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Four-stroke SI engine working and components

Short answer

Suction: inlet valve open, piston descends, air-fuel mixture enters. Compression: both valves closed, piston rises, mixture compressed, spark near TDC. Power: burning mixture expands, forcing piston down. Exhaust: exhaust valve opens, piston rises, expelling gases. Main components: cylinder block, head, piston, rings, connecting rod, crankshaft, flywheel, camshaft, valves, sump, gaskets.

Explain the working of a four-stroke spark-ignition engine with a neat sketch of the strokes. List the major engine components and state the function of each.

A four-stroke SI engine completes one power cycle in four piston strokes, i.e. two crankshaft revolutions (720 degrees).

 1 SUCTION    2 COMPRESSION  3 POWER      4 EXHAUST
 IV open      both closed    both closed  EV open
 |  |v| |     |  |  |  |     |  |* |  |   |  |  |^|
 |_ | |_|     |_ |  |_ |     |_ |  |_ |   |_ |  | |
  | |            |||           |||           | |
  piston down    piston up     piston down   piston up
  mixture in     mixture       spark,        burnt gas
                 compressed    expansion     out
  1. Suction: the piston moves from TDC to BDC, the inlet valve is open and a petrol-air mixture from the carburettor or injector enters.
  2. Compression: both valves are closed, the piston rises and compresses the mixture (r = 8-10). Near the end the spark plug ignites it.
  3. Power (expansion): the burning gas pushes the piston down. This is the only stroke giving work.
  4. Exhaust: the exhaust valve opens and the rising piston pushes out the burnt gases.

Major components and functions

ComponentFunction
Cylinder block and linerHolds the cylinder, guides the piston
Cylinder headCloses the cylinder; holds valves, spark plug, combustion chamber
PistonReceives gas force and transmits it to the connecting rod
Piston ringsSeal the gas, control oil film, conduct heat to the wall
Gudgeon pinConnects piston to the small end of the connecting rod
Connecting rodTransmits the piston force to the crankshaft
CrankshaftConverts the reciprocating motion into rotary motion and delivers torque
FlywheelStores energy, smooths speed fluctuation
Camshaft, valvesOpen and close the inlet and exhaust ports at the correct time
Crankcase / sumpEncloses the crankshaft, stores lubricating oil
GasketsSeal joints against gas, oil and water leakage

Materials: cast iron or aluminium alloy for block and piston, forged steel for the crankshaft and connecting rod.

✓ Answer checked by Claude Opus 5.5 (high effort)

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