"Today, the top mobility of Passat car series is equipped with two-stage turbocharged in-line 4-cylinder 2.0L dual turbocharged direct injection diesel engine. At the speed of 4000 R / min, the power is 176 kW. In the speed range of 1750 ~ 2500 R / min, the maximum torque is 500 N · m and the power rise is as high as 88 kW / L. it has the highest power rise among mass-produced 4-cylinder diesel engines. Based on the modular standard parts launched by Volkswagen in 2012 [1], the new diesel engine has developed a compact supercharging unit with two exhaust gas turbochargers, with a supercharging pressure of up to 0.38 MPa (absolute pressure).
1. Technical specifications and features
The technical features of the new Passat car equipped with 2.0L twin turbocharged direct injection diesel engine are as follows (Fig. 1): ⑴ the power is 176 kW, which is equivalent to 88 kW / L of the 4-cylinder diesel engine; (2) the torque is 500 N · m, which is equivalent to 250 n · M / L; (3) achieve the level of sporty driving performance and best acoustic characteristics of high-class cars; (4) reach Euro 6 emission limit; (5) low fuel consumption; (6) use modular standard parts to effectively reduce costs; (7) it is suitable for transverse installation. The main specifications of 2.0 l dual turbocharged direct injection diesel engine are shown in Table 1.
Figure 1 - technical data of a new 2.0l-tdi dual turbocharged diesel engine with a power of 176kwd
2 basic engine
The 2.0L twin turbocharged direct injection diesel engine is designed based on the single turbocharged model, and the cylinder block crankcase is cast with gjl-250 sheet graphite cast iron. In order to obtain the best friction performance under the condition of low oil consumption and low crankcase ventilation, the working surface of the cylinder is honed with a bolt fixed honing center frame.
Different from the original technical basic state, the crankcase of cylinder block of twin turbocharged direct injection diesel engine is improved to make all parts bear higher load. The diaphragm connected with the main bearing seat has optimized the stress in the opening range of its upper part to reduce the transverse pulse flow loss of crankcase gas by increasing the wall thickness, and the lengthened crankshaft main bearing cap bolt can bear the increased tensile force. The oil supply of the two exhaust gas turbochargers is improved by improving the arrangement of the oil passage. With the help of other structural measures, the strength and noise radiation of the support fastening point of the booster unit are optimized.
Due to the high maximum combustion pressure, the compression ratio was reduced to 15.5 (16.2 for 110 kW single turbocharged models) and a new combustion process was adopted. The newly developed piston is used in the new diesel engine. The area with the highest load at the lip of the combustion chamber pit on the top is remelted to further improve the strength. The cooling channel formed by salt core casting at the head of the new piston is optimized for the shape of the new piston. The second piston ring is designed as a nose shaped inclined ring, while the third piston ring adopts a double chamfered hose spring oil ring with a height reduced from 3.0 mm to 2.0 mm, which can reduce the tangential force.
Increasing the piston pin diameter from 26 mm to 29 mm reduces the stress and surface pressure on the piston pin axis. Like all diesel engine standard combination components, the piston pin coated with diamond-like graphite has become a standardized part. The connecting rod body is partially strengthened. The crankshaft is forged from 42crmos4 high-strength alloy steel, while the 8-hole flange still uses the design of 2.0L turbocharged direct injection (TDI) diesel engine with a power of 140 kW.
The load of 2.0L dual turbocharged direct injection diesel engine is higher than that of single turbocharged model, so it is necessary to match and adjust the oil budget, and increase the flow of cooling oil by increasing the piston cooling nozzle to meet the increased cooling demand of piston. At the same time, in view of the increase of oil volume flow, the pressure level of oil should be reduced, To reduce pressure fluctuations in the oil circulation circuit.
The oil pump is a two-stage vane pump with adjustable volume flow and seven oil chambers. The oil pressure of the low-pressure stage is 0.18 MPa and that of the high-pressure stage is 0.33 MPa. Its speed is 10% higher than that of the single turbocharged model, so as to optimize the oil supply at low speed. The oil pressure switch has been adjusted accordingly, and the oil filter and oil cooler are the acceptance parts of the base engine.
3 cylinder head
In view of the intake charge swirl design and two-stage pressurization in the combustion process, the variable valve mechanism can no longer be used. The valve star arrangement on the dual turbocharged diesel engine adopts the structural type parallel to the cylinder centerline (Fig. 2). This integral valve mechanism module is an important component of modular diesel engine, which remains basically unchanged. The same cylinder head adopts the same high temperature resistant material.
Figure 2 - high power cylinder head
The design scheme for adjusting the high-power cylinder head first involves obtaining better cooling through the additional cooling water flow guidance between the intake valves, and strengthening the cylinder head bottom plate, the range of oil chamber, around the fuel injector and the cylinder head bolt boss. The cylinder head bolt adopts 12.9 strength grade (grade 10.9 for single turbocharged models). The oil separation chamber on the cylinder head cover is enlarged, and the method of oil separation is modified due to the increase of crankcase ventilation share.
