How a Four Stroke S.I.(Spark Ignition) Engine work // Types of Vehicle || knowledgeaboutautomobile957

  Four Stroke Spark Ignition Engine:

In a four-stroke engine, the cycle of operations is completed in four strokes of the piston or two revolutions of the crankshaft.and in this system spark plug is mounted on cylinder head.

 During the four strokes,there are five events to be completed, viz., suction, compression, combustion,expansion and exhaust. 

 Each stroke consists of 180◦of crankshaft rotation and hence a four-stroke cycle is completed through 720◦of crank rotation.The cycle of operation for an ideal four-stroke SI engine consists of the fol-lowing four strokes : 

(i) suction or intake stroke; 

(ii) compression stroke; 

(iii)expansion or power stroke and 

(iv) exhaust stroke.

Working Principle of 4-Stroke S.I. Engine

Suction or Intake Stroke: 

In p-v diagram, Suction stroke 0→1  starts when the piston is at the top dead centre and about to move downwards.The inlet valve is assumed to open instantaneously and at this time the exhaust valve is in the closed position.

 Due to the suction created by the motion of the piston towards the bottom dead centre, the charge consisting of fuel-air mixture is drawn into the cylinder.

 When the piston reaches the bottom dead centre the suction stroke ends and the inlet valve closes instantaneously.

Compression Stroke :

The charge taken into the cylinder during the suction stroke is compressed by the return stroke of the piston 1→2.

 During this stroke both inlet and exhaust valves are in closed position,. The mixture which fills the entire cylinder volume is now compressed into the clearance volume. At the end of the compression stroke the mixture is ignited with the help of a spark plug located on the cylinder head. 

In ideal engines it is assumed that burning takes place instantaneously when the piston is at the top dead centre and hence the burning process can be approximated as heat addition at constant volume.

 During the burning process the chemical energy of the fuel is converted into heat energy producing a temperature rise of about 2000◦C (process 2→3), The pressure at the end of the combustion process is considerably increased due to the heat release from the fuel.

Expansion or Power Stroke :

The high pressure of the burnt gases forces the piston towards the BDC, (stroke 3→4) Both the valves are in closed position,  Of the four-strokes only. 

during this stroke power is produced. Both pressure and temperature decrease during expansion.

Exhaust Stroke :

At the end of the expansion stroke the exhaust valve opens instantaneously and the inlet valve remains closed, The pressure falls to atmospheric level a part of the burnt gases escape.

The piston starts moving from the bottom dead centre to top dead centre (stroke 5→0),and sweeps the burnt gases out from the cylinder almost at atmospheric pressure. The exhaust valve closes when the piston reaches T DC. at the end of the exhaust stroke and some residual gases trapped in the clearance volume remain in the cylinder.These residual gases mix with the fresh charge coming in during the following cycle, forming its working fluid.

 Each cylinder of a four-stroke engine completes the above four operations in two engine revolutions,first revolution of the crankshaft occurs during the suction and compression strokes and the second revolution during the power and exhaust strokes. Thus for one complete cycle there is only one power stroke while the crankshaft makes two revolutions. For getting higher out put from the engine the heat addition (process 2→3) should be as high as possible and the heat rejection (process 3→4) should be as small as possible. Hence, one should be careful in drawing the ideal p-V diagram, which should depict the processes correctly.

Different Types of  vehicle are-

1.  Spark-Ignition Engines with Port Fuel Injection.
2.  SI Engines for Hybrid Electric Vehicles.
3.  Boosted SI Engines.
4.  Direct-Injection SI Engines.
5.  Prechamber SI Engines.
6.  Rotary Engines