F. Bottiglione 1 Assistant Professor e-mail: f.bottiglione@poliba.it G. Mantriota Full Professor e-mail: mantriota@poliba.it Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari, Bari (BA), Viale Japigia, 182, Italy Effect of the Ratio Spread of CVU in Automotive Kinetic Energy Recovery Systems 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 per- formance 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 ef- ficiency and to minimize wear. Extended range shunted CVT (Power Split CVT or PS- CVT), 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 ba- sic efficiency. For these reasons, in the literature they are sometimes addressed as devi- ces 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 capa- bilities 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 genera- tion high-efficiency CVTs can change the scenario. [DOI: 10.1115/1.4024121] 1 Introduction Improving the performance of ground vehicles in terms of reduction of fuel consumption and pollutant emissions is presently one of the most interesting challenges of the vehicle industry. A great research and development effort is spent towards this sub- ject, looking for short, medium and long range solutions. Electric vehicles are the most favored candidates for the long range, whereas in the short-medium, hybrid vehicles are a more feasible solution. Presently, the key winning features of hybrid vehicles are basically three: (1) driving range is comparable to traditional internal combustion engine vehicle; (2) power can be managed from two (or more) energy sources to get the optimal efficiency and pollutant emissions for any given torque-speed demand of the driver; (3) it is possible to recover the energy in braking and to reuse it when necessary. KERS can be based upon different prin- ciples [1–3] (basically hydraulic, mechanical, electrical,...) but share the same purpose: the kinetic energy of the vehicle is pre- cious and should not be dissipated in brakes. The KERS must take the kinetic energy from the vehicle during braking, store it in a storage device and then reuse it to speed-up the vehicle again. Presently the electronic system is the commonest, being the only one that is used in Formula 1. In the electronic KERS, the kinetic energy of the vehicle is converted during braking into electrical energy (ac) by means of an electric motor/generator, then by a dc/ac converter it is driven in the battery and stored as chemical energy. The process is then repeated the opposite way during speed-up. Because of the several transformations involved, the overall round-trip efficiency is about 31–34% [4]. For this reason, the mechanical KERS, which involves no energy transformation, could be one promising solution in mainstream automotive appli- cations. In the mechanical KERS, the high-speed rotating flywheel is the storage device and a stepless transmission is used to manage the power flow from-to the flywheel. Originally developed for Formula 1 motor sport [1], the CVT/flywheel system provides a highly efficient hybrid with half the weight and size than the con- ventional battery-based system. The original concept must obey the Formula 1 regulation: the KERS can recover up to 400 kJ per lap and can deliver additional power at a maximum rate of 60 kW. In the flywheel KERS, the energy transfer does not imply any energy conversion: the kinetic energy is simply removed from one source and then re-allocated to the other. This working principle gives the best round-trip efficiency when compared to other sys- tems in which energy conversions are involved. For instance, it is claimed [1,5] that the round-trip efficiency of a mechanical KERS can be of order 70% [6], about twice the efficiency of electronic KERS, in which three energy conversions are involved in recov- ery mode and three in re-use mode. Several investigations have been made to understand the effec- tive benefits that such a system at the present state-of-the-art can give in mainstream cars and trucks with different engines [3,5,7–9]. Computational results [5] demonstrate that a fuel econ- omy improvement up to 25% can be obtained in mainstream pas- senger cars and a similar result can also be obtained in trucks [7]. Moreover, the KERS is an additional (temporary) source of power that facilitates of engine downsizing with good results in terms of reduction of fuel consumption and CO 2 emissions. In Refs. [8,9], a reduction of 30% of fuel consumption is claimed when KERS is adopted on a 1.2 l TDI engine compact car, with advantages in terms of CO 2 emissions and fuel energy usage that are 10% better than the most efficient hybrid electric vehicle currently on the market. The Continuously Variable Unit (CVU) plays a key role for an effective operation of mechanical hybrid systems. As shown in Refs. [4,6], the full toroidal traction drive [10] is a suitable choice for application to KERS. Tests were performed with the XTrac P662 full toroidal traction drive with a ratio spread of about 6. Ef- ficiency up to circa 90% were measured with a power capacity of 110 kW claiming that it is feasible for application to KERS for both F1 and mainstream vehicles. However, the limited extent of ratio range limits the energy recovery capabilities in mainstream passenger cars, where, in con- trast to motor sports, there is no rule limiting the energy which 1 Corresponding author. Contributed by the Power Transmission and Gearing Committee of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received January 25, 2012; final manuscript received March 14, 2013; published online May 2, 2013. Assoc. Editor: Avinash Singh. Journal of Mechanical Design JUNE 2013, Vol. 135 / 061001-1 Copyright V C 2013 by ASME Downloaded From: http://mechanicaldesign.asmedigitalcollection.asme.org/ on 06/03/2013 Terms of Use: http://asme.org/terms