The Kinetic Energy Recovery Systems (KERS) are being considered as promising short-range solution to improve the fuel economy of road vehicles. The key element of a mechanical hybrid is a Continuously Variable Unit (CVU), which is used to drive the power from the flywheel to the wheels and vice versa by varying the speed ratio. The performance of the KERS is very much affected by the efficiency of the CVU in both direct and reverse operation, and the ratio spread. However, in real Continuously Variable Transmissions (CVT), the ratio spread is limited (typical value is 6) to keep acceptable efficiency and to minimize wear. Extended range shunted CVT (Power Split CVT or PSCVT), made of one CVT, one fixed-ratio drive and one planetary gear drive, permit the designer to arrange a CVU with a larger ratio spread than the CVT or to improve its basic efficiency. For these reasons, in the literature they are sometimes addressed as devices for proficient application to KERS. In this paper, two performance indexes have been defined to quantify the effect of the ratio spread of PS-CVT on the energy recovery capabilities and overall round-trip efficiency of KERS. It is found that no substantial benefit is achieved with the use of PS-CVT instead of direct drive CVT, because the extension of the speed ratio range is paid with a loss of efficiency. It is finally discussed if new generation high-efficiency CVTs can change the scenario.

We analyse in terms of efficiency and traction capabilities a recently patented traction drive, referred to as the double roller fulltoroidal. variator (DFTV). We compare its performance with the single roller full-toroidal variator (SFTV) and the single roller. half-toroidal variator (SHTV). Modeling of these variators involves challenging tribological issues; the traction and efficiency. performances depend on tribological phenomena occurring at the interface between rollers and disks, where the lubricant. undergoes very severe elastohydrodynamic lubrication regimes. Interestingly, the DFTV shows an improvement of the mechanical. efficiency over a wide range of transmission ratios and in particular at the unit speed ratio as in such conditions in which the DFTV. allows for zero-spin, thus strongly enhancing its traction capabilities.The very highmechanical efficiency and traction performances. of the DFTV are exploited to investigate the performance of a flywheel-based Kinetic Energy Recovery System (KERS), where. the efficiency of the variator plays an important role in determining the overall energy recovery performance. The energy boost. capabilities and the round-trip efficiency are calculated for the three different variators considered in this study.The results suggest. that the energy recovery potential of the mechanical KERS can be improved with a proper choice of the variator.

Recent applications of continuously variable transmissions with large ratio spread, such as mechanical Kinetic Energy Recovery Systems or recent hybrid architectures, need the transmission to be perfectly reversible. This short paper deals with the mechanical efficiency of power-split continuously variable transmissions with particular emphasis on the switching from forward to reverse power flow. Forward and reverse transmission efficiency are calculated and compared, and the conditions which make it impossible to switch to reverse mode are studied. In particular, it is suggested that, although less efficient at high transmission ratios, a forward power circulation should be preferred because it has almost the same efficiency in forward and reverse operation.