"The engine of compressed natural gas (CNG) is an important part of the vehicle power plant project to continuously reduce emissions. Therefore, Volkswagen and its partners began to focus on CNG, with the goal of large-scale application of this filling station in Germany and increasing the number of natural gas vehicles in the German domestic market. The new 1.5 l-ea211-tgi-evo supercharged direct injection compressed natural gas engine introduced in this paper can operate with liquid and gaseous fuels. In addition to the high compression ratio, the most important feature of this kind of engine is the efficient use of TGI Miller combustion process and its combination with VTG turbocharging.
Key words: pressurized direct injection compressed natural gas combustion
1 background
Volkswagen Group plans to use compressed natural gas (CNG) as a sustainable vehicle fuel, and cooperates with suppliers, network operators and gas station operators. In 2007, it began to study CNG. By taking various measures, it realized the use of CNG as an alternative fuel in only one year, The research objectives are more specific: by 2025, about 2000 CNG gas stations will be built in Germany, and the supply will be increased to millions of CNG vehicles. Since the reduction of the tax burden on CNG fuel will continue until 2026, this policy will benefit relevant users greatly. Driving CNG vehicles is both economical and environmentally friendly.
Volkswagen Group expects that the use of CNG fuel will have various meanings in the future. Among them, natural gas produced from liquid energy will bear a large share of global energy supply: the International Energy Agency (IEA) predicts that in some cases, its share will even rise to more than 25% by 2040. Conventional natural gas based on fossil natural gas has not only provided good potential for reducing CO2 emissions due to its chemical characteristics, but also opened new opportunities for synthetic gaseous fuels. The so-called new synthetic energy (e-fuels) fuels have the potential to significantly reduce emissions. They are synthetic fuels, such as renewable current production through water and carbon dioxide.
2 based on ea211-tsi-evo gasoline engine
By studying the technical potential of CNG automotive and e-fuels to improve the environment, Volkswagen believes that it is obliged to improve the possibility of its production and promotion. Subsequently, Volkswagen provided a CNG power unit for golf class cars in its product series, which can improve the supply of modern CNG power units (Figure 1), which is characterized by high efficiency and relatively low fuel consumption.
Figure 1 CNG engine of Volkswagen Group
With the launch of 1.5 l-tsi-evo turbocharged stratified combustion direct injection (TSI) gasoline engine in 2006, Volkswagen put the turbocharged stratified direct injection Miller cycle (TSI Miller) combustion process into mass production for the first time. When developing TSI power plant, it has been considered to be used in TSI Miller cycle combustion process. In addition, the combustion process, combined with variable turbine geometry section (VTG) pressurization, can be ideally suitable for binary operation (liquid or gaseous fuels can be used respectively). Coupled with camshaft phase adjustment, it ensures efficient operation whether using CNG or gasoline. 1.5 l-tgi-evo supercharged direct injection compressed natural gas engine (Figure 2) has become the technical basis for further development. In the further expansion of vehicle production business, Volkswagen provides CNG and e-fuels fuel that can improve the environment (Figure 3). Therefore, this kind of power unit is an important contribution of Volkswagen.
Figure 2 new 1.5 l-ea211-tgi-evo CNG Engine
Figure 3 CNG models of Volkswagen Group
3 new ea211-tgi-evo CNG Engine
1.5 l-tsi-eva supercharged stratified combustion direct injection (TSI) gasoline engine has provided a good foundation for the transformation of the power unit into CNG model in its basic structural design. Therefore, only a few modules need to be added or adjusted when developing 1.5 l-tgi-evo supercharged direct injection compressed natural gas engine, At the same time, the technical basis of tsi-evo model still needs to be used to give full play to the potential of CNG fuel as much as possible. The focus is to achieve as high engine efficiency or as low fuel consumption as possible while having excellent acceleration response characteristics and expanding power.
4cng special parts
Modular standard parts are enough to match CNG special parts with 1.5 l-tgi-evo supercharged direct injection compressed natural gas engine (Figure 4). Only its cylinder head needs to be further developed and can also be applied to the gasoline engine variant of tsi-evo standard parts in the near future. The basic modification is the newly designed flow optimized cooling water jacket, which helps to reduce the pressure loss of the cooling system. In addition, the intake camshaft has been further adjusted, and a fast hydraulic camshaft phase regulator with a central regulating valve is adopted. Its high adjustment speed helps to improve the inflation adjustment process.
