During which stroke of the four-stroke engine cycle does the spark plug ignite the fuel-air mixture?

Power stroke. The spark plug fires at the top of the compression stroke, just before the power stroke begins. This ignition causes the rapid expansion of gases that pushes the piston down during the power stroke, creating the mechanical force that turns the crankshaft.

What component converts the up-and-down motion of the pistons into rotational motion?

Crankshaft. The crankshaft converts the linear (up-and-down) motion of the pistons into rotational motion that can be used to turn the wheels. The connecting rod links the piston to the crankshaft, while the flywheel helps maintain momentum between power strokes.

What is the primary function of the thermostat in a cooling system?

To regulate coolant flow based on engine temperature. The thermostat regulates coolant flow based on engine temperature. It stays closed when the engine is cold (allowing faster warm-up) and opens at approximately 195F-220F to allow coolant to flow through the radiator for cooling.

What does the alternator do when the engine is running?

Generates electricity to power systems and recharge the battery. When the engine is running, the alternator generates electricity (13.5-14.5V) to power the vehicle's electrical systems and recharge the battery. The battery is used primarily for starting, while the alternator handles electrical needs once the engine runs.

Automotive Basics

Quick Answer: The Auto Information (AI) subtest has 11 questions in 7 minutes on CAT-ASVAB. It covers engine components, ignition systems, fuel systems, cooling systems, and automotive electrical systems.

Engine Components

The internal combustion engine converts fuel into mechanical motion. Understanding its parts is essential for the ASVAB.

Engine Block and Cylinders

Component	Function
Engine Block	Main structure housing cylinders; made of cast iron or aluminum
Cylinders	Hollow tubes where combustion occurs; typically 4, 6, or 8 in a row or V-configuration
Cylinder Head	Covers the top of cylinders; contains valves and spark plugs
Head Gasket	Seals the connection between block and head; prevents leaks

Pistons and Crankshaft

Component	Function
Piston	Moves up and down inside cylinder; compresses fuel-air mixture
Piston Rings	Seal between piston and cylinder wall; prevent oil leakage
Connecting Rod	Links piston to crankshaft; transfers linear to rotary motion
Crankshaft	Converts piston's up-down motion into rotational motion
Flywheel	Heavy wheel attached to crankshaft; maintains momentum between power strokes

The Four-Stroke Cycle

Most automotive engines use the four-stroke cycle:

Stroke	Piston Direction	Valve Position	What Happens
1. Intake	Down	Intake open	Air-fuel mixture enters cylinder
2. Compression	Up	Both closed	Mixture compressed (10:1 ratio typical)
3. Power	Down	Both closed	Spark ignites mixture; expansion pushes piston
4. Exhaust	Up	Exhaust open	Burned gases expelled from cylinder

Memory Tip: "Suck, Squeeze, Bang, Blow" - Intake sucks in fuel, Compression squeezes it, Power is the bang, Exhaust blows out gases.

Camshaft and Valves

Component	Function
Camshaft	Rotating shaft with lobes that open and close valves
Intake Valve	Opens to allow air-fuel mixture into cylinder
Exhaust Valve	Opens to allow burned gases to exit
Valve Springs	Return valves to closed position
Timing Belt/Chain	Synchronizes camshaft with crankshaft rotation

Ignition System

The ignition system delivers spark at precisely the right moment to ignite the compressed fuel-air mixture.

Ignition Components

Component	Function
Battery	Provides initial electrical power (12 volts DC)
Ignition Switch	Controls power to ignition system
Ignition Coil	Transforms 12V to 20,000-40,000V for spark
Distributor	Routes high-voltage spark to correct cylinder (older vehicles)
Spark Plugs	Create spark to ignite fuel mixture; gap typically 0.028-0.060 inches

Modern Ignition Systems

Type	Description
Distributor Ignition	Traditional system with rotating distributor cap
Distributorless (DIS)	Electronic coils fire directly; no mechanical distributor
Coil-on-Plug (COP)	Individual coil for each spark plug; most efficient

Fuel System

The fuel system delivers the correct amount of fuel mixed with air to the engine.

Fuel System Components

Component	Function
Fuel Tank	Stores gasoline or diesel
Fuel Pump	Moves fuel from tank to engine; electric in modern vehicles
Fuel Filter	Removes contaminants from fuel
Fuel Injectors	Spray precise amounts of fuel into intake or cylinder
Throttle Body	Controls airflow into engine based on accelerator position

Fuel Injection vs. Carburetor

Feature	Fuel Injection	Carburetor
Fuel Delivery	Electronic, precise	Mechanical, less precise
Efficiency	Higher MPG	Lower MPG
Cold Starting	Excellent	Can be difficult
Common In	All modern vehicles	Pre-1990 vehicles

Cooling System

Engines generate tremendous heat and must be cooled to prevent damage.

Cooling System Components

Component	Function
Water Pump	Circulates coolant through engine and radiator
Thermostat	Regulates coolant flow; opens at ~195F-220F
Radiator	Heat exchanger; coolant releases heat to air
Radiator Cap	Maintains system pressure (14-18 PSI typical)
Coolant	Mixture of water and antifreeze; prevents freezing and boiling
Cooling Fan	Pulls air through radiator; electric or belt-driven
Heater Core	Small radiator inside cabin for interior heat

Coolant Flow Path

Water pump pushes coolant through engine block
Coolant absorbs heat from cylinders
Hot coolant flows through radiator
Heat transfers to air passing through radiator fins
Cooled coolant returns to engine

Electrical System

The electrical system powers everything from starting the engine to running accessories.

Starting System

Component	Function
Battery	Stores electrical energy; 12V, rated in CCA (Cold Cranking Amps)
Starter Motor	High-torque motor that cranks the engine
Solenoid	Engages starter gear with flywheel ring gear
Ignition Switch	Activates starter circuit

Charging System

Component	Function
Alternator	Generates electricity when engine runs (13.5-14.5V output)
Voltage Regulator	Maintains consistent voltage; prevents overcharging
Drive Belt	Connects alternator pulley to crankshaft

Common Electrical Terms

Term	Definition
Ampere (Amp)	Unit of electrical current flow
Volt	Unit of electrical pressure/potential
Watt	Unit of electrical power (Volts x Amps)
Ohm	Unit of electrical resistance
Ground	Negative connection; usually to vehicle frame

ASVAB

1Introduction

2Verbal Domain (WK + PC)

3Mathematics Domain (AR + MK)

4Science & Technical Domain (GS + EI + MC)

5Technical Skills Domain (AI + SI + AO)

Key Takeaways