Practice problems for this study guide can be found at:
Auto Information Practice Problems for McGraw-Hill's ASVAB
The automobile questions that appear on the ASVAB measure how much you understand about automobile components and systems, and how much you know about maintaining and repairing them. The questions may ask you to describe the function of a particular part, to tell what might be causing a given problem, or to explain how to repair a given malfunction. You won't necessarily be asked to explain why auto parts function as they do, but you will be asked what to do and how to do it when a part needs maintenance or repair. If you own a car and maintain it yourself, you may already be familiar with many of the topics covered on the test. You may also have learned about cars by watching or helping family members or friends maintain an automobile, or by working in a garage yourself.
On the paper-and-pencil version of the ASVAB, automobile questions are one part of the Auto and Shop Information test. On the CAT-ASVAB, they form a separate test of their own.
Whichever ASVAB version you take, you'll have only about a minute to answer each automobile question, so you'll have to work fast if you want to get a good score. That's why it pays to spend time studying the test topics and tackling plenty of sample ASVAB automobile questions.
The topic review that follows will help prepare you to answer ASVAB automobile questions. It describes each of the major systems of today's automobiles and reviews the functions of all the most important automobile parts.
Automobile Engines
The engine is the heart of an automobile. Cars use an internal-combustion engine, meaning that the fuel is burned inside the engine. (Steam engines are "external-combustion engines" that burn fuel outside the engine; steam is piped to a turbine that creates the rotary motion.) All car engines, including diesel engines, use the Otto cycle, named for Nicholas Otto, the German who invented the four-stroke gasoline engine in the 1870s.
Here is an overview of what happens inside an Otto-cycle engine: A mix of fuel and air is brought inside a closed space, called a cylinder. The mix is compressed and then explodes. The explosion moves a piston, which rotates the crankshaft. The crankshaft is connected through the drive train to the driving wheels, which move the car. Waste heat from the explosions is removed by the cooling system.
Cylinders are located in the large, cast-iron engine block. Cylinders are laid out in a straight line or a V shape. Straight-line engines usually have four cylinders. For a six- or eight-cylinder engine, the V design (called a V-6 or V-8) saves space.
Cylinder and Piston
The cylinder is the heart of the internal combustion engine, since it's where combustion takes place. The cylinder is a finely machined chamber that holds a piston as it slides up and down. Thin rings called piston rings seal the gap between the cylinder and the piston, containing the explosions and increasing efficiency.
- If the piston rings wear, oil can enter the cylinder. Burning oil makes blue smoke and cuts power output. When an engine starts to burn oil, a major repair called an engine overhaul is needed.
- Changing the oil regularly is the best way to prevent excess wear to piston rings.
Cylinder Head
The cylinder head is a complex metal casting that closes the top of the cylinders. The head is bolted to the engine block. A head gasket separates the head and the block. Like all gaskets, the head gasket creates a seal between two rigid objects that would leak if there were not something compressible between them.
Each cylinder needs at least one intake valve and one exhaust valve. These valves close off a port that allows intake gases to enter or exhaust gases to leave. Many engines increase their power output by using two intake valves and/or two exhaust valves. The cylinder head also has passages for coolant and holes for the bolts that hold it to the engine block.
When you bolt a cylinder head to the engine block, both the order of tightening and the torque (tightening force) are important. Tightening bolts in the correct order prevents the head from warping. Tightening to the right torque ensures that the head is tight enough to seal the gasket evenly. For American cars, torque is measured in foot-pounds. For other cars, it is measured in newton-meters.
The cylinder head also has threaded holes for the spark plug. These electrical devices create a spark when they get a high-voltage jolt of electricity from the ignition system. Spark plugs are screwed into the cylinder head and should be replaced periodically.
Crankshaft
Pistons move in a straight line, but the engine produces rotating motion. The connecting rods and crankshaft change linear motion into rotary motion. Think of the knee of a bicyclist. It moves up and down in a straight line, just like a piston. The knee is connected to the pedal by the lower leg, which acts like a connecting rod. The pedals and cranks act like a crankshaft to convert linear motion to rotary motion.
Connecting rods are attached to the crankshaft by the main bearings. The crankshaft itself rotates on journal bearings attached to the engine block. The crankshaft is housed inside an oil pan, and the bearings also get lubrication from oil tubes or channels in the block.
