1? Initial condition
Due to the stricter emission limit of CO2, the share of heavy-duty vehicles in SUV models is increasing, and the further reduction of diesel engines requires an alternative drive scheme based on internal combustion engine in addition to electrification. Hybrid power system has a good application prospect for CO2 balance. Hybrid power system can integrate the latest new technologies, such as electric supercharger technology, which needs to be supplemented by a brand-new exhaust gas turbocharging design strategy. All schemes require high-efficiency supercharging within the working range which is significant for engine fuel consumption while being flexible enough, so that the engine can run reliably at full load and partial load working points. In addition, the hybrid scheme requires the internal combustion engine to have as high efficiency as possible, so that the whole scheme can achieve a high CO2 emission value. An excellent engine scheme to improve efficiency is a gasoline engine with Miller cycle, and the variable turbine geometry section (VTG) supercharger is the best supercharging system for this cycle. In addition, it is necessary to study the structure of VTG supercharger of gasoline engine in detail, especially to optimize the efficiency of parts to make gasoline engine suitable for Miller cycle.
The possibility of further improving the efficiency is to use ball bearings on the turbocharger, which can improve the flow geometry and improve the efficiency by minimizing the friction power. The details of the improvement will also be introduced below.
2? motive
In order to achieve the required CO2 emission target value after 2023, it is necessary to improve the thermodynamic efficiency of internal combustion engine and combine the power system with hybrid power. Improving geometric compression ratio, lean burn, Miller cycle and their combinations are important ideas for the future development of gasoline engines, with the goal of making the working process efficiency of gasoline engines close to that of diesel engines.
With the increase of electric mileage of full hybrid and plug-in hybrid, it is possible to optimize the scheme of hybrid internal combustion engine. Compared with the best naturally aspirated hybrid engine, the turbocharged gasoline engine with high compression ratio, Miller combustion process and external cooling EGR can obtain significantly higher specific power spectrum with lower fuel consumption. Because the optimal fuel consumption operation range can be adjusted and expanded under the condition of supercharged engine, the advantages of operation strategy and energy management can also be obtained, so that the gearbox will not be too complicated and require less electrical assistance.
Due to lean inflation and intermediate expansion, the combination of improving the efficiency of internal combustion engine and higher supercharging pressure demand poses a new challenge to supercharging system. Under this boundary condition, gasoline engine VTG has more advantages than the traditional exhaust valve technology. Compared with the exhaust gas bleeder valve supercharger, VTG gasoline engine can not only increase the power by about 15 ~ 20? In addition to kW, the fuel consumption at rated power can be reduced by 7? %, which is basically achieved by using the total enthalpy of exhaust gas to improve Miller cycle.
In addition, VTG gasoline engine can make the temperature difference between pre-turbine temperature T3 and pre-catalytic converter temperature T4 about 25? ℃, is the power level of high temperature VTG technology above 100? KW/L engine, which can also meet the requirements of RDE emission standards (Figure 1).
In the hybrid scheme, the efficiency of internal combustion engine depends on the maximum charge lean degree. In the range of moderate load (PME = 1.0 ~ 1.4? MPa) reduces the efficiency of turbocharger and increases EGR rate, but this will limit the optimization potential of fuel consumption. Through the design of corresponding pneumatic components, the efficiency of turbocharger can be improved by 5? %。 In addition, due to the use of rolling bearings, the variability of the turbine and the reduction of friction power help to compensate for the delayed load establishment caused by the new combustion process. Compared with the exhaust valve turbocharger, increasing Miller cycle to improve combustion efficiency or using ball bearings can improve the power performance by up to 20% under the condition of similar Miller cycle. Figure 2 shows a 48? Simulation results of fuel saving potential of V-P2 hybrid C-Class vehicle in WLTC test cycle. Because the best Miller cycle is used, the assistance of cooling EGR and reducing P2 mixing mode is about 3? % carbon dioxide emission advantage.
3? VTG supercharger for gasoline engine
VTG technology has been applied in the field of diesel engine since 1997 (Figure 3), and it has achieved great success in the field of car diesel engine as a supercharging system. At present, Miller cycle has been applied to gasoline engines under similar conditions. In 2006, the gasoline engine VTG used the Borgwarner turbocharger on Porsche for the first time. 9 1 1? Turbocharger 3.6? L on the engine. With the application of Miller technology, this technology is also essential for a large number of markets. With continuous development and years of experience, the development of diesel engines from VTG has greatly reduced the cost.