Figure 4 CNG special components and modules
5 adaptability of structural design
The high explosion resistance of methane fuel allows the selection of ignition timing with optimized efficiency, so the peak pressure is as high as 13 MPa. However, the earlier position of combustion center of gravity leads to higher wall heat conduction, coupled with the lack of evaporation latent heat, resulting in higher component temperature. Due to this series of changes, tgi-evo models must be qualified for these requirements.
6cng matching
On tgi-evo engine, the materials of valve and its valve guide and valve seat ring need to adapt to the poor lubrication performance of CNG. With the help of nitriding process, the valve and valve seat ring are matched with the higher thermal hardness required for CNG operation. In addition, the wear resistance of the intake valve guide should be improved.
The cylinder head gasket consists of three materials. The final 0.2 mm range of the seating slope on the intake and exhaust convex contour line is flattened so that the valve can be closed more slowly, so as to reduce wear. Due to the poor ignitability of methane, the ignition coil shall provide higher ignition voltage than TSI model.
In the crank connecting rod mechanism, the connecting rod bearing should adapt to a higher pressure level. In order to improve the wear resistance, the second ring groove of the piston is optimized by additional process steps anodizing for CNG operation. Similarly, the natural gas common rail and intake pipe have been further developed and optimized for tgi-evo model on the basis of previous projects. The basic structure and two-stage pressure reducing function of the natural gas pressure regulator are taken from the 3-cylinder ea211-1.0 l-tgi model, but it has been further developed in terms of natural gas flow and heating.
7 combustion process and pressurization
The basic principle of Volkswagen TSI Miller cycle combustion process is also applied to 1.5 l-tgi-evo engine, which is based on the following four points (Figure 5):
(1) The geometric compression ratio is increased to 12.5:1 to improve the efficiency of the operating range;
(2) The end of compression temperature is reduced by early closing of inlet valve (FES) and subsequent expansion cooling in inlet stroke;
(3) In order to use the flame propagation process with FES to realize ventilation optimization, so as to reduce the knock tendency in the torque range under low-speed working conditions;
(4) VTG pressurization is used for the first time in mass production to achieve a high degree of pressurization to compensate for the inflation loss in the FES process and the loss of the highest total efficiency determined by the pressurization and combustion process.
Figure 5 1.5 valve timing of l-tgi-evo engine
This new combustion process shows decisive advantages even when CNG is used on 1.5 l-tgi-evo engine. The early closing of the intake valve and effective charge air cooling reduce the charge temperature, and then reduce the compression end temperature of the cylinder charge. Together with the optimized combustion chamber cooling and the peak pressure of the engine up to 13 MPa, it leads to the location of the combustion center of gravity with optimized efficiency and reduces the exhaust gas temperature. The favorable expansion ratio (caused by FES and 12.5:1 high compression ratio) and the exhaust manifold integrated in the cylinder head also help to reduce the exhaust gas temperature. Just like the conventional CNG engine, the 1.5 l-tgi-evo engine runs in the whole characteristic curve field λ= 1 excess air coefficient operation, the maximum exhaust gas temperature is only 880 ℃.
VTG exhaust gas turbocharger can be used at low exhaust gas temperature. It plays an important role in the combustion process of TGI Miller cycle together with the inflation adjustment of intake camshaft with adjustable phase. In the wide speed range up to the rated speed, it can obtain favorable scavenging pressure difference and reduce air exchange work. The effect of eliminating throttling produced by this combustion process under partial load has the potential to significantly reduce CO2 emission, and it also has advantages in low speed and high load (low speed torque (let) range), Because the low knock tendency of CNG fuel allows the use of very high exhaust back pressure, sufficient turbine power can be established to compensate for inflation defects caused by the formation of external mixture as much as possible during MPI multi-point valve injection.
Therefore, the low fuel consumption of 1.5 l-ea211-tgi-evo supercharged Direct Injection CNG engine is attributed to two important effects of TGI Miller cycle combustion process: reducing compression loss and reducing air exchange work. Fig. 6 shows the effect of improving efficiency compared with 1.4 l-tgi supercharged direct injection compressed natural gas engine in a wide range of characteristic curve field, especially in the range of partial load and let operating conditions.
Figure 6 relative efficiency advantage (left) of 1.5 l-tgi-evo engine (96 kW) compared with 1.4 l-tgi engine (81 kW) and CO2 emission advantage (vortex) of 1.5 l-tgi-evo engine (96 kW) compared with 1.5 l-tsi-evo engine (96 kW) (right)
In addition, the advantages of TGI Miller cycle combustion process are shown in terms of full load and dynamic characteristic values (Fig. 7). The torque has reached 200 N · m at 1400 R / min and the power is 96 kW at 5000 R / min. When CNG engine runs on gas fuel, it reaches the same value as the corresponding state of TSI gasoline engine, and is not inferior in mobility.