How the Four-Cycle Engine Works
Understanding an Otto-cycle engine starts with firing order. Memorize this order: intake, compression, power, exhaust.
- Intake. The piston moves down, creating a partial vacuum in the cylinder. The fuel-air mixture enters the cylinder through the open intake valve. The exhaust valve is closed.
- Compression. Both valves are closed. The piston moves up, compressing the fuel-air mixture to about 10 times atmospheric pressure.
- Power. The spark plug fires, starting an explosion inside the cylinder. The resulting high pressure pushes the piston down.
- Exhaust. The piston moves up again, with the exhaust valve open and the intake valve closed. The piston pushes burned exhaust gases into the exhaust manifold and out of the engine.
Valves and Valve Train
Valves play a critical part in the Otto engine because they admit fresh fuel and air and discharge burned fuel and air. Valves must open and close precisely and quickly, several thousand times a minute when an engine is running at full throttle. Valves, particularly the exhaust valve, are in the hottest part of the engine. They cannot be cooled by water, but must conduct away their heat by contact with the valve seat in the cylinder head.
Helix-shaped valve springs hold valves against the valve seat, a polished, sloping surface that closely fits the outside edge of the valve.
In overhead-camshaft engines, the top of the valve rides against the camshaft. Once every two revolutions of the crankshaft, the camshaft lobe pushes against the valve stem, opening the valve.
In conventional-camshaft engines, the camshaft pushes against a push rod, which pushes the rocker arm. The rocker arm pivots on the rocker-arm shaft, so the ends move in opposite directions. When the push rod raises one end of the rocker arm, the other end pushes down on the valve, so it opens.
The camshaft is driven by a timing chain from a sprocket on the crankshaft. The chain keeps the camshaft perfectly in sync with the crankshaft. A broken timing chain is one of the most serious of all problems that can affect an Otto-cycle engine.
Firing Order
To make the engine run more smoothly, nearby cylinders do not ignite in sequence. Instead, the firing is spread around the engine. A typical four-cylinder engine might fire in the order 1-3-2-4. Firing order gets a bit more complicated in V-type engines, but the general rule is the same.
Lubrication System
Engines contain hundreds of metal parts that rotate or slide against metal. Pistons and piston rings slide against the cylinder. The crankshaft rotates inside journal bearings. Camshafts, push rods, rocker arms, and valves must also be protected against friction with a thin film of lubricant—engine oil.
Engines have a complicated set of tubes and internal passages that bring oil to the contact points. A gear-driven oil pump pushes oil through these passages. Oil also splashes onto the cylinder walls, lubricating the piston rings and making compression more effective.
- Oil gets dirty and wears out, so it must be replaced periodically. Oil filters clean the oil, but they must also be replaced.
- Oil and filter manufacturers estimate their product lifetime in miles and/or months. Replace the oil or filter when you reach the first of these milestones.
- To change oil, warm up the engine, place a pan under the oil plug in the crankcase, remove the plug, and drain the oil. Replace the plug and refill the oil through the filler cap on top.
- Recycle used oil; do not dump it down the drain or in a field.
- New cars should not need a top-up of oil between oil changes. But as cars age, engines wear and oil consumption increases, so it makes sense to check the oil periodically. Much of this wear occurs at the piston rings and cylinders.
- When a car starts "burning oil," blue smoke indicates wear of the rings and/or cylinders.
- If the oil pressure light comes on while you are driving, pull over as soon as possible and check for trouble. If you are lucky, the oil level may simply be low, and adding oil should take care of the problem. Otherwise, to prevent severe engine damage, get the car towed to a shop. Sudden loss of oil pressure can also result from oil-pump failure, or other serious engine problems.
Viscosity Oil, like most fluids, gets thicker as the temperature drops. You can see this with molasses. In the refrigerator, it is almost solid. But if you heat molasses on the stove, it starts to flow like water. The viscosity, or thickness, of oil is measured by S.A.E. numbers, which range from a very light S.A.E. 5 to a molasses-like S.A.E. 90. Typically, for summer driving, oil is rated at S.A.E. 30 to 40.
