How an internal combustion engine works
An internal combustion engine burns fuel inside a closed cylinder. The hot gas pushes a piston. A connecting rod turns the piston's up-down motion into rotation of the crankshaft.
The four strokes
- Intake: intake valve opens, piston moves down, air (and fuel) enters.
- Compression: valves closed, piston moves up and squeezes the mixture.
- Power: a spark (petrol) or the heat of compression (diesel) ignites it; expanding gas pushes the piston down.
- Exhaust: exhaust valve opens, piston moves up and pushes burnt gas out.
Two crankshaft turns give one power stroke per cylinder. A two-stroke engine does it in one turn, but is dirtier.
Main parts
Block, cylinder head, pistons and rings, connecting rods, crankshaft, camshaft, valves, timing belt or chain, flywheel, gaskets, oil pan.
Cylinder configurations
- Inline: all cylinders in one row. Simple, common in small cars.
- V: two rows at an angle. Shorter for many cylinders (V6, V8).
- Flat / boxer: cylinders lie flat, opposite each other. Low and smooth.
- Others: rotary (Wankel), radial (old aircraft).
Fuel, lubrication, cooling, ignition and timing
- Fuel system: tank → pump → filter → injectors spray a fine mist; the engine computer sets how much.
- Lubrication: an oil pump pushes oil between moving metal parts to cut friction and wear, carry heat away and clean. Low oil quickly ruins bearings.
- Cooling: coolant flows through the engine, then the radiator, where air removes heat. A thermostat keeps the temperature steady.
- Ignition: a coil makes a very high voltage; the spark plug fires across a small gap.
Timing
Valves must open and close, and the spark must fire, at exactly the right crank angle. The timing belt/chain links crankshaft and camshaft (cam turns at half crank speed). The spark fires a little before top dead centre because the flame needs time to spread; at higher rpm it fires earlier (advance). Wrong timing means low power, knocking, high fuel use, or broken valves.
Power transfer: torque, power and gear ratios
Torque is turning force (N·m). Power is how fast work is done: P (W) = torque (N·m) × angular speed (rad/s) = 2π × N × T ÷ 60, with N in rpm.
- Petrol engines give peak torque in a middle rpm band and peak power at high rpm.
- Diesel engines give high torque at low rpm, good for trucks and tractors.
- Electric motors give full torque from 0 rpm, so EVs accelerate quickly and often need only one gear.
The powertrain
Engine → clutch (or torque converter in automatics) connects/disconnects → gearbox picks a ratio → drive shaft → differential (final drive, lets wheels turn at different speeds on bends) → axles → wheels. Front-wheel-drive cars combine gearbox and differential in a transaxle.
Gear ratio
Gear ratio = driven teeth ÷ driver teeth. Output speed = input speed ÷ ratio. Output torque ≈ input torque × ratio (minus small losses). Overall ratio = gearbox ratio × final drive ratio.
Vehicle body, frame and modifications
- Body-on-frame: a strong ladder frame carries engine and body. Tough, easy to repair; used for trucks and many SUVs.
- Unibody (monocoque): the body itself is the frame. Lighter, stiffer, with crumple zones; used in most cars.
- Aircraft use a light skin over frames and stringers; boats use a hull shaped for stability and low drag.
Body parts: bonnet/hood, doors, fenders, bumpers, roof, floor pan, pillars, windscreen.
Effects of modifications
Bigger wheels change speedometer reading and gearing; lowering changes handling and ground clearance; spoilers add downforce but also drag; more engine power needs better brakes and cooling. Some changes affect safety, emissions, insurance and legality, so check local rules.
Service, diagnosis and safe workshop practice
Systematic troubleshooting
- Confirm the complaint (drive or run it).
- Look up specifications, service manual and trouble charts; read fault codes with a scan tool.
- Test the simplest likely cause first.
- Repair, then verify the fix.
Measuring wear
Use a micrometer for crankshaft journals, a bore gauge for cylinder wear and taper, feeler gauges for ring gap and valve clearance, a compression tester to compare cylinders, and a torque wrench to tighten bolts to spec.
Common symptoms
| Symptom | Possible cause |
|---|---|
| Blue smoke | Burning oil: worn rings or valve seals |
| White sweet smoke | Coolant leak, head gasket |
| Grinding when shifting | Worn clutch or synchronisers |
| Clunk on bends, CV joint click | Worn CV joint or differential |
| Vibration at speed | Unbalanced wheel, bent drive shaft, worn U-joint |
Safety
Wear eye protection; never work under a car held only by a jack (use stands); disconnect the battery before electrical work; let engines cool; dispose of oil and coolant correctly.
Key formulas and definitions
- Displacement = π/4 × bore² × stroke × number of cylinders
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume
- Gear ratio = driven teeth ÷ driver teeth
- Output speed = input speed ÷ ratio; output torque ≈ input torque × ratio
- Power (W) = 2π × N (rpm) × T (N·m) ÷ 60
- Road speed (km/h) = wheel rpm × 2π × r (m) × 60 ÷ 1000
Worked examples
1. A single cylinder has bore 5 cm and stroke 6 cm. Find its swept volume.
V = 0.785 × 5² × 6 = 0.785 × 25 × 6 ≈ 118 cm³.
2. A 4-cylinder engine has bore 8.6 cm and stroke 8.6 cm. Find the displacement in litres.
One cylinder = 0.785 × 8.6² × 8.6 ≈ 499.6 cm³. × 4 ≈ 1998 cm³ ≈ 2.0 L.
3. Swept volume 500 cm³, clearance volume 50 cm³. Find the compression ratio.
CR = (500 + 50) ÷ 50 = 11, written 11:1.
4. A 12-tooth gear drives a 36-tooth gear. Input 900 rpm, 20 N·m. Find output speed and torque.
Ratio = 36 ÷ 12 = 3. Speed = 900 ÷ 3 = 300 rpm. Torque ≈ 20 × 3 = 60 N·m.
5. An engine makes 200 N·m at 3000 rpm. Find its power in kW.
P = 2π × 3000 × 200 ÷ 60 ≈ 62 832 W ≈ 62.8 kW.
6. First gear 3.6, final drive 4.1, engine 3000 rpm, wheel radius 0.3 m. Find road speed.
Overall ratio = 14.76. Wheel rpm = 3000 ÷ 14.76 ≈ 203. Speed = 203 × 2π × 0.3 × 60 ÷ 1000 ≈ 23 km/h.
Common mistakes
- Thinking a higher gear number means more torque. Higher gears have lower ratios: more speed, less torque.
- Forgetting the 'number of cylinders' in displacement.
- Mixing up radius and diameter in π/4 × bore²: the bore is a diameter.
- Tightening head or wheel bolts by feel. Use a torque wrench and the specified sequence.