An in-wheel motor unit includes a reduction gear mechanism such as a planetary gear set in addition to the above-described electric motor, as described in, for example, a patent document 1. These are constituted as one unit. The in-wheel motor unit decreases a speed of the rotational power from the electric motor by the reduction gear mechanism, and transmits this reduced power to a wheel to drive the wheel.
Accordingly, the in-wheel motor unit needs to lubricate the above-described reduction gear mechanism.
However, when the above-described lubrication is depend on the oil raised up by rotation members within the in-wheel motor unit, oil stirring resistance increases consumed electric power of the in-wheel motor unit (the electric motor), this causes large deterioration of electricity cost (electricity economy) which is a highest priority issue for an electric vehicle.
Accordingly, in a conventional art, as described in the patent document 1, the oil in the lower portion within the in-wheel motor unit is sucked by using an oil pump whose the consumed electric power is much smaller than the consumed electric power by the oil stirring resistance. A predetermined lubrication is often performed by supplying this oil to lubrication necessary portions (portions in which the lubrication is needed).
In a lubrication control art described in the patent document 1 at this lubrication, when oil temperatures within the in-wheel motor units of a pair of left and right wheels are different from each other, the lubrication oil amount of the high temperature side in-wheel motor unit (the oil pumping amount by the oil pump) is increased than the lubrication oil amount of the low temperature side in-wheel motor unit (the oil pumping amount by the oil pump), so that the oil temperature difference between the left and right in-wheel motor units is eliminated.
In this lubrication control art of the in-wheel motor unit, the driving force difference between the left and right wheels due to the oil temperature difference between the left and right in-wheel motor units is relieved. Accordingly, it is possible to improve the running stability of the in-wheel motor drive electric vehicle.
However, in the art in which the lubrication oil amounts of the left and right in-wheel motor units (the oil pumping amounts by the oil pumps) are different from each other as described in the patent document 1, the oil levels in the lower portions within the left and right in-wheel motor units are different from each other, so that below-described problems are caused.
That is, the electric motor within the in-wheel motor unit has a diameter larger than a diameter of the reduction gear mechanism, as described in the patent document 1. When it is considered that a required mountability needs to be ensured by decreasing a radial size of the in-wheel motor unit as much as possible, it is not possible to set a large gap between the electric motor and the in-wheel motor unit case.
On the other hand, the oil pump needs to pump (suck and transfer) the oil in the lower portion within the in-wheel motor unit case even at vibration and inclination of the vehicle. The oil level in the lower portion within the in-wheel motor unit case needs to be considerably high so that the oil pump can suck the oil even at the vibration and the inclination.
For these reasons, it is unavoidable that the electric motor (rotational rotor) having the large diameter is dipped in the oil in the lower portion within the in-wheel motor unit.
By the way, in a case where (in a method that) the lubrication oil amounts of the left and right in-wheel motor units (the oil pumping amounts by the oil pumps) are different from each other as described in the patent document 1, the levels of the oil in the lower portions within the left and right in-wheel motor units are different from each other, so that below-described problems are caused.
That is, a case that the oil levels in the lower portions within the left and right in-wheel motor unit cases are different from each other means that the oil dipping amounts of the electric motors (the rotational rotors) which are positioned below the oil levels are different between the left and right in-wheel motor units.
In this way, when the oil dipping amounts of the electric motors (the rotational rotors) are different between the left and right in-wheel motor units, the oil stirring resistances by the electric motors (the rotational rotors) are different between the left and right in-wheel motor units, so that the driving force difference is generated between the left and right wheels. Accordingly, there is generated a problem that the running stability of the in-wheel motor drive vehicle is deteriorated.