Continuously Variable Transmission Provided With a Torque Converter

The invention relates to a continuously variable transmission provided with a primary shaft with a primary pulley mounted thereon, a secondary shaft with a secondary pulley mounted thereon, each pulley comprising two conical discs, at least one of the two pulleys being axially movable by means of a hydraulic control cylinder controlled by a control system, a driving belt provided between the secondary and the primary pulley, the primary pulley being drivable by an ingoing shaft via a torque converter.

Such a transmission is known from the Dutch patent application 8104001. In such a transmission it is usual that the transmission ratio is adjusted by means of pressure regulation in the hydraulic control cylinder of one of the pulleys, while the tension of the driving belt is controlled by means of regulation of the pressure in the hydraulic control cylinder, such that hardly any slip can occur between the driving belt and the pulleys and that the entire torque transmitted by an external drive on the driving shaft and increased by the torque converter can be transmitted to the driving belt.

A drawback of this known transmission is that the characteristic of the torque maximally to be transferred by the transmission with the control system does not match the characteristic of the torque delivered by the torque converter. The control system is therefore usually strongly overdimensioned for one or more speed ranges of the ingoing shaft in order to be able to lead the torque delivered by the torque converter through the transmission for other speed ranges as well. This results in loss of power and a high pressure on the driving belt so that it needs to be overdimensioned in relation to the torque delivered by the transmission.

The purpose of the invention is to obviate these drawbacks. For this purpose the invention is characterized in that the hydrodynamic torque converter has such torque converting factors at various speeds of the ingoing shaft that the torque characteristic of the torque converter approximates the characteristic of the maximum torque to be transmitted by means of the transmission and the control system for a substantial part of the speed range.

As a result of this measure advantageous use is made, at low speeds of the ingoing shaft, of an increased torque which is transmitted by the torque converter to the primary pulley and therewith by the driving belt to the secondary pulley. Said increased torque is greatly desirable, especially as a driving off torque in a motor vehicle. On the other hand the control system does not have to be overdimensioned as a result of the use of the torque converter with said characteristic for higher operating speeds, as a result of which the loss of power is reduced considerably.

In general the combustion engine of a motor vehicle has an operating speed range of about 1000 to about 6000 revolutions per minute. The normal operating speed lies roughly between 2500 and 4000 revolutions per minute. In that situation it is preferably provided that the converting factor of the hydrodynamic torque converter is 2 at most of an engine speed range of 1000-2000 revolutions per minute and approximates the value 1 in the engine speed range of 2000-4000 revolutions per minute, while it has become apparent that the most advantageous gearing of the characteristics of engine and torque converter on the one hand and the continuously variable transmission on the other hand is obtained when said converting factor lies between 1.5 and 1.8 in the engine speed range of 1000-2000 revolutions per minute. Further a so called lock-up of the torque converter can avantageously be used.

The invention will be explained with reference to a drawing in which:

FIG. 1 is a simplified diagram of the continuously variable transmission with torque converter and

FIG. 2 is a graphic illustration of the torque-speed characteristics of engine and torque converter as well as the characteristic of the control system.

A primary shaft 15 is drivably coupled to an ingoing shaft 1 by means of a torque converter 2. On the primary shaft 15 there is mounted a primary pulley 3 which comprises two conical discs 6 and 7, the disc 6 being provided axially movably on the primary shaft 15. On a secondary shaft 10 there is provided a secondary pulley 4 comprising two conical discs 8 and 9, the disc 9 being provided axially movably on the secondary shaft 10. The axially movable discs 6 and 9 form, together with a wall 16, 17 respectively, a pair of hydraulic control cylinders 18 and 19. Between the pulleys 3 and 4 there is stretched a driving belt 5. The pressure can be regulated separately in the hydraulic cylinder 18 of the primary pulley 3 and in the hydraulic cylinder 19 of the secondary pulley by means of a control system (not shown). One of the two pressures, the line pressure, thereby determines the tension in the driving belt 5 and therewith the maximum torque to be transmitted by means of said driving belt. The pressure in the other hydraulic cylinder determines the radial position of the driving belt 5 between the pulleys, and therewith the transmission ratio.

FIG. 2 illustrates a number of torque characteristics as a function of the speed of the ingoing shaft. Curve 20 thereby illustrates the maximum torque to be produced with the control unit and to be transmitted by the transmission and the driving belt. Curve 11 is the torque characteristic of the ingoing shaft 1 driven by means of an external drive, whilst curve 14 illustrates the torque characteristic after a conventional torque converter such as e.g. disclosed in the Dutch patent application 8104001. From FIG. 2 it is apparent that especially in the normal and the high operating speed ranges there is a considerable difference between the torque 14 of the torque converter and the torque 20 maximally to be transmitted with the transmission. The control unit is overdimensioned for this speed range in order to be able to transmit the torque 14 delivered by the conventional torque converter at the lower speeds. The characteristic of the torque converter 2 such as used in the invention is illustrated by means of curve 12. The corresponding characteristic 13 of the maximum torque to be transmitted by the transmission and the control unit can always be chosen much lower thereby and is at no point overdimensioned in relation to the torque characteristic of the torque converter. In this torque converter the increased torque delivered by the torque converter in the low speed range is used advantageously for an increased drive-off torque. On the other hand the torque characteristic 12 of the torque converter also matches the characteristic 13 of the maximum torque to be transmitted with the transmission for the normal and high speeds, as a result of which there will be hardly any loss of power. Moreover, the driving belt is not excessively loaded hereby so that an overdimensioning of the driving belt is not necessary.

It will be apparent that the invention has not been described exhaustively with reference to the embodiment and that modifications of the invention will be obvious to a person skilled in the art. Such modifications are considered to fall within the scope of the invention.