Driveline losses, which include the transmission, differential, and final drive gear, consume 5 to 6 percent of the energy input to the vehicle. The energy distribution in a typical gasoline vehicle is shown in Figure 5.2. TRANSMISSION FUNDAMENTALS FOR ACHIEVING FUEL CONSUMPTION REDUCTIONS The chapter concludes with the committee’s findings and recommendations on transmissions.
In the second section, the committee’s estimates of the effectiveness of each technology are presented, along with the committee’s approach to estimating costs. The second section discusses specific transmission-related technologies, some considered and others not considered by the Corporate Average Fuel Economy (CAFE) rule analysis. The first section discusses the fundamentals of transmissions and covers design architecture, number of ratios and ratio spreads, and parasitic losses within various transmissions. These approaches are being pursued by vehicle manufacturers and suppliers and are discussed in this chapter.
H FORCE TRANSMISSION FULL
Although the CVT provides the ideal ratio for any operating condition, the full potential of this attribute is not realized because the parasitic losses can exceed those of a conventional automatic transmission.
The continuously variable transmission (CVT) has seen a recent resurgence, and its penetration in the new vehicle fleet has increased significantly. However, drivability and consumer acceptance issues remain for the most efficient dry-clutch DCTs. Dual-clutch transmissions (DCTs), with significantly lower parasitic losses, have already been introduced in some production vehicles and are providing reductions in fuel consumption. In addition to pursuing improvements in conventional automatic transmissions, which dominate light-duty vehicles in the United States, a variety of alternative transmission designs are being developed and introduced into production. In addition to these considerations, adapting transmissions to new fuel-efficient, turbocharged, downsized engines with fewer cylinders and to diesel engines with higher torque fluctuations generally necessitates additional torsional vibration damping. Second, reducing parasitic losses within the transmission improves transmission efficiency and reduces vehicle fuel consumption. These design features concurrently provide smaller change steps to maintain nearer-to-optimum engine speeds. Transmission design affects vehicle fuel consumption in two ways: First, increasing the number of gear ratios and providing a larger ratio spread allows the internal combustion engine to operate more often in regions of high efficiency. Integral to many automatic transmissions are hydraulic torque converters, which also provide significant torque multiplication under starting and low speed conditions. Figure 5.1 illustrates a six-speed transmission enabling engine torque variation across changing engine speeds.
The basic function of a transmission, together with the differential, is to reduce the relatively high engine output speeds to the vehicle’s slower wheel speeds and to increase the torque applied to the wheels.
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