ASVAB · Auto Information · 7 min read
Fuel, Ignition and Exhaust Systems
- Explain the components and operating principles of automotive fuel delivery and metering systems.
- Describe the function of ignition coils, distributors, spark plugs, and timing adjustments in gasoline engines.
- Identify key exhaust, emissions, and diagnostic symptoms, including smoke colors and component malfunctions.
The Fuel System: Delivery, Metering, and Air-Fuel Ratios
The primary purpose of an automobile's fuel system is to store, filter, and deliver fuel to the engine. Inside the engine, the fuel mixes with incoming air before combustion takes place. For a gasoline engine, the chemically balanced, or stoichiometric, air-fuel ratio is approximately 14.7 parts air to 1 part fuel by weight. While 14.7 to 1 represents the ideal chemical balance, the best ratio for an engine can vary depending on actual operating conditions, such as idling, accelerating, or cruising.
When an engine receives an air-fuel mixture with too much fuel, it is referred to as a rich mixture. Conversely, an air-fuel mixture containing too much air is called a lean mixture. Fuel delivery begins at the fuel tank. The fuel pump moves gasoline from the fuel tank forward to the engine's fuel metering system. In modern fuel-injected vehicles, you will typically find an electric fuel pump submerged directly inside the fuel tank. Before the fuel reaches the delicate components that spray it into the engine, it passes through a fuel filter. The fuel filter traps dirt, rust, and debris, protecting the downstream components from damage or clogs. In addition, a fuel pressure regulator helps maintain the required fuel pressure throughout the system; depending on the vehicle's design, this regulator may be located near the fuel rail or inside the fuel tank.
Over time, automotive engineering has evolved several methods to meter fuel into the airstream. Older vehicles often use carburetors: airflow through a narrow venturi creates a pressure drop that draws fuel into the incoming air. Modern vehicles employ electronic fuel injection, which uses computer-controlled solenoid valves called injectors to spray pressurized fuel into the engine. Electronic systems differ primarily by where the injectors are placed and how much pressure they utilize.
| System Type | Injector Location and Operation |
|---|---|
| Carburetor | Uses the pressure drop created as air flows through a venturi to draw fuel into the airstream, without electronic injectors. |
| Throttle Body Injection | Uses one or two central fuel injectors positioned on top of the intake manifold. |
| Multi-Port Fuel Injection | Places an individual fuel injector near the intake valve of each cylinder. |
| Gasoline Direct Injection | Sprays highly pressurized fuel directly into the combustion chamber rather than into the intake port. |
Common mistakes
- Confusing rich and lean mixtures: a rich mixture has too much fuel, whereas a lean mixture has too much air.
- Mixing up backfire and afterfire locations: a backfire occurs when fuel ignites in the intake, while an afterfire occurs when unburned fuel ignites in the exhaust.
- Confusing the roles of emission valves: the exhaust gas recirculation valve lowers combustion temperatures to control nitrogen oxides, whereas the positive crankcase ventilation valve vents blow-by gases from the crankcase.
- Misidentifying exhaust smoke colors: black smoke indicates excess fuel or incomplete combustion, blue or gray-blue indicates burning oil, and thick white sweet-smelling smoke indicates coolant from an issue such as a blown head gasket.
Key points
- The fuel system stores, filters, and delivers gasoline, with 14.7 parts air to 1 part fuel by weight representing the chemically balanced stoichiometric ratio for gasoline.
- Modern fuel delivery relies on an electric fuel pump submerged in the tank, fuel filters to protect injectors, and systems ranging from throttle body to direct injection.
- The ignition coil has fewer primary turns and many more secondary turns; switching off primary current produces a high-voltage pulse for the spark plugs.
- Ignition timing describes when the spark fires relative to top dead center; advancing timing fires the plug earlier, while retarding timing fires it later.
- The exhaust manifold routes toxic gases away from the cabin, while the catalytic converter uses platinum, palladium, and rhodium to reduce harmful emissions.
- Exhaust smoke colors indicate specific problems: black indicates an overly rich mixture, blue or gray-blue indicates burning oil, and thick white with a sweet smell indicates burning coolant.
- Automotive Information is a technical subtest that does not affect your AFQT score, and calculators are never permitted during the exam.
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