising for a further increase in the low-end torque. Various investigations have already indicated that, in some cases, the torque in the lower engine-speed range can be boosted by up to 40 % [4], [5] by “scavenging” if dual-flow housings are used and simple VVT systems, e.g. camshaft phase adjusters at the inlet and outlet ends. Adequately large effective valve opening cross-sections and a positive, dynamic flushing gradient applied across the cylinders during the valve overlap phase are required for a flushing charge cycle. The far higher air demand λa which occurs during “scavenging” improves both the SI engine work process and the operating behavior of the turbocharger (see Figure 2.2) in this case. On the one hand, through-flushing with fresh air, besides the clear reduction in residual gas content xRG, contributes towards an increase in recharge mass flow rate mLZ, and, on the other hand, the turbine mass flow and, thus, ultimately the charge pressure can be increased or the pressure upstream of the turbine can be reduced for the same turbine performance PT. 7 Design and properties of the dual-volute VTG Surge charging and multi-scroll turbine housings Both the turbine efficiency and the turbine output can be influenced greatly by the design of exhaust gas routing through to the turbine rotor. It is possible to distinguish between build-up charging and surge charging depending on the design of the exhaust gas manifold and that of the turbine housing (volumes, diameters and lengths). The latter has become established to an increasing extent in the case of passenger cars, among them, particularly in the case of SI engines. The geometrically short link of the turbine to the engine allows a higher charge of kinetic exhaust energy to be transmitted to the rotor. However, the exhaust gas pulsation occurring over a work cycle and the resultant unequal application of the pressure on the turbine do have a disadvantageous effect. This leads to incorrect incident flow to the rotor as the result of the turbocharger speed which does not follow equally quickly and thus leads to a reduction in turbine efficiency. Despite impaired turbine efficiency, it is ultimately possible to raise the turbine output with the risen available exhaust gas energy, allowing advantageous transient behavior (abrupt load change, acceleration) and high turbine outputs at low speeds (low-end torque). A more extensive increase in utilization of exhaust gas energy can be achieved by using a multi- scroll turbine housing instead of a single-scroll turbine housing. On these, the aim is to achieve separate exhaust gas routing of the individual cylinders so that the pressure waves are routed to a point directly in front of the turbine rotor wherever possible [6]. Separate-scroll turbine housings are frequently technically implemented in the form of twin-scroll housings on which both scrolls are arranged adjacently and thus supply the exhaust gas over the entire circumference of the rotor. The principle may involve disadvantages in efficiency since the rotor incident flow adjacently and intermittently causes differing section pressure conditions. The resultant disturbance to incident flow or partial application of pressure to the turbine rotor can be reduced by creating, in the design, a transfer compartment in the nozzle area upstream of the rotor. The associated mixing of exhaust gas pulsations and mass flow rates may lead to reductions in efficiency owing to the dissipation losses. Besides this type of construction, dual-volute housings on which the flow per volute is over part of the circumference are also used. The volutes are separated from each other by a separating web which, similar to the separating lug on single-scroll flow housings – projects as far as the rotor. Figure 3.1.1 schematically shows the design differences between both versions. 8 Figure 3.1.1: Multi-scroll turbine housings [7] In the case of dual-volute housings, separation between the scrolls leads to different thermodynamic states in two adjoining blade channels if the rotor moves over one of the separating lugs. The resultant application of pressure which changes greatly may lead to critical vibration excitations of the blades, which is why, in the past, twin-flow housings were given preference over dual-volute housings. A reduction in unequal application of pressure over the circumference of the rotor can be achieved by positioning guide vanes upstream of the rotor. This design variant is known primarily from the commercial vehicle sector. The dual-volute VTG The advantageous operating behavior of dual-volute or twin-flow turbines at low engine speeds and the variable pressure build-up behavior of the VTG at moderate to high engine speeds have each been described several times as a separate measure [1], [3], [4]. One obvious combination of surge charging, dual-volute or twin-flow turbine and variable guide vanes to utilize the synergism effects has already been investigated in simulation calculations. The combination of twin-flow turbine housing and variable guide vanes has proven to be superior in this case [8]. The above-described transitional chamber between the two scrolls as far as the turbine rotor is increased in size if using variable guide vanes in the case of twin-flow housings. This increases the capability of pressure equalization or reduction in pulsation between both scrolls. A dual- volute turbine, for reasons relating to its principle, affords advantages over a twin-flow turbine owing to the incident flow over the entire width of the rotor. For this reason, it was decided to develop and analyze a dual-volute VTG in a potential analysis. The specifications contained the following essential requirements: • Consistent scroll separation in the exhaust gas manifold and turbine housing 9 • Implementing scroll separation either on entry into the guide vanes or on exit to the guide blades • Compact design of the turbocharging assembly of the turbine and cylinder head