The continuing pursuit of more fuel efficient motor vehicles has led to the development of continuously variable transmissions by a number of manufacturers. The major difference between a continuously variable transmission and a conventional automatic transmission is that automatic transmissions use planetary gear sets to accomplish speed changes, whereas a continuously variable transmission uses pulleys and a belt to change speed. A conventional automatic transmission normally has 3, 4 or 5 fixed reduction ratios or "speeds", e.g., a 5-speed automatic transmission. The operating system of the transmission selects the appropriate reduction ratio, or speed, based on engine rpm, ground speed and throttle position. In a continuously variable transmission an almost infinite number of reduction ratios, within fixed limits, can be achieved by changing the relative radius of travel of the driving belt on the driving and driven pulleys.
The critical mechanism in the CVT is the variator. The variator is composed of two steel pulleys and a steel belt. The pulleys can be opened and closed thereby allowing the belt to travel at different radiuses. When the driving pulley is fully opened (small radius of belt travel) and the driven pulley is fully closed (large radius of belt travel) very high reduction ratios are achieved (yielding low ground speeds). Conversely, when the driving pulley is fully closed (large radius of belt travel) and the driven pulley is fully opened (small radius of belt travel) increases in output speed over input speed are achieved (yielding high ground speeds).
The novelty of this design is that the belt is made of steel. Two types of CVT transmissions exist. In one design, the belt is "pushed" or compressed to transmit power, and in the other the belt is pulled, as is more common with a V-belt. Since in both designs steel belts are used in contact with steel pulleys, the lubrication requirements are identical for both design types.
There are two critical requirements for the lubricants used in CVT transmissions: (1) control of wear and (2) control of friction. Since steel-on-steel coefficients of friction tend to be very low, e.g., 0.03 to 0.15, extremely high closing forces are applied to the pulley sides to keep the belt from slipping. Any slippage of the belt causes catastrophic wear, which quickly leads to failure. The pulleys are made to exacting limits and have a precise surface finish to allow optimum operation. No wear of these surfaces can be allowed. Therefore, an appropriate lubricant must have excellent wear control. The frictional characteristics of the belt-pulley interface are also critical. The friction must be very high to prevent slippage of the belt during transmission of high torque from the engine to the drive wheels. Too high a static coefficient of friction, however, can cause "slip-stick" behavior of the belt which leads to oscillation and audible noise in the passenger compartment of the vehicle. This "whistling" of the belt is highly undesirable.
As indicated above, fluids with too high a static, or low speed coefficient of friction are likely to cause stick-slip behavior in the transmission. Since the objective of using a CVT is to produce a vehicle with improved fuel efficiency, they are often fitted with a slipping torque converter clutch. The fuel efficiency gains possible with slipping torque converter clutches are well documented. Stick-slip behavior, when not prevented by the lubricant, manifests itself as whistling noise in the belt or vibration in the slipping torque converter clutch.
In order to successfully prevent stick-slip behavior in the slipping torque converter clutch or variator it is essential that the lubricant have excellent control of friction at low sliding speeds. More specifically the lubricant must provide a non-stick-slip friction environment at low sliding speeds. This friction characteristic is determined by calculating the friction versus velocity relationship or d.mu./dV [the change of friction coefficient (.mu.) with changing velocity (V)] of the system, where the system is defined as the lubricant and friction material being used. To successfully control stick slip behavior, this relationship, the d.mu./dV, must always be positive, i.e. the friction coefficient must always increase with increasing sliding speed or velocity. Moreover, the more positive the d.mu./dV the greater safety margin the lubricant provides against stick-slip behavior.
Since transmissions in motor vehicles are used over a wide range of ambient temperatures it is not only important for the lubricant to possess a positive d.mu./dV at one temperature, but also over a wide range of temperatures. It is this aspect of fluid performance, the control of d.mu./dV over a wide range of temperatures, more specifically at lower temperatures, in the range of about 40.degree. C., that this invention addresses.
Prior attempts have been made to formulate a continuously variable transmission fluid which provides the appropriate amount of lubrication, while allowing sufficient friction between the belt and the pulleys to avoid slippage of the belt during transmission of high torque from the engine. One such lubricating fluid is disclosed in WO 98/39400, published Sep. 11, 1998, which describes a lubricating composition comprising a mixture of: (1) a major amount of a lubricating oil; and (2) an effective amount of a performance enhancing additive combination comprising: (a) an ashless dispersant, (b) a metallic detergent, (c) an organic phosphite, (d) an amine salt of an organic phosphate, and (e) one or more friction modifiers, e.g., an amide friction modifier, a succinimide friction modifier and an ethoxylated amine friction modifier. See also U.S. Pat. No. 5,750,477 (Sumiejski et al.), which issued on May 12, 1998, and which is incorporated herein by reference. These lubricants however have not addressed the control of d.mu./dV, especially at low temperatures.
We have now found a unique combination of additives and friction modifiers that solve the difficult lubrication problems created by combination of the steel-on-steel pulley system and slipping torque converter clutch in a continuously variable transmission. In particular, the present inventors have discovered a unique zinc-free continuously variable transmission (CVT) fluid which exhibits substantially improved friction characteristics (d.mu./dV) at low temperatures (e.g. 40.degree. C.) That is, the lubricant of the present invention is particularly suited for CVT applications due its ability to provide high steel-on-steel friction coefficients and its ability to maintain a positive d.mu./dV over an expanded temperature range. This improvement in operating temperature range is accomplished by the addition of a primary amide of a long chain carboxylic acid into the additive.