A simple supercharging system uses an exhaust pressure reducing valve to bypass part of the exhaust gas of the turbine wheel. For example, the multi-channel turbine casing matched with the double volute turbine can expand the operating range of this technology, but it still cannot reach the operating range of VTG application. This VTG turbine uses an adjustable guide vane ring in front of the turbine wheel to adjust the exhaust gas mass flow, so the turbine can work with high efficiency in a wider engine working range. In addition to turbine impeller, adjustable guide vane is also the core technology of turbine aerodynamics. These guide vanes are supported in a nozzle ring, which can rotate and come into direct contact with high-temperature exhaust gas. Because the material performance is very demanding, it can be used for a long time mainly when the exhaust temperature is below 860? On the diesel engine at 40℃, the requirements for materials are obviously higher than 900? Can Borgwarner learn from Porsche in its further development? 9 1 1? Turbine cars are applied to gasoline engines in batches. 050? The experience of℃ high temperature engineering, not only in terms of materials, but also in terms of cost and using Miller combustion process, the highest temperature of its exhaust gas can reach 950? Great technological progress has been made in the fields of℃ and so on, which makes the application of VTG technology in the hybrid power system of gasoline engines very interesting in the future. It can be predicted that in this case, the boundary conditions of the engine are the increase of exhaust gas temperature and air demand, and the working range which is of great significance to RDE. The technical challenge to the exhaust gas turbocharger turbine is to have high reliability and good adjustability under the condition of excellent aerodynamic efficiency.
The aerodynamic efficiency of VTG turbines can be simplified to an efficiency parabola as shown in Figure 4. The adjustable guide vane cascade can obtain a wider flow span, which can be larger than that of the two-state turbine compared with the fixed volute turbine with similar impeller diameter and exhaust gas relief valve. When the guide vanes are completely closed and fully opened, the efficiency will be lower than the maximum. The closed guide vane plays the role of nozzle in aerodynamics, but the effect of deflecting the cascade in the opening direction will become stronger and stronger, and the incident angle on the turbine wheel will change throughout the adjustment range. A proper combination of vane angles and turbine impeller blade angles can achieve a wide and effective operating range, and a parabolic decrease in efficiency caused by aerodynamics near the edge of the flow range will move out of the engine operating range.
Therefore, the turbine provides high flexibility in the adjustment of back pressure, power and related boost pressure, thus making an important contribution to reducing exhaust emissions under the condition that the efficiency characteristics are ideally matched with each engine target, and the components that have an important impact on aerodynamics are the decisive components.
The factors that have an important influence on the efficiency parabola include the arrangement and support of turbine impeller and guide vane in nozzle ring, the geometry of guide vane, the design of volute and special joint for engine. All turbines shall be dynamically optimized to suit the flange position of engine and exhaust system applied by users. If the surface wetted by exhaust gas is less, the better, and the structural space at the front is smaller, which depends on the design of the volute, both of which should be well coordinated with the aerodynamic target of the flow to the guide cascade.
Proper flow guidance is helpful to reduce the thermo-mechanical deformation of nozzle ring, so more structural design measures should be taken to minimize the blade gap. The design of the blade itself is also helpful to reduce the gap flow loss (Figure 5), which can improve the efficiency of the engine's low-speed angular torque range. VTG guide vanes have a patented S-shape, which can achieve two main goals: low flow deflection loss and low blade rotation torque, but always open the blades (fail-safe function). The structure of nozzle ring not only considers the aerodynamic efficiency target and cost, but also is designed according to the concept of assembly, in which the main size is selected to optimize the modeling for the thermodynamic target of current and future important engine grades, and the additional intermediate structure size matches the production variety. In addition, the introduction of new turbine impeller series can use the same VTG nozzle ring, and the efficiency parabola can be specially adapted to different engine targets according to the different efficiency parabola curves of diesel engine and gasoline engine. For example, as shown in fig. 4, green and yellow intermediate parabolas are realized by using different types of turbine wheels, respectively.
Most impellers of VTG turbines of Borgwarner Company adopt radial structure, which is still the standard structure of diesel engines. Because of its high deflagration tendency, gasoline engines need less turbine back pressure and gradually increased flow. However, in order to obtain low-speed angular torque, especially the acceleration response performance of the engine, it is necessary to have good efficiency even in a small flow range of about 30% ~ 40% of the maximum flow. In this respect, radial impeller has obvious advantages. Compared with the small impeller fixed turbocharger with the best inertia, the inertia moment of the turbine impeller can be reduced by about 10% ~ 20%. Coupled with excellent efficiency, VTG turbine can obtain highly competitive acceleration response performance, and the maximum flow rate in the same nozzle ring is about 20% higher than that of the standard impeller used in diesel engines, which is realized through the special design of impeller blade shape (Figure 6). The blade shape and impeller disk are carefully designed from the perspectives of aerodynamics, mechanics and manufacturing technology. Thermal optimization of impeller disk can reduce the moment of inertia, improve the mass flow, and reduce the stress to a safe level that can obtain a long service life.
