IC engines

In short

An internal combustion (IC) engine burns fuel inside its cylinders and turns the pressure of the hot gases into rotation of a crankshaft. Petrol (spark-ignition) and diesel (compression-ignition) engines power India's two-wheelers, cars, tractors, trucks, buses, gensets and pumps. IC engines are the single most asked topic in automotive and core mechanical interviews (Maruti, Tata Motors, Mahindra, Bajaj, TVS, Hero, Ashok Leyland, Cummins, Kirloskar) and carry steady marks in university exams and GATE.

Explained

Main parts and terms

• Cylinder, piston, piston rings (compression rings seal gas; oil ring scrapes oil), connecting rod, crankshaft, cylinder head, valves (inlet and exhaust), camshaft (opens valves; turns at half crank speed in four-stroke engines), flywheel (smooths the uneven torque between power strokes).
• TDC / BDC: top and bottom dead centre - the piston's extreme positions. Stroke (L): distance between them = 2 × crank radius. Bore (d): cylinder diameter.
• Swept volume Vs = (π/4)d²L; clearance volume Vc = volume above the piston at TDC; compression ratio r = (Vs + Vc)/Vc.
• Square engine: bore = stroke; over-square (bore > stroke) revs higher - many motorcycle engines; under-square (long stroke) gives more low-speed torque - many diesels.

Four-stroke cycle (one power stroke every two crank revolutions)

1. Suction (intake): piston moves TDC → BDC, inlet valve open; an SI engine draws air-fuel mixture (or air, in direct-injection petrol engines); a CI engine draws air only.
2. Compression: both valves closed, piston moves up; pressure and temperature rise. Diesel air reaches roughly 500-700 °C - hot enough to ignite fuel.
3. Power (expansion): a spark (SI) or injected fuel self-igniting (CI) starts combustion near TDC; gas pressure drives the piston down - the only stroke that produces work.
4. Exhaust: exhaust valve open, piston pushes burnt gases out.

Two-stroke cycle (one power stroke every revolution)

• Intake and exhaust happen through ports in the cylinder wall uncovered by the piston, while the crankcase pre-compresses the fresh charge; scavenging pushes exhaust out with fresh charge.
• Advantages: simpler, lighter, roughly double the power strokes for the same speed - used in small chainsaws, outboard motors and, at the other extreme, huge marine diesels.
• Disadvantages: part of the fresh charge escapes with the exhaust (short-circuiting), so fuel consumption and HC emissions are high; oil mixed with petrol for lubrication. Two-stroke petrol scooters and motorcycles disappeared in India as emission norms tightened.

SI (petrol) versus CI (diesel) combustion

• SI: a premixed air-fuel mixture near stoichiometric (about 14.7:1 by mass for petrol) is ignited by a spark plug; a flame front spreads across the chamber. Load is controlled by the throttle (quantity governing).
• CI: fuel is sprayed into hot compressed air and ignites after a short ignition delay; combustion is heterogeneous and runs lean overall. Load is controlled by the amount of fuel injected (quality governing) - no throttle, so less pumping loss at part load.
• Why diesels are more efficient: much higher compression ratio (about 15-22 against 9-12), no throttling losses, lean operation.
• Knock in SI engines: the unburnt end-gas self-ignites before the flame arrives, creating pressure waves (pinging) that damage pistons. Prevented by higher-octane fuel, lower compression ratio, retarded ignition, good cooling, and knock sensors that retard timing.
• Knock in CI engines: a long ignition delay lets too much fuel accumulate and burn at once (diesel knock). Prevented by higher-cetane fuel, higher compression temperature, pilot injection.
• So the remedies are opposite: SI engines want fuel that resists self-ignition; CI engines want fuel that self-ignites readily.

Valve timing (four-stroke)

• Valves do not open and close exactly at TDC and BDC: the inlet opens before TDC and closes well after BDC (using the inertia of the incoming charge); the exhaust opens before BDC (blowdown) and closes after TDC.
• Valve overlap: both valves open together near TDC to help scavenging - larger in high-speed engines. Variable valve timing (VVT) changes timing with speed for both torque and economy.
• Ignition advance: the spark fires before TDC (about 10-40° depending on speed and load) so peak pressure occurs just after TDC.

