Practice problems for this study guide can be found at:
Auto Information Practice Problems for McGraw-Hill's ASVAB
Drive Train
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.
Manual Transmission
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 Transmission
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.
Differential
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.
Front-Wheel or Rear-Wheel Drive
Traditionally, cars steered with the front wheels and drove with the rear wheels. Driving with the front wheels is more complicated, but it offers many advantages:
- The engine weight is on the driving wheels, increasing traction.
- The steering wheels also drive, pulling the car to follow the steering wheels on ice, snow, or sand.
- There is no driveshaft, saving interior room.
The disadvantage is complexity. Stacking the transmission and differential together in a "transaxle" makes front-wheel-drive systems harder to design and repair.
Cars with four-wheel drive, of course, have both a conventional driveshaft and a transaxle, and a front-back differential. They have the most complicated drive trains.
Electrical System
The first cars had barely any electrical system. A hand crank started them, and gas or oil lamps lit the road for those foolhardy enough to drive after dark. The only electrical mechanism was the magneto that fed electric current to the spark plugs. Let's start our examination of the electrical system by looking at the ignition system.
Ignition System
The key parts of the ignition system are the breaker points, the coil, the distributor cap, and the distributor rotor. The breaker points ride against a lobed shaft inside the distributor. The shaft completes one revolution for every two strokes of the crankshaft (intake, compression, power, exhaust).
When the breaker points close, they complete a circuit, and a pulse of 12-volt current goes through the primary winding of the ignition coil, creating a brief magnetic field. This changing magnetic field induces a current in any nearby wire. The voltage of this induced (output) current depends on the ratio of the primary and secondary windings. In an ignition coil, the primary winding has few loops, and the secondary winding has many.
The ignition coil is a direct-current transformer. The induced current, which creates the spark at the spark plugs, is at 10,000 volts or higher. This high-voltage output goes via heavy, high-voltage cable to the center of the distributor cap.
The distributor cap and rotor direct the high-voltage current to the spark plugs. The rotor is on top of the distributor shaft. The rotor receives the high-voltage current from the coil wire and directs it to the spark plug wires connected to the distributor cap. Because the shaft rotation is synchronized with the crankshaft rotation, the spark occurs at the right time.
All the important parts of the electrical system—the breaker points, rotor, distributor cap, and high-voltage wires—can wear out. That's why tune-ups are needed every few thousand miles. Distributor ignition has largely been replaced by electronic ignition, which we'll get to shortly.
Spark Plug The spark plug receives the high-voltage spark current from the distributor and creates an electric spark that sets off the explosion in the cylinder. Spark plugs operate in hot conditions, and they must be replaced occasionally.
- If a spark plug is coated with a greasy black substance, the cylinder is leaking oil, probably because of worn piston rings. Oil burns incompletely in the cylinders, leaving this black deposit.
- Spark plugs must have the correct gap, measured in thousandths of an inch. Feeler gauges are used to set the correct gap.
- When spark plug electrodes get thin, the plug should be replaced.
Diesel engines need no spark plug because the fuel-air mixture ignites when it is compressed. Diesels don't need a coil or distributor. They are somewhat more fuel-efficient, but also more polluting, than gasoline engines. Diesels require fuel injectors and special fuel, but otherwise are quite similar to gasoline engines.
Electronic Ignition
Electronic ("solid-state") ignitions have been introduced over the last 20 years to eliminate the many weak points of distributor ignition. Instead of one centrally located coil, there is a coil at each spark plug, and instead of a mechanical distributor, an electronic unit directs a low-voltage current to those coils. The system allows precise spark timing, which increases power and gasoline mileage, while reducing pollution.
Battery
Cars use a 12-volt battery to store electric energy for starting. These lead-acid batteries are compact and affordable electricity storage devices that last for several years. Lead-acid batteries can quickly deliver the large current needed by the starter motor. They are easily recharged, because the electrochemical reaction that makes electricity is reversible, allowing electricity to be stored as chemical energy.
Inside a lead-acid battery, a chemical reaction between sulfuric acid (the electrolyte) and lead plates (the electrodes) creates extra electrons at the negative pole. When you connect the starter motor to the negative and positive electrodes, a current flows, discharging the battery and turning the starter motor. Each battery cell makes about 2 volts, so a 12-volt battery has six cells. During charging, a 12-volt battery requires about a current of about 14 volts.
