Going through the Gears

• Transmission—Another handy item in today’s EV conversions, the transmission’s gears not only match the vehicle you are converting to a variety of off-the-shelf electrical motors, but also give you a mechanical reversing control that eliminates the need for a two-direction motor and controller—again simplifying your work. In the future, when widespread adoption of AC motors and controllers provides directional control and eliminates the need for a large number of mechanical gears to get the torques and speeds you need, today’s transmission will be able to be replaced by a greatly simplified (and even more reliable) mechanical device. • Driveshaft, Differential, Drive Axles—These components are all used intact in today’s EV conversions. Because contemporary, built-from-the-ground-up electric vehicles like General Motors’ Impact use two AC motors and place them next to the drive wheels, it’s not too difficult to envision even simpler solutions for future EVs, because electric motors (with only one moving part) are so easily designed to accommodate different roles.

Going through the Gears

The transmission gear ratios, combined with the ratio available from the differential (or rear end, as it’s sometimes called in automotive jargon), adapt the internal combustion engine’s power and torque characteristics to maximum torque needs for hill-climbing or maximum economy needs for cruising. Figure 5-8 shows these at a glance for a typical internal combustion engine with four manual forward gears—horsepower/ torque characteristics versus vehicle speed appear above the line and RPM versus vehicle speed appear below. The constant engine power line is simply equation 5, hp 5 FV/375 (V in mph), less any drivetrain losses. The tractive force line for each gear is simply the characteristic internal combustion engine torque curve (similar to the one shown in Figure 5-7) multiplied by the ratios for that gear. The superimposed incline force lanes are the typical propulsion or road-load force components added by acceleration or hill-climbing forces (recall the shape of this curve in Figure 5-5). The intersection of the incline or road-load curves and the tractive-force curves are the maximum speed that can be sustained in that gear. The upper half of Figure 5-8 illustrates how low first gearing for startup and high fourth gearing for high-speed driving apply to engine torque capabilities.

The lower part of Figure 5-8 shows road speed versus engine speed—for each gear appears. The point of this drawing is to illustrate how gear selection applies to engine speed capabilities. Normally, the overall gear ratios are selected to fall in a geometric progression: 1st / 2nd 5 2nd / 3rd , etc. Then individual gears are optimized for starting (1st), passing (2nd or 3rd), and fuel economy (4th or 5th).

Table 5-9 shows how these ratios turn out in an actual production car—in this case a Ford 1989 Taurus SHO. Notice the first two gear pairs are in a 1.5 ratio, whereas the next two move to 1.35. Table 5-9 also calculates the actual transmission, differential, and overall gear ratios (overall equals transmission times differential) for the 1987 Ford Ranger pickup truck that will be later used in the design section. Notice that the Ranger is optimized at both ends of the range but lower in the middle versus the Taurus, reflecting the difference in car versus truck design.