1  engine term

Linking the piston by a connecting rod to a crankshaft causes the gas to rotate the shaft through half a turn. The power stroke“uses up”the gas , so means must be provided to expel the burnt gas   and recharge the cylinder with a fresh petrol-air mixture :this control of gas movement is the duty of the valves；an inlet valve allows the new mixture to enter at the right time and an exhaust valve lets out the burnt gas afterthe gas has done its job. Engine terms are：

1)  TDC(Top Dead Center):the position of the crank and piston when the piston is farther away from the crankshaft.

2) BDC(Bottom Dead Center):the position of the crank and piston when the piston is nearest to the crankshaft.

3) Stroke : the distance between BDC and TDC; stroke is controlled by the crankshaft.

4）Bore : the internal diameterof the cylinder.

5）Swept volume : the volume between TDC and BDC.

6）Engine  capacity  :  this  is  the  swept  volume  of  all  the  cylinders.

e.g.  a  four-cylinder  engine having a capacity of two liters(2000cm3) has a cylinder swept volume of 500cm3.

7）Clearance volume: the volume of the space above the piston when it is at TDC.

8）Compression ratio έ= (swept vol + clearance vol)∕(clearance vol)

9）Two-stroke : a power stroke every revolution of the crank.

10）Four-stroke: a powerstroke every other revolution of the crank.

2   The Four-stroke Spark-ignition Engine Cycle

The spark-ignition engine is an internal-combustion engine with externally supplied in ignition ,which converts the energy contained in the fuel to kinetic energy.The cycle of operations is spread over four piston strokes. To complete the full cycle it takes two revolutions of the crankshaft. the operating strokes are:

1) Intake(induction) stroke

The downward-moving piston increases the volume in the cylinder and draws in fresh air-fuel mixture through the open intake valve.

2) Compression stroke

The  upward-moving  piston  reduces  the  volume  in  the  cylinder  and  compresses  the  air-fuel mixture.Shortly before TDC is reached ,the spark plug ignites the compressed air-fuel mixture and thus initiates the combustion process. A higher compression ratio means better utilization of thefuel. The extent of compression is restricted by the knock limit.

3) Power stroke

After  the  ignition  spark  at  the  spark  plug  has  ignited  the  compressed  air-fuel  mixture,the temperature increases as the result of combustion of the mixture.The pressure in the cylinder increases and forces the piston downwards. The piston transfers power to the crankshaft via the connecting rod.

4) Exhaust stroke

The upward-moving piston expels the combusted gases （exhaust gas） through the open exhaust valve. After this 4th stroke,the cycle is repeated.

3  Engine Overall Mechanics

The engine has hundreds of other parts . The major parts of engine are engine block , engine heads,  pistons,  connecting  rods,  crankshaft  and  valves.  The  other  parts  are  joined  to  make systems.  These  systems  are  the  fuel  system,  intake  system,  ignition  system,  cooling   system,   lubrication  system  and  exhaust  system.  Each  of  these  systems  has  a  definite  function.  These systems will be discussed in detail later。