Auto Information Study Guide 2 for McGraw-Hill's ASVAB (page 2)
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.
Automatic transmissions change gears automatically to suit driving conditions. Modern automatics often have four forward gears, plus reverse, park, and neutral. The transmission itself uses a complex set of gears to do the shifting.
Instead of the manual clutch just described, automatics link the engine to the transmission with a hydraulic mechanism called a torque converter. The torque converter has two blades that look much like propellers. The drive blade, attached to the engine output shaft, churns up hydraulic fluid the way a propeller moves water. The driven blade is caught by the stream of hydraulic fluid and turns like a pinwheel in a breeze.
When the engine is idling at a stoplight, little hydraulic fluid is pumped, little energy is transmitted to the driven blade, and just a bit of pressure on the brake holds the car still. When the engine speeds up, the drive blade pumps more fluid, and more energy is transferred to the driven blade. At highway speeds, almost all of the energy is transmitted to the driven blade. Because of inefficiencies, automatics waste more gasoline than manual transmissions, but they are "hands-free" and much more popular.
When a car turns a corner, the outside wheels must drive further than the inside wheels. If both driving wheels were locked to the axle, the tires would be forced to slip against the pavement. The differential solves this problem.
The differential has three jobs:
- Allow different (differential) movement of the two axles, so that the car can turn a corner.
- Change drive directions. In rear-wheel-drive cars, the differential connects to a driveshaft that runs from front to back and powers the axles, which run from side to side.
- Increase power. The differential usually reduces the drive speed, so that three to four input revolutions create one turn of the axles.
Normally, if one wheel is slipping, the differential will direct all power to it, allowing the other wheel to sit still without doing any work. This causes problems when traction is poor. "Limited-slip" or "positraction" differentials limit this slip.
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