Fuel supply and engine systems

• Petrol: carburettors have given way to MPFI (multi-point fuel injection into each intake port) and GDI (gasoline direct injection into the cylinder), controlled by the ECU using oxygen (lambda), throttle, temperature and airflow sensors.
• Diesel: CRDI (common rail direct injection) stores fuel at very high pressure (well over 1,000 bar) and injects precisely, with pilot and multiple injections for quieter, cleaner combustion.
• Air: naturally aspirated, turbocharged (exhaust-driven compressor; an intercooler cools the compressed air) or supercharged (crank-driven).
• Ignition: coil-on-plug electronic ignition in modern petrol engines.
• Cooling: water cooling (radiator, water pump, thermostat, coolant) in cars and trucks; air cooling (fins) in many two-wheelers.
• Lubrication: wet sump with oil pump and filter; reduces friction and wear, carries heat away, keeps parts clean.
• Starting: electric starter; glow plugs help cold-start diesels.

Emissions and Indian norms

• Pollutants: CO (incomplete combustion), HC (unburnt fuel), NOx (high combustion temperature), particulate matter (soot - mainly diesels), CO₂ (proportional to fuel burnt).
• After-treatment: three-way catalytic converter for petrol engines (works only near stoichiometric mixture, controlled by the lambda sensor); diesel oxidation catalyst (DOC), diesel particulate filter (DPF) and selective catalytic reduction (SCR) with urea solution (AdBlue/DEF) for diesels; EGR (exhaust gas recirculation) reduces NOx by lowering combustion temperature.
• India's norms: the country moved from BS-IV directly to BS-VI in April 2020; BS-VI Phase 2 (2023) added real driving emission (RDE) checks and on-board diagnostics; E20 petrol (20% ethanol) is now widely sold.

Key points

• A four-stroke engine fires each cylinder once every two revolutions; a two-stroke once every revolution.
• Compression ratio = (Vs + Vc)/Vc; raising it raises efficiency until knock (SI) or mechanical stress (CI) limits it.
• Indicated power is developed by the gas on the pistons; brake power is delivered at the crankshaft; friction power = IP − BP.
• Mechanical efficiency = BP/IP, typically about 75-90% at full load and much lower at part load.
• Brake thermal efficiency = BP/(fuel energy input) - roughly 25-35% for petrol and 35-45% for modern diesel engines.
• BSFC (brake specific fuel consumption, g/kWh or kg/kWh) is the practical efficiency measure - lower is better.
• Volumetric efficiency = actual air drawn / swept volume at intake conditions - about 75-90% naturally aspirated, above 100% with turbocharging.
• Mean effective pressure links work to engine size; brake mean effective pressure (BMEP) compares engines of different sizes.
• Torque peaks at a lower speed than power; power = torque × angular speed.
• Petrol engines are throttled (quantity governing); diesel engines vary fuel quantity (quality governing).
• Octane rating measures resistance to knock (petrol); cetane rating measures ease of self-ignition (diesel).
• A turbocharger recovers exhaust energy to push more air in, raising power from the same displacement (downsizing).
• Roughly one-third of fuel energy becomes useful work; the rest leaves through exhaust and cooling - shown in a heat balance sheet.
• The stoichiometric air-fuel ratio is about 14.7:1 for petrol and about 14.5:1 for diesel; diesels run much leaner overall.
• Firing order (for example 1-3-4-2 in many four-cylinder engines) is chosen for balance and smooth torque.

Formulas and examples

• Displacement and compression ratio: a four-cylinder engine with bore 75 mm and stroke 84 mm → Vs = 0.7854 × 7.5² × 8.4 ≈ 371 cm³ per cylinder, about 1,484 cm³ total (a "1.5-litre" engine). If Vc = 37 cm³, r = (371 + 37)/37 ≈ 11.0.
• Brake power, torque and efficiency: a diesel engine on a dynamometer gives 250 N·m at 2,000 rpm while burning 13 kg/h of diesel (CV 42,500 kJ/kg).
  • BP = 2π × 2,000 × 250/60 ≈ 52.4 kW.
  • Fuel power = 13 × 42,500/3,600 ≈ 153.5 kW → ηbth ≈ 34.1%.
  • BSFC = 13/52.4 ≈ 0.248 kg/kWh (248 g/kWh).
• Indicated power and mechanical efficiency: a single-cylinder four-stroke engine, bore 100 mm, stroke 120 mm, indicated mean effective pressure 7 bar, 1,500 rpm.
  • A = 0.007854 m², L = 0.12 m, n = 750 power strokes/min.
  • IP = 7 × 10⁵ × 0.12 × 0.007854 × 750/60 ≈ 8.25 kW.
  • If BP = 6.6 kW, FP = 1.65 kW and ηm = 80%.
• Volumetric efficiency: a 1.2-litre four-stroke engine at 3,000 rpm draws 1.5 m³ of air per minute (at intake conditions). Swept volume drawn per minute = 1.2 litres × 1,500 suction strokes = 1.8 m³ → ηv ≈ 83%.
• Heat balance (per hour, diesel test):

