Proc. Natl. Sci. Counc. ROC(A) Vol. 23, No. 6, 1999. pp. 716-727 New Automatic Hybrid Transmissions for Motorcycles KUEN-BAO SHEU*, SHEN-TARNG CHIOU**, WEN-MING HWANG**, TING-SHAN WANG**, AND HONG-SEN YAN**,† *Department of Vehicle Engineering National Huwei Institute of Technology Yuenlin, Taiwan, R.O.C. **Department of Mechanical Engineering National Cheng Kung University Tainan, Taiwan, R.O.C. (Received August 19, 1998; Accepted May 19, 1999) ABSTRACT This paper presents a systematic approach to designing new automatic transmission systems for motorcycles, including conceptual design, kinematic design, efficiency analysis, engine and transmission matching, and prototype developing and testing. The basic concept is to combine a stepped design and a stepless one into a hybrid transmission system. Four hybrid transmission systems, consisting of a two degrees of freedom planetary gear train and a rubber V-belt drive unit, are synthesized. Two operating regimes over the full speed ratio range are obtained by means of engaged clutches. Kinematic design of the hybrid transmission systems is employed to obtain the major dimensions. Based on the power flow and efficiency analysis, formulas for the mechanical efficiency of the transmission systems are derived. The transmission theory of the V-belt drive unit is studied. The design procedure for matching the power source and transmission is presented. A testing rig was designed and constructed. A prototype of this new hybrid transmission system was developed to measure the transmission efficiency and test the operat- ing characteristics. The results show that the proposed design is theoretically correct and practically feasible. Key Words: motorcycle, hybrid transmissions, conceptual design, kinematic design, mechanical efficiency I. Introduction The transmission devices used in motorcycles can be divided into two categories: (1) stepped transmis- sion devices that work by alternating the gear drives, and (2) continuously variable transmissions (CVT) that transmit power by using a rubber V-shaped belt. CVTs have the advantages over gear drives of being simple Fig. 1. An input-coupled differential transmission. in construction, smooth in operation, easy to drive, low in cost, etc. However, the overall efficiency of CVTs is lower than that of gear drives. When a rubber V- CVU changes the overall speed ratio and the division belt CVT with a mechanical-type feedback control of power between the differential gear and CVU paths. system is used in a motorcycle, the transmission ef- Such a system can raise the overall transmission ef- ficiency is poor, especially during start-up and low ficiency while sacrificing some of the speed ratio range speed travel. of the CVU for higher overall efficiency. To obtain certain characteristics that are superior The concepts behind and applications of differ- to those of existing CVTs while retaining the variable ential transmissions date back to the early days of motor speed ratio property, the differential transmission has vehicles. In the Thomas transmission (Thomas, 1914- been used for many decades. It consists of a continu- 1915), used in locomotives and trucks, power from an ously variable unit (CVU) and a differential gear, as engine was split into two paths by a differential gear. shown in Fig. 1. Altering the speed ratio across the Subsequent development focused on automotive †To whom all correspondence should be addressed. − 716 − Hybrid Transmissions for Motorcycles systems. In an input-coupled system, as shown in Fig. 1, one shaft of the differential gear is linked to the input side. In an output-coupled system, one shaft of the differential gear is linked to the power output side. Theoretically six connection arrangements for the input- coupled and output-coupled system are possible, pro- vided that all three members of a simple planetary gear train can change positions. Figure 2 shows an example of a differential transmission with an input-coupled configuration. Fig. 2. An example of an input-couple differential transmission. To enhance the limited speed of motorcycles and provide excellent start-up acceleration, the concepts behind a hybrid transmission system can be used to applications. MacMillan and Davies (1965) modeled overcome the limitations placed on the speed ratio span six configurations of differential transmissions by using of the mechanical-type CVU used in differential block diagrams. White (1970, 1976) developed the transmissions. Rearrangement of the differential trans- multi-stage differential transmission and presented two mission system shown in Fig. 1 using clutches and families of differential transmissions that utilized two brakes results in the system shown in Fig. 3, where Ca coaxial differential gears. Mucino et al. (1995) pro- (Cv), B, and G are the clutches, brake, and chain device, posed a family of transmission which use two com- respectively. Alternative hybrid transmissions can be pound planetary gear trains as the differential gear