ASVAB Auto Information — Transmission, Drivetrain, and Suspension Systems

Automotive electrical system: BATTERY provides power to START the engine and run electronics when engine is off; ALTERNATOR generates power once engine is running, recharging the battery and supplying all electrical loads. Alternator (modern AC generator) components: (1) ROTOR — electromagnet driven by accessory belt from engine crankshaft; (2) STATOR — stationary three-phase windings; rotor's magnetic field induces AC voltage in stator; (3) DIODE BRIDGE/RECTIFIER — converts AC output to DC (vehicle systems are DC); (4) VOLTAGE REGULATOR — controls rotor field strength to maintain output voltage at ~13.5-14.5V (above battery voltage so battery charges, but not so high as to damage components); modern: built into alternator. Output: typically 60-200+ amps depending on vehicle's electrical demands; modern vehicles with many electronics need higher capacity. Belt: driven by accessory drive belt (serpentine belt) from crankshaft pulley; same belt typically drives water pump, power steering pump, A/C compressor, idler/tensioner pulleys. BATTERY: lead-acid in most vehicles (some newer: AGM/absorbed glass mat or lithium). Voltage: nominally 12V (six 2V cells in series); typical fully charged: 12.6-12.8V at rest; while charging from alternator: 13.5-14.5V. Capacity rated in: COLD CRANKING AMPS (CCA — current available at 0°F for 30 seconds, important for cold-weather starting), RESERVE CAPACITY (minutes battery can supply load if alternator fails). Battery dies: (1) Old age (typical lifespan 3-5 years in moderate climates, shorter in extreme); (2) Parasitic drain (lights left on, electronics drawing current with engine off); (3) Faulty alternator not charging; (4) Cold weather (chemistry slows in cold, but cold-weather demands highest — cars hardest to start when battery weakest); (5) Loose/corroded connections. Symptoms of dead battery: slow cranking, clicking when starting, no lights/electronics, jump start gets car running. Symptoms of bad alternator: battery doesn't hold charge after jump, dim lights at idle, dashboard battery warning light, electronics misbehaving. STARTER MOTOR: high-current DC motor; engages flywheel ring gear via SOLENOID; turns engine until it starts; very high current draw (typical 100-300+ amps for brief seconds); reason batteries need high CCA. Starting circuit: ignition key/button → starter relay/solenoid → starter motor (drawing power from battery). Without functional battery, starter can't crank engine. CHARGING SYSTEM testing: check battery voltage at rest (should be >12.6V healthy); check voltage with engine running (should be 13.5-14.5V — too low = alternator weak; too high = regulator failure); check 'voltage drop' across cables; load test battery to see if it holds voltage under high current. MODERN VEHICLES: many sophisticated electronics — engine ECU, transmission ECU, ABS module, airbag module, body control module, infotainment, sensors, lights, heated seats, power locks/windows/mirrors, etc. Total electrical draw on modern luxury vehicles can exceed 100 amps. EVs/HYBRIDS use larger battery systems, DC-DC converters supplying conventional 12V system from main high-voltage battery, regenerative braking partially recharges main battery.

