ASVAB Auto Information — Brakes, Suspension, and Steering Systems

The four-stroke cycle (Otto cycle, used in most gasoline car engines) operates in this sequence: (1) INTAKE — piston moves DOWN; intake valve OPEN, exhaust valve CLOSED; air-fuel mixture drawn into cylinder; (2) COMPRESSION — piston moves UP; both valves CLOSED; air-fuel mixture compressed to high pressure (8:1 to 12:1 compression ratio typical for gasoline, higher for high-performance); (3) POWER (combustion/expansion) — spark plug fires just before piston reaches top dead center; ignited mixture expands rapidly, forcing piston DOWN; this is the only stroke that produces power; (4) EXHAUST — piston moves UP; exhaust valve OPEN, intake CLOSED; burnt gases pushed out through exhaust port. Memory aid: 'Suck, Squeeze, Bang, Blow.' Each cylinder fires once every TWO crankshaft revolutions (the crankshaft turns 720° per complete cycle since each stroke = 180°). Multi-cylinder engines: cylinders fire in a specific sequence (firing order) staggered so power strokes overlap, providing smoother power. Common firing orders: 4-cylinder typically 1-3-4-2 or 1-2-4-3; V8 typically 1-8-4-3-6-5-7-2. CRANKSHAFT converts the up-down reciprocating motion of pistons into rotational motion. CAMSHAFT operates the valves; rotates at half crankshaft speed (one cam rotation per two crank rotations); driven by timing belt, chain, or gears. Two-stroke engines (older motorcycles, chainsaws, outboard motors) combine the four strokes into two using ports and crankcase compression — simpler but less efficient and more polluting. Diesel engines also four-stroke but use COMPRESSION IGNITION (no spark plug; high compression heats air enough to ignite injected fuel) — compression ratios 14:1 to 22:1.

Major moving parts of a piston engine: (1) PISTONS — slide up and down in cylinders; compress mixture; receive force from combustion; equipped with PISTON RINGS that seal against the cylinder wall, control oil, and conduct heat to the cylinder; pistons made of aluminum alloy (light and conducts heat well); (2) CYLINDERS — bored holes in the engine block where pistons travel; cylinder walls finished to specific tolerances; (3) CONNECTING RODS (CON RODS) — link pistons to crankshaft; convert piston's linear motion to crank's rotational motion; (4) CRANKSHAFT — rotating shaft; offset journals (crankpins) attach connecting rods at different positions, creating rotational motion; bottom of engine; (5) CAMSHAFT(s) — operates valves; can be in the block (OHV/pushrod engines) or in the cylinder head (OHC/overhead cam, more common today); SOHC = Single overhead cam, DOHC = Double overhead cam; (6) VALVES — control flow of air-fuel mixture IN (intake valves) and exhaust OUT (exhaust valves); typical modern engine has 2 or 4 valves per cylinder; (7) TIMING BELT or TIMING CHAIN — connects crankshaft to camshaft, keeping their rotation synchronized; (8) FLYWHEEL — heavy disk on crankshaft; stores rotational energy between power strokes; smoothens power delivery; on manual transmissions, the clutch engages with the flywheel. Engine displacement: total volume swept by all pistons = (π × bore²/4) × stroke × number of cylinders. Typically quoted in liters (L) or cubic inches (CID); 2.0L engine = 2000 cc. Bore = cylinder diameter; stroke = piston travel distance. Engines are classified by configuration: INLINE (cylinders in a row, like I4, I6), V (two banks of cylinders in V-shape, V6, V8, V12), FLAT/BOXER (opposed cylinders, used by Subaru, Porsche), W (multiple V banks). More cylinders generally = smoother running but more complex and heavier.

