Auto Information Study Guide 2 for McGraw-Hill's ASVAB
Practice problems for this study guide can be found at:
The drive train gets power from the engine to the wheels. The drive train includes the transmission, driveshaft, differential, and axles on the driving wheels, which may be in front, in back, or both. In cars with front-wheel drive, the transmission and differential are combined in a "transaxle."
The drive train must
- Provide different gears so that the engine can always work at an efficient rpm (revolutions per minute), no matter what the driving speed.
- Allow the car to move backward (in reverse gear).
- Allow the engine to run when the car is not moving.
- Drive two or four wheels.
- Allow the car to turn without tire slippage.
In a manual transmission, you change gears with a clutch and gearshift. Manuals, which once had only two or three speeds, today generally have five forward speeds plus reverse. In general, transmissions have an input shaft, a layshaft, and an output shaft. The input shaft is connected to the clutch, and the output shaft is connected to the driveshaft and eventually the driving wheels.
When a transmission "changes gears," it changes the ratio of input speed to output speed. In first gear, roughly four revolutions of the input shaft turn the output shaft once. First gear is used to accelerate from a stop, and the engine must turn fast while the wheels turn slowly. First gear allows fast acceleration because it multiplies torque at slow driving speed.
When you shift gears in a manual transmission, the gears do not engage or disengage; all the gears are always engaged. Instead, the gears are shifted by a small collar attached to the output shaft. Dog teeth on the side of this collar catch holes in the side of the gear, connecting the gear to the output shaft. When the collar is disengaged, the gears spin freely on the output shaft.
The gearshift moves the collar, and synchronizers between the collar and the gear allow the dog teeth to engage the gears. If you shift too fast, these synchronizers don't have time to engage, and the gears clash.
To change direction for reverse gear, there is an idler gear between the layshaft and the output shaft. In forward gears, the input and output shafts rotate in opposite directions. But the idler gear causes the output shaft to turn in the same direction as the input shaft.
Clutch The clutch disconnects the engine from the transmission, so that you can shift gears. The clutch also allows you to idle at a traffic light without shifting into neutral. A clutch usually has three plates, which are controlled by the clutch pedal.
The clutch is engaged when the clutch pedal is up and disengaged when the pedal is down.
When the clutch is engaged, springs push the pressure plate against the clutch disk, pressing the clutch disk against the flywheel. Because of friction between the pressure plate and the flywheel, the input shaft and flywheel rotate together. In this position, neither the throwout bearing nor the clutch plates wear.
When the clutch is disengaged, a cable or hydraulic piston moves the release fork, pressing the throwout bearing against the diaphragm spring, moving the pressure plate away from the clutch disk, and disconnecting the flywheel from the input shaft. The throwout bearing does get wear in this position. The pressure plate wears when the clutch is partly engaged, mainly when starting from a standstill in first gear.
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