Mechanical
Four-Stroke Engine
Intake, compression, power, exhaust — Otto cycle
A four-stroke internal combustion engine completes one power cycle per two crankshaft revolutions, executing four piston strokes: intake, compression, power, exhaust. Invented by Nikolaus Otto (1876). Dominant in automobiles, motorcycles, generators. Each cylinder fires once every two revolutions, so multi-cylinder engines stagger firing order for smooth torque. Compression ratio, valve timing, and ignition timing govern efficiency, power, and emissions.
- StrokesIntake, compression, power, exhaust
- CycleTwo crankshaft revolutions per power stroke
- InventorNikolaus Otto, 1876
- Compression ratio8 to 12 (gasoline), 14 to 22 (diesel)
- Thermal efficiency25 to 40% (gasoline)
- Valve countTypically 2 to 4 per cylinder
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Why the four-stroke engine matters
- Automotive. Powers most cars, trucks, motorcycles.
- Power generation. Backup generators, off-grid power.
- Marine. Inboard and outboard boat engines.
- Aviation. Light aircraft piston engines.
- Industrial. Pumps, compressors, construction equipment.
- Agriculture. Tractors, combines, farm machinery.
- Education. Canonical thermodynamic cycle (Otto cycle).
Common misconceptions
- Power stroke equals one revolution. Each cylinder fires every two revolutions.
- Higher compression always better. Knock and emissions limit it.
- Spark and timing same thing. Spark advance differs from valve timing.
- Octane equals quality. Octane is knock resistance, not energy content.
- Engines are mechanical only. Modern engines depend on ECU, sensors, actuators.
- Friction is minor. Roughly a third of fuel energy lost as heat plus friction.
Frequently asked questions
What are the four strokes?
Intake — piston descends, intake valve opens, fresh air-fuel mixture fills cylinder. Compression — both valves close, piston rises, mixture compressed. Power — spark ignites mixture, expanding gases drive piston down. Exhaust — exhaust valve opens, rising piston pushes spent gases out. Cycle repeats every two crankshaft revolutions.
How does compression ratio affect performance?
Compression ratio is cylinder volume at bottom dead center divided by volume at top dead center. Higher ratio extracts more work per cycle (better thermal efficiency) but risks knock — premature autoignition damaging the engine. Higher-octane fuel resists knock. Diesel uses very high compression to autoignite without spark plugs.
What is valve timing?
When intake and exhaust valves open and close relative to piston position. Valve overlap, when both are briefly open near top dead center, lets exhaust scavenge fresh charge. Variable valve timing (Honda VTEC, BMW VANOS) optimizes timing across RPM range. Cam profile shape and lift govern airflow and power character.
How is firing order chosen?
To balance the engine, distribute torque evenly across the crankshaft, and minimize vibration. Inline-four typical firing order is 1-3-4-2. V8s commonly use 1-8-4-3-6-5-7-2. Choices balance crankshaft loads and exhaust pulse separation. Wrong firing order causes severe vibration and uneven combustion.
What's thermal efficiency?
Useful work output divided by chemical energy input. Modern gasoline engines achieve 25 to 35% peak. Diesel hits 40 to 45%. The remainder leaves as exhaust heat (~30%), coolant heat (~30%), and friction (~5%). Theoretical Otto-cycle efficiency depends only on compression ratio and specific heat ratio.
How do diesels differ?
Compression ignition rather than spark. Air alone is compressed to 14:1 to 22:1 ratios, raising temperature above fuel autoignition. Fuel injects directly into hot air and burns immediately. Higher compression yields higher efficiency. Tradeoffs include heavier construction, slower response, higher NOx and particulate emissions.
Why two-stroke alternatives exist?
Two-strokes complete a power cycle every revolution: more power per displacement, mechanical simplicity (no valves), but worse emissions and fuel economy due to scavenging losses. Used in chainsaws, outboards, small engines. Four-stroke dominates automotive due to efficiency, longevity, and emissions compliance.