through to the guide vane assembly of the turbine • Maximum exhaust temperature T3= 980 °C • Modular design so as to incorporate existing series components Figure 3.2.1 below shows a view of the twin-volute VTG together with optimized exhaust gas manifold. Figure 3.2.1: View of dual-volute VTG with exhaust gas manifold It can be clearly seen that a joint gas supply flange was selected in order to implement a compact design. Details of the aerodynamics in the turbine housing can be seen in Figure 3.2.3. Both flow scrolls route the exhaust gas through a deflection angle of 180° in each case to the 10 guide vanes before it is routed through the guide blades to the turbine rotor. The view clearly indicates that the guide blades in this position of rotor incident flow impress a high circumferential component for high turbine outputs. Figure 3.2.2: Deflection through a dual-volute-turbine housing with VTG guide vanes The illustration below shows a component listed with integrated bores for accommodating the measuring systems. Figure 3.2.3: View through dual-volute turbine housing with VTG guide vanes 11 Test engine and boundary conditions Test engine The basic engine selected for the tests was a modified SI engine with direct injection and camshaft phase adjusters at the inlet and outlet sides Table 1 provides an overview of the most important engine details. Table 1: Technical data of the test engine Engine type - In-line 4-cylinder Ignition sequence - 1-3-4-2 Combustion method - DI-SI engine Valve timing gear - DOHC, camshaft phase adjuster at inlet and outlet camshaft Compression ratio - lowered to 9.5 Specific output kW/l @ rpm 85@ 5500 Max. medium pressure pme bar @ rpm 21 @ 1500-4800 Investigated turbocharger and exhaust gas manifold variants Analysis of various charging systems also includes correspondingly designed exhaust gas manifolds. A separated-scroll exhaust gas manifold the combines the scrolls of cylinders 1+4 and 2+3 and implements the separation by the turbine housing until directly upstream of the guide blades was set up in order to represent decoupling of pressure pulsation next in the ignition sequence in the exhaust gas for the dual-volute VTG. This dual-volute manifold was also used for the single-flow VTG in order to investigate the influence of a scroll-separated manifold in conjunction with the exhaust gas build-up behavior in the case of closed guide blades. Moreover, a single-scroll congestion-type manifold was investigated with the VTG in order to analyze the lack of decoupling of the exhaust gas pulsations. The dual-volute VTG was designed for the same turbine throughput characteristic as the single-scroll VTG. 12 Table 2: Overview of the turbocharger variants examined Turbocharger variant Turbine Exhaust gas manifold Designation 1 2 3 4 Dual-volute VTG Dual-volute VTG Single-scroll VTG Single-scroll VTG Dual-volute Dual-volute, with scroll connection Dual-volute Congestion-type manifold, single-scroll DVTG+DSK DVTG+DSK+FV VTG+DSK VTG+SK Test boundary conditions, measured data acquisition and analysis The test engine was equipped with a high-pressure and low-pressure indication system in order to determine the dynamic pressures in the exhaust and intake system and to monitor the engine for knocking. The characteristic parameters obtained with the indication system were then edited by computer using pressure progression analysis in order to determine fundamental assessment variables of the charge cycle. Essential results of this calculation shown here are as follows: • Residual gas content xRG • Fresh air mass in the cylinder after Inlet Closes mLZ • Mass flow functions over the inlet and outlet valves • Dynamic flushing radiant A universal controller for controlling all relevant actuators was available in order to be able to implement the degrees of freedom resulting from camshaft phase adjusters and exhaust turbocharging in the engine. The overall system of engine and turbocharger is operated under defined adjusting conditions in order to guarantee reproducible results: • Target intermediate pressure: 21 bar (to be achieved as early as possible) • Nominal output: 85 kW/l • Air ratio: λ= 1 (wherever possible) • Maximum exhaust gas temperature: T3 = 950 °C (enrichment if required) 13 • Ignition point: efficiency-optimum for maximum intermediate pressure or limited by knock limit • Minimum air ratio: λ= 0.75 (output reduction if necessary) • In the case valve timing variations: control of the air ratio by lambda probe upstream of catalytic converter Measured results and analysis Analysis of the full-load behavior with no variation in engine timing A first step comprised the investigations with minimum valve overlap in order to preclude the transverse influence of the flushing charge cycle (“scavenging”) at low engine speeds. The following Figure 5.1.1 provides an overview of important engine characteristic parameters at full load. All charger variants achieve the required rated output, but there are clear differences at low engine speeds. The dual-volute VTG with and without scroll connection features an intermediate pressure advantage over the single-scroll VTG constantly. At the very lowest engine speeds in particular (1000, 1200 rpm), it is possible to boost the intermediate pressure by approx. 0.7 bar with this variant, and the advantage at 1800 rpm is approx. 2 bar pme. The DVTG+DSK achieves the target intermediate pressure as early as 1800 rpm whilst the VTG variant with congestion- type manifold does not reach this value until upwards of 2500 rpm. The air ratio curve follows the maximum permitted exhaust gas temperature. There are lower exhaust gas temperatures on the variants with scroll separation than is the case on the variant with congestion-type manifold. Splitting the exhaust gas heat flow over two scrolls and the associated, increased wall surface area in the exhaust-gas line means a lower measured temperature. Owing to this geometry-related advantage, the limit temperature o