Auto Information Study Guide 1 for McGraw-Hill's ASVAB (page 3)
Practice problems for this study guide can be found at:
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.
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.
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.
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.
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