7 3/8 x 9 1/4 T echnical / Build Your Own Electric Vehicle / Leitman / 373-2 / Chapter 5
Chapter 5:
Chassis and Design
engines. This section will discuss the basic components; cover differences in motor versus engine performance specifications; discuss transmission gear selection; and look at the trade-offs of automatic versus manual transmission, new versus used, and heavy versus light fluids on drivetrain efficiencies.
Drivetrains Let’s start with what the drivetrain in a conventional internal combustion engine vehicle must accomplish. In practical terms, the power available from the engine must be equal to the job of overcoming the tractive resistances discussed earlier for any given speed. The obvious mission of the drivetrain is to apply the engine’s power to driving the wheels and tires with the least loss (highest efficiency). But overall, the drivetrain must perform a number of tasks: • Convert torque and speed of the engine to vehicle motion-traction • Change directions, enabling forward and backward vehicle motion • Permit different rotational speeds of the drive wheels when cornering • Overcome hills and grades • Maximize fuel economy The drivetrain layout shown in simplified form in Figure 5-6 is most widely used to accomplish these objectives today. The function of each component is as follows: • Engine (or Electric Motor)—Provides the raw power to propel the vehicle. • Clutch—For internal combustion engines, separates or interrupts the power flow from the engine so that transmission gears can be shifted and, once engaged, the vehicle can be driven from standstill to top speed. • Manual Transmission—Provides a number of alternative gear ratios to the engine so that vehicle needs—maximum torque for hill-climbing or minimum speed to economical cruising at maximum speed—can be accommodated. • Driveshaft—Connects the drive wheels to the transmission in rear-wheel-drive vehicles; not needed in front-wheel-drive vehicles. • Differential—Accommodates the fact that outer wheels must cover a greater distance than inner wheels when a vehicle is cornering, and translates drive force 90 degrees in rear-wheel-drive vehicles (might or might not in frontwheel-drive vehicles, depending on how engine is mounted). Most differentials also provide a speed reduction with a corresponding increase in torque. • Drive Axles—Transfer power from the differential to the drive wheels. Table 5-8 shows that you can typically expect 90 percent or greater efficiencies (slightly better for front-wheel-drive vehicles) from today’s drivetrains. Internal combustion engine vehicle drivetrains provide everything necessary to allow an electric motor to be used in place of the removed engine and its related components to propel the vehicle. But the drivetrain components are usually complete overkill for the EV owner. The reason has to do with the different characteristics of internal combustion engines versus electric motors, and the way they are specified.
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