"1 basic scheme of all in-line engines
BMW put the newly developed engine series on the market of the new mini car, and finally expanded it to include all 3-cylinder, 4-cylinder and 6-cylinder gasoline and diesel engines. All in-line engines adopt the same basic engine scheme and unified peripheral auxiliary equipment.
BMW's new gasoline engine has three 3-cylinder models and a 4-cylinder variant, while the 3-cylinder engine has two displacements of 1.2 L and 1.5 L. The rated power of the 1.2L model starts from 75 kW, covering the power positioning of the mini one car. The 1.5 l gasoline engine is equipped on the Mini Cooper car with 100 kW rated power, while its special high-power variant is used on the new BMW I8 hybrid sports car with 170 kW rated power. The new 4-cylinder gasoline engine takes 140 kW rated power as the power unit of mini cooper s car for the first time, and will be installed on BMW and mini car with significantly higher power [1].
2 goal setting
The main goal of the new engine family is to develop efficient, compact and powerful engines and meet the increasingly stringent emission regulations in the world in the future. At the same time, the following performance objectives are specified in the design specification: ⑴ in top applications, the maximum power rise can reach 115 kW / L; (2) high torque (low-speed torque) can be exerted by slightly exceeding the idle speed; (3) the dynamic response performance is comparable to that of the naturally aspirated engine with the same power; (4) the fuel consumption of standard test cycle and actual use of users is low; (5) it has the potential to meet the limits of the world's most stringent exhaust emission regulations; (6) aluminum cylinder block crankcase with light structure; (7) minimize the friction loss by optimizing the structural design of the basic engine; (8) through the mass balance system, both 3-cylinder engine and 4-cylinder engine can obtain high operation stability. In addition, the following effects can be achieved with the universality of gasoline engine and diesel engine series: ⑴ flexibly organize production in multiple production bases (3-cylinder, 4-cylinder and 6-cylinder gasoline engine and diesel engine); (2) launch different technical schemes easily and quickly on the same basic engine platform; (3) easily and quickly push out the variant machine; (4) all variants have a unified interface with the vehicle. Other preconditions and challenges are that even with the greatest degree of versatility, all variants can achieve the best performance design to ensure their top position in the competitive models.
Table 1 main technical specifications of 3-cylinder and 4-cylinder gasoline engines
project
parameter
Number of cylinders
3-cylinder engine
4-cylinder engine
low power
high power
Maximum power / kw
100(4 500 r/min)
170(5 800 r/min)
141(5 700 r/min)
Maximum torque / (n · m)
220(1 250 r/min)
320(3 500 r/min)
280(1 250 r/min)
Oil cut-off speed / (R · min-1)
6 500
6 500
6 500
Liter power / (kW · L-1)
sixty-six point seven
one hundred and thirteen point three
seventy point five
Lifting torque / (n · m) · L-1)
one hundred and forty-six point six
two hundred and thirteen point three
one hundred and forty
Maximum specific work / (kJ · L-1)
one point eight two
two point three five
one point seven
Displacement / ml
one thousand four hundred and ninety-eight point eight
one thousand nine hundred and ninety-eight point three
Cylinder diameter / mm
eighty-two
eighty-two
Stroke / mm
ninety-four point six
ninety-four point six
Stroke cylinder diameter ratio
one point one five
one point one five
Single cylinder displacement / ml
four hundred and ninety-nine point six
four hundred and ninety-nine point six
Connecting rod length / mm
one hundred and forty-eight point two
one hundred and forty-eight point two
Connecting rod crank ratio
zero point three one nine
zero point three one nine
Cylinder center distance / mm
ninety-one
ninety-one
live
stopper
Compression height / mm
thirty-three point two
thirty-three point two
Fire shore height / mm
seven
seven
piston pin
Diameter / mm
twenty-two
twenty-two
Length / mm
fifty-five
fifty-five
valve
Inlet / exhaust valve diameter / mm
30.0/28.5
30.0/28.5
Intake / exhaust valve lift / mm
9.9/9.7
9.9/9.7
Inlet / exhaust valve rod diameter / mm
5.0/5.0
5.0/5.0
Compression ratio
eleven
eleven
3 design scheme
The new 3-cylinder and 4-cylinder gasoline engines (Figure 1) still follow the design dimensions of BMW's in-line engine, with a cylinder center distance of 91 mm and a single cylinder displacement of 0.5 L. The 3-cylinder engine scheme for miniOne car is a special case, with a single cylinder displacement of 0.4 L and a total displacement of 1.2 l (Table 1). In terms of structural design and peripheral auxiliary equipment, these engines basically adopt supercharging. In order to create the best preconditions for the best performance and vehicle integration, the charge replacement is designed as a cross flow type, with intake from the left side of the engine and exhaust from the right side. The chain drive mechanism is arranged at the rear end of the engine, so that all auxiliary equipment can be concentrated on the inlet side. The right side can be used to arrange the exhaust turbocharger and the exhaust aftertreatment device near the engine. Fig. 2 shows a vertical and horizontal sectional view of a 4-cylinder direct injection gasoline engine.
