Auto and Shop Information Study Guide for the ASVAB

Auto: Combustion Systems

The combustion system in a car is responsible for converting fuel into energy to power the vehicle. A properly functioning combustion system is critical for the smooth operation of a car, and an understanding of the various components of this system is essential to do well on the ASVAB Auto and Shop section.

Fuel System

The fuel system in a vehicle is responsible for delivering fuel from the fuel tank to the engine. It is designed to ensure that the engine receives the proper amount of fuel and air to operate efficiently and provide power to the vehicle. The fuel system is also responsible for filtering impurities from the fuel, regulating the fuel pressure, and controlling the fuel delivery based on driving conditions to optimize engine performance.

Types of Fuel Systems

There are two types of fuel systems: carburetor and electronic fuel injection.

Carburetor

A carburetor is an older type of fuel system that mixes fuel and air in the carburetor before delivering the mixture to the engine. The carburetor regulates the amount of fuel that enters the engine based on the amount of air that enters the carburetor. Carbureted engines require regular maintenance to ensure proper performance.

Electronic Fuel Injection

An electronic fuel injection system is a newer type of fuel system that uses electronic sensors and a computer to deliver fuel to the engine. The system monitors the air-to-fuel ratio and adjusts the fuel delivery accordingly. Electronic fuel injection systems provide better fuel economy and reduced emissions compared to carbureted engines.

Parts of a Fuel System

The fuel system is composed of several parts that work together to regulate the flow of fuel and ensure that the engine is receiving the proper amount of fuel. These parts include:

electric fuel pump—The electric fuel pump is responsible for pumping fuel from the fuel tank to the engine. It is typically located inside or near the fuel tank and is controlled by the powertrain control module (PCM).

fuel filter—The fuel filter is responsible for removing impurities from the fuel before it reaches the engine. It is usually located along the fuel lines and can become clogged over time, reducing fuel flow to the engine and affecting performance.

fuel rail—The fuel rail is a tube or pipe that delivers fuel to the fuel injectors. It is located on the engine and is designed to maintain a constant pressure and flow rate to ensure that each injector receives the proper amount of fuel.

fuel pressure regulator—The fuel pressure regulator is located along the fuel lines and is responsible for maintaining a consistent fuel pressure in the system. It regulates the amount of fuel that enters the engine based on the engine’s demand for fuel.

fuel injector—Fuel injectors are responsible for spraying fuel into the engine cylinders. They are located in the engine and are controlled by the PCM, which adjusts the fuel delivery based on driving conditions to optimize engine performance.

intake manifold—The intake manifold is responsible for distributing the air-fuel mixture to the engine cylinders. It is located on top of the engine and is designed to optimize airflow to the cylinders and improve engine performance.

intake air filter—The intake air filter is responsible for filtering out dirt, dust, and other particles from the air entering the engine. It is usually located in the air intake system, which brings air into the engine to mix with the fuel.

powertrain control module (PCM)—​​The PCM is the computer that controls the fuel system and adjusts the fuel delivery based on driving conditions. It receives input from sensors throughout the vehicle and uses that information to optimize engine performance.

throttle body/plate—The throttle body or throttle plate is located in the intake manifold and regulates the amount of air that enters the engine. The throttle plate opens and closes in response to the driver’s accelerator pedal input, which controls the engine’s speed and power output.

Fuel Injection Systems

Fuel injection systems are a type of electronic fuel delivery system that delivers fuel directly to the engine cylinders. There are three types of fuel injection systems: throttle body injection, multiport fuel injection, and direct injection.

Throttle Body Injection (TBI)

TBI is a type of fuel injection system that uses a single fuel injector to deliver fuel to the engine. TBI systems are simpler than other fuel injection systems, and they are often found on older vehicles.

Multiport Fuel Injection (MFI)

MFI systems use multiple fuel injectors to deliver fuel to each cylinder. These systems are more complex than TBI systems but provide better fuel efficiency and performance.

Direct Injection

Direct injection systems spray fuel directly into the combustion chamber, resulting in improved fuel efficiency and performance. Direct injection systems are the most advanced type of fuel injection system, and they are commonly found in newer vehicles.

