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