An internal combustion engine (IC engine) is a heat engine that converts the chemical energy stored in fuel into mechanical energy by burning the fuel inside a cylinder. The combustion of fuel (petrol, diesel, or gas) produces high-pressure gases that push a piston, which turns a crankshaft to produce rotational mechanical work. IC engines power cars, motorcycles, ships, and generators, with thermal efficiencies typically between 25% and 40%.
An IC engine converts chemical energy of fuel into mechanical energy via combustion inside a cylinder.
4-stroke engine cycle: Intake β Compression β Power β Exhaust (2 crankshaft revolutions per cycle).
2-stroke engine: one power stroke per crankshaft revolution β more power, less efficient.
Petrol engines use spark ignition (spark plug); diesel engines use compression ignition.
Diesel engine compression ratio (~14β22:1) is higher than petrol (~8β12:1).
Typical efficiency: petrol engine 25β30%, diesel engine 30β40%.
Energy losses: heat to coolant (~30%), exhaust gases (~30%), friction (~5β10%).
The fundamental working of an internal combustion engine:
Energy conversion chain: Chemical energy (fuel) β Thermal energy (combustion) β Mechanical energy (piston motion) β Rotational energy (crankshaft)
The working medium is a fuel-air mixture that undergoes combustion inside a closed cylinder.
Key components: β’ Cylinder: the chamber where combustion takes place. β’ Piston: moves up and down inside the cylinder, converting pressure to linear motion. β’ Crankshaft: converts linear (reciprocating) piston motion into rotational motion. β’ Valves: intake valve (admits air/fuel) and exhaust valve (removes burnt gases). β’ Spark plug (petrol engine) or fuel injector (diesel engine): ignites or introduces fuel. β’ Connecting rod: links piston to crankshaft. β’ Camshaft: controls the timing of valve opening and closing.
Thermodynamic cycle: β’ Petrol engines follow the Otto cycle (constant-volume combustion). β’ Diesel engines follow the Diesel cycle (constant-pressure combustion).
A 4-stroke engine completes one power cycle in 4 piston strokes (2 full crankshaft revolutions):
Stroke 1 β Intake (Suction): β’ Piston moves DOWN from TDC (Top Dead Centre) to BDC (Bottom Dead Centre). β’ Intake valve OPEN; exhaust valve CLOSED. β’ Air-fuel mixture (petrol engine) or air only (diesel engine) is drawn into the cylinder.
Stroke 2 β Compression: β’ Piston moves UP from BDC to TDC. β’ Both valves CLOSED. β’ The mixture is compressed:
Stroke 3 β Power (Combustion/Expansion): β’ At or near TDC, the mixture is ignited:
Stroke 4 β Exhaust: β’ Piston moves UP from BDC to TDC. β’ Exhaust valve OPEN; intake valve CLOSED. β’ Burnt gases are expelled from the cylinder. β’ Cycle repeats.
Summary: Intake β Compression β Power β Exhaust
A 2-stroke engine completes one power cycle in just 2 piston strokes (1 crankshaft revolution):
Stroke 1 β Power + Exhaust: β’ Mixture is compressed and ignited at TDC. β’ Piston moves DOWN β power stroke. β’ As the piston moves down, exhaust ports are uncovered β burnt gases escape. β’ Fresh mixture enters through transfer ports (or via crankcase).
Stroke 2 β Compression + Intake: β’ Piston moves UP, compressing the fresh mixture. β’ Intake and exhaust ports are covered by the piston. β’ At TDC, ignition occurs again.
Advantages of 2-stroke: β’ Simpler design (no separate valves needed in basic designs). β’ One power stroke per revolution β more power for the same engine size. β’ Lighter and cheaper.
Disadvantages of 2-stroke: β’ Less fuel-efficient (some fresh charge is lost with exhaust). β’ More polluting (unburnt fuel exits with exhaust gases). β’ Higher wear rate. β’ Not used in modern cars due to emission regulations.
Applications: mopeds, lawnmowers, chainsaws, two-wheelers (older models), marine outboard motors.
Petrol (Gasoline) Engine: β’ Fuel: petrol (gasoline) β’ Ignition: spark ignition (SI) β a spark plug ignites the air-fuel mixture. β’ Compression ratio: ~8:1 to 12:1 (lower, as petrol is prone to pre-ignition/knocking at high compression). β’ Thermodynamic cycle: Otto cycle β’ Typical efficiency: ~25β30% β’ Uses: cars, motorcycles, aircraft (small)
Diesel Engine: β’ Fuel: diesel β’ Ignition: compression ignition (CI) β air is compressed until hot enough (~500β700Β°C) to ignite diesel injected at that point; no spark plug needed. β’ Compression ratio: ~14:1 to 22:1 (higher compression β higher efficiency). β’ Thermodynamic cycle: Diesel cycle β’ Typical efficiency: ~30β40% β’ Uses: trucks, buses, ships, trains, tractors, generators
Key differences: Property | Petrol Engine | Diesel Engine Ignition | Spark plug | Compression Fuel | Petrol | Diesel Efficiency | ~25β30% | ~30β40% Compression | 8:1 to 12:1 | 14:1 to 22:1 Speed (RPM) | Higher | Lower Cost | Lower | Higher
Thermal efficiency of an IC engine is the ratio of useful mechanical work output to the heat energy supplied by the fuel:
Efficiency (Ξ·) = Work output / Heat input Γ 100%
Typical efficiencies: β’ 2-stroke petrol engine: ~15β20% β’ 4-stroke petrol engine (Otto cycle): ~25β30% β’ 4-stroke diesel engine (Diesel cycle): ~30β40% β’ Turbo-diesel engine: up to 45%
Why is efficiency less than 100%? β’ Heat is lost to the engine coolant (cooling system) β ~30%. β’ Heat is lost in the exhaust gases β ~30β35%. β’ Friction losses in pistons, bearings, etc. β ~5β10%. β’ Only ~25β40% of fuel energy becomes useful mechanical work.
Ideal Otto cycle efficiency: Ξ·_Otto = 1 β (1/r^(Ξ³β1)) Where r = compression ratio, Ξ³ = ratio of specific heats (β1.4 for air)
Higher compression ratio β higher efficiency (limited by fuel knocking for petrol). This is why diesel engines, with higher compression ratios, are more efficient.
An internal combustion engine (IC engine) is a heat engine that burns fuel inside a cylinder to convert chemical energy into mechanical energy. The expanding combustion gases push a piston, which drives a crankshaft to produce rotational motion.
The four strokes are: (1) Intake β piston moves down, drawing in air-fuel mixture; (2) Compression β piston moves up, compressing the mixture; (3) Power β ignition pushes piston down, producing work; (4) Exhaust β piston moves up, expelling burnt gases.
Spark ignition (SI): used in petrol engines β a spark plug creates an electric spark to ignite the compressed air-fuel mixture. Compression ignition (CI): used in diesel engines β air is compressed to such a high temperature (~500β700Β°C) that diesel fuel auto-ignites when injected, with no spark plug needed.
Petrol engines have a thermal efficiency of about 25β30%, and diesel engines about 30β40%. The rest of the fuel energy is lost as heat to the cooling system (~30%), exhaust gases (~30%), and friction (~5β10%).
A 4-stroke engine has one power stroke every 2 crankshaft revolutions (4 strokes), is more fuel-efficient, and is less polluting. A 2-stroke engine has one power stroke per crankshaft revolution, is simpler and lighter, but is less efficient and more polluting.
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