Power unit

The invention relates to a power unit, in particular for a hybrid vehicle, having a two-cylinder reciprocating piston engine which comprises two pistons guided in two cylinders in a tandem arrangement and two counter-rotating crankshafts connected to the pistons by connecting rods, and having an alternator that can rotate in the opposite direction to the first crankshaft and in the same direction as the second crankshaft. The invention is characterized in that the alternator is in driving engagement with at least the first crankshaft via a traction mechanism and comprises a counterbalance, wherein the first crankshaft is connected via a timing chain or a timing belt to a balancing shaft which carries a further balancing mass. The invention also relates to a vehicle, in particular a hybrid vehicle, having such a power unit.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application of International Application No. PCT/EP2015/072198, filed 28 Sep. 2015 and published as WO 2016/058811 A1 on 21 Apr. 2016, in German, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

The invention relates to a power unit for a hybrid vehicle having a two-cylinder reciprocating piston engine which comprises two pistons guided in two cylinders in a tandem arrangement and two counter-rotating crankshafts connected to the pistons by connecting rods, and having a alternator that can rotate in the opposite direction to the first crankshaft and in the same direction as the second crankshaft. The invention additionally relates to a vehicle having such a power unit. A power unit of the above type is known, for example, from patent WO 2012/056275 A1.

The known power unit has a two-cylinder reciprocating piston engine with counter-rotating crankshafts, which are coupled to each other at their shaft journals by spur gears. The spur gears are in engagement with each other and synchronize the counter-rotating crankshafts. The reciprocating piston engine drives an alternator which is used for electricity generation. The alternator comprises an alternator gear wheel which meshes with one of the two crankshafts. In operation, the alternator rotates in the opposite direction to a first crankshaft and in the same direction as a second crankshaft.

The known power unit is used in particular in a hybrid vehicle, and therefore high running smoothness, low-vibration and a compact design are all important factors. These characteristics are already satisfied by the known power unit. Efforts are nevertheless being made to further improve these properties.

SUMMARY

The object of the invention is therefore to further develop the known power unit in such a way that it has improved properties with regard to running smoothness and low vibration, while in particular having compact dimensions. A further object of the invention is to specify a motor vehicle having such an improved power unit.

In accordance with the invention, this object is achieved in relation to the power unit by the subject matter set forth herein and in relation to a motor vehicle.

Thus the invention is based on the idea of proposing a power unit, in particular for a hybrid vehicle, having a two-cylinder reciprocating piston engine that has two pistons guided in two cylinders in a tandem arrangement. The two-cylinder reciprocating piston engine also has two counter-rotating crankshafts connected to the piston by connecting rods. The power unit comprises an alternator, which rotates in the opposite direction with respect to the first crankshaft and in the same direction with respect to the second crankshaft. In accordance with the invention the alternator is in driving engagement with at least the first crankshaft via a traction mechanism, and has a counterbalance. The first crankshaft is also connected via a timing chain or a timing belt to a balancing shaft, which carries a further counterbalance.

By virtue of the alternator carrying a balancing mass, the alternator already acts as the first balancing shaft. The alternator therefore fulfils a dual function, namely the production of electricity on the one hand and the balancing of inertial forces on the other. Consequently, the alternator thus designed contributes to improved running smoothness. To completely compensate for the second order inertial forces, only a single separate balancing shaft is therefore required. A compact structure of a power unit is therefore implemented, which in spite of a complete Lanchester balancing, still shows significant improvements in terms of running smoothness and low vibration levels.

In a preferred variant it can be provided that the traction mechanism is a toothed belt or toothed chain. The use of a toothed belt enables a particularly low-noise design. The use of a toothed chain also facilitates another noise-reducing design, while also providing a high wear resistance. In addition, the toothed chain has advantages in terms of oil lubrication of the reciprocating piston engine.

In the case of a variant of the invention, the first crankshaft and the second crankshaft can be in direct gear engagement with each other. In other words, on each of their end faces the crankshafts can comprise sprockets or toothed crank cheeks that are in engagement with each other. The first crankshaft is additionally connected to the alternator via the traction mechanism and is connected to the balancing shaft via the timing chain.

A particularly preferred alternative provides that the traction mechanism is a toothed chain having teeth on both sides, wherein an outer side of the toothed chain engages with a sprocket of the first crankshaft and an inner side of the toothed chain engages with a sprocket of the second crankshaft. The toothed chain thus follows a relatively simple course, which is advantageous in terms of the force transmission from the sprockets to the alternator. In particular, the chain has a small number of deflection points under load. In addition, a direct gear engagement of the crankshafts with one another, or with the alternator, is avoided, which benefits the smooth running of the reciprocating piston engine.

In other preferred variants of the power unit according to the invention, it can be provided that the first crankshaft, or the balancing shaft, carries a flywheel mass. The function of the flywheel mass is to compensate for the alternating torques. Preferably, the flywheel mass is arranged on the first crankshaft, i.e. the crankshaft which rotates in the opposite direction to the alternator. Alternatively, the flywheel mass can be arranged on the balancing shaft, which depending on the installation position of the power unit in a motor vehicle, can be advantageous in order to provide a minimum possible width of the power unit in the plane of the crankshaft. It may also be provided that both the first crankshaft and the balancing shaft each carry a partial flywheel mass.

To compensate for the second-order inertial forces, it is advantageous if the translation ratio of the crankshaft to the alternator and/or to the balancing shaft is 2:1. Since the alternator with the balancing mass also acts as a balancing shaft, the translation ratio of 2:1 which is advantageous for the Lanchester balancing, i.e. the balancing of the free second-order inertial forces, should also be maintained here. This means that the balancing shaft and the alternator each rotate at double the speed relative to the crankshafts.

