Page 2 - Auto and Shop Information Study Guide for the ASVAB

Auto: Combustion Systems

In this section, you’ll learn about the fundamentals of combustion within an internal combustion engine, along with basic functions and the major components of the fuel system.

Fuel System

For effective engine performance, the fuel system is responsible for maintaining the proper air-fuel mixture. Drivability, emission control, fuel efficiency, and engine service life can all suffer if the air-fuel mixture isn’t adjusted properly.

Types of Fuel Systems

The carburetor and electronic fuel injection are two mechanisms used to determine the air-fuel mixture. The carburetor has mostly been phased out in favor of electronic fuel injection, primarily due to emission control regulations.

Carburetor

Mechanical methods were most commonly used to determine the air-fuel mixture until around 35 years ago. This was achieved by using a carburetor, which was quite reliable but lacked the precision required in modern fuel systems.

Electronic Fuel Injection

In modern fuel systems, the air-fuel mixture is determined by electronic fuel injection. This system is managed by an onboard computer and features a feedback function that allows it to adapt to changing engine conditions quickly.

Parts of a Fuel System

The major components of an electronic fuel injection system are as follows:

Electric fuel pump—The electric fuel pump is positioned in the vehicle’s fuel tank and delivers fuel to the fuel injectors under pressure.

Fuel filter—Before the fuel enters the fuel rail, the fuel filter removes impurities.

Fuel rail—The fuel rail is a manifold that provides all of the engine’s fuel injectors with fuel under pressure.

Fuel pressure regulator—The fuel pressure regulator adjusts the fuel rail pressure based on the vacuum in the intake manifold. Excess fuel is bled to a fuel return line, where it is returned to the tank.

Fuel injector—The fuel injector sprays fuel into the intake air stream, as the powertrain control module (PCM) sends electrical signals to it. The type of fuel injection system used on the engine is determined by the location of the injector.

Powertrain control module (PCM)—The vehicle’s central computer is known as the powertrain control module (PCM). All functions related to the engine and transmission are controlled by the PCM.

Intake manifold—The intake manifold is responsible for distributing air to the cylinder heads’ intake ports.

Intake air filter—Airborne pollutants that could damage internal engine parts are removed by the intake air filter. The air filter filters all of the air that enters the engine.

Throttle body/throttle plate—The throttle body/throttle plate is attached to the throttle pedal and regulates engine speed and torque output.

Fuel Injection Systems

When fuel injection systems originally became popular, they were designed to employ one or two injectors positioned in a throttle body, which replaced the carburetor. This is referred to as a throttle body injection system (TBI). While this system was dependable, it was unable to provide a high enough level of fuel control to meet pollution control criteria.

Multiport Fuel Injection

This system has an injector for each engine cylinder. The injectors in multiport fuel injection are positioned in the intake manifold and direct their spray toward the intake valves. Air enters the intake manifold and flows all the way to the cylinder head before fuel is injected into it.

Direct Injection

In this system, fuel is injected into the combustion chamber directly. A high-pressure fuel injector is used in direct injection to spray highly pressurized fuel directly into the combustion chamber. As a result, only air enters the combustion chamber after flowing through the intake valves. A fuel spray can then be directed directly into the combustion chamber at an optimal point during the compression stroke.

Care of the Fuel System

The fuel injection system has a low maintenance requirement. According to the manufacturer’s guidelines, the filters (both air and fuel filters) must be changed on a regular basis. Manufacturers may also recommend that the injection system be cleaned on a regular basis. Aside from that, fuel injection systems are designed to provide dependable service with minimal upkeep.

Ignition System

The ignition system is one of the most important vehicle systems. Since combustion cannot actually occur without it, the engine will not start. In order for the engine to function effectively and efficiently, the ignition system must produce high-voltage sparks at a specific time.

The primary ignition system and the secondary ignition system are two distinct subsystems of the ignition system. The primary is the system’s low-voltage part, while the secondary is the system’s high-voltage part.

Parts of the Primary Ignition System

The primary ignition system of a basic electronic ignition system consists of the following key components:

Battery—The battery powers the ignition system, which allows the engine to start.

Ignition Switch—By switching power to the ignition system, the ignition switch turns the engine on and off.

Primary coil winding—The primary coil winding in the ignition coil is the low-voltage winding. Several hundred turns of relatively heavy wire make up this winding.

Ignition module—The primary current is turned on and off by the ignition module, which is a transistorized switch.

Reluctor and pickup coil—The ignition module is operated by a signal generated by the reluctor and pickup coil. The reluctor is positioned on the distributor shaft and rotates with the distributor, generating a signal in the static pickup coil.

Distributor—The distributor, which is driven by the engine’s camshaft, is in charge of timing the spark and further distributing it to the proper cylinder. The distributor, like the camshaft, rotates at half the engine’s speed.