Engine displacement: total volume swept by all cylinders during one engine cycle. Calculation: displacement = (π × bore²/4) × stroke × number of cylinders. BORE = cylinder diameter; STROKE = piston travel distance from top dead center to bottom dead center. Units: LITERS (L) (1L = 1000 cc) — modern standard; CUBIC CENTIMETERS (cc) — same as liters but for small engines (motorcycles); CUBIC INCHES (CID) — older US system. Conversions: 1L ≈ 61 CID. Examples: 2.0L = 2000 cc ≈ 122 CID; 5.0L V8 ≈ 305 CID; 350 CID ≈ 5.7L. Larger displacement engines typically (but not always) produce more power, use more fuel, and weigh more. Modern trends: TURBOCHARGED smaller engines produce power similar to older larger engines with better fuel economy (a 2.0L turbo can match a 3.5L naturally aspirated). PISTON CYCLE in terms of displacement: piston travels from TOP DEAD CENTER (TDC) to BOTTOM DEAD CENTER (BDC) and back; full displacement is swept twice per complete four-stroke cycle (each stroke is half the displacement). NATURAL ASPIRATION vs FORCED INDUCTION: (1) NA engines breathe air at atmospheric pressure; (2) TURBOCHARGED engines use exhaust-driven turbine to spin compressor that forces more air in — more air = more fuel = more power; (3) SUPERCHARGED engines use belt-driven compressor for same effect; more responsive but consumes engine power. Boost pressures typically 6-20+ psi above atmospheric. ENGINE OUTPUT: HORSEPOWER (HP) — rate of work output; usually peaks at higher RPM; TORQUE (lb-ft or Nm) — rotational force; typically peaks at lower-mid RPM. HP = (Torque × RPM) / 5252; both curves cross at 5252 RPM on a dyno graph. Modern car engines: 100-200 HP typical for small cars; 250-400 HP common; performance/luxury 400-700+; supercars 700-1500+ HP. RPM ranges: idle 600-900; cruising 1500-3000; redline 5000-8000 (gasoline), 3000-5000 (diesel). COMPRESSION RATIO: ratio of cylinder volume at BDC to volume at TDC; higher compression = more efficient, more power, requires higher-octane fuel; gasoline 8:1 to 12:1 typical (high-performance up to 14:1); diesel 14:1 to 22:1 (uses compression for ignition). OCTANE RATING: gasoline's resistance to KNOCK/PRE-IGNITION; higher octane (87, 89, 91, 93 in US) tolerates higher compression and timing advance; use what manufacturer specifies; using higher than required doesn't improve performance; using lower than required can damage engine. KNOCK: premature ignition causing pressure waves; can damage pistons; detected by knock sensors which retard timing.

Exhaust system components: (1) EXHAUST MANIFOLD (or HEADERS) — collects exhaust gases from each cylinder and combines them; cast iron (manifold) or steel tubing (headers — performance, equal-length tubes for optimal scavenging); (2) DOWN PIPE — connects manifold to rest of system; (3) CATALYTIC CONVERTER — converts toxic gases to less toxic ones using precious metal catalysts (platinum, palladium, rhodium); converts CO (carbon monoxide) to CO₂, HC (hydrocarbons) to CO₂ + H₂O, NOx (nitrogen oxides) to N₂ + O₂; requires unleaded gasoline (lead destroys catalyst); typically two converters in modern cars (close-coupled near engine for fast warmup, and under-floor); operates 500-1000°F+; expensive ($500-2500+); commonly stolen for precious metals; (4) OXYGEN SENSORS — measure oxygen content in exhaust; ECU uses signal to maintain optimal air-fuel ratio (14.7:1 stoichiometric); typically one upstream of catalytic converter (controls fueling), one downstream (monitors catalyst function); modern wideband sensors more accurate than older narrowband; (5) RESONATOR — smaller muffler-like device; cancels specific frequencies; reduces drone and harshness; (6) MUFFLER — reduces noise via expansion chambers, perforated tubes, sound-absorbing material; quieter but adds some backpressure; (7) TAIL PIPE — final exit. Sensors: oxygen sensors, air-fuel ratio sensors, temperature sensors, sometimes pressure sensors — ECU uses all data for fuel/spark control. Emissions standards: Tier 3 / EPA regulations require very low pollutant levels; combustion alone produces NOx, CO, HC, particulates; catalytic converter, EGR (exhaust gas recirculation, reduces NOx by reducing peak combustion temperature), EVAP system (captures fuel tank vapors), and good combustion control combined achieve modern standards. DIESEL EMISSIONS: additionally use DPF (Diesel Particulate Filter) to trap soot; SCR (Selective Catalytic Reduction) with DEF (Diesel Exhaust Fluid, urea solution) to convert NOx; EGR; and oxidation catalysts. Diesel engines without these systems pollute significantly more — modern diesel meets the same emissions targets as gasoline. BACKPRESSURE: exhaust gases face resistance flowing through the system; some backpressure can actually help low-end torque; performance modifications often reduce backpressure with larger pipes, better mufflers, headers — but excessive reduction can hurt low-end performance. Cat-back vs full system: 'cat-back' replaces from catalytic converter back; doesn't affect emissions. HEADER WRAPPING / INSULATION: keeps heat in exhaust gases so they exit faster (Bernoulli/Venturi), and protects nearby components. CHECK ENGINE LIGHT: often related to emissions systems; oxygen sensors, catalytic converter efficiency, EVAP leaks are common causes; OBD-II diagnostic codes (P0xxx series) pinpoint issues; readers ($20-200) plug into port under dash. Common exhaust problems: rust through (especially in salt-belt regions); broken/cracked components from impacts or thermal cycling; oxygen sensor failure; catalytic converter failure (clogged from rich-running engine; melted from excessive temperatures); exhaust leak (causes loud engine, can cause emissions test failure).