Care of the Fuel System

Proper maintenance of the fuel system is essential to ensure optimal performance and longevity of the system. This includes regularly replacing the fuel filter, cleaning the throttle body and intake manifold, and keeping the fuel tank clean and free of contaminants. Regular inspections of the fuel system by a qualified mechanic can also help identify potential problems before they become major issues.

Ignition System

The ignition system in a vehicle is responsible for igniting the fuel-air mixture in the combustion chamber of the engine. It does this by producing a high-voltage electrical spark that jumps across the gap between the electrodes of a spark plug, igniting the fuel-air mixture.

The Primary Ignition System

The primary ignition system is responsible for generating the high voltage needed to create the spark that ignites the fuel-air mixture. The components of the primary ignition system include the battery, ignition switch, primary coil winding, ignition module, regulator and pickup coil, and distributor.

Parts of the Primary Ignition System

battery—The battery in a car is a rechargeable device that supplies electrical energy to the vehicle. It is usually a lead-acid battery and is located under the hood of the car. The battery is responsible for providing the initial electrical charge that starts the engine, as well as powering the lights, radio, and other electrical components when the engine is not running. It is charged by the alternator when the engine is running, and it stores electrical energy that can be used later when needed.

ignition switch—The ignition switch is responsible for activating the electrical system and starting the engine. When the key is inserted and turned, the ignition switch sends an electrical signal to various components in the car’s ignition system, including the starter motor and the ignition coil, to start the engine. The switch also controls the power to the car’s accessories, such as the radio and the lights, and can be used to lock or unlock the steering wheel.

primary coil winding—The primary coil winding is a coil of wire that is connected to the battery, ignition switch, and ignition module. The primary coil winding functions as an electromagnet, which is activated when the ignition switch is turned on. It produces a magnetic field that induces a high voltage in the secondary coil winding, which in turn produces the spark that ignites the fuel-air mixture in the combustion chamber. The primary coil winding is typically made of copper wire with a low number of turns and a thick diameter to handle the high current flow.

ignition module—An ignition module is an electronic component whose primary function is controlling the timing and strength of the spark delivered to the spark plugs by regulating the flow of current to the primary coil winding in the ignition system. It is typically located within the distributor and can be a separate component or integrated with the coil. The module receives signals from the engine control module or other sensors and adjusts the timing and duration of the spark accordingly.

regulator and pickup coil—A regulator and pickup coil is a component found in the primary ignition system of some vehicles. Its main function is to regulate the voltage going to the ignition coil by adjusting the resistance in the primary circuit. This helps to ensure that the correct amount of power is sent to the spark plugs at the right time. The pickup coil works in conjunction with the ignition module to determine the position of the crankshaft and to signal the module when to fire the spark plugs. Together, the regulator and pickup coil play important roles in the operation of the ignition system and the vehicle’s overall performance.

distributor—The distributor is a component in older ignition systems that directs high voltage from the ignition coil to the correct spark plug at the correct time in order to ignite the fuel mixture in the engine’s cylinders. It includes a rotor, which rotates inside a distributor cap, distributing the high voltage to the spark plug wires leading to each spark plug.

Operation of the Primary Ignition System

The primary ignition system works by using electromagnetic induction to create a high voltage in the secondary coil winding. When the ignition switch is turned on, electrical current flows from the battery to the primary coil winding. As current flows through the coil, it creates a magnetic field around the core. When the ignition module determines that the timing is correct, it interrupts the flow of current to the primary coil winding. This sudden current interruption creates a high-voltage surge in the secondary coil winding, which is sent to the distributor and then to the spark plugs.

The Secondary Ignition System

The secondary ignition system is responsible for distributing the high voltage from the ignition coil to the spark plugs, where it ignites the fuel-air mixture in the combustion chamber. The components of the secondary ignition system include the secondary coil winding, coil wire, distributor cap and rotor, spark plug wires, and spark plugs.

Parts of the Secondary Ignition System

secondary coil winding—This is a coil of wire that is wound around the primary coil and is used to create a high voltage when the primary coil is energized.

coil wire—This high-voltage wire connects the coil to the distributor cap.

distributor cap and rotor—This component distributes the high voltage from the coil to the spark plug wires.

spark plug wires—These wires connect the distributor cap to the spark plugs.

spark plugs—These components create sparks to ignite the fuel-air mixture in the combustion chamber.