and formed by employing different clutches and/or brakes a CVT unit as the CVU without using clutches and/ as described below. or chain drives. Other investigators, such as Beachley and Frank (1980), Stubbs (1981), Shockton (1984), and 1. Fixed Gear Ratio and Differential Transmis- Mucino et al. (1994), succeeded in designing differ- sion System ential transmission systems. Moreover, the configu- ration of many commercial differential transmissions A hybrid transmission system operates in two can be found in various US patents (Abbott, 1981; different regimes by engaging different clutch Gizard, 1985; Hirosawa, 1986; Itoh and Okada, 1986; sequences. Figure 4 shows an example of a fixed gear Macey and Vahabzadeh, 1987; Stockton, 1989; ratio and differential transmission system. In start-up Sakakibara and Hattori, 1989; Tervola, 1993; Brambilla, and low speed situations, power is transmitted via the 1994). Analyses of differential transmissions have high efficiency gear device by engaging clutch Ca and been numerous, including power flow analysis (Mac- applying brake B while clutch Cv is disengaged to Millan, 1961; White, 1967, 1977) and mechanical operate a fixed gear ratio regime. As vehicle speed efficiency analysis (Yu and Beachley, 1985; Hsieh and increases to and passes a medium speed, both clutches Yan, 1990; Hedman, 1993; Yan and Hsieh, 1994). Ca and Cv are engaged and brake B is released. The However, studies on differential transmissions for input power is transmitted to the chain device assembly motorcycles are not available. while the power flow is transferred from the variable The purpose of this work is to present a design ratio drive assembly to the planetary gear set to act as procedure for developing a new class of automatic a differential transmission regime. motorcycle transmissions, including a conceptual design, kinematic design, efficiency analysis, engine 2. Differential Transmission and CVT System and transmission matching, and prototype development and testing. Figure 5 shows an example of a differential II. Conceptual Design To obtain the benefits of low cost, small size, and manufacturing simplicity, the mechanical-type CVT is adopted here as the CVU of the differential transmission, and the simple planetary gear train (PGT) is adopted as the differential gear. The differential transmission systems can be grouped into input-coupled systems and output-coupled Fig. 3. A concept behind the hybrid transmission system. − 717 − K.B. Sheu et al. speed running. Therefore, the concepts behind the hybrid trans- mission system provide a new direction for studying the design motorcycle transmissions. The idea is to combine the stepped transmission function together with the stepless transmission function to obtain a hybrid system. In this way, the transmitting efficiency of the entire device can be significantly increased. A detailed discussion of these design procedures was given by Sheu et al. (1996). III. Kinematic Design For the input-coupled configuration of the differ- ential transmission, the kinematic relationship among the speed ratio of the transmission (r), the relative Fig. 4. A fixed gear ratio and differential transmission system. speed ratio of the differential gear (R), the speed ratio of the chain device (K), and the speed ratio of the CVU (V) is (Sheu and Yan, 1996) r ± KR V = . (1) 1±R When the CVU of the differential transmission reaches the peak speed ratio, V=Vmax (V=Vmin), the transmission system achieves r=rmax (r=rmin). From Eq. (1), the speed range of the CVU, Vt, can be written as V max r max ± RK V t = = . (2) V min r min ± RK Let (Din)max ((Din)min) and (Dout)max ((Dout)min) be the maximum (minimum) pulley diameters of the input axis Fig. 5. A differential transmission and CVT system. transmission and CVT system. As clutch Ca is engaged and clutch Cd is disengaged, the transmission system first operates in a differential transmission mode. Then, as clutch Cd is activated while clutch Ca is disengaged, the transmission system operates in a CVT mode with the planetary gear train locked at a 1:1 ratio. 3. Fixed Gear Ratio and CVT System The feature of this design is that the transmission system operates in two modes, a fixed gear ratio mode and a continuously variable speed ratio mode. Figure 6 shows an example. In start-up and low/medium speed situations, the transmission first operates in the 1st gear range of the fixed gear ratio and then operates in the 2nd gear range. Finally, the V-belt device with a stepless transmission function is employed for high Fig. 6. A fixed gear ratio and CVT system. − 718 − Hybrid Transmissions for Motorcycles and the output axis of the CVU, respectively; we have ± T (ω ± ω ) ηc r r c ω −ω sr = ω ω for Ts( s c)>0, (7) × T s( s ± c) V max (D in)max (D out)max V t = = . (3) V (D ) × (D ) ηc ηc ω −ω min in min out min sr = rs =1 for Ts( s c)=0, (8) According to the performance requirements of ω ω ± T s( s ± c) motorcycles and the space constraints placed on the ηc = for T (ω −ω )<0.