Differential: a gear arrangement in the drive axle that allows the two drive wheels to rotate at DIFFERENT SPEEDS while still receiving power from the driveshaft/transaxle. Why needed: when a vehicle turns, the OUTSIDE wheel must travel a longer distance than the INSIDE wheel (larger radius), so it must rotate FASTER. Without a differential, both wheels would be forced to spin at the same speed, causing tire skidding/wear, steering difficulty, and stress on drivetrain components. Operation: input from driveshaft → ring gear (large bevel gear) → pinion gear (smaller, drives ring gear); the differential carrier inside the ring gear houses smaller bevel gears (side gears and spider/pinion gears) that allow the two output shafts (to the wheels) to turn at different rates while the total drives the carrier. OPEN DIFFERENTIAL (standard): simplest, lightest, cheapest; sends EQUAL TORQUE to both wheels (but torque follows path of least resistance — so if one wheel has no traction, that wheel spins, the other gets little/no torque); great for cornering on dry roads; terrible for low-traction situations (mud, snow, ice — one wheel slips, vehicle stuck). LIMITED-SLIP DIFFERENTIAL (LSD): uses clutches, gears, or fluid coupling to provide some torque transfer to the wheel with traction when the other slips; reduces wheel-spin loss; types include: CLUTCH-PACK LSDs, HELICAL/TORSEN (gear-based, smooth), VISCOUS-COUPLING (fluid-based, less aggressive). LOCKING DIFFERENTIAL (locker): mechanically links both wheels to spin together; either FULLY LOCKED (no differential action; great traction on/off-road but causes wheel-skidding on turns; off-road vehicles), or SELECTABLE (driver engages when needed). ELECTRONIC LIMITED-SLIP / BRAKE-BASED traction control: uses ABS sensors to detect wheel slip; applies brakes to spinning wheel, transferring torque to the other side via the open differential's torque-equality principle (one wheel braked = more torque to other). Modern AWD/4WD: combinations of front, rear, and CENTER differentials. CENTER DIFFERENTIAL allows front and rear drive shafts to rotate at different speeds (needed for turning since front and rear wheels travel different paths). VISCOUS CENTER DIFFERENTIALS or computer-controlled clutches manage front-rear torque distribution. TORQUE VECTORING: high-performance systems actively control torque to individual wheels for cornering performance. Differential gear ratio: drive shaft RPM / wheel RPM; e.g., 3.73:1 means driveshaft spins 3.73 times for each wheel rotation; affects acceleration vs top speed (lower numerical ratio = higher gearing, better fuel economy at speed; higher numerical = quicker acceleration). Differential fluid: special gear oil (often 75W-90 or 80W-90); usually contains friction modifiers for LSDs; change interval varies (often 30,000-100,000 miles; severe duty more frequent). FWD vehicles have differential integrated into transaxle. RWD vehicles have separate rear differential. AWD/4WD: front differential, rear differential, and transfer case (which routes power between them). RWD and 4WD have a driveshaft running under the vehicle.

