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Chapter 5: Chassis and Design 113engines. This section will discuss the basic components; cover differences in motorversus engine performance specifications; discuss transmission gear selection; and lookat the trade-offs of automatic versus manual transmission, new versus used, and heavyversus light fluids on drivetrain efficiencies.DrivetrainsLet’s start with what the drivetrain in a conventional internal combustion engine vehiclemust accomplish. In practical terms, the power available from the engine must be equalto 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 thewheels and tires with the least loss (highest efficiency). But overall, the drivetrain mustperform 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 economyThe drivetrain layout shown in simplified form in Figure 5-6 is most widely used toaccomplish 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 powerflow from the engine so that transmission gears can be shifted and, onceengaged, the vehicle can be driven from standstill to top speed.• Manual Transmission—Provides a number of alternative gear ratios to theengine so that vehicle needs—maximum torque for hill-climbing or minimumspeed to economical cruising at maximum speed—can be accommodated.• Driveshaft—Connects the drive wheels to the transmission in rear-wheel-drivevehicles; not needed in front-wheel-drive vehicles.• Differential—Accommodates the fact that outer wheels must cover a greaterdistance than inner wheels when a vehicle is cornering, and translates driveforce 90 degrees in rear-wheel-drive vehicles (might or might not in frontwheel-drivevehicles, depending on how engine is mounted). Most differentialsalso 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. Internalcombustion engine vehicle drivetrains provide everything necessary to allow an electricmotor to be used in place of the removed engine and its related components to propelthe vehicle. But the drivetrain components are usually complete overkill for the EVowner. The reason has to do with the different characteristics of internal combustionengines versus electric motors, and the way they are specified.