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RESEARCH PRODUCT

Efficiency of Split-Way CVT's. A simplified model

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ABSTRACT In this paper, a general formula is obtained for the efficiency of all types of Split-Way CVT, consisting of one or two epicyclic trains, a number of fixed ratio gears and one variable speed unit (CVU). It is based on the application of the principle of virtual works and on the simplifying hypothesis that all the dissipation is lumped inside the CVU. The comparison with some experimental results from the literature shows a surprisingly good fit, also in the neighbourhoods of zero speed ratio. INTRODUCTION The possibility of changing the speed ratio continuously between the primary and the secondary shaft of a mechanical transmission is a very interesting topic, mainly in the automotive field. With the aim at overcoming the main drawbacks of mechanical variators, in terms of transmissible power, width of the speed ratio range and efficiency, the idea of using the continuously variable unit (CVU) combined with a differential drive has stimulated the researchers in the last two decades. Although the names of this kind of mechanical transmissions are somewhat different in the literature (Split-Way CVT’s, Split Power CVT’s, Infinitely Variable Transmission IVT, Split Path Transmission, et cetera), they consist always of one or two epicyclic trains, a number of fixed ratio gears and one CVU. Broadly speaking, they can either amplify the overall speed ratio range in comparison with the simple variator, though entailing a higher variator power ratio (i.e. power recirculation in the CVU) and a lower efficiency, or can be designed to get opposite features. Anyway, the combination of a large speed range and a low variator power ratio, with a higher transmission efficiency, can be obtained by using several modes (that is associating two or more split way layouts which commute to each other by brakes or clutches). There are numerous examples of Split-Way CVT (SW-CVT) in literature, also of the multi mode type, but they use only one epicyclic train in general (for example [1] [2] [3] [4] [5], as well as many others). On the contrary, applications with two epicyclic trains are rare, though, the use of a second epicyclic train can improve the variator power ratio substantially, as shown in references [6][7][8][9]. In any case, the theoretical models proposed to describe the kinematical behaviour and to evaluate the performances in terms of power flows and efficiency, are based in general on the knowledge of the structural characteristics of each transmission unit. Nevertheless, assuming ideal working, it has been recently proved, that the overall behaviour of these drives, either with one or two epicyclic trains, depends only on three functional parameters, which can be defined regardless of the drive layout and of its structural components [10]. These three parameters, also named apertures