IC Engines & Steam Boilers Notes

Last reviewed 16 Sept 2026 · 9 min read

Internal combustion engines

In an internal combustion (IC) engine, fuel burns inside the cylinder, and the high-pressure gases directly push the piston. (In external combustion engines such as steam engines/turbines, combustion occurs outside the working fluid.)

Classification

Basis	Types
Ignition	Spark ignition (SI) — petrol, CNG, LPG; compression ignition (CI) — diesel
Working cycle	Four-stroke (one power stroke per two crank revolutions); two-stroke (one power stroke per revolution)
Thermodynamic cycle	Otto (constant volume), Diesel (constant pressure), dual
Cooling	Air-cooled, water-cooled
Cylinder arrangement	Inline, V, opposed, radial
Fuel supply	Carburettor, port/direct fuel injection
Use	Automotive, stationary (gensets), marine, construction equipment

Main parts

Cylinder block, cylinder head, piston with rings, connecting rod, crankshaft, flywheel (smooths speed fluctuations), camshaft and valves (inlet, exhaust), spark plug (SI) / fuel injector (CI), crankcase, sump, manifolds.

Terminology

FormulaEngine geometry

Bore $D$ — cylinder diameter; stroke $L$ — piston travel between TDC and BDC
Swept (displacement) volume $V_{s} = \frac{π}{4} D^{2} L$
Clearance volume $V_{c}$ — volume above piston at TDC
Compression ratio $r = \frac{V _{s} + V _{c}}{V _{c}}$
Mean piston speed $= 2 L N /60$ (m/s)

Typical compression ratios: SI engines about 6–11; CI engines about 14–22 (higher, since diesel ignites by compression heat).

Four-stroke engines

Stroke	Petrol (SI) engine	Diesel (CI) engine
1. Suction (intake)	Air–fuel mixture drawn in (inlet valve open)	Only air drawn in
2. Compression	Mixture compressed; spark ignites near TDC	Air compressed to high pressure and temperature; fuel injected near TDC and self-ignites
3. Power (expansion)	Hot gases push piston down	Same
4. Exhaust	Burnt gases expelled (exhaust valve open)	Same

One power stroke every two revolutions of the crankshaft; valves operated by the camshaft at half crank speed.

Two-stroke engines

All events in two strokes (one revolution) — ports instead of valves (in simple designs); scavenging by incoming charge.
Advantages: higher power-to-weight, simpler, lighter. Disadvantages: higher fuel consumption and emissions (charge loss during scavenging), poorer lubrication (oil mixed with fuel in small petrol two-strokes).
Uses: small motorcycles (historically), chainsaws, outboard motors, large marine diesel engines.

SI vs CI engines

Aspect	SI (petrol)	CI (diesel)
Fuel	Petrol, CNG, LPG	Diesel
Charge admitted	Air–fuel mixture	Air only
Ignition	Spark plug	Heat of compression
Compression ratio	Lower	Higher
Thermal efficiency	Lower	Higher
Weight/cost	Lighter, cheaper	Heavier (strong construction)
Torque at low speed	Lower	Higher — suits heavy vehicles and construction equipment
Noise/vibration	Smoother	More
Main emissions of concern	CO, HC, NOx	NOx, particulate matter

Supporting systems

Fuel system — carburettor (older SI), multi-point/direct fuel injection; CI: fuel injection pump and injectors, common rail direct injection (CRDI).
Ignition system (SI) — battery/coil, magneto, electronic ignition.
Cooling — air cooling (fins) or water cooling (radiator, pump, thermostat).
Lubrication — splash, pressure feed; reduces friction and wear, cools, cleans.
Governing — speed control under varying load (quantity governing in SI, quality governing in CI).
Turbocharging/supercharging increases power by compressing intake air.
Emission control — catalytic converters (SI), diesel particulate filters, SCR with urea (DEF), EGR; vehicles and construction equipment must meet applicable emission norms (e.g. Bharat Stage/CEV stage standards).

Engine performance

FormulaPerformance parameters

Indicated power (from indicator diagram):

I P = \frac{p _{m} L A nk}{60}

$p_{m}$ = indicated mean effective pressure (Pa); $L$ = stroke (m); $A$ = piston area (m²); $n$ = working strokes per minute ( $N /2$ for four-stroke, $N$ for two-stroke); $k$ = number of cylinders.

Brake power (measured by dynamometer): $B P = \frac{2 π N T}{60}$

Friction power $F P = I P - B P$

Mechanical efficiency $η_{m} = B P / I P$

Brake thermal efficiency $η_{b t h} = \frac{B P}{m ˙ _{f} \times C V}$ ; indicated thermal efficiency uses IP

Brake specific fuel consumption $B S F C = \frac{m ˙ _{f}}{B P}$ (kg/kWh)

Volumetric efficiency $= \frac{actual air volume inducted}{swept volume}$

Heat balance sheet — heat supplied distributed into brake power, cooling water, exhaust gases and unaccounted losses.
Morse test (multi-cylinder engines) — finds IP by cutting off cylinders one at a time.

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