Spark plug: produces an electric spark in the combustion chamber to ignite the air-fuel mixture. Located in each cylinder head, screwed into the engine. Structure: central electrode (high voltage), ground electrode (return path), ceramic insulator (between electrodes), metal shell (grounds to engine). High voltage (~15,000-40,000V) from the ignition system jumps the gap between electrodes, creating a spark. SPARK PLUG GAP: distance between central and ground electrodes; typically 0.028-0.060 inch depending on engine; affects spark intensity; checked with feeler gauge. HEAT RANGE: how quickly the plug dissipates heat; 'hot' plugs run hotter (good for cold engines, low-speed); 'cold' plugs run cooler (high-performance, high-RPM); using wrong heat range causes problems: too hot = pre-ignition / detonation, too cold = fouling. Spark plug condition reveals engine health: light brown/tan deposits = normal; oily/wet = oil leak past rings or valve seals; sooty black = rich mixture (too much fuel); white/blistered = lean mixture or overheating. IGNITION SYSTEM components: (1) BATTERY — provides 12V DC; (2) IGNITION COIL — transformer that steps up 12V to 15,000-40,000V; (3) DISTRIBUTOR (older systems) — routes high voltage to correct cylinder in firing order; modern engines use DISTRIBUTORLESS or COIL-ON-PLUG (each plug has its own coil) — fewer parts, more reliable, precise timing; (4) IGNITION CONTROL MODULE / ENGINE CONTROL UNIT — controls timing electronically; (5) SPARK PLUG WIRES (older systems) — high-voltage cables from coil/distributor to plugs; modern coil-on-plug eliminates wires. IGNITION TIMING: spark fires SLIGHTLY BEFORE the piston reaches top dead center (before TDC) on compression stroke; allows time for combustion to develop pressure as piston is at top; typically 10-30° BTDC at idle; advances with engine speed. Faulty spark plugs cause misfires (cylinder doesn't fire properly), rough idle, hesitation, poor fuel economy, hard starting. Modern long-life plugs (iridium, platinum) last 100,000+ miles; older copper plugs need replacement every 30,000 miles.

Fuel delivery systems, evolution: (1) CARBURETOR (older, pre-1990s) — uses venturi effect (Bernoulli's principle) and mechanical metering; air drawn through narrowing passage creates low pressure, drawing up fuel from float bowl; mixed with air, drawn into intake; simple, no electronics needed, but imprecise control of fuel-air ratio across operating conditions; harder to meet emissions standards; affected by altitude, temperature; (2) FUEL INJECTION (modern, EFI/electronic fuel injection) — electronic control unit precisely meters fuel via INJECTORS (electrically-operated valves that spray fuel); much more accurate, adapts to conditions; required to meet emissions and fuel economy standards; subtypes: (a) THROTTLE BODY INJECTION (TBI) — one or two injectors at throttle body (early EFI); (b) MULTI-PORT INJECTION (MPI/MPFI) — one injector per cylinder, sprays into intake port near valve; (c) DIRECT INJECTION (DI/GDI) — sprays directly into cylinder at very high pressure; better atomization, more efficient, more power; common in modern engines; (d) PORT + DIRECT (some modern engines combine both). FUEL SYSTEM components: FUEL TANK; FUEL PUMP (electric, in tank usually); FUEL FILTER (removes contaminants); FUEL LINES; FUEL RAIL (manifold supplying injectors); INJECTORS; FUEL PRESSURE REGULATOR; RETURN LINE (sends excess fuel back to tank). Pressures: port injection 35-65 psi; direct injection 500-3000+ psi. ENGINE CONTROL UNIT (ECU/PCM/ECM) computes fuel needs based on: air flow (MAF or MAP sensor), oxygen sensor feedback (closed-loop control), engine speed, temperature, throttle position, accelerator, knock sensor, and many others. Stoichiometric ratio: 14.7:1 air:fuel by mass for gasoline = chemically optimal complete combustion; ECU varies this for starting (richer), warmup (richer), wide-open throttle (slightly rich for power), cruising (stoichiometric for efficiency and emissions). DIESEL fuel injection: very high pressure (15,000-30,000 psi modern common rail); diesel injected late in compression stroke; compression ignition (no spark plug). Carburetors still used in: lawn mowers, small engines, motorcycles (some), older cars. Diagnosing fuel system problems: weak fuel pump, clogged filter, bad injector, faulty sensor — affecting performance, economy, emissions, drivability.

