AFCT Auto and Shop Information: Automotive Systems 2 — Questions and Answers

Question 1: What is the primary function of a catalytic converter in an automobile's exhaust system?

To reduce engine noise
To convert harmful exhaust gases into less toxic emissions (Correct answer)
To increase fuel efficiency
To cool exhaust gases before release

Correct answer: To convert harmful exhaust gases into less toxic emissions

A catalytic converter uses chemical catalysts (platinum, palladium, rhodium) to convert harmful pollutants like carbon monoxide, hydrocarbons, and nitrogen oxides into less harmful carbon dioxide, water vapor, and nitrogen.

The catalytic converter is a critical emissions control device located in the exhaust system between the exhaust manifold and the muffler. It contains a honeycomb structure coated with precious metal catalysts. A three-way catalytic converter simultaneously performs three reactions: oxidizing carbon monoxide to carbon dioxide, oxidizing unburned hydrocarbons to carbon dioxide and water, and reducing nitrogen oxides back to nitrogen and oxygen. Modern vehicles require properly functioning catalytic converters to meet EPA emissions standards.

Question 2: In a hydraulic braking system, what happens when you press the brake pedal?

Brake fluid is compressed to create stopping force
A cable mechanically pulls the brake pads
Hydraulic pressure is transmitted equally through brake fluid to all brake calipers (Correct answer)
An electric motor engages the brake rotors

Correct answer: Hydraulic pressure is transmitted equally through brake fluid to all brake calipers

Pressing the brake pedal activates the master cylinder, which pressurizes brake fluid. Per Pascal's law, this hydraulic pressure is transmitted equally through the brake lines to calipers or wheel cylinders at each wheel.

Hydraulic braking systems work on Pascal's principle, which states that pressure applied to a confined fluid is transmitted undiminished in all directions. When the driver presses the brake pedal, it pushes a piston in the master cylinder, pressurizing the brake fluid. This pressure travels through steel and flexible brake lines to calipers (disc brakes) or wheel cylinders (drum brakes) at each wheel. The fluid is not compressed; rather, it transmits force. Disc brake calipers squeeze brake pads against a rotor, while drum brakes push shoes outward against a drum. Dual-circuit systems ensure that if one circuit fails, the other still provides braking.

Question 3: What component in an automatic transmission allows the engine to idle without stalling when the vehicle is stopped?

The clutch disc
The torque converter (Correct answer)
The flywheel
The differential

Correct answer: The torque converter

The torque converter is a fluid coupling that connects the engine to the transmission. It allows the engine to spin independently of the transmission at idle, preventing stalling when the vehicle is stationary.

The torque converter replaces the manual clutch in automatic transmissions. It consists of three main components: the impeller (pump), which is connected to the engine; the turbine, connected to the transmission input shaft; and the stator, which redirects fluid flow to multiply torque. At idle, the impeller spins slowly and the fluid coupling is loose enough that the turbine can remain stationary. As engine speed increases, the impeller drives transmission fluid faster, spinning the turbine and transferring power to the transmission. A lock-up clutch inside the converter engages at highway speeds to eliminate slippage and improve fuel efficiency.

Question 4: What is the purpose of the alternator in a vehicle's electrical system?

To start the engine
To store electrical energy for later use
To convert mechanical energy into electrical energy to charge the battery and power accessories (Correct answer)
To regulate fuel injection timing

Correct answer: To convert mechanical energy into electrical energy to charge the battery and power accessories

The alternator converts mechanical energy from the engine's serpentine belt into alternating current (AC) electrical energy, which is then rectified to direct current (DC) to charge the battery and power the vehicle's electrical systems.

The alternator is the primary source of electrical power in a running vehicle. Driven by the serpentine belt connected to the engine's crankshaft pulley, it contains a rotor with electromagnets that spin inside a stator with copper windings. This produces alternating current, which internal diodes convert to direct current suitable for the vehicle's 12-volt system. A voltage regulator maintains output between 13.5 and 14.5 volts. The alternator charges the battery, powers the ignition system, lights, infotainment, and all other electrical accessories. A failing alternator may cause dimming lights, a dead battery, or warning lights on the dashboard.

Question 5: What does the thermostat in a vehicle's cooling system control?

The speed of the radiator fan
The flow of coolant between the engine and radiator based on temperature (Correct answer)
The amount of antifreeze in the system
The pressure inside the cooling system

Correct answer: The flow of coolant between the engine and radiator based on temperature

The thermostat is a temperature-sensitive valve that remains closed when the engine is cold, allowing it to warm up quickly. Once the coolant reaches the set temperature (typically 195-220°F), it opens to allow coolant flow to the radiator for cooling.

The thermostat is typically located where the upper radiator hose meets the engine. It contains a wax pellet that expands when heated. When the engine is cold, the thermostat stays closed, so coolant circulates only within the engine block and heater core, allowing the engine to reach optimal operating temperature quickly. This improves fuel efficiency and reduces emissions. Once coolant temperature reaches the thermostat's rated opening temperature, the wax expands, pushing the valve open and allowing hot coolant to flow to the radiator where it is cooled by airflow. A stuck-closed thermostat causes overheating, while a stuck-open thermostat prevents the engine from reaching proper operating temperature.

Question 6: In a four-stroke gasoline engine, during which stroke does combustion occur?

Intake stroke
Compression stroke
Power stroke (Correct answer)
Exhaust stroke

Correct answer: Power stroke

Combustion occurs at the beginning of the power stroke. After the air-fuel mixture is compressed during the compression stroke, the spark plug ignites it, and the expanding gases push the piston down during the power stroke.

The four strokes of a gasoline engine are intake, compression, power, and exhaust. During the intake stroke, the piston moves down and the intake valve opens, drawing in an air-fuel mixture. During compression, both valves close and the piston moves up, compressing the mixture to about a 10:1 ratio. Near the top of the compression stroke, the spark plug fires. The resulting combustion rapidly expands the gases, forcing the piston down during the power stroke, which is the only stroke that produces work. Finally, during the exhaust stroke, the exhaust valve opens and the rising piston pushes spent gases out of the cylinder. The cycle then repeats at speeds of thousands of revolutions per minute.

AFCT Auto and Shop Information: Automotive Systems 2

What is the primary function of a catalytic converter in an automobile's exhaust system?