- Lead-acid batteries eventually wear out. However, the biggest problem with a lead-acid battery is often the simplest. Corrosion on the battery terminals or battery cables can break the circuit, causing a fully charged battery to appear "dead."
- Corrosion-preventing chemicals can avoid problems blamed on "dead batteries." These chemicals often appear as a red spray on the terminal.
- Older lead-acid batteries had a tendency to run dry, but modern batteries really need no checking. They do wear out after a few years, however. Repair shops have battery testers that can measure how much life is left in a battery.
Starter Motor
The battery is connected to a large direct-current motor called the starter motor. When the starter spins, its gear is inserted against teeth on the flywheel, cranking the engine until (we hope) it starts. When you release the ignition key, the starter motor disengages from the flywheel and the motor stops.
If you've ever tried to start a car that was already running, you've heard a loud grinding noise. This racket comes from the starter-motor gear clashing against the spinning flywheel.
Starter motors require a very large current, which requires a large cable. Instead of running a large cable from the battery to the ignition switch and then to the starter, cars use a relay. This electromagnetic switch closes a circuit between the battery and the starter motor. Because they save weight and money, these solenoid relays are also used in other circuits.
Alternator
Batteries don't create electricity, they only store it, and they must be recharged while the engine runs. On early cars, a generator made electricity to run the lights and recharge the battery. These generators made direct-current (DC) electricity, the kind used in a car, and were most effective at higher engine rpms. Nowadays, the alternator handles this essential task. Alternators make alternating-current (AC) electricity, which must be "rectified" to DC for use in a car. Alternators, however, make more power at lower rpms.
An alternator is basically an electric motor working in reverse. Instead of converting electricity into rotary motion, it makes electricity from rotary motion. The rotation comes from the engine, via a fan belt.
The principle of an alternator is similar to the principle of an ignition coil: Electricity is induced in a wire that moves in a magnetic field. In a coil, although the wires don't move, the magnetic field does.
To make an electric current, you can rotate a magnet inside a coil or rotate a coil inside a magnet. In an alternator, a magnet rotates inside a stationary coil, called a stator. The magnet is called the rotor because it rotates.
As the name implies, alternators create AC. To make the DC that the car needs, the AC goes through a device called a diode. Diodes, housed inside the alternator, are a common source of trouble. The rest of the alternator is more reliable, because it has three separate circuits for creating AC.
- If an alternator circuit fails, you may not notice it until you place a heavy demand on the electrical system.
- Alternators can fail suddenly if a belt breaks.
- If the alternator goes out, the battery must supply all electricity. By cutting off all unnecessary electrical devices, you may be able to get home or to a repair shop before the engine dies.
Engine Control Unit
Auto engines are now run by a small computer called the engine control unit, or ECU. The ECU is the car's brain, and while it is not serviceable, technicians can diagnose trouble by connecting the ECU to a computer.
ECUs get information from sensors that detect
- The mass of air going into the engine
- Engine speed
- The position of the throttle (showing how much power the driver wants)
- The amount of oxygen in the exhaust (showing whether the fuel mix is right)
- Coolant temperature (cold engines need a different mix from warm ones)
- Pressure in the intake manifold (showing how hard the engine is working)
- Alternator voltage (whether the engine needs to speed up to make more current)
Electrical System Troubleshooting
Electrical problems were once about the most common source of driving complaints. Fortunately, all of the big problems have been improved over the years through better materials, better design, and the elimination of problematic parts, especially the distributor and coil. Here are a few troubleshooting points for electrical systems:
- Loose and corroded connections are a major source of intermittent problems. Corrosion is an insulator, remove it with fine sandpaper to ensure a good contact.
- Many times, the best diagnosis for electrical problems comes from hooking a car up to a diagnostic computer.
Practice problems for this study guide can be found at:
Auto Information Practice Problems for McGraw-Hill's ASVAB
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From ASVAB: Armed Services Vocational Aptitude Battery. Copyright © 2009, 2006 by The McGraw-Hill Companies, Inc. All Rights Reserved.
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