Transmission: provides selectable gear ratios between the engine's crankshaft and the drive wheels. Without a transmission, engines (which need to spin in a specific RPM range — typically 600-7000 RPM) couldn't efficiently propel a car across all speeds (0-80+ mph). Lower gears multiply torque (more force, less speed) for starting from rest, climbing hills, accelerating; higher gears allow cruising at low RPM (better fuel economy, less noise). MANUAL TRANSMISSION (stick shift) parts: (1) CLUTCH — friction disk between engine flywheel and transmission input shaft; engaged/disengaged by clutch pedal; (2) GEARS — sets of meshed gears in different ratios (1st, 2nd, 3rd, 4th, 5th, 6th, plus reverse); shift fork engages chosen pair; (3) SYNCHRONIZERS — match speeds between gears before engagement to prevent grinding; (4) SHIFT MECHANISM — linkage from gear selector lever to transmission; (5) DIFFERENTIAL — splits torque between drive wheels, allowing them to turn at different speeds in turns. Typical gear ratios: 1st = 3.5-4.5 (high torque multiplication); 2nd = 2.0-2.5; 3rd = 1.5; 4th = 1.0 (1:1, direct drive); 5th = 0.8 (overdrive, reduces engine RPM); 6th-10th = lower for fuel economy. AUTOMATIC TRANSMISSION (most US cars): (1) TORQUE CONVERTER — fluid coupling between engine and transmission; allows engine to keep running with vehicle stopped; some torque multiplication at stall; replaces clutch in conventional automatics; (2) PLANETARY GEAR SETS — multiple ratios via sun-planet-ring gear configurations; selected by friction clutches and bands; (3) HYDRAULIC CONTROL UNIT — fluid pressure controls clutch/band engagement; modern: electronically controlled; (4) TORQUE CONVERTER LOCKUP — locks input-output for direct drive at cruise (eliminates fluid coupling slippage for efficiency); (5) MODES — typically P (park), R (reverse), N (neutral), D (drive, all forward gears), 2 or L (limited gear range). Other transmission types: (1) CVT (Continuously Variable Transmission) — uses belts and variable-diameter pulleys; infinite ratios within range; smooth, efficient; (2) DUAL-CLUTCH (DCT/DSG) — like two manual transmissions; odd and even gears each have own clutch; ultra-fast shifts; sporty; (3) AUTOMATED MANUAL — manual transmission with computer-actuated clutch and shifts; (4) ELECTRIC VEHICLES — typically single-speed (electric motors deliver max torque from 0 RPM); some performance EVs use 2-speed. AWD/4WD: power to all wheels; AWD typically constant or variable, 4WD typically driver-engaged for off-road. DIFFERENTIAL: allows wheels to rotate at different speeds in turns (outside wheel travels farther); standard 'open' differential sends power to wheel with least traction; limited-slip and locking differentials counteract this for traction.

Clutch: a friction-based coupling that connects/disconnects the engine to/from the transmission. When the driver presses the clutch pedal, the clutch DISENGAGES (engine spins independently of transmission), allowing: (1) Smooth start from stop without stalling (engine maintains running RPM while car begins from 0); (2) Gear changes (transmission must be unloaded to shift gears); (3) Idling at red lights with transmission in gear. When pedal released, clutch ENGAGES, locking engine to transmission via friction. Clutch components: (1) FLYWHEEL — heavy disk attached to crankshaft (rotates with engine); also stores rotational energy; (2) CLUTCH DISC (friction disc) — splined to transmission input shaft; sandwiched between flywheel and pressure plate; covered with friction material; (3) PRESSURE PLATE — spring-loaded plate that clamps clutch disc against flywheel when clutch is engaged; (4) RELEASE BEARING (throwout bearing) — pushed by pedal mechanism to compress pressure plate springs, releasing clamping force, disengaging clutch; (5) CLUTCH PEDAL and HYDRAULIC SYSTEM (or cable in older cars) — transmits driver's force; (6) MASTER and SLAVE CYLINDERS — in hydraulic systems. Wear: clutch friction material wears over time, especially with aggressive driving (riding the clutch, slipping, holding partial engagement on hills). Replacement typically every 60,000-150,000 miles depending on driving style. Symptoms of worn clutch: slipping (engine RPM rises but vehicle doesn't accelerate proportionally), shuddering on engagement, hard pedal, inability to disengage. Common driving mistakes that wear clutches: (1) RIDING THE CLUTCH (resting foot on pedal causing partial engagement); (2) USING CLUTCH TO HOLD ON HILLS (use brake instead); (3) AGGRESSIVE STARTS with slipping; (4) DOWNSHIFTING TOO LOW for current speed. Proper technique: press fully when shifting; release smoothly while applying throttle; rest foot off pedal when not shifting. Automatic transmissions use TORQUE CONVERTERS instead of clutches — fluid coupling that allows similar function without driver input (engine can spin while car is stopped due to fluid slip). Modern automatics often have LOCKUP CLUTCH within the torque converter that locks input-output at cruise speed for efficiency. Dual-clutch transmissions (DCT) have two clutches handling odd and even gears, alternating for ultra-fast shifts.