Tire tread: the patterned rubber that contacts the road. Functions: (1) CHANNELS WATER away from the tire-road interface, preventing HYDROPLANING (tire riding on top of water film); deeper tread = more water displaced; (2) PROVIDES MECHANICAL GRIP on irregular surfaces (gravel, snow, ice, off-road); tread blocks bite into irregularities; (3) DISSIPATES HEAT from rubber working against road; (4) GENERATES NOISE-MASKING patterns (good designs minimize road noise); (5) REDUCES ROLLING RESISTANCE through optimized contact area. Tread depth: new tires typically have 10/32 to 12/32 inch tread depth; legal minimum: 2/32 inch in most US states (when wear bars are flush with tread surface); 4/32 considered prudent minimum for safety in wet conditions; 6/32 for winter conditions. Tread wear indicators (WEAR BARS): small raised bars between tread blocks; become flush when tread reaches 2/32. Penny test: insert penny upside down into tread; if Lincoln's head is fully visible, tread is 2/32 or less (replace); quarter test: Washington's head fully visible = 4/32 or less. Tire types: (1) ALL-SEASON — compromise design; good for most conditions except deep snow/ice; most common original equipment; (2) SUMMER/PERFORMANCE — softer rubber, optimized for dry/warm; poor cold/snow performance; (3) WINTER/SNOW — siped (small slits) tread blocks, soft rubber compound that stays pliable in cold; symbol: snowflake-and-mountain peak; significantly better cold/ice traction; wear faster on dry warm pavement; (4) ALL-TERRAIN/MUD-TERRAIN — aggressive tread for off-road; noisier on pavement; (5) TOURING — comfort, longevity; (6) HIGH-PERFORMANCE — sport handling, shorter life. Tire markings: P (passenger), LT (light truck); WIDTH (e.g., 215); ASPECT RATIO (sidewall height as percentage of width, e.g., 60); CONSTRUCTION (R for radial); WHEEL DIAMETER (e.g., 17); LOAD INDEX (numerical, weight capacity); SPEED RATING (letter, max speed for sustained operation: S=112 mph, T=118, H=130, V=149, W=168, Y=186). Example 215/60R17 95H: 215 wide, 60% aspect ratio, radial, 17-inch wheel, 95 load index (~1521 lbs), H rated (130 mph). MAINTENANCE: (1) PRESSURE — check monthly when cold; follow placard in driver's door jamb (not the tire sidewall, which shows MAX pressure); underinflation increases rolling resistance, wear, and overheating risk; overinflation reduces contact, increases harshness; modern TPMS warns at 25% low; (2) ROTATION — every 5000-7500 miles; promotes even wear (front tires wear faster, especially on FWD); typical patterns: front-to-rear straight (non-directional), front-to-rear with side swap, X-pattern; (3) ALIGNMENT — when uneven wear noticed; (4) BALANCE — when vibration; weights added to wheel; (5) AGE — replace tires older than 6-10 years even with good tread, as rubber degrades; check DOT code (last 4 digits = week/year of manufacture). HYDROPLANING: tire rides on water film; loss of traction and steering control; minimize by: maintaining tread depth, slowing in heavy rain, avoiding standing water; if happens: don't brake or steer hard; ease off accelerator until traction returns.

Gasoline (Otto cycle) vs Diesel engine differences: (1) IGNITION: gasoline uses SPARK IGNITION (spark plug fires the air-fuel mixture); diesel uses COMPRESSION IGNITION (high compression heats air to 700-900°F, sufficient to ignite injected fuel — no spark plug); (2) FUEL DELIVERY: modern gasoline uses port or direct injection; diesel always uses high-pressure DIRECT INJECTION (15,000-30,000+ psi) into the cylinder near top dead center; (3) COMPRESSION RATIO: gasoline 8:1 to 12:1 typical (higher would cause knock); diesel 14:1 to 22:1 (the high compression is essential for ignition); higher compression = more thermal efficiency; (4) FUEL: gasoline is more volatile (vaporizes easily); diesel is heavier oil-like fuel (slower-burning, more energy per gallon — about 10% more); (5) THROTTLE: gasoline engines control power by RESTRICTING AIR with throttle plate (varies air-fuel mixture); diesels typically have NO THROTTLE PLATE — control power by VARYING FUEL INJECTION amount; (6) POWER CHARACTERISTICS: gasoline peaks at higher RPM (5000-7000), produces more HP per liter; diesel peaks at lower RPM (2500-4500), produces more TORQUE per liter — great for heavy loads, hauling, low-end pulling; (7) FUEL ECONOMY: diesels typically 20-30% better fuel economy than gasoline (more efficient combustion + higher energy density of fuel); (8) EMISSIONS: diesel produces more NOx and particulates (soot); modern diesels require complex aftertreatment (DPF, SCR with DEF, EGR); gasoline produces more CO and HC; both meet modern standards with appropriate emission systems; (9) DURABILITY: diesel engines built sturdier for high compression and pressures; typically last longer (300,000-500,000+ miles common for diesel trucks); (10) COLD STARTING: diesels harder in cold (less volatile fuel); glow plugs preheat combustion chamber; cold-weather gelling of diesel fuel a concern in winter (winter blends with anti-gel additive); (11) APPLICATIONS: gasoline for most cars and light-duty trucks; diesel for heavy-duty trucks (semi-trucks, buses, vans), agricultural equipment, locomotives, marine, generators, some passenger cars (more common in Europe). TURBOCHARGING: nearly universal on modern diesels (atmospheric diesel is rare in modern vehicles); also increasingly common on gasoline engines; turbo uses exhaust energy to compress intake air → more air = more fuel = more power. MAINTENANCE: diesels need more frequent oil changes (combustion byproducts contaminate oil faster), fuel filter changes, DPF regeneration cycles, DEF refilling. Diesel fuel + gasoline engine = NO START (diesel doesn't vaporize); gasoline + diesel engine = ENGINE DAMAGE (lubricity issues, premature combustion timing, particulate damage to DPF). HCCI (homogeneous charge compression ignition) and other advanced combustion: experimental engines combining best traits of gasoline and diesel.