In order to achieve the target power, the inlet and exhaust ports are designed to achieve the minimum pressure loss and the maximum flow. Therefore, the flow of the ports is increased by about 30% compared with the single turbocharged models to achieve the target power. On the high-power cylinder head with reduced eddy current strength, the curve shape and cross section of the inlet and exhaust ports greatly eliminate throttling (Fig. 3).
Figure 3 - flow optimization in cylinder head
The newly developed Bosch Fuel injection system with a maximum injection pressure of 250 MPa is responsible for the preparation of the mixture. The matched charging movement is generated by the vortex chamfering on the inlet valve seat, and the number of vortices is reduced by more than 50% compared with the single turbocharged model.
The inlet valve of dual turbocharged diesel engine is made of x85 valve steel, while the exhaust valve is made of bimetallic valve, in which the valve stem is x45 valve steel and the valve disc is 3015d special alloy steel. Both intake valve lift and exhaust valve lift are increased by 0.5 mm, i.e. 9.5 mm.
The geometry and size of the belt transmission mechanism and toothed belt remain unchanged. By improving the stiffness design, the belt can transmit greater torque, which is achieved by using tightly wound glass fiber tension belt and synthetic rubber mixture belt teeth that can bear greater load. The belt spring tensioner has also been properly improved.
When the main cooling water pump is cut off, the coolant microcirculation circuit used to cool the cylinder head is the acceptance part of the single turbocharged model. In addition to the EGR cooler and heating heat exchanger, it is also used to cool the bearing shell of the low-pressure exhaust gas turbocharger. The coolant circulation of the coolant microcirculation circuit is realized by the electric pump, and its power is greater than that of the electric pump of the single turbocharged model.
4 common rail fuel injection system
The innovation of the 2.0 l twin turbocharged direct injection diesel engine lies in the Bosch injection system with a maximum injection pressure of up to 250 MPa (Fig. 4). The target power was successfully achieved by using this fuel injection system. Meanwhile, Volkswagen Company applied this fuel injection system to the new Passat car for the first time and put it on the market
Figure 4 - common rail fuel injection system injector and high pressure fuel pump
Bosch-cp4 series double plunger high-pressure fuel pump is used to generate common rail pressure, which is driven by the engine toothed belt transmission mechanism, in which the two pump oil plungers are arranged at 90 ° to each other, and the camshaft has two lift per revolution, so the oil supply and injection are synchronized. In order to reduce CO2 emission, the injection pressure can be reduced to about 23 MPa in the range close to idle operation, which has a favorable impact on reducing fuel leakage. The whole fuel injection system is optimized to high strength. The common rail and its high-pressure oil pipe are made of high-strength steel. When making the common rail, the automatic friction corrosion process is also used to improve its strength again.
The piezoelectric actuator in the common rail injector has the maximum flow accuracy and good execution force. The hydraulic extension rod is also used to compensate the error. It transmits the force generated by the piezoelectric actuator to the on-off valve (distribution valve).
The non pressure chamber injector has 10 conical orifices, which can obtain favorable mixture preparation and homogeneous fuel atomization and mixing. Each working cycle can be injected up to 8 times: 2 pre injection, 1 main injection and 5 post injection. The minimum injection quantity is about 0.mm3.
5 booster unit
The 2 exhaust gas turbochargers (Figure 5) of the 2.0 l dual turbocharged direct injection diesel engine are located between the engine block and the cowl panel. The high-pressure exhaust gas turbocharger adopts a variable turbine section (VTG) turbocharger with a boost pressure of up to 0.15 MPa (relative pressure) and a maximum speed of up to 240 000 R / min. it takes up to 300 ms for its electric actuator to fully open the guide vane.
Figure 5 - exhaust gas turbocharger unit
The low-pressure exhaust gas turbocharger can produce a boost pressure of up to 0.38 MPa (absolute pressure), and its maximum rotor speed is up to 165 000 R / min. In order to avoid overspeed and excessive boost pressure, a pneumatically operated exhaust gas bleeding regulating valve is equipped. The compressor casing is provided with a cooling water jacket, which can pre cool the pressurized air. The turbine impellers of the two superchargers are milled, while the compressor impellers are coated with a thickness of about 25 μ M, which can resist the excessive heat load caused by low-pressure EGR. Four small chambers are integrated in the airflow muffler, which are connected with the air gap through the gap.
D5s high heat resistant steel is used as the material of exhaust manifold. The T3 sensor for measuring the exhaust gas temperature in front of the high-pressure exhaust gas turbocharger has been replaced with a reliable unilateral segmented transmission protocol.