Automotive braking: applies friction between stationary parts (pads/shoes) and rotating parts (rotors/drums attached to wheels). This converts the vehicle's KINETIC ENERGY (½mv²) into HEAT through friction — energy is conserved, just transformed. The heat is dissipated to surrounding air. Two main brake types: (1) DISC BRAKES — most common today, especially on front wheels; ROTOR (disc) attached to wheel hub rotates with wheel; CALIPER straddles rotor with PISTONS that push BRAKE PADS against both sides of rotor when pressure applied; provides strong, consistent braking; cools well due to airflow; recovers quickly from heat fade. Components: rotor (often vented or drilled for cooling), caliper (single or multi-piston), pads (friction material), pad backing plates, retaining hardware, sometimes a separate parking brake mechanism; (2) DRUM BRAKES — older design, still used on rear wheels of many vehicles (cheaper, integrates with parking brake well); BRAKE DRUM attached to wheel; SHOES inside drum push outward against drum's inner surface when pressure applied; cheaper to manufacture, more susceptible to fade (heat builds up inside drum). Hydraulic system: when you press the brake pedal, MASTER CYLINDER (piston in a fluid reservoir) generates hydraulic pressure; pressure transmitted through BRAKE LINES (steel pipes and flexible rubber hoses) to brake calipers/wheel cylinders at each wheel; pistons in calipers/wheel cylinders multiply pedal force (mechanical advantage from area ratios). BRAKE FLUID: hydraulic fluid (DOT 3, DOT 4, DOT 5.1, DOT 5); incompressible; high boiling point (so brake heat doesn't vaporize it, which would cause spongy pedal and brake failure); hygroscopic (absorbs water, which lowers boiling point — fluid should be flushed every 2-3 years). POWER BRAKES: vacuum from engine intake (or electric pump on hybrids/EVs) assists pedal force via a BOOSTER — makes braking effort manageable. ABS (Anti-lock Braking System): sensors at each wheel detect lockup; computer modulates brake pressure rapidly (10-15 times per second) to prevent wheel lockup, maintaining steering control during emergency braking; standard on virtually all modern vehicles. ELECTRONIC STABILITY CONTROL (ESC) extends ABS — selectively brakes individual wheels to counteract skidding/understeer/oversteer. REGENERATIVE BRAKING (hybrids/EVs): electric motor acts as generator when slowing, converting kinetic energy back to electrical energy stored in battery; supplements friction brakes; recovers energy normally lost as heat; reason hybrid/EV brake pads often last 100,000+ miles. PARKING BRAKE: separate mechanical system; operates rear brakes (or sometimes a separate brake on driveshaft) via cable or electric motor; holds vehicle stationary when parked. BRAKE FADE: brakes lose effectiveness when overheated (long mountain descents); the friction coefficient of pad material drops at high temperatures; pump brakes rhythmically or downshift to use engine braking instead of riding the brakes continuously. Engine braking: in lower gears, engine compression slows the vehicle without using brakes; useful on long descents to prevent brake overheating.