Operation of the Primary Ignition System

Current flows from the battery, via the ignition switch, and into the primary coil winding when the driver turns the ignition switch to the “run” state. The same current goes from the coil winding to the ignition module and then back to the battery via the vehicle ground circuit. The low-voltage (primary) system controls the high-voltage (secondary) system in the ignition system as a whole.

The ignition system operates based on an effect known as electromagnetic induction. Voltage is induced in a stationary wire when a magnetic field moves across it.

Parts of the Secondary Ignition System

The secondary ignition system of a basic electronic ignition system contains the following key components:

Secondary coil winding—The secondary coil winding in the ignition coil is the high-voltage winding. Several thousand turns of thin wire are wound around the primary coil winding to make up the secondary one.

Coil wire—The Coil wire transmits high voltage to the distributor cap from the secondary coil winding.

Distributor cap and rotor—High voltage from the coil wire is directed to each cylinder in the firing order through the distributor cap and rotor. This is a switching process that permits a single ignition coil to serve all of the engine cylinders.

Spark plug wires—High voltage is transmitted from the distributor cap to each spark plug through spark plug wires.

Spark plugs—Spark plugs produce the spark that starts the combustion process.

Operation of the Secondary Ignition System

A large voltage is induced in the secondary winding when the magnetic field in the primary coil winding collapses. Secondary system voltages normally vary between 20,000 and 40,000 volts, with newer systems capable of going considerably higher. Because of the massive number of turns of wire in the secondary winding compared to the primary, the ignition coil can accomplish such a large increase in voltage. The primary winding magnetic field cuts across a substantially larger number of wire turns in the secondary, causing a step-up effect in the coil.

The secondary winding’s high-voltage current must be sent to the proper cylinder in the firing order.

Recent Developments in Ignition Systems

Because engine performance is strongly reliant on precise ignition timing, engineers are looking for ways to reduce the number of moving parts in the ignition system. As a result, the distributor was eliminated in a design known as the distributorless ignition system (DIS). In the most contemporary ignition system designs, the spark plug wires have been eliminated completely. Such systems are called coil-on-plug ignition systems.

Distributorless Ignition Systems (DIS)

Because DIS systems use a single ignition coil to power two spark plugs, a V-8 engine needs four separate coils rather than one. The increased number of coils gives each coil enough time to build up its electric charge, resulting in a more powerful spark when it is time to discharge.

Coil-on-Plug Ignition Systems

Individual ignition coils are directly installed on the spark plugs in this system, and the vehicle computer system controls the coils’ discharge. While the addition of computer control has increased the complexity of the ignition system, the removal of moving and high-maintenance elements has resulted in very precise spark timing control, which is crucial for higher engine efficiency and emissions, as well as increased ignition system reliability.

Exhaust Systems

The exhaust system is in charge of eliminating the engine’s waste gases. In order to accomplish this, the system must allow these gases to flow freely, muffle the exhaust sound, and keep gases and heat from entering the vehicle cabin.

Parts of Exhaust Systems

The following are typical components included in an exhaust system:

Exhaust manifolds—Exhaust manifolds connect directly to the cylinder head’s exhaust ports. The exhaust manifolds absorb the majority of the exhaust heat and noise. For durability under high-temperature situations, these are usually manufactured with cast iron.

Catalytic converter—The harmful components of engine exhaust are converted into relatively harmless substances such as carbon dioxide and water by the catalytic converter.

Muffler—The muffler includes an expansion chamber and sound absorption material to mitigate loud exhaust noises.

Tailpipe—The tailpipe is the equipment through which exhaust gases exit the engine and enter the open atmosphere. In most cases, the tailpipe exits at the rear of the vehicle.

Header pipes—The exhaust system can be modified to match the engine configuration to improve engine efficiency and power. Exhaust manifolds that have been “tuned” are commonly referred to as header pipes. The goal is to improve engine breathing by allowing exhaust gases to flow more freely.

Exhaust System Operation

The exhaust manifold collects gases that are released from the engine’s exhaust ports. On a V-type engine, each cylinder head has its own exhaust manifold. These manifolds direct the gases into the steel exhaust pipes that connect the key components of the exhaust system.

After that, the exhaust gases are routed to the catalytic converter. The catalytic converter generates a lot of heat as it converts the hazardous components of the exhaust into less toxic gases.

Exhaust gases are routed into the muffler after exiting the catalytic converter. Expansion chambers are incorporated inside the muffler to absorb the strong noises produced by the engine’s combustion. The muffler is usually found in the rear of the vehicle, between the catalytic converter and the tailpipe.

Care of Exhaust Systems

Physical damage to the system should be inspected on a regular basis; there should be no restrictions or leaks. The mounting brackets and clamps on the system should also be secure.

Any damage should be rectified as soon as possible. The catalytic converter will have a long service life if the engine is kept in good working order, but it will be damaged by an engine that burns oil or has ignition or fuel system issues.

All Study Guides for the ASVAB are now available as downloadable PDFs