Auto engines start out cold and warm up as they operate. Thick, cold oil creates a lot of resistance when you try to start a cold engine. But if the oil is too thin at operating temperature, it won't separate the metal parts. To resolve this dilemma, lubrication engineers created "multiweight" oil, which is rated with two S.A.E. numbers, for low/high-temperature viscosity. Thus S.A.E. 10W-40 flows more easily at low temperatures (where it's S.A.E. 10) and is thicker when it warms up (where it's S.A.E. 40).
Transmissions and differentials also need lubrication, usually from heavier oil, such as S.A.E. 80 or 90. (These transmission oils or transmission greases are different from automatic transmission fluid, which is used only in automatic transmissions.)
Cooling System
Internal-combustion engines develop a great deal of heat from all those explosions. Only about 30 percent of the energy in gasoline is converted into energy to drive the car. The rest becomes waste heat. Unless the engine can get rid of this heat quickly, it will overheat. Waste heat is removed and delivered to the atmosphere by coolant. At first, water was used as the coolant. But water freezes in winter, and the expansion when it does so can crack the engine block. Chemicals called antifreeze are added to prevent freezing. Dissolving most chemicals in water usually lowers the freezing point. Water can also rust—corrode—the iron in an engine. Coolants also contain a chemical that halts rust, called, logically, rust inhibitor. Because rust inhibitor eventually breaks down, coolant must be replaced every few years.
Coolant flows through hollow passages in the engine block and cylinder head. These water jackets must bring enough coolant to every part of the engine block and cylinder head. From the engine block, coolant flows to the radiator, which has many small tubes covered by fins. The radiator is located on the front of the engine, where it can get plenty of fresh air.
A radiator fan pulls air through the radiator, removing heat from the tubes. The radiator is connected to the engine block through a series of hoses, which can wear out or leak.
- Checking hose and fan belt condition is good preventive maintenance, although modern engines are packed so tightly that the hoses and belts may be hard to see.
- Don't fill the radiator through the cap. Instead, add coolant to the translucent reservoir that's attached by hoses to the radiator. The reservoir has "low" and "full" markings; just keep coolant between these marks.
Repair shops have testers to determine the freezing point of coolant. If you always add coolant (50-50 antifreeze and water), your coolant should stay liquid down to about –30° F. But if you top up the radiator with water, get the freezing point checked in the fall; you may have to replace the coolant to protect the engine.
A water pump, often located on the front of the crankshaft, circulates coolant through the engine. If the water pump fails for even a few minutes, overheating and severe engine damage can result. To increase heat removal, the coolant is pressurized. Pressurized liquids stay liquid at higher temperatures, and hotter liquids can move more heat.
Although we've talked about the need to cool an engine, engines operate best after they have warmed up. To help an engine reach operating temperature, a thermostat prevents the circulation of cold coolant. Thermostats are usually located on the top of the engine, inside a housing that connects the engine block to the top radiator hose. Inside the thermostat, a metal valve opens when the coolant reaches operating temperature.
In newer cars, the fan may also help with engine warm-up. In older cars, the fan was driven from a pulley on the crankshaft by a fan belt. These fans rotated when the engine ran. Now, electrically driven fans start running only when the engine is warm.
- Cars have either a warning light to indicate engine overheating or a gauge that measures engine temperature. Another sign of overheating is the smell of antifreeze; overheating raises engine pressure, and eventually coolant blows out through the radiator cap, releasing a cloud of sweet-smelling, greasy steam.
- A stuck thermostat is probably the largest cause of engine overheating. If you don't drive until you correct the problem, it's usually an inexpensive one as well. Problems with fan belts, radiators, coolant level, and sensors can also cause overheating.
Engine Troubleshooting
Engines are getting more complex all the time, but it helps to know a few troubleshooting hints to score big on the ASVAB:
- A "ping" or "knocking" sound on acceleration usually means that you need a higher-octane gasoline. The noise indicates that the fuel-air mixture is igniting too soon inside the cylinder.
- A squealing noise that increases with engine speed indicates a loose or worn fan belt.
- An engine that runs extremely rough may have a failed spark plug or some other problem in the ignition system.
- General sluggishness, roughness, or poor fuel mileage all indicate the need for a tune-up.
- Loud clanking sounds may indicate major engine problems that will only get worse if you ignore them.
Practice problems for this study guide can be found at:
Auto Information Practice Problems for McGraw-Hill's ASVAB
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