Suspension system: the components between the vehicle's body/frame and the wheels that allow the wheels to move up and down independently while supporting the vehicle, absorbing road shocks, and maintaining tire contact with the road. Key components: (1) SPRINGS — SUPPORT the vehicle's weight; absorb impact energy of bumps; types: COIL SPRINGS (most common today, helical wound spring steel), LEAF SPRINGS (older cars, still on most trucks rear; layered steel strips), TORSION BARS (some trucks/SUVs; bar twists when wheel moves), AIR SPRINGS (luxury vehicles, large trucks; pressurized air bag, adjustable height). Springs alone would cause continuous bouncing — they don't dissipate energy, just store and release it; (2) SHOCK ABSORBERS / DAMPERS — DAMP spring motion; oil-filled (or gas-charged) cylinders with pistons; resist motion through oil flow through small orifices; convert kinetic energy of suspension motion into heat in the oil; without shocks, springs would oscillate after each bump, making the car unstable and uncontrollable. Tires would lose contact with the road, reducing steering, braking, and traction. (3) STRUTS — combined spring and shock absorber in one unit; also serve as structural component of the suspension (locating the wheel); MacPherson strut is most common front suspension design; (4) CONTROL ARMS — link wheel hub to chassis; allow up-down motion; control wheel position; (5) BALL JOINTS — allow control arms to pivot; (6) BUSHINGS — rubber/polyurethane cushions in suspension joints; isolate vibration; (7) SWAY BAR (anti-roll bar) — torsion bar connecting left and right wheels; resists body roll in turns; (8) STEERING LINKAGES — connect steering wheel input to front wheels (tie rods, etc.). Common suspension designs: (1) MACPHERSON STRUT — strut + lower control arm; simple, compact; most cars; (2) DOUBLE WISHBONE / DOUBLE A-ARM — upper and lower control arms; better handling; performance and luxury cars; (3) MULTI-LINK — multiple control arms; precise wheel control; performance and luxury; (4) SOLID AXLE — older trucks, off-road; whole axle moves together; durable but less independent. ALIGNMENT: angles of wheels relative to chassis. CAMBER — tilt inward/outward viewed from front; negative (top in) for cornering; CASTER — steering axis tilt viewed from side; affects steering feel and self-centering; TOE — wheels pointing in/out viewed from above; even slight misalignment affects tire wear and handling. Worn shocks: bouncing more than 1-2 times after compression; uneven tire wear; nosediving under braking; poor handling. Tested with 'bounce test': push down corner of car, release, observe if it returns to rest in 1-2 bounces (good) or bounces continuously (worn). Modern systems: ADAPTIVE/ACTIVE SUSPENSION (electronically variable damping); MAGNETORHEOLOGICAL (oil viscosity changes with electric field); AIR SUSPENSION (variable height); each adapts to road and driving conditions.

Most modern engines use LIQUID COOLING because internal combustion creates enormous heat (combustion gases reach 2000°F+ internally); without cooling, engine components would melt or seize. Liquid cooling components: (1) COOLANT — typically a 50/50 mix of WATER and ETHYLENE GLYCOL ANTIFREEZE (also propylene glycol, less toxic); antifreeze: lowers freezing point (pure water freezes at 32°F and expands, cracking engine block), raises boiling point (delays boil-over), inhibits corrosion of metal components, lubricates water pump; (2) WATER PUMP — driven by accessory belt (or sometimes timing belt or electric motor); circulates coolant through engine, hoses, and radiator; (3) RADIATOR — heat exchanger; coolant flows through tubes with fins; airflow (from vehicle moving forward and from electric or belt-driven cooling FAN behind it) carries heat away to surrounding air; (4) THERMOSTAT — temperature-sensing valve; CLOSED when engine is cold (allows engine to warm up quickly for efficiency and emissions); OPENS at specific temperature (typically 195°F / 90°C for modern cars) to allow coolant flow to radiator; failure modes: stuck closed = overheating; stuck open = engine slow to warm up, poor heater performance, slightly reduced efficiency; (5) RADIATOR PRESSURE CAP — pressurizes the system (typically 15 psi); raises boiling point of coolant (each 1 psi raises boiling point about 3°F; 15 psi raises water's boiling point from 212°F to about 257°F); (6) OVERFLOW RESERVOIR (expansion tank) — captures coolant that expands as it heats; coolant returns to system when cooled; (7) HEATER CORE — small radiator inside dashboard; uses engine heat to warm cabin (free heating!); blower fan circulates cabin air past it; (8) HOSES — upper radiator hose, lower radiator hose, heater hoses, bypass hoses; rubber, fail with age (swelling, cracks, leaks); (9) FAN(S) — electric fan(s) (most modern cars) controlled by ECU based on coolant temperature; mechanical fan with clutch on older RWD cars and trucks. Operation cycle: COLD START — coolant inside engine; thermostat closed; small circuit between engine and bypass; engine warms up quickly. AT OPERATING TEMP — thermostat opens; full flow through radiator; equilibrium maintains operating temperature. Common cooling failures: COOLANT LEAK (gasket, hose, water pump, radiator) → low level → overheating; THERMOSTAT STUCK CLOSED → overheating; WATER PUMP failure → no circulation → overheating; RADIATOR clogged → reduced cooling; FAN failure → overheating in slow traffic; HEAD GASKET LEAK → coolant in oil (milky oil), oil in coolant, white exhaust smoke; AIR POCKET in system → hot spots → overheating. Overheating: STOP DRIVING IMMEDIATELY when temp gauge reaches red or 'HOT' warning; continued operation can warp head, damage gaskets, ruin pistons. AIR COOLING (less common today): air flows directly across finned cylinders; simpler but less effective; used on motorcycles, lawn equipment, older VW Beetle, Porsche 911s through mid-1990s, and aircraft engines.

