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Fuel System
The fuel system contains the fuel tank, the fuel filter, and the carburetor or fuel injector. To increase reliability and reduce pollution, fuel systems have changed radically over the years.
The fuel tank, usually located at the rear of the car, stores gasoline. A charcoal filter in the tank absorbs gasoline fumes, reducing pollution. A fuel pump delivers gas through a different fuel filter to the fuel injectors.
A throttle plate controls how much air enters the engine. In older cars, the plate was connected directly to the gas pedal. The plate controls how fast the engine is running and how much power it puts out.
Carburetor
Carburetors are another "old-style" component that has largely been replaced. Carburetors combine gasoline with air in a venturi, where a rapid stream of air flows past a small fuel port. The partial vacuum in the fast-moving air draws fuel into the airstream, where it vaporizes and mixes with air.
The exact ratio of fuel to air is critical to performance. A mixture with too much fuel (a "rich" mixture) will waste gas and increase pollution. A mixture with too little fuel (a "lean" mixture) will burn too hot and be short on power. Carburetors always struggled to create the perfect mix for constantly varying driving conditions. Now, fuel injectors have solved that problem through a combination of electronic control and quick response.
Fuel Injector
Fuel injectors are electronically controlled valves that squirt fuel into the cylinder. The valve is closed until the injector receives an electric current and an electromagnet opens the valve. Fuel sprays into the cylinder until the valve closes.
The first fuel injectors were called throttle-body injectors. One injector sprayed fuel into the intake manifold. Newer, multiport injectors spray fuel directly upstream of the intake valve. The injectors, one per cylinder, get fuel from the fuel rail, a supply pipe filled with high-pressure liquid fuel.
The key advantages of injectors over carburetors are accuracy and reliability. By precisely timing the moment and quantity of spray, injectors allow engine designers to improve fuel mileage and performance while cutting pollution. And since injectors have few moving parts, they are more reliable than carburetors.
Fuel systems have improved greatly over the decades. Most problems stem from dirty or stale gasoline. While problems are rare, they still arise:
- Water or dirt in the fuel can clog lines or filters.
- Water can freeze in the fuel line.
- Vapor lock, a condition caused by overheated fuel lines, is no longer a problem with modern vehicles.
Exhaust System
After combustion, burned gases enter the exhaust system. The exhaust manifold connects the cylinders to the muffler, tailpipe, and parts of the pollution control equipment. Exhaust systems operate at high temperatures, and must protect the car and its occupants from heat.
The muffler is a chamber with baffles that deaden the noise of the explosions inside the engine. It's something that you take for granted until you hear a car with a hole in the muffler—and then you realize how loud an internal-combustion engine can be!
Pollution Control
When gasoline burns cleanly, it produces mainly carbon dioxide and water vapor. While carbon dioxide is a cause of global warming, it is not usually considered a pollutant. However, gasoline engines make several types of pollutants, especially when combustion is incomplete. One goal of the ECU is to cut pollution by allowing complete combustion.
Engines can produce these kinds of pollution:
- Carbon monoxide, a product of partial combustion, is a colorless, odorless, but poisonous gas.
- Partially burned hydrocarbons, also known as particulate matter, include various pollutants. In large quantities, small pieces of hydrocarbon make black soot that you can see.
- Volatile organic compounds are another group of pollutants made by partial combustion. Some cause cancer, others irritate lung tissue, and many are a source of smog.
- Air is 79 percent nitrogen, and several nitrogen oxides are created in the high temperatures and pressures inside an engine. Sunlight can convert nitrogen oxides into ozone, a molecule with three oxygen atoms that is a key part of smog. Ozone damages the lungs. Nitrogen oxides also cause acid rain, which damages forests and surface waters.
Positive Crankcase Ventilation
The bottom of an internal-combustion engine is full of a smoky, polluted gas made from the burning of oil and gasoline. To prevent crankcase gases from polluting the atmosphere, the positive crankcase ventilation system pipes this gas to the intake manifold, where the gas is burned in the cylinders. These simple, effective controls were the first pollution controls on gasoline engines.
Catalytic Converter
Catalysts—rare metals like platinum, rhodium, and palladium—cause other chemicals to react without being consumed themselves. Catalysts are embedded in heat-resistant ceramic elements. Pollutants in the exhaust gases are converted to simpler, less toxic compounds in the catalytic converter. From the outside, catalytic converters look like mufflers.
A three-way catalytic converter deals with carbon monoxide, volatile organic compounds, and nitrogen oxides. Often, two separate catalysts are needed to perform these chemical reactions:
A reduction catalyst reduces nitrogen oxides to oxygen and nitrogen.
An oxidation catalyst oxidizes carbon monoxide into carbon dioxide, and volatile organic compounds into carbon dioxide and water.
Suspension
The job of the suspension is to hold the wheels to the road and make the ride safe and comfortable. Axles connect the car to the wheels. Driving axles move the wheels and must rotate themselves. In rear-wheel-drive cars, the driving axles connect the differential to the wheel. In front-wheel-drive cars, the axles are part of the transaxle. These are quite complicated.
Steering Gear
Steering gear connects the steering wheel to the front wheels. In rack-and-pinion steering, a small pinion gear at the end of the steering shaft turns against a flattened gear called a rack. In other systems, a steering gearbox converts rotary motion of the steering wheel to a linear motion that steers the wheels.
Modern steering equipment can be quite complex, especially with front-wheel or all-wheel drive.
