192 clutches and power-stop control – Transmission control and clutch control – Common control
Reexamination Certificate
2000-07-06
2002-07-02
Lorence, Richard M. (Department: 3681)
192 clutches and power-stop control
Transmission control and clutch control
Common control
C192S1030FA, C474S008000
Reexamination Certificate
active
06412617
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a continuously variable transmission provided with:
a driving shaft connectable to an engine;
a driven shaft connectable to a load;
transmission means for transmitting a driving power between said shafts;
a clutch associated with at least one of said shafts and having an in-going shaft and an out-going shaft; and
control means at least capable of controlling said clutch.
The present invention also relates to a clutch suited for use in a continuously variable transmission.
In U.S. Pat. No. 4,606,446 a continuously variable transmission is described. In the known transmission a torque transmitting belt is clamped between the conical discs of two pulleys. The force with which the belt is clamped is controlled to transmit torque between said pulleys without slip of the belt between said discs occurring. Such slip is highly undesirable, because it leads to excessive wear of the belt and could considerably reduce the working life of the transmission. The transmission is provided with a magnetic particle clutch, which is electrically controlled to slip at a clutch slip torque lower than the belt slip torque at which the belt commences to slip. Based on measurement of variables, such as rotation speed of the engine, rotation speed of in-going and out-going clutch shafts, engine throttle opening, vehicle speed, et cetera, a clutch slip torque is calculated to be a slightly higher than a calculated developed engine torque. The belt slip torque is controlled to be a slightly higher than the clutch slip torque. In this manner it is achieved, that actual transmitted torques exceeding the clutch slip torque advantageously lead to a temporary slip of the magnetic particle clutch instead of leading to slip of the torque transmitting belt between the discs of a pulley. The actual transmitted torques may exceed the calculated engine torque due to unknown or unforeseen influences, such as torque jumps emanating from the driven shaft or inaccuracy of the calculated engine torque.
A continuously variable transmission is known from EP-A-0 446 497, which discloses a hydraulically controllable clutch as a belt slip preventing means. The transmission is constructed such that the clutch will slip at a lower torque than the belt, thereby preventing excessive belt wear. This is achieved by controlling the clutch slip torque—which is the torque at which the clutch shows a tendency to slip—to be slightly higher than the developed engine torque, but slightly lower than the belt slip torque. Although in principle the transmission should function satisfactory, in practice it has proven to be very difficult to control the clutch slip torque such that it is at all times only slightly higher than the developed engine torque. This is due to fact that the clutch slip torque is dependent on several parameters, such as temperature and rotational speeds, and furthermore may vary over a prolonged period of use. The result of which is, that either the clutch slip torque can become higher than the belt slip torque, possibly leading to belt slip, or that the efficiency of the transmission is reduced, due to the need for a higher force for clamping the belt. In EP-A-0.446.497 it is also recognised that in case a bridging clutch of a lock-up means of a torque converter is integrated in the slip preventing means, such lockup is arranged for being controlled in a pulsating way, that is either fully closed or open, i.e. either bridging the torque converter or not. This provides for a controlled continuous slip, which however pulsates. This type of pulsating control is undesirable since it leads to excess vibrations of the transmission and vehicle which influences driving comfort in a negative way and in addition leads to a poor transmission efficiency. Furthermore, the known lock-up means are not designed for such kind of operation and would yield a shortened working life.
A general disadvantage of the known solutions is, that during normal operation the means adapted for preventing belt slip have a positive torque capacity, i.e. the clutch slip torque is slightly higher than the developed engine torque. This means that, when a torque jump exceeding said clutch slip torque arrives at the transmission, the torque transferred by belt suddenly rises from the developed engine torque to the clutch slip torque. Due to inertia effects this sudden rise of transferred torque might still lead to undesired belt slip.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a continuously variable transmission wherein transmission belt slip is prevented in a robust and optimal manner, preferably with an improved efficiency, and to provide a motor vehicle incorporating such a continuously variable transmission, which possesses a decreased fuel consumption and an increased driving comfort.
Thereto the continuously variable transmission according to the present invention is characterised in that the control means are arranged to control a marginal though notional amount of essentially continuous clutch slip, such that the driving power is virtually but not entirely transmitted through the clutch during operation of the transmission for protecting the transmission against torque jumps emanating from either one of the engine and the load.
By this measure the invention vis-à-vis the state of the art overcomes the technical prejudice, that the most efficient means for preventing belt slip will be a clutch arranged and controlled such that the clutch slip torque is slightly higher than the driving torque. The present invention also overcomes the psychological prejudices that continuous slip over a clutch would lead to unacceptably poor efficiency, burned clutch plates and/or wear.
The transmission according to the invention has the advantage, that when a torque jump arrives at the transmission the clutch is already slipping. Due to the torque jerk it will slip even more, but there will thus be no threshold value of torque to be overcome by the belt clamping force before the clutch starts its protecting slipping action. In other words there will be no noticeable rise in torque transferred by the clutch and therefore by the belt, yielding an efficient protection against belt slip. According to the invention the belt slip torque may be chosen essentially equal to the developed engine torque, yielding an efficient transmission.
It is an advantage of the continuously variable transmission according to the present invention that the clutch can advantageously be used to smoothly protect the continuously variable transmission against abrupt torque jumps emanating from either a vehicle engine and/or the load, such as the wheels of a vehicle. This in addition to protecting the friction type transmission from slipping improves the driving comfort of a vehicle having the present transmission.
As a rule the clutch will already be slipping continuously before it will slip even more if said jumps arise. So the clutch will not have to be pulled loose if said torque jumps arise, which promotes a smooth and elegant accommodation of torque jumps even further.
The inventor found out further that the amount of energy, which is necessary to have the transmission means transmit a required driving torque can be reduced if some continuous, generally a marginal though notional amount of slip, is allowed to occur in the clutch, in particular by allowing the clamping or pinching force applied on a transmission means, such as a belt in the transmission means to decrease for example during transmission periods—being relatively long periods of smooth varying torque transmission, such as during normal operation or driving—the fuel consumption can be reduced noticeably. This is due to the fact that the increase of fuel loss because of some generally small allowed slip in the clutch surprisingly appears to be more than offset by a generally simultaneous decrease of required clamping force. So a net positive reduction of fuel consumption arises during a load process, such as in a vehicle incorporati
Bäuerle Peter
Spijker Engbert
Lorence Richard M.
Van Doorne's Transmissie B.V.
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