In addition to the experience gained from many large-scale production projects, the most modern digital methods, including automatic optimization process, have also been used in the design of turbine components with great aerodynamic significance.
4? Ball bearing of turbocharger
Compared with the sliding bearing with the same structural size, the ball bearing in the turbocharger (Figure 7) has obviously lower mechanical loss, and because of its good rotor stability, the profile clearance of the compressor side and the turbine side can be optimized to the best degree, thus further improving the overall efficiency of the turbocharger (Figure 8).
The specially developed ball bearing scheme optimizes the sound propagation path and rotor dynamics (shaft trajectory stability). Compared with sliding bearing, the higher bearing stiffness of ball bearing needs to optimize the transmission path of vibration energy to the surrounding, so it is necessary to suppress vibration as much as possible to achieve the highest stability of the bearing. Through the cooperation of simulation and experiment, a design scheme with the best balance of these target sizes can be developed. First of all, we should pay special attention to the application of low viscosity engine oil (HTHS≈2.0? MPa seconds).
The ball bearing with sleeve structure is obtained through optimization design, and the outer sleeve of the bearing floats on a layer of oil film (see Figure 7). The damping of the rotor system is guaranteed by the extrusion damping oil film formed. In order to ensure the establishment of damping oil film, an innovative system composed of isolation sealing rings is adopted, which can make the bearing sleeve in the center of the bearing channel, improve the impact resistance of the bearing (this is the key factor of acoustic performance), and at the same time play a role in sealing the damping pressure range of the squeezed oil film relative to the pressureless space in the bearing box, so it is necessary to guide the oil flow and reduce the flow loss. Through the optimal design of squeeze film damping, the unique bending determined by the design of ball bearings can be resisted.
The complex relationship in ball bearing support needs a lot of optimization or new parts design scheme, parts processing and assembly strategy. Through the combination of these optimization schemes, for example, improving the stiffness of the bearing bush, the acoustic level of the sliding bearing can be achieved even under cold start conditions, and besides reducing the axis trajectory deviation, higher reliability can be achieved relative to higher radial and axial loads. In the case of using ball bearings, it is always necessary to re-evaluate the acoustic performance, because the sensitivity of the vehicle has a great influence on the acoustic performance besides the turbocharger, so it is necessary to optimize the transmission characteristics of the vibration propagation path.
Fig. 8 shows the steady-state measurement efficiency advantage of the ball bearing relative to the sliding bearing under the same aerodynamic components on the compressor side and the turbine side. The left figure shows the comprehensive efficiency of the turbine measured when the expansion ratio is 1.5. Compared with sliding bearings, the efficiency can be improved by up to 4%, and this efficiency advantage decreases slightly with the increase of turbine power. The diagram on the right side of Figure 8 shows the low efficiency characteristic curve field of turbocharger in the compressor characteristic field, especially in the partial load range which is important for fuel consumption. Due to the improvement of rotor stability (the reduction of profile clearance) and the reduction of bearing power loss, ball bearings can obtain an efficiency advantage of 5% at most. Fig. 9 shows the engine speed of 1 by way of example. 500? Comparing the load mutation of two kinds of bearings at r/min, it is obvious that the pressurization pressure of ball bearings is established faster, and the average effective pressure of 2MPa is 0.7? Before the sliding bearing. S, so the acceleration response performance is obviously improved, the efficiency advantage also improves the fuel consumption rate and reduces the exhaust emission.
5? conclusion
For gasoline engines, the mixing of Miller combustion process and power system is helpful to achieve the specified CO2 emission target, among which the supercharging system optimized by VTG is an important module. Starting from the field of diesel engine, the development of VTG, especially the optimization and development of turbine end, has further adapted to the demand. Borgwarner's gasoline engine VTG supercharger can be used in all typical displacement vehicles, especially the highest exhaust temperature can reach about 1? 020~ 1? 050? The gasoline engine with high specific power and high supercharging runs at λ= 1 at℃.
In addition to all product series of all displacement and power levels, we can also provide product series with ball bearings (BB0 1, BB02, BB03), which can further improve efficiency. In view of the advantages of ball bearings in total efficiency, rotor stability and transient performance, rolling bearings that can be used in future hybrid applications are recommended. When low viscosity engine oil and low engine oil pressure are used at the same time, the requirements for start-stop performance are improved.
These two technologies can gradually optimize the supercharging system according to the future CO2 emission target and the development of hybrid and electrification of power system.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.