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Research Papers

Effect of the Ratio Spread of CVU in Automotive Kinetic Energy Recovery Systems

[+] Author and Article Information
F. Bottiglione

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

1Corresponding 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.

J. Mech. Des 135(6), 061001 (May 02, 2013) (9 pages) Paper No: MD-12-1075; doi: 10.1115/1.4024121 History: Received January 25, 2012; Revised March 14, 2013

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 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 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.

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Figures

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Fig. 1

Schematic of the driveline of the mechanical hybrid vehicle. Sections of the driveline are tagged with numbers. FR is the Fixed Ratio final drive of the vehicle driveline, FM is the Final Multiplier. The variator can be of two types: single-unit mechanical CVT (toroidal traction drive for instance) or PS-CVT architecture, where PG is the planetary gear. The KERS is plugged in the vehicle propshaft through a friction clutch. A second clutch (not shown in the figure) can disconnect the FM from the variable drive when the flywheel is in idle rotation.

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Fig. 2

Schematic of the power flows (arrows) in the shunted CVT architecture. PG is the planetary gear drive, FR is the fixed-ratio drive. In the forward mode or direct operation the power input is the shaft 4 and the output is the shaft 3. In the reverse operation, the power input is the shaft 3 and the output is the shaft 4.

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Fig. 3

The efficiency of PS-CVT as a function of the τPS with internal power recirculation of (a) type I and (b) type II in direct (thick dashed line) and reverse (thick continuous line) operation. The efficiency of the CVT (thin line) is constant ηCVT=0.92, the lower and upper bounds of the CVT ratio are τCVTmin=0.4,τCVTmax=2.5, the lower and upper bounds of the PS-CVT ratio (vertical thin dashed lines) τPSmin=0.2,τPSmax=2.5 corresponding to rrPS=12.5, and ηFR=0.97.

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Fig. 9

The KERS boost as a function of the lower bound of the variable drive ratio τPSmin at different values of the upper bound of the flywheel angular velocity. In the comparison the maximum energy which can be stored in the flywheel is kept constant and equal to 178 kJ. Simulations have been performed with PS-CVT with power flow of type II or III and with direct drive CVT.

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Fig. 10

(a) The KERS boost and (b) the round-trip efficiency in the FTP-75 driving schedule as a function of the lower bound of the PS-CVT ratio τPSmin. The results are shown considering the internal power circulations of types I, II, and III and the direct drive CVT (marked with a dot). In these simulations, the CVT efficiency has been supposed constant and equal to 0.97.

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Fig. 8

(a) The average power loss in the variator and (b) in the clutch in the FTP-75 driving schedule as a function of the lower bound of the PS-CVT ratio τPSmin. Internal power flows of types I, II, and III and direct drive CVT are considered. The horizontal dotted line emphasizes the value given by the system with direct drive CVT (marked with a dot).

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Fig. 7

(a) The KERS boost and (b) the round-trip efficiency in the FTP-75 driving schedule as a function of the lower bound of the PS-CVT ratio τPSmin. The results are shown for internal power flows of types I, II, and III and the direct drive CVT. The horizontal dotted line emphasizes the value given by the system with direct drive CVT (marked with a dot).

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Fig. 6

KERS boost as a function of the τPSmin in FTP-75 driving shedule. The variator is ideal (unitary efficiency) in this simulation. The horizontal dotted line emphasizes the value given by the system with direct drive CVT (marked with a dot).

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Fig. 5

The average power loss in the clutch as a function of the minimum value of the τPS in FTP-75 driving schedule. The variator is ideal (unitary efficiency) in this simulation. The horizontal dotted line emphasizes the value given by the system with direct drive CVT (marked with a dot).

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Fig. 4

The federal test procedure driving schedule. The first part is the urban FTP-75, followed by the extra-urban HFET driving schedule.

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