• Air-standard comparison: an Otto cycle with r = 10 and γ = 1.4 gives η = 1 − 1/10^0.4 ≈ 60% - real engines achieve about half of this because of heat loss, friction, real gas properties and incomplete combustion.

Where it is used

• Two-wheelers and small cars: air- or water-cooled petrol engines, 100-1,500 cm³, MPFI with BS-VI catalytic converters.
• Commercial vehicles, tractors and buses: turbocharged CRDI diesels with SCR and DPF; CNG engines in city buses and autos.
• Gensets and pumps: constant-speed diesel engines (Kirloskar, Cummins, Mahindra Powerol); agricultural pump sets.
• Railways and ships: large diesel engines in locomotives and slow-speed two-stroke marine diesels.
• Testing and development roles: engine test beds, calibration engineers tuning ECU maps for power, economy and emissions; validation against BS-VI and RDE requirements.
• Transition: hybrids pair IC engines with motors; flex-fuel engines (E20 to E100) and hydrogen IC engines are being developed.

Interview questions

• Q: Why does a four-stroke engine have one power stroke every two revolutions? A: The cycle needs four piston strokes - suction, compression, power and exhaust - and each revolution gives two strokes.
• Q: Why is a diesel engine more fuel-efficient than a petrol engine? A: It runs at a much higher compression ratio, has no throttling losses at part load and burns a lean mixture.
• Q: What is knocking in a petrol engine and how is it controlled? A: Self-ignition of the end-gas ahead of the flame front, causing pressure waves; controlled with higher-octane fuel, lower compression, retarded spark timing, better cooling and knock sensors.
• Q: Why are the remedies for knock opposite in petrol and diesel engines? A: Petrol knock is premature self-ignition, so fuel must resist ignition (high octane); diesel knock comes from late self-ignition, so fuel must ignite quickly (high cetane).
• Q: What is valve overlap and why is it used? A: The period near TDC when inlet and exhaust valves are both open; outgoing exhaust helps draw in the fresh charge, improving scavenging at high speed.
• Q: What is the difference between indicated power and brake power? A: Indicated power is produced by the gas on the pistons; brake power is the usable output at the crankshaft after friction and pumping losses.
• Q: How does a turbocharger increase power? A: Exhaust gas drives a turbine connected to a compressor that forces more air into the cylinders, so more fuel can be burnt per cycle.
• Q: Why is an intercooler used with a turbocharger? A: Cooling the compressed air makes it denser (more oxygen per cycle) and reduces knock and NOx.
• Q: What is the function of a flywheel? A: It stores energy during the power stroke and releases it during the other strokes, smoothing crankshaft speed.
• Q: What do the DPF and SCR do in a BS-VI diesel vehicle? A: The DPF traps soot particles (burnt off periodically by regeneration); SCR uses urea solution to convert NOx into nitrogen and water.
• Q: What is the difference between MPFI and CRDI? A: MPFI injects petrol into each intake port at low pressure; CRDI injects diesel directly into the cylinder from a high-pressure common rail with precise multiple injections.

Common mistakes

• Using N instead of N/2 for power strokes in four-stroke power calculations.
• Confusing octane (petrol) with cetane (diesel), or applying the wrong knock remedy.
• Comparing air-standard efficiency directly with real engine efficiency.
• Forgetting the clearance volume when calculating compression ratio.
• Mixing units - kJ/h with kW, cm³ with m³, bar with Pa - in performance calculations.
• Saying a two-stroke engine produces exactly double the power of a four-stroke of the same size (scavenging losses reduce it).

Related topics

• Power cycles: Rankine, Otto, Diesel and Brayton · Thermodynamics: laws and properties · Turbomachinery: turbines, pumps and compressors · Heat transfer: conduction, convection and radiation · Automobile engineering (vehicle systems) · Emissions and Bharat Stage norms.