Introduction
The four-stroke engine is the most widely used internal combustion engine in the world. It powers millions of vehicles, agricultural machines, industrial equipment, generators, construction machinery, and marine vessels. Compared to a two-stroke engine, the four-stroke engine offers better fuel efficiency, lower emissions, improved durability, and smoother operation, making it the preferred choice for modern transportation and power generation.
The name "four-stroke engine" comes from the fact that one complete operating cycle requires four piston strokes and two complete revolutions of the crankshaft (720°). During these four strokes, the engine performs all four essential processes of an internal combustion cycle: intake, compression, power, and exhaust.
Although the four-stroke engine produces a power stroke only once every two crankshaft revolutions, its superior combustion efficiency, effective lubrication system, and lower maintenance requirements have made it the dominant engine design in automobiles and heavy-duty machinery.
This article provides a comprehensive overview of the four-stroke engine, including its history, construction, working principle, valve timing, thermodynamic cycle, performance characteristics, advantages, disadvantages, maintenance practices, and modern technological developments.
What is a Four-Stroke Engine?
A four-stroke engine is an internal combustion engine in which one complete operating cycle is completed in four piston strokes:
- Intake Stroke
- Compression Stroke
- Power (Expansion) Stroke
- Exhaust Stroke
These four strokes require two complete revolutions (720°) of the crankshaft.
The engine uses inlet and exhaust valves, operated by a camshaft, to control the flow of air, fuel, and exhaust gases. Unlike a two-stroke engine, the intake and exhaust processes occur separately, leading to more complete combustion, higher thermal efficiency, and cleaner exhaust emissions.
Historical Development
The four-stroke engine has a long history that shaped modern transportation.
- 1862 – French engineer Alphonse Beau de Rochas described the theoretical principles of the four-stroke cycle.
- 1876 – German engineer Nikolaus Otto successfully built the first practical four-stroke engine, leading to the development of the Otto Cycle.
- 1890s – Four-stroke engines became widely used in automobiles.
- 20th Century – Rapid improvements in fuel systems, ignition systems, lubrication, and cooling technologies increased engine efficiency and reliability.
- Present Day – Modern four-stroke engines incorporate electronic fuel injection, turbocharging, variable valve timing, direct fuel injection, and hybrid technologies to improve performance while reducing fuel consumption and emissions.
Classification of Four-Stroke Engines
Four-stroke engines can be classified based on several criteria.
Based on Ignition Method
Spark Ignition (SI) Engine
- Uses petrol (gasoline)
- Ignition by spark plug
- Operates on the Otto cycle
Examples:
- Cars
- Motorcycles
- Small generators
Compression Ignition (CI) Engine
- Uses diesel fuel
- Fuel ignites due to high compression temperature
- Operates on the Diesel cycle
Examples:
- Tractors
- Trucks
- Agricultural machinery
- Marine engines
Based on Number of Cylinders
- Single-cylinder engine
- Twin-cylinder engine
- Three-cylinder engine
- Four-cylinder engine
- Six-cylinder engine
- Eight-cylinder engine
- Multi-cylinder engine
Based on Cooling Method
- Air-cooled engine
- Water-cooled engine
Based on Fuel Supply
- Carbureted engine
- Port Fuel Injection (PFI)
- Gasoline Direct Injection (GDI)
- Common Rail Direct Injection (CRDI) for diesel engines
Four stroke Engine
Main Components of a Four-Stroke Engine
1. Cylinder Block
The cylinder block forms the main body of the engine. It houses the cylinders, coolant passages, and lubrication passages.
2. Cylinder Head
The cylinder head seals the top of the cylinder and contains:
- Combustion chamber
- Inlet valve
- Exhaust valve
- Spark plug (SI engine)
- Fuel injector (CI engine)
3. Piston
The piston moves vertically inside the cylinder, transmitting combustion force to the connecting rod.
Functions:
- Compresses air or air-fuel mixture
- Receives combustion pressure
- Transfers force to crankshaft
4. Piston Rings
Piston rings provide:
- Gas sealing
- Heat transfer
- Oil control
Types:
- Compression rings
- Oil control ring
5. Connecting Rod
Connects the piston to the crankshaft and converts reciprocating motion into rotary motion.
6. Crankshaft
The crankshaft converts piston motion into rotational motion that powers the vehicle or machine.
7. Camshaft
The camshaft controls valve opening and closing.
It rotates at half the speed of the crankshaft because the engine cycle requires two crankshaft revolutions.
8. Inlet Valve
Allows fresh air or air-fuel mixture into the cylinder.
9. Exhaust Valve
Allows burnt gases to leave the cylinder.
10. Flywheel
Stores rotational energy and maintains smooth engine operation between power strokes.