Catalytic converter: emission control device that converts toxic combustion byproducts into less toxic substances. Located in the exhaust system after the manifold (often two — one close to engine for fast warmup, another under floor). Catalyst materials: PLATINUM, PALLADIUM, RHODIUM — precious metals that facilitate chemical reactions without being consumed (true catalysts). Coated on a ceramic honeycomb substrate for maximum surface area. Three-way catalyst (most modern gasoline cars) handles three pollutants simultaneously: (1) CARBON MONOXIDE (CO) — toxic, odorless, poisonous (binds to hemoglobin, displacing oxygen) → CO₂ via oxidation (CO + O → CO₂); (2) HYDROCARBONS (HC) — unburned fuel, contributes to smog → CO₂ + H₂O via oxidation; (3) NITROGEN OXIDES (NOx) — form at high combustion temperatures (atmospheric N₂ + O₂); cause smog, acid rain, respiratory damage → N₂ + O₂ via reduction. The 'three-way' name reflects handling all three pollutants. Effectiveness: well-functioning modern systems convert >95% of these pollutants. Operating temperature: catalyst must be hot (500-1000°F) to work effectively; most emissions during cold start occur before catalyst warms up; modern designs include close-coupled converters that warm quickly. Requires LEADED GASOLINE? Absolutely not. UNLEADED gasoline is essential — lead poisons the catalyst, destroying its function. This is why leaded gasoline was phased out (US: completely ~1996). Modern fuel is unleaded. RICH-RUNNING engines (too much fuel) overload converter, can melt the substrate, ruining it. Excess fuel during misfires (faulty spark plug, ignition coil) can also destroy converter — fix engine issues promptly to protect $1000-3000+ converter. SYMPTOMS of failing catalyst: check engine light (P0420 'catalyst efficiency below threshold' is common code); rotten egg smell (sulfur compounds not converted); restricted exhaust (clogged catalyst); reduced power; failed emissions test; rattling sound (broken internal substrate); reduced fuel economy. CATALYTIC CONVERTER THEFT: due to precious metal content (rhodium has exceeded $20,000/oz at peaks; small amounts in each converter); thieves cut converters out of vehicles (especially Toyota Prius, large trucks); replacement is very expensive. Protective measures: catalytic converter shields, identification etching, parking in secure locations. AFTERMARKET / DELETE: removing or 'gutting' a converter is illegal under federal Clean Air Act ($25,000+ fines for tampering with emission controls). Diesel: separate system with DPF (Diesel Particulate Filter) for soot, SCR (Selective Catalytic Reduction) with DEF for NOx, oxidation catalyst for HC/CO. DEF (Diesel Exhaust Fluid, urea solution) injected into exhaust where it reacts with NOx → N₂ + H₂O. EVs and pure hybrids in EV mode produce no exhaust — no converter needed. WHY EMISSIONS MATTER: untreated combustion byproducts contribute to: smog (HC, NOx), acid rain (NOx, SOx), greenhouse effect (CO₂, methane), respiratory disease, ozone layer issues. Vehicles are major emission sources; modern emissions controls have dramatically improved air quality in urban areas where adopted; significant air quality crises remain in places without strict emissions standards.

