Engine Basics and the Four-Stroke Cycle: Practice Questions & Explanations

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.

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.

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.

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.

Top Dead Center (TDC): the position of a piston at the highest point in the cylinder, where it momentarily stops before reversing direction downward. The volume in the cylinder is at its MINIMUM at TDC (combustion chamber volume). Bottom Dead Center (BDC): the lowest piston position; volume at MAXIMUM (combustion chamber + displaced volume). These reference points are essential for engine timing and measurements. STROKE: distance piston travels from TDC to BDC (and same back). On crankshaft, the piston is at TDC when crank pin is at TOP of its rotation circle; at BDC when at BOTTOM. Piston moves slowly near TDC and BDC (changing direction); fastest at mid-stroke. BTDC (Before TDC): position is approaching TDC on compression stroke; ignition timing usually specified BTDC (e.g., '10° BTDC' means spark fires when piston is 10° of crank rotation before TDC); ATDC (After TDC): after passing TDC on power stroke. VALVE TIMING events relative to TDC/BDC: (1) IVO (intake valve opens) — typically a few degrees BTDC on exhaust stroke to start opening before piston reaches top; (2) IVC (intake valve closes) — typically a few degrees ABDC on compression stroke, after piston has started up but inertia still drawing in air; (3) EVO (exhaust valve opens) — typically BBDC (before bottom dead center) on power stroke; (4) EVC (exhaust valve closes) — typically ATDC on intake stroke. VALVE OVERLAP: brief period when both intake and exhaust valves are open simultaneously, around TDC at end of exhaust / start of intake; helps with cylinder scavenging at high RPM but can hurt low-end performance. Performance engines have more overlap; passenger car engines have less. CAMSHAFTS DETERMINE VALVE TIMING — different cam profiles (duration, lift) optimize for different RPM ranges. PERFORMANCE CAMSHAFTS have higher lift and longer duration, sacrificing low-end for high-end. VARIABLE VALVE TIMING (VVT/VVT-i/VTEC/VANOS etc.) — adjusts cam timing electronically/hydraulically to optimize across RPM range; common modern technology. IGNITION TIMING: when the spark plug fires relative to piston position; typically 10-30° BTDC at idle; advances (fires earlier) with RPM (giving combustion time to develop pressure as piston is at top); advances with engine demand. Excessive advance causes KNOCK/PING (premature ignition); too retarded causes power loss and overheating. KNOCK SENSORS detect knock and ECU retards timing automatically. IGNITION TIMING is often diagnosed with a TIMING LIGHT (strobe synchronized with #1 cylinder spark) aimed at timing marks on the crankshaft pulley/harmonic balancer. Many modern engines run distributorless ignition with timing entirely controlled by ECU based on sensor inputs.

The four strokes of a four-stroke engine, in order, are: Intake (the piston moves down, drawing in the air-fuel mixture), Compression (the piston moves up, compressing the mixture), Power (the spark ignites the mixture, forcing the piston down), and Exhaust (the piston moves up, pushing out the burned gases). A common memory aid is 'Suck, Squeeze, Bang, Blow.' Each stroke is one movement of the piston, and the cycle repeats continuously. Knowing the correct order of the four strokes is one of the most fundamental Auto Information topics.

Engine displacement is the total volume swept by all the pistons moving inside the cylinders during one stroke, usually expressed in liters or cubic centimeters (cc). It is a basic measure of an engine's size and a rough indicator of its potential power and air-fuel intake — a larger displacement generally means more power but often lower fuel economy. For example, a '2.0-liter' engine has cylinders whose combined swept volume is two liters. Displacement is determined by the bore (cylinder diameter), stroke (piston travel), and number of cylinders, and it is a standard engine specification.

A gasoline engine uses a spark plug to ignite the compressed air-fuel mixture at the right moment. A diesel engine has no spark plugs; instead it compresses air to a very high pressure, which makes it hot enough that injected diesel fuel ignites on contact (compression ignition). Diesel engines run at higher compression ratios and are generally more fuel-efficient and produce more low-end torque, while gasoline engines rev higher and run more smoothly at high speed. Knowing this key difference — spark ignition versus compression ignition — is a frequently tested engine-basics concept.