Operation of the Secondary Ignition System

The secondary ignition system operates in conjunction with the primary ignition system to deliver high voltage electrical energy to the spark plugs, which ignite the fuel mixture in the engine. The process of the secondary ignition system starts when the primary ignition system triggers the ignition coil to build up energy in the primary coil winding.

Once the energy in the primary coil winding reaches a certain level, the ignition module signals the coil to release the energy. The energy is then transferred to the secondary coil winding through a high-voltage wire. This transfer of energy causes a step-up effect, increasing the voltage to the necessary level required to create a spark at the spark plug.

The high voltage then travels through the coil wire to the distributor cap, which distributes the energy to the appropriate spark plug wire, depending on the engine’s firing order. The energy travels through the spark plug wire to the spark plug, where it creates a spark that ignites the fuel mixture.

This process repeats continuously, allowing the engine to run smoothly. It is important to note that the secondary ignition system must be properly maintained to ensure that the electrical energy is delivered efficiently and effectively to the spark plugs.

Recent Developments in Ignition Systems

Advancements in technology have led to the development of new ignition systems that are more efficient and reliable than traditional systems. Two examples of recent developments in ignition systems are distributorless ignition systems and coil-on-plug ignitions.

Distributorless Ignition System (DIS)

The DIS was developed to replace traditional distributor-based ignition systems. A DIS uses sensors to determine the position of the crankshaft and camshaft to determine the precise timing for ignition. The system then sends a signal directly to the ignition coil to produce a spark, eliminating the need for a distributor. This system provides a stronger, more consistent spark to the spark plugs and can improve overall engine performance.

Coil-on-Plug (COP) Ignition

COP ignitions are another recent development in ignition systems. This system places an ignition coil directly on top of each spark plug, eliminating the need for spark plug wires and a distributor. This system provides a stronger spark to the spark plug and can improve fuel efficiency and reduce emissions. COP systems are becoming more common in modern vehicles due to their efficiency and reliability.

Exhaust Systems

The exhaust system consists of several parts that work together to remove exhaust gases from the engine and reduce harmful emissions. The exhaust system parts include the exhaust manifolds, catalytic converter, muffler, tailpipe, and header pipes. Understanding the function and importance of each part is essential in maintaining the exhaust system’s proper function and ensuring the vehicle’s safe operation.

Parts of Exhaust Systems

exhaust manifolds—The exhaust manifold is a set of pipes that collects exhaust gases from each cylinder and channels them into a single outlet that leads to the exhaust system.

catalytic converter—The catalytic converter is a critical component of the exhaust system responsible for reducing harmful emissions. It converts pollutants, such as carbon monoxide and nitrogen oxides, into less harmful gases.

muffler—The muffler is a chamber designed to reduce engine noise by absorbing sound waves produced by the engine as exhaust gases pass through it. It’s an important component that helps to minimize the noise level of the vehicle.

tailpipe—The tailpipe is the final part of the exhaust system responsible for releasing the treated exhaust gases into the atmosphere. It directs the gases away from the vehicle, reducing the likelihood of them entering the cabin.

header pipes—Header pipes connect the exhaust manifold to the catalytic converter. They’re designed to improve exhaust flow, allowing the engine to perform more efficiently.

Exhaust System Operation

The operation of the exhaust system begins at the exhaust manifold, where exhaust gases from each cylinder are collected and channeled into a single outlet. The gases then flow through the header pipes and enter the catalytic converter, where harmful pollutants are converted into less harmful gases. From there, the gases pass through the muffler, which helps to reduce engine noise, before finally exiting the vehicle through the tailpipe.

Care of Exhaust Systems

To keep the exhaust system functioning properly, it’s important to perform regular maintenance. This includes checking for leaks, replacing damaged components, and cleaning the exhaust pipes to prevent the buildup of harmful deposits. It’s also essential to avoid driving over rough terrain or in areas with a lot of debris, which can damage the exhaust system.