Development of new generation split drive CVT

1. Overview

Regarding trend of global environmental protection is increasing worldwide, several company are becoming an electrified, but due to its high price, we forecast that many inexpensive small gasoline vehicles still be demanded in the future. Therefore, the fuel efficiency improvement technology of the continuously variable transmission (CVT) mounted on the vehicle is very important. The weak point of the conventional CVT is the low power transmission efficiency in the high transmission ratio range by the belt driving. Moreover, expansion of the pulley diameter is required for widing the transmission ratio of driving force when vehicle departure and fuel economy by decreasing engine rotation required.
In order to solve these problems, we have developed the CVT that achieves high efficiency, wide ratio, small size, and light weight by performing torque split drive and configuration of element parts. (Fig. 1)

Fig.1 Newly developed CVT

2. Technical content

2.1 Basic cross-section and power flow
The basic cross section and power flow of the newly developed CVT are shown in Fig. 2.
There are two modes of driving, belt mode (shifting part is driven only by belt) and split mode (shifting part is driven by both of belt and gear). When it is low speed, it runs in belt mode and when it becomes high speed, it is shifted to be split mode by switching the multi-plate wet clutch.
The main specifications are shown in Tab. 1

Fig.2 Basic cross-section and rotation direction


Tab.1 Main specifications

2.2 Split drive and torque sharing ratio
In the high shift ratio region of conventional CVT, 80% torque loss of the entire transmission is from operation of oil pump and belt, improvement of the part is important.

In the oil pump, torque loss occurs during generating hydraulic pressure sandwiching the pulley to make friction force to the belt. In the belt part, torque loss occurs regarding relative slippage on each part of the belt by the hydraulic pressure, torque, and reaction force during rotation. To improve above loss, the split drive of belt and gear through planetary gear mechanism, makes transmission torque of the belt and the required generated hydraulic pressure of the pump was reduced. As a result, torque loss by belt and pump was reduced.
Fig. 3 shows the torque sharing ratio of belts and gears in split mode. This sharing ratio is decided by the elements connected to the planetary gear, the planetary gear ratio, and the shifting ratio. By studying the structure, selecting and adjusting the gear ratio, the driving belt was compared with a low share ratio of -0.2 to -0.6 (negative is the driven state).

Fig.3 Torque sharing ratio in split mode


2.3 Parallel axis pre-reduction gear
Parallel axis pre-reduction gears runs an important role in the structure of newly developed CVT. Our conventional CVT adopts a planetary gear type pre-reduction gear which is a 3-axis structure. In the case of split drive CVT, while the input is connected to the pulley the 3-axis structure, 2 more axes are required for matching the rotation of the power split gear and the output that normally should be 5 axes. By the way, the parallel shaft type front reduction gear makes it possible to adjust the rotation direction, and it can be configured with only 4 axes as shown in Fig. 2. By this smallest element configuration, reduction of transmission loss, cost reduction, and compactness were realized.

2.4 Crank coaxial small oil pump
In order to reduce the shear resistance of oil during drive, we have developed a small pump with a crank coaxial shaft drive that utilizes the space between the pre-reduction gear and the primary pulley. In each company, the use of small oil pumps is mostly chain drive, but on the other hand, the structure with low torque loss was realized at low price.

2.5 Power transmission efficiency and transmission ratio width
While maintaining the short interaxial distance of our conventional CVT which is mounted in kei- vehicles, the transmission ratio width was improved about 25% compared to the conventional CVT as shown in Fig. 4, and the transmission efficiency was improved about 8% over the entire gear ratio, especially in the high gear ratio region of the split mode.

Fig.4 Transmission ratio and transmission efficiency

3. Summary

By developing a split mechanism with its original layout, making coaxial to crank with small oil pump, etc., we have succeeded in development to contribute the customer a newly CVT which are high efficiency, large shift range and compact sizing for kei-vehicles together with protection of the global environment.

Soichiro Okudaira*1
Kyota Matsumoto*2
Yuki Yoneda*2
Daisuke Kishi*2
Naoki Ooji*2