The first crankshaft can also comprise a control gear, which is coupled via the timing chain or the timing belt to an idler gear of a timing drive. The idler gear has a diameter equal to twice the diameter of the control gear. In other words, it is provided that the timing chain or timing belt couples the control gear of the first crankshaft to both the balancing shaft and the idler gear. The idler gear can be arranged on the end face of the timing drive. The use of double the size of the idler gear relative to the timing gear enables a translation ratio of 1:2 to be obtained between the crankshafts and the timing drive. The timing drive therefore rotates at half the speed of the crankshafts. This timing drive can be a deep-lying timing drive, which is arranged below the level of the cylinder seal of the reciprocating piston engine.

In an advantageous embodiment, the power unit according to the invention is a single alternator. The restriction to a single alternator not only benefits the compactness of the power unit, but also lowers the costs of production. Overall, a particularly efficient, low-noise and smooth-running power unit is created, which can be produced cost-effectively. At the same time the small number of whole components improves the maintainability of the power unit.

In accordance with a secondary aspect, a vehicle, in particular a hybrid vehicle, having a power unit as mentioned above is disclosed and claimed.

DETAILED DESCRIPTION

The invention is described hereafter in greater detail on the basis of an exemplary embodiment and by reference to the attached drawing. The single FIGURE shows a schematic side view of a power unit according to the invention, wherein components that are inessential to the invention are hidden.

The power unit shown in the FIGURE comprises a reciprocating piston engine with two cylinders1,2(two-cylinder reciprocating piston engine), in which pistons3,4are guided. The pistons3,4are connected to crankshafts7,8by means of connecting rods5,6. Each piston3,4is assigned to one crankshaft7,8each. The reciprocating piston engine therefore comprises two crankshafts7,8, arranged next to one another in parallel. The crankshafts7,8rotate in opposite directions. This means that the two-cylinder reciprocating piston engine is implemented in a tandem design.

The crankshafts7,8each carry sprockets9,10. The sprockets9,10are spaced apart from one another and coupled together by a toothed chain15. The toothed chain15is toothed on both sides, wherein an outer side15aof the toothed chain15is in engagement with the sprocket9of the first crankshaft7. An inner side15bof the toothed chain15is in engagement with the sprocket10of the second crankshaft8. The toothed chain15also passes to an alternator11, the inner side15bof the toothed chain15engaging in a gear wheel of the alternator11. The alternator11is therefore coupled to the crankshafts7,8by means of the toothed chain15and is driven by the crankshafts7,8. The alternator11in this case rotates in the opposite direction with respect to the first crankshaft7and in the same direction with respect to the second crankshaft8.

In some sections the toothed chain15passes between the sprockets9,10in a straight line, the straight section of the toothed chain15between the sprockets9,10being preferably at least 30 mm long, in order to ensure an exact engagement of the individual teeth of the toothed chain15in the sprockets9,10.

As is clearly visible in the FIGURE, a driving side16of the toothed chain15runs between the alternator11and the first crankshaft7, or sprocket9of the first crankshaft7. A slack side17of the toothed chain15runs between the second crankshaft8or sprocket10of the second crankshaft8and the alternator11. Since the distance between the sprocket10of the second crankshaft8and the alternator11is comparatively large, it is advantageous to provide a chain guide23between the alternator11and the second crankshaft8. The chain guide23substantially prevents any sagging of the slack side17and thus contributes to the smooth running of the power unit.

On a shaft journal of the first crankshaft7, a timing gear21is arranged. The timing gear21can be rigidly connected to the sprocket10of the first crankshaft7. In any event, it is provided that a connection between the timing gear21and the sprocket10of the first crankshaft7is rotationally fixed. The timing gear21engages with a timing chain18that couples the timing gear21to a balancing shaft12and a timing drive19. In particular, the timing chain18passes from the timing gear21to the balancing shaft12and further to an idler gear20that is seated on the front face of the timing drive19. The idler gear20is preferably designed twice as large as the timing gear21, so that a translation ratio of 2:1 is set between the timing drive19and the crankshafts7,8. The timing drive19can be advantageously designed as a deep-lying timing drive. Specifically, the timing drive19is preferably arranged below the level of the cylinder seal. This reduces the overall height of the power unit.

To balance out the second-order inertial forces, i.e. to provide the so-called Lanchester compensation, balancing masses13,14are provided. One of these balancing masses13is arranged on the alternator11. Rotating in phase with this but in the opposite direction, a further balancing mass14is arranged on the balancing shaft12. Between the balancing shaft12or the alternator11and the crankshafts7,8there exists a translation ratio of 1:2. In other words, both the alternator11and the balancing shaft12rotate twice as fast as the crankshafts7,8. The rotation of the balancing shaft12and the alternator11are each in opposite directions to one another, so as to ensure an efficient balancing of the second-order inertial forces.

To compensate for the alternating torques, which occur due to the ignition order of the reciprocating piston engine in 4-cycle operation, a flywheel mass22is provided, which is connected to the first crankshaft7. The flywheel mass22is preferably arranged on a reverse side of the reciprocating piston engine. In other words, at its longitudinal ends the first crankshaft7comprises a shaft journal, wherein the sprocket9and the timing gear21are located on a first, frontal shaft journal. On a second shaft journal, opposite to the first, the flywheel mass22is rotationally fixed. A further flywheel mass24can be connected to the second crankshaft8. The flywheel mass24which is assigned to the second crankshaft8is preferably smaller than the flywheel mass22of the first crankshaft7.

LIST OF REFERENCE NUMERALS