IC Engines

Internal combustion engines turn fuel into shaft power. Learn the four-stroke cycle, SI vs CI engines, displacement and compression ratio, torque, power and BMEP, fuel consumption (BSFC) and efficiency, turbocharging and emissions control.

For over a century, the internal combustion engine has powered cars, trucks, ships and generators. Fuel burns inside the cylinder; the expanding gas pushes a piston; a crankshaft converts that into rotation.

Understanding engine performance numbers — displacement, torque, power, BMEP, BSFC — lets engineers compare engines, match them to vehicles and hybrid systems, and see where efficiency is lost.

An analogy

An engine 'breathes': suck (intake), squeeze (compression), bang (power), blow (exhaust). Bigger lungs (displacement) and faster breathing (rpm) mean more power.

Four-stroke cycle

Intake (piston down, valve open, air/fuel in), compression (both valves shut), power (spark or auto-ignition, gas expands), exhaust (piston up, gases out). One power stroke every two crank revolutions per cylinder. Two-stroke engines fire every revolution (small engines, large marine diesels).

Spark vs compression ignition

Petrol (SI) engines mix fuel and air, ignite with a spark, and control load with a throttle; knock limits compression ratio (≈ 10–13). Diesel (CI) engines compress air to ~15–20:1 so injected fuel self-ignites; no throttle, higher efficiency, more torque, but NOx and soot need after-treatment.

Torque, power and BMEP

Torque shows the engine's pulling force; power = torque × speed. BMEP normalises torque by displacement so different-sized engines can be compared: naturally aspirated petrol ≈ 1.0–1.3 MPa, turbo petrol 1.8–2.5 MPa, truck diesels 2–2.5 MPa.

Efficiency, boosting and emissions

Only 30–45 % of fuel energy becomes shaft work; the rest leaves as exhaust heat and coolant heat, plus friction and pumping losses. Turbocharging with downsizing, direct injection, variable valve timing, Atkinson/Miller cycles, cooled EGR and cylinder deactivation raise efficiency. Three-way catalysts (petrol), DPF + SCR with urea (diesel) and GPFs clean the exhaust to meet Euro 6/7 and EPA limits.

Formulas

• Displacement: V_d = n_cyl · (π/4) B² S
• Power (four-stroke): P = BMEP · V_d · N / (2 · 60)
• Torque: T = BMEP · V_d / (4π)
• Brake thermal efficiency: η = 3600 / (BSFC[g/kWh] · LHV[MJ/kg])
• Mean piston speed: U_p = 2 S N / 60

Worked example: Engine size and output

Given: 4 cylinders, bore = stroke = 86 mm; BMEP = 1.2 MPa at 5000 rpm

Find: Displacement, power, torque, piston speed.

• V_d = 4 × π/4 × 0.086² × 0.086 = 2.0 L
• P = 1.2 × 10⁶ × 0.002 × 5000/120 = 100 kW
• T = 1.2 × 10⁶ × 0.002/(4π) = 191 N·m
• U_p = 2 × 0.086 × 5000/60 = 14.3 m/s

Answer: 2.0 L, 100 kW, 191 N·m, 14.3 m/s

Worked example: Efficiency from BSFC

Given: BSFC = 250 g/kWh, petrol LHV = 43 MJ/kg

Find: Efficiency and fuel flow at 100 kW.

• η = 3600/(250 × 43) = 33.5 %
• Fuel = 250 × 100 = 25 kg/h (≈ 34 L/h)

Answer: 33.5 %, 25 kg/h

Common mistakes

• Wrong: Using N instead of N/2 for four-stroke power strokes. — Right: Four-stroke: one power stroke per two revolutions — divide by 2.
• Wrong: Comparing engines by peak power only. — Right: Look at the torque curve, BMEP and BSFC map across the operating range.

Applications

• Passenger cars: Downsized turbo petrol engines and hybrid-optimised Atkinson engines.
• Trucks & off-highway: Diesels with 45–50 % peak efficiency and SCR after-treatment.
• Power generation & marine: Gas engines and two-stroke marine diesels > 50 % efficient.

Key takeaways

• Intake–compression–power–exhaust.
• P = BMEP·V_d·N/120 (four-stroke).
• η = 3600/(BSFC·LHV).
• Boosting and downsizing raise efficiency.

Going further

• Engine maps: BSFC contour maps over speed–torque guide gearbox and hybrid strategies to operate engines near their sweet spot.
• Combustion modes: HCCI, spark-assisted CI and lean-burn pre-chamber ignition push efficiency toward 45–50 % in cars.
• Hydrogen & e-fuels: IC engines running on hydrogen or synthetic fuels can be near-zero CO₂.