Four Stroke Petrol Engine. Image credit petrolheadkenya.com

Four Stroke Petrol Engine. Image credit petrolheadkenya.com

ZIMSEC O Level Combined Science Notes: The Petrol engine

  • Four strokes are involved in the operation of most petrol engines
  • They are thus known as four-stroke engines
  • The first stroke is known as the inlet stroke
  • During the inlet stroke the piston moves downwards
  • Drawing in air and petrol vapor from the carburetor through the open inlet valve
  • The compression stroke 
  • During this stroke the piston moves upwards and the all the valves are closed
  • This compresses the air and petrol mixture
  • The power stroke comes next
  • During this phase the gas is ignited by a spark plug
  • The gas mixture explodes giving off a lot of heat energy
  • This causes a large rise in pressure in the cylinder and the piston is forced downwards
  • The last stroke is known as the exhaust stroke
  • The piston rises and the exhaust valve opens
  • The exhaust gas is pushed out of the cylinder
  • The piston is kept moving between power strokes by the flywheel
  • A flywheel is a heavy revolving wheel that machine which is used to increase the machine’s momentum
  • and they provide greater stability or  reserve of available power in an engine
  • The crankshaft connects the movement of the pistons to the driving shaft through a system of gears
  • There are at least four cylinders in a motor car
  • During every half-revolution of the crankshaft the cylinders “fire” in turn
  • This results in a much smoother running operation and evens out the distribution of power
  • Lubrication of the engine results in higher speed in the pistons
  • Oil is ed under pressure into the moving parts

To access more topics go to the Combined Science Notes page.


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