Hydraulic brake systems use Pascal's Principle: pressure in a confined fluid is transmitted equally in all directions. Force amplification: small force on small-area piston creates pressure (P = F/A); same pressure on large-area piston creates large force (F = P × A). This allows reasonable pedal pressure to generate enormous braking force. System pressure typically 800-1500 psi during normal braking, can reach 2000 psi in emergency. Master cylinder + brake pedal: when you push the pedal, mechanical linkage compresses MASTER CYLINDER piston, which pushes brake fluid out into the brake lines. The reservoir on top of master cylinder holds extra fluid (also accommodates fluid level changes from temperature and pad wear). DUAL-CIRCUIT brakes (standard since 1967 in US): master cylinder has TWO internal chambers, each serving a separate brake circuit; typically split FRONT-REAR or DIAGONAL (left-front + right-rear together, right-front + left-rear together — diagonal split maintains some braking even if one circuit fails); if one circuit fails (leak), the other still works; you'll feel a longer pedal travel but still have some braking. BRAKE LINES: rigid steel lines for most of the run; flexible rubber hoses near wheels (allow steering and suspension movement); under high pressure during operation; can corrode (steel) or deteriorate (rubber bulging/cracking). CALIPER PISTONS: receive hydraulic pressure; push outward to press brake pads against rotors. Floating caliper has one or two pistons on one side; the caliper itself slides on pins; pad pressure on one side pulls the caliper which pulls the opposite pad against the rotor. Fixed caliper has pistons on both sides; doesn't slide; usually higher performance (track cars). Caliper sizes: typical car 40-60mm piston; performance cars 50-70mm; some race cars have multiple smaller pistons. WHEEL CYLINDERS (drum brakes): smaller pistons on each end push out shoes against drum. BRAKE FLUID: hydraulic fluid; KEY PROPERTIES: incompressible (otherwise pedal would feel spongy), high boiling point (brake heat would otherwise boil it, causing fluid to vapor-lock — gas IS compressible, leading to spongy pedal and brake failure), low freezing point, lubricates seals, compatible with rubber components. TYPES: DOT 3 (glycol-based, 401°F dry / 284°F wet boiling point); DOT 4 (446°F dry / 311°F wet, more common modern); DOT 5.1 (similar to DOT 4 but higher quality); DOT 5 (SILICONE-BASED, NOT compatible with glycol fluids; some military and classic car applications). HYGROSCOPIC: glycol fluids ABSORB MOISTURE from air over time; this lowers boiling point progressively; fluid should be flushed every 2-3 years (or per manufacturer); brake fluid turns from clear-amber to dark with age. POWER BRAKE BOOSTER: between brake pedal and master cylinder; uses ENGINE VACUUM (gasoline engines) or ELECTRIC PUMP/MOTOR (diesel, hybrid, EV) to amplify pedal force; without booster you'd need to push very hard — typically 4-8x assistance. ABS (Anti-lock Brakes): adds electronic control; sensors detect wheel speeds; computer modulates pressure rapidly via valves to prevent wheel lockup; maintains steering control during emergency braking. Stability control extends this — selective wheel braking to counteract skidding.

Intake system components: (1) AIR INTAKE — opening at the front of the engine bay; modern designs are 'cold air intakes' drawing from cooler areas (denser air = more power); (2) AIR FILTER — paper or oiled gauze filter; traps dirt, dust, debris; replace per maintenance schedule (typically 15,000-30,000 miles, or sooner in dusty conditions); essential — engines breathe enormous volume of air (a 2.0L engine at 3000 RPM ingests over 3000 L of air per minute); even small particles damage engines; (3) MASS AIRFLOW SENSOR (MAF) or MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP) — measures airflow into engine; ECU uses this to calculate proper fuel injection amount; MAF measures airflow directly with a heated wire; MAP measures vacuum in intake manifold (correlates with airflow); both critical to engine operation, faulty sensor → incorrect fueling → poor running; (4) INTAKE AIR TEMP SENSOR (IAT) — air temperature affects density; cold air is denser (more mass per volume) so requires more fuel; sensor allows fueling adjustment; (5) THROTTLE BODY — contains THROTTLE PLATE (butterfly valve) that controls airflow into engine; on gasoline engines, throttle position determines engine power (more throttle = more air, ECU adds more fuel proportionally); modern cars use DRIVE-BY-WIRE/ETC (Electronic Throttle Control): no mechanical link from pedal to throttle, pedal sends electronic signal to ECU which positions throttle motor; allows traction control, cruise control, stability control