11. Spark Plug (SI Engine)
Produces an electric spark to ignite the compressed air-fuel mixture.
12. Fuel Injector (CI Engine)
Injects diesel fuel into the combustion chamber at high pressure.
Working Principle of a Four-Stroke Engine
A complete engine cycle consists of four piston strokes.
First Stroke: Intake Stroke
The piston moves from Top Dead Centre (TDC) to Bottom Dead Centre (BDC).
Valve Position
- Inlet valve: Open
- Exhaust valve: Closed
Process
As the piston moves downward, the cylinder volume increases, creating a partial vacuum. Fresh air (diesel engine) or an air-fuel mixture (petrol engine) is drawn into the cylinder through the open inlet valve.
At the end of the stroke:
- Cylinder is filled with fresh charge.
- Inlet valve closes.
Second Stroke: Compression Stroke
The piston moves from BDC to TDC.
Valve Position
- Inlet valve: Closed
- Exhaust valve: Closed
Process
The piston compresses the trapped charge, increasing its pressure and temperature.
Near the end of compression:
Petrol Engine
The spark plug ignites the compressed air-fuel mixture.
Diesel Engine
High-pressure diesel fuel is injected into the hot compressed air, where it ignites spontaneously due to the high temperature.
Compression is one of the most important stages because it determines the engine's thermal efficiency and power output.
Third Stroke: Power (Expansion) Stroke
This is the only stroke that produces useful mechanical work.
Valve Position
- Inlet valve: Closed
- Exhaust valve: Closed
Process
Combustion generates extremely high-pressure gases that force the piston downward from TDC to BDC.
This downward movement rotates the crankshaft and delivers power to the transmission, machinery, or equipment connected to the engine.
The flywheel stores excess energy during this stroke to help drive the remaining three non-power strokes.
Fourth Stroke: Exhaust Stroke
The piston moves from BDC to TDC.
Valve Position
- Exhaust valve: Open
- Inlet valve: Closed
Process
The upward-moving piston pushes the burnt combustion gases out of the cylinder through the open exhaust valve.
At the end of the stroke:
- Exhaust valve closes.
- Inlet valve opens.
- The cycle repeats.
Valve Timing
Although the ideal engine cycle assumes valves open and close exactly at TDC and BDC, actual engines use valve timing to improve breathing and efficiency.
Typical valve events include:
- Inlet valve opens slightly before TDC.
- Inlet valve closes after BDC.
- Exhaust valve opens before BDC.
- Exhaust valve closes after TDC.
This intentional overlap, known as valve overlap, improves cylinder filling at higher engine speeds and enhances volumetric efficiency.
Thermodynamic Cycles
The thermodynamic cycle depends on the type of engine.
Spark Ignition Engine
Operates approximately on the Otto Cycle, characterized by constant-volume heat addition.
Compression Ignition Engine
Operates approximately on the Diesel Cycle, characterized by constant-pressure heat addition.
Some modern high-speed diesel engines are better represented by the Dual Cycle, which combines constant-volume and constant-pressure heat addition.
Lubrication System
Unlike two-stroke engines, four-stroke engines have a separate lubrication system.
Engine oil is stored in an oil sump and circulated by an oil pump through oil galleries to lubricate moving parts such as bearings, piston rings, crankshaft, camshaft, and valve train.
Benefits include:
- Reduced friction
- Lower wear
- Improved cooling
- Longer engine life
- Cleaner combustion (oil is not burned with fuel)
Cooling System
Combustion generates large amounts of heat that must be removed to prevent engine damage.
Air Cooling
Heat is dissipated through cooling fins attached to the cylinder and cylinder head.
Applications:
- Motorcycles
- Small engines
Water Cooling
Coolant circulates through water jackets in the engine block and cylinder head, transferring heat to the radiator.
Applications:
- Cars
- Tractors
- Trucks
- Industrial engines
Fuel Supply Systems
Modern four-stroke engines use advanced fuel delivery systems.
Petrol Engines
- Carburetor (older engines)
- Multi-point Fuel Injection (MPFI)
- Gasoline Direct Injection (GDI)
Diesel Engines
- Mechanical fuel injection
- Unit injectors
- Common Rail Direct Injection (CRDI)
These systems improve fuel atomization, combustion efficiency, and emissions performance.
Performance Characteristics
The performance of a four-stroke engine is evaluated using several parameters:
- Brake Power (BP)
- Indicated Power (IP)
- Friction Power (FP)
- Brake Mean Effective Pressure (BMEP)
- Indicated Mean Effective Pressure (IMEP)
- Brake Thermal Efficiency
- Mechanical Efficiency
- Volumetric Efficiency
- Specific Fuel Consumption (SFC)
These parameters help engineers assess engine efficiency, power output, and fuel economy.