Steering may be power-assisted, where the steering wheel does only part of the work, or manual, in which case the driver provides all the effort. Generally, larger cars have power steering, while in smaller cars, the driver's force is sufficient. Power steering can supply a varying level of assistance. A type of hydraulic fluid called power steering fluid is used in the hydraulic system in power steering.
Suspension
Autos use many types of suspension, but the two key categories are leaf spring or coil spring. Springs are made of a kind of steel that is tough and resilient.
Shock absorbers ("shocks") prevent axles from bouncing back, which makes for an uncomfortable ride. Springs resist movement when the car is moving downward, and shocks resist the rising motion. Ideally, after hitting a bump, the axle will move up toward the frame, then return to its original position.
When shocks wear out, a car tends to wander on the road because the springs aren't limited by the shocks.
Bearings
Bearings are mechanical gadgets that allow a part to rotate with almost no friction. Cars have bearings wherever rotating parts are found: in the steering wheel, engine, transmission, drive shaft, differential, and axles. Wheel bearings hold a wheel to a stationary (not driving) axle. Axle bearings hold driving axles. The individual bearings may be shaped like balls (ball bearings) or like tiny cylinders (roller bearings). Many individual bearings are housed together in an assembly that is also called a bearing.
Wheels and Tires
As we reach the end of the suspension, we meet two familiar objects: the wheel and the tire. Wheels are usually made of steel, and are bolted to the axle (for a driving wheel) or the brake apparatus (for a non-driving wheel). Most tires are called radial-ply, meaning that their plies (the invisible reinforcing belts that hold the tire together) run radially—in a line starting at the center of the wheel. Radials offer better life and performance than older bias-ply tires.
Tires are made of artificial rubber, inflated to a pressure of 30 or 40 pounds per square inch. Air pressure holds the tire's shape, and also permits it to absorb road shock.
- Proper inflation is important for critical gas mileage, long tire life, and safe handling. The suggested pressures are listed on a tire sidewall or inside the glove compartment. Normally, hot air expands. But air trapped inside a tire cannot expand, so its pressure rises instead. Suggested tire pressures apply only to a cold tire. Do not release air from a hot tire if it seems overpressurized. Instead, wait until the tire is cold to check the pressure.
- Tires may gradually lose pressure without having a major problem. But if you have to pump up a tire frequently, it's got a problem that needs attention.
Brake System
Mechanically, brakes are the opposite of engines. Engines convert heat that originates in fuel into motion; brakes use friction to convert motion into heat. Brake systems have two key parts: the hydraulics and the braking mechanism at the wheel that does the actual stopping. The hydraulics transfer motion from the driver's foot to the brake mechanism.
Hydraulic systems are based on the fact that hydraulic fluid cannot be compressed. In the simplest hydraulic mechanism, two cylinders are connected by a tube. When a piston moves in the first cylinder, the piston in the second cylinder must also move. Why? Because the fluid is not compressible and must always occupy the same volume.
Air is easily compressed. If air enters the hydraulic system, the brake pedal will get "spongy." The cure is to "bleed" out the air through bleeder valves located at each brake.
In a car, the brake pedal is connected to the master cylinder, and four tubes run to the slave or wheel cylinders. These are located in the brake mechanism at the wheels. Cars now have dual master cylinders and dual hydraulic lines. If one part of the system fails, the other half should still work.
Drum Brakes
Drum brakes operate by expanding two brake shoes inside a large cylinder called a drum. The drum rotates with the wheel, but the shoes do not rotate. A wheel cylinder operates the brake by moving the shoes toward the drum. Friction between the shoes and the drum creates the stopping power.
- When the shoes wear, a self-adjusting mechanism moves them closer to the drum. Wheel cylinders have rubber pistons that must be replaced when they wear out.
- Brake shoes and drums must also be replaced occasionally. If you let the shoes wear too far, they will damage the drums, forcing a more expensive repair.
Disk Brakes
A disk brake has a rotor, or disk, that revolves with the wheel and a pair of stationary pads that create friction when they press against the rotor. Disk brakes are a big improvement over drum brakes because they offer shorter stopping distances and better control.
The horseshoe-shaped caliper holds the pads. Hydraulic force squeezes the pads against the disk.
- Over time, pads wear out. Many disk brakes make a warning noise when the pads are wearing out. You usually will hear the noise when your foot is not on the brake pedal.
- Eventually, the pads wear concentric grooves in the disk. Before putting on new pads, the disks must be machined flat so that the new pads press against a flat surface. After being machined several times, the disks become too thin, risking a heat buildup and reduced stopping power ("fading"). For safety's sake, these disks must be replaced.
Parking Brakes
Parking brakes need a separate mechanical system to operate the brakes; they cannot use hydraulics. Usually, cables from the parking-brake lever to a drum brake operate the parking brake. Disk brakes require a more complicated parking brake. They may have a miniature drum brake built inside them. In other cases, a corkscrew mechanism pushes the brake pads against the rotor.
Antilock Brakes
When tires skid, they lose braking power, and steering becomes difficult or impossible. To improve safety during sudden stops, many cars have an antilock braking system (ABS). An ABS uses sensors to detect wheel rotation. If a wheel starts to skid during braking, the ABS reduces pressure to that brake, then starts pumping the brake. Pumping may produce a vibration in the brake pedal. Other wheels get full braking pressure until they too start to skid. ABS is a good feature on ice or snow.
Practice problems for this study guide can be found at:
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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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