4 structural design of basic engine
The cylinder block crankcase with all aluminum structure is designed for all models (Fig. 3). The camshaft chain drive mechanism is arranged on the flywheel side. The working surface of the cylinder liner adopts an innovative arc metal wire spray coating with a thickness of only 0.3 mm, which is extremely wear-resistant, and has good heat dissipation performance compared with the traditional gray cast iron cylinder liner. All models adopt forged steel crankshaft and induction quenching at the bearing part. The balance shaft is integrated in the crankcase of the cylinder block. On the 3-cylinder engine, it is driven by the front end of the crankshaft, while on the 4-cylinder engine, the driving gears of the two balance shafts are integrated on the crank arm at the rear end of the crankshaft. The main bearing adopts bimetallic aluminum bearing, while the connecting rod bearing adopts trimetallic bearing coated with polymer coating. The forged steel connecting rod is designed as a stepped structure, and the small end of the connecting rod is cast and rolled into a solid bronze sleeve.
In order to balance the first-order free inertia moment, all 3-cylinder engines adopt the forged steel balance shaft supported in the crankcase of the cylinder block. It has two counter arranged balance weights, one of which is directly forged on the balance shaft. The gear integrated on the crankshaft meshes with the transmission gear at the front end of the balance shaft to directly drive the balance shaft. The transmission gear is sintered, the hub has elastic damping that can isolate noise, and integrates a second-order balance mass. In order to reduce transmission power, the balance shaft is supported by rolling bearings.
All 3-cylinder gasoline engines and diesel engines adopt the same structural layout to reduce the vibration of the crankshaft. However, considering that the combustion process and the motion quality of crank connecting rod mechanism are different, the unbalanced mass and its transmission mechanism on gasoline engine and diesel engine have been specially matched.
In order to 100% balance the second-order inertial force causing vibration, the 4-cylinder gasoline engine uses two forged steel balance shafts supported in the crankcase of the cylinder block, which rotate at twice the crankshaft speed. In order to reduce the transmission power, the two balance shafts of the 4-cylinder machine are also supported by rolling bearings.
Through the height difference of two balance axes with the same structure, the second-order inertia moment is added. Because its height difference is slightly smaller than that of the old 4-cylinder model, the generated alternating torque moves to the direction of low speed, which has advantages for operation comfort close to the low-speed torque range.
The 3-cylinder and 4-cylinder gasoline engines use the same swing slide valve oil pump that adjusts the volume flow according to the characteristic curve field to supply oil. It is combined with the vacuum pump integrated in the same shell. The tandem pump is arranged in the oil sump of the oil pan and driven by a chain. The engine oil pressure is adjusted by a proportional solenoid valve according to the characteristic curve field stored in the electronic control unit. A combined oil pressure and temperature sensor is installed on the main oil passage, and its signal can be used for the oil pump to adjust according to the characteristic curve field and the thermal management system. The oil level sensor in the oil pan constantly monitors the oil level. The oil filter module made of plastic has an integral oil cooler.
The Valvetronic variable valve mechanism is further developed on the basis of the old model. In order to move the position of the eccentric, look for a position on the front side of the intake side of the cylinder head and integrate it into the intake device housing. By further developing the intake valve transmission mechanism, the structural space is significantly reduced, and the position of the intake camshaft and the eccentric is adjusted to each other, which significantly reduces the structural height (Fig. 4).
