The present invention relates to a continuously variable transmission means for use in a vehicle. The continuously variable transmission (CVT) varies transmitting speed ratios of a revolution number of a driven pulley to that of a driving pulley. More particularly, the present invention relates to a transmission means having a control means which permits a low engine RPM, even when an engine experiences a large load.
FIG. 1 shows a graph illustrating a relationship between an engine torque and an engine speed (RPM) of a vehicle in which a continuously variable transmission means is mounted. Heretofore, the RPM of an engine equipped with a CVT was designed such that it assumed a value determined by the curves "a" and "b" in FIG. 1. It is preferable that an engine RPM is determined so that the amount of fuel consumption be minimized over the range. The entire curve "a" shown in FIG. 1 is determined such that the amount of fuel consumed may be minimized. On the other hand, in general, vibrations in the engine increase when the engine is at a at a low RPM, while experiencing a high load. This results in the occurrence of undesirable vibration in vehicle body, and noise in a passenger compartment. Further, this vibration shortens the endurance of driving parts employed in a vehicle. It is quite difficult to determine an engine RPM especially under the condition that an engine RPM is low and an engine load is high. Hence, an engine RPM must be varied from an engine speed curve, indicated by the reference "a", to an another engine speed curve indicated by a reference "b" during the time when engine RPM is low. A point D1 on the curve "a" is a point where the curve "a" intersects with the curve "b". The curve "b" is determined independently from the minimum fuel consumption curve "a", as shown in FIG. 1. The preferable engine characteristics at a low engine RPM under a high engine load varies, according to the type of engine mounting structure to a vehicle body, the vehicle body structure and the engine operating conditions. Hence, in order to obviate the above-mentioned drawbacks the engine characteristic curve at a low engine speed is determined by the curve "b", which is located at relatively higher engine RPM's than that of the curve "c". This results in a large amount of a gasoline consumption and impairs efficient operation.