intervention; (6) THROTTLE POSITION SENSOR (TPS) — tells ECU current throttle position; (7) INTAKE MANIFOLD — distributes air from throttle body to each cylinder; usually plastic or aluminum; runners (passages to each cylinder) are tuned in length and shape to optimize airflow at specific RPM ranges; some modern engines have VARIABLE-LENGTH INTAKE RUNNERS that change for low-end vs high-end power; (8) IDLE AIR CONTROL valve (older systems) — bypasses throttle plate at idle to maintain idle RPM; modern systems do this with throttle plate control directly. AIR-FUEL RATIO: stoichiometric (chemically balanced) for gasoline = 14.7 parts air to 1 part fuel by mass. ECU varies this: 12-13:1 for power (rich, more fuel for cooling and complete combustion under load); 14.7:1 for cruise (efficient, meets emissions); 16-18:1 for max fuel economy in some conditions (lean burn). RICH = more fuel (black smoke, poor economy, fouled plugs, increased emissions); LEAN = less fuel (white/blue exhaust possible, hot combustion, ping/knock risk, possible damage if extreme). FORCED INDUCTION: TURBOCHARGER uses exhaust gas to spin a turbine which spins a compressor that forces more air into engine; SUPERCHARGER does the same but driven mechanically by the engine; both INCREASE air mass = more power output for given engine size. INTERCOOLER: cools the compressed air before it enters engine (compression heats air; cooler air is denser); typically air-to-air or air-to-liquid. Boost pressure: how much above atmospheric (psi or bar); modern: 6-30 psi typical; race: higher. CRANKCASE VENTILATION: PCV (Positive Crankcase Ventilation) valve allows blowby gases (combustion gases that leak past piston rings into the crankcase) to be drawn back into intake and burned; reduces emissions and prevents pressure buildup in crankcase. EVAP system: captures fuel tank vapors in CHARCOAL CANISTER, releases them into intake when engine running, prevents fuel vapor emission to atmosphere.

Horsepower (HP): unit of POWER, originally defined by James Watt in the late 1700s to compare steam engine output to draft horses. He calculated that a mill horse could do 33,000 foot-pounds of work per minute (lift 33,000 lbs one foot in one minute), so 1 HP = 33,000 ft-lbs/min = 550 ft-lbs/second = 745.7 watts. Power = Work / Time = Force × Velocity (for constant force). DIFFERENT from TORQUE: TORQUE is twisting force (rotational); POWER is rate of doing work. Engine produces torque; how quickly that torque is delivered (RPM) determines power. RELATIONSHIP: HP = (Torque × RPM) / 5252, where torque is in lb-ft and RPM is rev per minute; 5252 is just a unit conversion factor. Consequences: (1) HP and torque are EQUAL in lb-ft and HP at 5252 RPM (always — torque-HP curves intersect at this RPM on a dyno graph); (2) Below 5252 RPM, torque exceeds HP numerically; above, HP exceeds torque; (3) HIGH-RPM engines (sports cars, motorcycles) produce more HP relative to torque (small displacement at high revs); (4) LOW-RPM engines (diesels, big-block V8s, trucks) produce more torque relative to peak HP (large displacement, low revs); (5) BOTH MATTER: torque accelerates the vehicle; horsepower determines top speed and how quickly that acceleration can be sustained. Common power figures: small car 100-150 HP; mid-size 150-250 HP; sports car 250-400+ HP; supercars 500-1500+ HP; semi-truck 400-600 HP (but enormous torque, 1500-2000+ lb-ft); 1 HP per cubic inch was a 1960s muscle car benchmark; modern engines easily exceed 100 HP per liter (some over 200 HP/L). HORSEPOWER MEASUREMENTS — different standards: (1) GROSS HP (older measurement) — engine alone on dyno, no accessories, optimized conditions; high numbers but unrealistic; pre-1971 US; (2) NET HP — engine WITH accessories (alternator, water pump, power steering, exhaust system) — realistic; standard in US 1971+ as SAE Net; (3) DIN HP / PS — German/European standard (Pferdestärke); PS = 1 PS = 735.5 W (slightly less than 1 HP = 745.7 W); cars built outside US often advertise PS; (4) WHEEL HP (WHP) or RWHP — measured at wheels on chassis dyno; lower than crankshaft HP due to drivetrain losses (typically 10-25% loss FWD/RWD, 20-30% AWD); (5) BRAKE HP (bhp) — at the crankshaft (engine output); 'brake' refers to the old method of measuring by braking the output shaft. ELECTRIC VEHICLES: power often quoted in HP OR kW (1 kW = 1.341 HP); EVs deliver max torque from 0 RPM, so feel very quick despite sometimes modest peak HP; Tesla Model S Plaid: 1,020 HP (rated). Output measurement: ENGINE DYNAMOMETER (engine alone) — most accurate; CHASSIS DYNAMOMETER (rolling road, measures wheel HP); track tests (acceleration timing). Manufacturer claims: subject to standardized test conditions; sometimes optimistic; usually realistic for modern cars.