Figure 7 1.5 l-tgi-evo engine and 1.4 l-tgi engine under full load and load at 1 500 R / min
Comparison of load sudden change (partial schematic)
8 dynamics challenges
In order to obtain excellent dynamic performance, the VTG characteristics, camshaft valve timing, exhaust back pressure limit and boost pressure adjustment are carefully calibrated. As a result, the load sudden change adjustment effect shown in Fig. 8 is obtained at 1500 R / min.
Fig. 8 dynamic regulation of 1 500 R / min load sudden change (schematic diagram)
8.1 stage 1: initial test status
The intake camshaft is in the early closing (FES) position of the intake valve with the best efficiency, and the VTG guide vane in the turbocharger is opened. At this time, the engine runs in the non pressurized state. Due to the small exhaust back pressure, the opened guide vane has a favorable effect.
8.2 phase 2: sudden load change
When the accelerator pedal is pressed, the boost pressure demand increases suddenly, and the VTG guide vane is completely closed, so as to achieve the maximum exhaust back pressure. The available compressor driving power can quickly establish the boost pressure. In order to further increase the torque, adjust the intake camshaft to the best inflation position. Based on the explosion resistance of CNG fuel, this strategy can be adopted without the limitation of detonation combustion. The stability of TGI Miller cycle combustion process allows the excessive exhaust gas back pressure to be reduced with the optimal position of combustion center of gravity.
Before the target boost pressure is reached, the dynamic exhaust back pressure is limited by opening the VTG guide vane in advance, and the VTG will be opened further and quickly in the process of adjusting the boost pressure, otherwise the driver will feel abnormal in the process of torque establishment. The reason is the sudden transition from strong negative sweep air pressure difference to obvious positive sweep air pressure difference. This regulation strategy will not adversely affect the establishment of boost pressure, because the good flow in the turbine greatly improves the turbine efficiency.
8.3 phase 3: steady state phase
Once the boost pressure is adjusted to the target value, the intake camshaft is adjusted to the best position of efficiency and fuel consumption, so as to achieve the highest turbine efficiency, so as to obtain a positive scavenging pressure difference in a wide range of engine characteristic curve field, and the exhaust back pressure is significantly lower than that in dynamic demand in the steady-state operation range.
9 Final dynamic characteristics
TGI Miller cycle combustion process can greatly reduce the excessive dynamic exhaust back pressure. Although there are many unstable flow states in the turbine, it can still achieve better dynamic response characteristics when burning gasoline. An important issue in the development is that the boost pressure can be adjusted during the rapid transition from maximum negative sweep pressure difference to positive sweep pressure difference without adverse impact on driving performance.
10 exhaust gas system
The exhaust gas system of 1.5 l-tgi-evo engine has been optimized for CNG operation. One of the major challenges is the heating of catalytic converter for emission optimization, just as Volkswagen has applied to 3-cylinder 1.0 l-tgi supercharged direct injection compressed natural gas engine for the first time. However, due to the low exhaust gas temperature on the 4-cylinder Miller cycle combustion process engine, this combustion process must be further developed. In particular, there are great differences in the low load range, which brings additional challenges to the operation preparation of the catalytic converter.
eleven λ Separate adjustment method
Due to the reaction characteristics of methane, it needs higher temperature in the catalytic converter for conversion and purification. In order to exceed the temperature threshold of 500 ℃ which is decisive for the conversion as soon as possible during the warm-up operation stage, a λ Separate adjustment method, i.e. lower than stoichiometric mixture is used for each two cylinders (until λ= 0.85) and above stoichiometric mixture (up to λ= 1.15) ignition operation, so that the normal heating stage of catalytic converter can be significantly shortened by adjusting the ignition angle later, which can obtain obvious benefits in terms of fuel consumption and exhaust emission. This function is more effective than other CNG models in a wider range on tgi-evo engines. This basic function has been used since the 3-cylinder 1.0 l-tgi engine and passed on to the 1.5 l-tgi engine.
12 vehicle structure layout
The golf TGI BlueMotion (BlueMotion = blue drive technical version, translation: "blue drive" means the blue sky effect driving device to reduce exhaust emissions) car is equipped with a 1.5 l-tgi-evo supercharged direct injection compressed natural gas engine for the first time, and is equipped with three cylindrical compressed natural gas tanks (Fig. 9). Car knot arranged in this way
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