Internal Combustion Engine — Working, Types and Efficiency

The fundamental working of an internal combustion engine:

1. Energy conversion chain: Chemical energy (fuel) → Thermal energy (combustion) → Mechanical energy (piston motion) → Rotational energy (crankshaft)

2. The working medium is a fuel-air mixture that undergoes combustion inside a closed cylinder.

3. Key components: • Cylinder: the chamber where combustion takes place. • Piston: moves up and down inside the cylinder, converting pressure to linear motion. • Crankshaft: converts linear (reciprocating) piston motion into rotational motion. • Valves: intake valve (admits air/fuel) and exhaust valve (removes burnt gases). • Spark plug (petrol engine) or fuel injector (diesel engine): ignites or introduces fuel. • Connecting rod: links piston to crankshaft. • Camshaft: controls the timing of valve opening and closing.

4. Thermodynamic cycle: • Petrol engines follow the Otto cycle (constant-volume combustion). • 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 — Intake (Suction): • Piston moves DOWN from TDC (Top Dead Centre) to BDC (Bottom Dead Centre). • Intake valve OPEN; exhaust valve CLOSED. • Air-fuel mixture (petrol engine) or air only (diesel engine) is drawn into the cylinder.

Stroke 2 — Compression: • Piston moves UP from BDC to TDC. • Both valves CLOSED. • The mixture is compressed:

• Petrol engine: compression ratio ~8:1 to 12:1
• Diesel engine: compression ratio ~14:1 to 22:1

Stroke 3 — Power (Combustion/Expansion): • At or near TDC, the mixture is ignited:

• Petrol engine: by a spark from the spark plug.
• Diesel engine: by the heat of highly compressed air (compression ignition; fuel injected at this point). • Burning gases expand rapidly, pushing the piston DOWN with great force. • This is the only stroke that produces power.

Stroke 4 — Exhaust: • Piston moves UP from BDC to TDC. • Exhaust valve OPEN; intake valve CLOSED. • Burnt gases are expelled from the cylinder. • Cycle repeats.

Summary: Intake → Compression → Power → Exhaust

A 2-stroke engine completes one power cycle in just 2 piston strokes (1 crankshaft revolution):

Stroke 1 — Power + Exhaust: • Mixture is compressed and ignited at TDC. • Piston moves DOWN — power stroke. • As the piston moves down, exhaust ports are uncovered → burnt gases escape. • Fresh mixture enters through transfer ports (or via crankcase).

Stroke 2 — Compression + Intake: • Piston moves UP, compressing the fresh mixture. • Intake and exhaust ports are covered by the piston. • At TDC, ignition occurs again.

Advantages of 2-stroke: • Simpler design (no separate valves needed in basic designs). • One power stroke per revolution → more power for the same engine size. • Lighter and cheaper.

Disadvantages of 2-stroke: • Less fuel-efficient (some fresh charge is lost with exhaust). • More polluting (unburnt fuel exits with exhaust gases). • Higher wear rate. • Not used in modern cars due to emission regulations.

Applications: mopeds, lawnmowers, chainsaws, two-wheelers (older models), marine outboard motors.

Petrol (Gasoline) Engine: • Fuel: petrol (gasoline) • Ignition: spark ignition (SI) — a spark plug ignites the air-fuel mixture. • Compression ratio: ~8:1 to 12:1 (lower, as petrol is prone to pre-ignition/knocking at high compression). • Thermodynamic cycle: Otto cycle • Typical efficiency: ~25–30% • Uses: cars, motorcycles, aircraft (small)

Diesel Engine: • Fuel: diesel • Ignition: compression ignition (CI) — air is compressed until hot enough (~500–700°C) to ignite diesel injected at that point; no spark plug needed. • Compression ratio: ~14:1 to 22:1 (higher compression → higher efficiency). • Thermodynamic cycle: Diesel cycle • Typical efficiency: ~30–40% • Uses: trucks, buses, ships, trains, tractors, generators

Key differences: Property | Petrol Engine | Diesel Engine Ignition | Spark plug | Compression Fuel | Petrol | Diesel Efficiency | ~25–30% | ~30–40% 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 (η) = Work output / Heat input × 100%

Typical efficiencies: • 2-stroke petrol engine: ~15–20% • 4-stroke petrol engine (Otto cycle): ~25–30% • 4-stroke diesel engine (Diesel cycle): ~30–40% • Turbo-diesel engine: up to 45%

Why is efficiency less than 100%? • Heat is lost to the engine coolant (cooling system) — ~30%. • Heat is lost in the exhaust gases — ~30–35%. • Friction losses in pistons, bearings, etc. — ~5–10%. • Only ~25–40% of fuel energy becomes useful mechanical work.

Ideal Otto cycle efficiency: η_Otto = 1 − (1/r^(γ−1)) Where r = compression ratio, γ = ratio of specific heats (≈1.4 for air)

Higher compression ratio → higher efficiency (limited by fuel knocking for petrol). This is why diesel engines, with higher compression ratios, are more efficient.