The new middle rod and slideway position reduce the force transmitted to the cylinder head, while the slideway is fixed on the bearing seat by only one screw and positioned by two precise positioning surfaces on the cylinder head. The return spring of the intermediate rod is supported between the cylinder head and the bearing seat, so it does not need a fixed point. The size of the eccentric shaft is the same as that of the inlet camshaft, while the exhaust camshaft drives the fuel high-pressure pump of the fuel injection system through a three-sided cam.
The driving force of the chain drive is transmitted through 2 camshaft phase regulators. The three wing rotor actuator in the camshaft phase adjuster enables the inlet camshaft to have a phase angle adjustment range of 70 º Ca and the exhaust camshaft to have a phase angle adjustment range of 60 º ca. The chain drive mechanism newly developed by BMW for gasoline engine is arranged at the power output end. The crankshaft drives the series pump (composed of oil pump and vacuum pump) downward through the rear short chain and drives the intermediate sprocket upward. The intermediate sprocket has two chain rails (24 / 32 teeth) to ensure that the transmission ratio from the crankshaft (24 teeth) to the camshaft phase adjuster (36 teeth) is 2:1, so as to reduce the diameter of the phase adjuster and reduce the structural height of the engine. The timing transmission mechanism above the intermediate sprocket drives the camshaft, and the chain guide rail screwed to the cylinder head between the two phase regulators prevents the chain from jumping. The intermediate drive chain and timing drive chain each have a chain rail. There is no guide or tensioning device in the chain drive mechanism of the oil pump.
5 pressurization
BMW's new engine is also innovative in supercharging technology. The 4-cylinder engine has a dual turbochannel turbine module integrated with the exhaust manifold. This structural type can ensure that the two waste gas streams can be reliably separated from the turbine, so that higher torque can be obtained after idling speed, and the potential of transient response performance can be further tapped. In order to make the exhaust manifold have enough room for expansion and can be installed in a compact space, the exhaust manifold is fastened on the cylinder head with a sliding pressing plate flange. Water cooling is no longer used in the structure of this integral double vortex exhaust turbocharger.
The structure design of the new 3 cylinder gasoline engine is quite different. In the mass production car, the water-cooled aluminum exhaust gas turbocharger was first made by casting technology. The structure of fastening the exhaust manifold to the cylinder head with a sliding pressing plate flange has a large degree of freedom in design. Through expensive computational fluid dynamics simulation and combined with part strength calculation, the wall heat flow is reduced. Although this structure type costs additional cost for cooling, it not only reduces the mass, but also obtains the potential to significantly reduce CO2 emission. Compared with the uncooled steel exhaust gas turbocharger, the exhaust gas temperature in front of the catalytic converter is significantly lower than 850 ℃, making the catalytic converter aging like unused.
The interface between the turbine module and the cylinder head is close to the flange surface of the cylinder head, so that the conventional steel exhaust turbocharger flange can also be installed on the same cylinder head on BMW I8 top models. In order to unify the assembly status of the exhaust device on the vehicle, the flange positions of the catalytic converter of the 3-cylinder and 4-cylinder models are the same (Fig. 5).
6 thermodynamics, combustion and fuel injection
On the basis of the old model, the new standard component gasoline engine has further developed the dual turbocharged combustion process to meet the more stringent exhaust emission regulations and further improve the efficiency. Therefore, the cylinder diameter is 2 mm smaller than that of the old model, i.e. 82 mm, and a significantly larger stroke cylinder diameter ratio, i.e. 1.15, is obtained, which is the best value for thermodynamics and friction.
The vertical profile of the combustion chamber shown in Fig. 6 shows that, compared with the combustion process of the old type, although the diameter of the piston is narrowed, the piston top pits are obviously wider, matching with the wide spray design formed by the porous injector with reduced flow rate, and the improved gas mixture is obviously improved under the combined action of increasing the charging movement, thereby achieving a more rapid combustion. Because of the reduction of the rated flow of the porous injector, the penetration depth of the spray has been greatly reduced.
In addition, the spray formation can be optimized through the distribution of the injection volume and the spray direction of the nozzle. It can find a good compromise between the requirements of the catalytic converter's heating (coating capacity) and the running state of the heat engine, and at the same time, it can also reduce the wetting of the piston and cylinder liner wall and intake valve to the minimum. The second generation variable valve mechanism can obtain greater freedom in fuel injection and can be applied to the minimum fuel injection in a wider range of injector characteristic curve
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