Advantages of Four-Stroke Engines
- Higher fuel efficiency due to complete combustion.
- Lower exhaust emissions.
- Better lubrication because engine oil is separate from the fuel.
- Longer engine life with reduced wear.
- Smoother and quieter operation.
- Lower fuel consumption than two-stroke engines.
- Greater reliability for continuous operation.
- Higher thermal efficiency.
- Better suitability for heavy-duty applications.
- Compliance with modern emission standards.
Disadvantages of Four-Stroke Engines
- More complex construction due to valves, camshaft, timing mechanism, and lubrication system.
- Higher manufacturing cost.
- Greater weight than equivalent two-stroke engines.
- Lower power-to-weight ratio because only one power stroke occurs every two crankshaft revolutions.
- More components increase maintenance complexity.
Applications
Four-stroke engines are used in a wide variety of applications, including:
- Passenger cars
- Motorcycles
- Tractors
- Agricultural machinery
- Trucks
- Buses
- Diesel generators
- Marine engines
- Construction equipment
- Irrigation pumps
- Lawn mowers
- Portable generators
- Industrial compressors
- Earthmoving equipment
- Railway locomotives (diesel engines)
Modern Technological Developments
To meet stricter emission regulations and improve efficiency, modern four-stroke engines incorporate advanced technologies such as:
- Electronic Fuel Injection (EFI): Delivers precise amounts of fuel for better combustion and fuel economy.
- Turbocharging and Supercharging: Increase air intake, resulting in higher power output without significantly increasing engine size.
- Variable Valve Timing (VVT): Adjusts valve opening and closing based on engine speed and load to improve efficiency and performance.
- Variable Valve Lift (VVL): Optimizes airflow by changing valve lift according to operating conditions.
- Gasoline Direct Injection (GDI): Injects fuel directly into the combustion chamber for improved power and reduced fuel consumption.
- Common Rail Direct Injection (CRDI): Provides high-pressure, electronically controlled diesel fuel injection for cleaner combustion.
- Cylinder Deactivation: Temporarily shuts down selected cylinders during low-load operation to save fuel.
- Start-Stop Systems: Automatically switch off the engine when the vehicle is stationary, reducing idle fuel consumption.
- Hybrid Powertrains: Combine four-stroke engines with electric motors to improve fuel efficiency and reduce emissions.
Maintenance Practices
Regular maintenance ensures reliable operation and extends engine life.
Recommended practices include:
- Change engine oil and oil filter at recommended intervals.
- Replace air filters to maintain proper airflow.
- Inspect and replace spark plugs (SI engines) when necessary.
- Check coolant levels and inspect the cooling system for leaks.
- Adjust valve clearances according to the manufacturer's specifications.
- Replace timing belts or timing chains as recommended.
- Inspect fuel injectors or carburetors for proper operation.
- Monitor compression pressure to detect engine wear.
- Use high-quality fuel and lubricants.
- Follow the manufacturer's maintenance schedule.
Environmental Considerations
Modern four-stroke engines are designed to reduce environmental impact through:
- Catalytic converters that reduce carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOₓ).
- Exhaust Gas Recirculation (EGR) systems that lower combustion temperatures and reduce NOₓ formation.
- Diesel Particulate Filters (DPF) that trap soot particles in diesel engines.
- Selective Catalytic Reduction (SCR) systems that convert NOₓ into harmless nitrogen and water using diesel exhaust fluid (DEF).
- Improved combustion chamber designs and electronic engine management systems that optimize fuel use and minimize emissions.
These technologies help modern engines comply with increasingly stringent global emission standards while maintaining high performance.
Conclusion
The four-stroke engine has become the foundation of modern transportation and power generation because of its efficiency, reliability, and environmental performance. By completing the intake, compression, power, and exhaust processes over four piston strokes, it achieves more complete combustion, better fuel economy, lower emissions, and a longer service life than the two-stroke engine.
Although its construction is more complex and heavier, the advantages of separate lubrication, precise valve control, and advanced fuel management systems make it the preferred choice for automobiles, tractors, industrial machinery, and heavy-duty equipment. Continuous innovations such as electronic fuel injection, variable valve timing, turbocharging, and hybrid integration ensure that the four-stroke engine remains a highly efficient and adaptable technology.
A solid understanding of the four-stroke engine—including its components, working principle, valve timing, thermodynamic cycles, performance characteristics, and maintenance—is essential for students, engineers, and professionals in mechanical, agricultural, automotive, and energy engineering. As engine technologies continue to evolve, the four-stroke engine will remain a key contributor to efficient and sustainable power generation for years to come.
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