Motor oil has five primary functions: (1) LUBRICATION — reduces friction between moving parts (pistons in cylinders, crankshaft and bearings, camshaft, valves, timing components); without lubrication, metal-on-metal contact causes rapid wear, heat buildup, and seizure; (2) COOLING — circulates through engine, absorbing heat from hot components (particularly piston undersides, where coolant doesn't directly reach); transfers heat to oil cooler or oil pan; complementary to coolant; (3) CLEANING — detergent and dispersant additives suspend contaminants (combustion byproducts, dirt, metal particles) so they can be carried to the oil filter; without these additives, sludge accumulates; (4) SEALING — fills tiny gaps between piston rings and cylinder walls, improving combustion pressure containment; (5) CORROSION PROTECTION — additives protect metal surfaces from acids formed during combustion and from moisture. Oil components: BASE OIL (typically refined petroleum, conventional; or synthetic — chemically engineered for better performance) plus ADDITIVE PACKAGE (detergents, dispersants, anti-wear, antioxidants, friction modifiers, viscosity index improvers, etc.). Viscosity ratings (SAE): expressed as 'WxxW-yyy' for multi-grade oils. Example 5W-30: '5W' is the cold-temperature (winter) viscosity rating; '30' is the operating-temperature viscosity rating. Lower numbers = thinner oil. Modern engines specify lower viscosity (0W-20, 5W-20, 5W-30) for fuel economy; older or high-performance engines may require thicker oils (10W-40, 5W-40, 20W-50). Always follow manufacturer specs — wrong viscosity causes engine damage or poor lubrication. SYNTHETIC oils: chemically manufactured to have more uniform molecules and better stability at temperature extremes; flows better when cold; resists thinning at high heat; lasts longer between changes; more expensive; recommended or required for many modern engines. CONVENTIONAL oils: refined petroleum; cheaper; adequate for older engines, requires more frequent changes. SYNTHETIC BLEND: mix of conventional and synthetic; intermediate. Oil change intervals: traditional 'every 3,000 miles' was for older oils; modern synthetics typically 5,000-15,000 miles per manufacturer; many cars have OIL LIFE MONITORS that calculate based on driving conditions. OIL FILTER: removes particulates; replace with oil change; oil pump pushes oil through filter before circulating to engine. OIL PRESSURE: monitored by ECU; warning light or gauge in dashboard; low pressure indicates problems (low level, worn bearings, failing pump) — STOP DRIVING IMMEDIATELY; engine damage from low oil pressure occurs in minutes. CHECKING OIL: pull dipstick, wipe clean, reinsert fully, pull again; level between minimum and maximum marks; color/condition: clean amber-brown is good; dark brown/black is normal aged oil; milky is coolant contamination; gritty is contamination. Oil burning: blue smoke from exhaust; rising oil consumption indicates worn rings, valve seals, or PCV issues. Oil leaks: spots on driveway from gaskets, seals; common spots: valve cover, oil pan, front/rear main seals.