The high-pressure and low-pressure exhaust gas turbochargers are connected on the turbine side through a pneumatically operated bypass valve with a diameter of 35 mm and position feedback (Fig. 6). Closing the bypass valve during two-stage operation at low speed causes the exhaust gas to first impact the high-pressure exhaust gas turbocharger. Fresh air enters the compressor of the low-pressure exhaust gas turbocharger, where it is slightly compressed, and then enters the high-pressure exhaust gas turbocharger for real compression.
Fig. 6 two stage and single stage operation of supercharger unit
In the speed range of about 2500 ~ 3500 R / min (Fig. 7), the turbine bypass valve is continuously opened according to the engine load, and the exhaust gas flow in the high-pressure exhaust gas turbocharger gradually decreases. At the same time, the compressor bypass valve passively bearing the spring load on the fresh air side is opened. From 4000 R / min, the single-stage operation starts when the turbine bypass valve is fully opened. At this time, Most of the exhaust gas directly reaches the low-pressure exhaust gas turbocharger, and the remaining exhaust gas flow continues to flow into the high-pressure exhaust gas turbocharger, but the high-pressure exhaust gas turbocharger is no longer compressed.
Figure 7 - operation strategy of supercharger in characteristic curve field
6 charge air cooler
The charge air cooler integrated in the intake pipe is one of the modular standard components of Volkswagen Company. In order to be used in 2.0L dual turbocharged direct injection diesel engine, the charge air cooler is enlarged to meet the improved requirements. Compared with the single turbocharged model with 10 cooling diaphragms, the new charge air cooler has 13 cooling diaphragms, the flow width has been increased from 200 mm to 250 mm, and the flow length is still 120 mm.
In the high-power scheme of dual turbocharged diesel engine, high-efficiency charge air cooler plays a decisive role. At full load, the charge air of about 210 ℃ enters the cooler, and the cooler with cooling power of 40 kW reduces the charge air temperature by 160 K to about 50 ℃.
The charge air cooler is connected to the low-temperature cooling circuit in the vehicle cooling system. The main task of the low-temperature cooling circuit is to meet the demand for charge air cooling. It uses the cooler located between the main radiator and the front air conditioning condenser. In part of its volume flow, in addition to the charge air cooler, it also includes the coolant flow in the AdBlue metering module and the compressor housing of the low-pressure exhaust gas turbocharger. On Passat car equipped with 2.0L twin turbocharged direct injection diesel engine, other innovations in the cooling water circulation circuit are the additional water cooler in the left wheelhouse and the enlarged balance water tank.
7 exhaust gas purification device arranged near the engine
The 2.0 l dual turbocharged direct injection diesel engine basically uses the same exhaust aftertreatment components as the single turbocharged model [2,3], but has been redesigned for high exhaust gas flow. The oxidation catalytic converter (DOC) and the diesel particle trap (DPF) coated with selective catalytic reduction (SCR) are connected into a compact unit through a throttling eliminating bell mouth (Fig. 8).
Figure 8 - SCR system components
The purification components arranged near the engine can ensure that the purification effect can be quickly played with high conversion after cold start, so there is no need to take measures to heat the catalytic converter. The 2.0L dual turbocharged direct injection diesel engine meets the Euro 6 emission limit. In addition, the SCR technology is designed for the actual vehicle driving emission regulations to be implemented in the future.
Because doc is arranged close to the engine, the purification and conversion of hydrocarbon (HC) / CO can be carried out soon after the engine cold start. In addition, it can also adjust the optimal no / NO2 ratio for the series SCR system. In order to be used on Passat car equipped with 2.0L dual turbocharged direct injection diesel engine, its volume has been increased by 40%, and the carrier matrix has been changed from ceramic matrix to metal matrix, so as to minimize the loss in the system.
The DPF is coated with SCR coating. Because it is close to the engine connection, it can quickly reach its working temperature after the engine cold start, and can always maintain this temperature during low load operation. The volume of DPF coated with SCR for dual turbocharged diesel engine is 10% larger than that of 2.0 l-tdi model, and the catalytic converter adopts Cu zeolite coating with high thermal stability.
A separate ammonia leakage catalytic converter is connected in series after the DPF coated with SCR under the automobile floor. In order to ensure high flow capacity, as with DOC, a metal carrier matrix optimized for pressure loss is applied.
The AdBlue metering module is modularized in the standard combination components of the diesel engine and is arranged on the transition bell between doc and SCR coated DPF. This installation position close to the engine needs to have a cooling water jacket and be included in the low-pressure circulation circuit of the engine cooling system.
Like the single turbocharged model, the dual loop EGR system in the standard combination of diesel engine consists of cooled low-pressure EGR system and uncooled high-pressure EGR system. High pressure EGR helps to reduce exhaust emissions after engine cold start, and can reduce the cooling of exhaust aftertreatment components under extremely low load. Low pressure EGR is used to reduce harmful emissions in the remaining driving range and have a favorable impact on engine noise.
8 engine management
In the modular standard combination components of diesel engine, all engine electronic control units are continuously modularized and graded to install software, and the gas system model based on it can be used for different emission levels and functions
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