HOW A ENGINE OF A CAR WORKS

There are two types of engines that are commonly used by the car industries and are proved to be efficient enough for marketing.

1.     Four-Stroke Petrol Engine or Spark Ignition Engine

2.     Four-Stroke Diesel Engine or Compression Ignition Engine

1.   FOUR-STROKE PETROL ENGINE OR ‘SPARK IGNITION ENGINE’

In a four stroke petrol engine the cycle of operation is completed in four strokes of the piston or two revolution of crankshaft. It involves following steps

i.                    Suction Stroke

ii.                  Compression Stroke

iii.                Expansion or Power Stroke

iv.               Exhaust Stroke

i.                  SUCTION STROKE

                              Fig: SUCTION

In the following figure when the piston is about to move downwards from top dead center, the inlet valve opens instantaneously and the exhaust valve is a closed position. So a suction pressure is created due to the downward motion of the piston and the charge consisting of air-fuel mixture is drawn into the cylinder. When the piston reaches bottom dead center the suction stroke ends and the inlet valve closes instantaneously.

ii.               COMPRESSION STROKE

                          Fig: COMPRESSION

In the following figure the charge that fills the entire cylinder is compressed by the return stroke of the piston i.e. the upward motion of the piston to clearance volume. During this stroke both the valves inlet and exhaust are in closed condition. At the end of the compression stroke , the compressed charge is ignited with the help of Spark Plug located on cylinder head.

iii.            EXPANSION OR POWER STROKE

                             Fig: EXPANSION

The ignited charge began to expand within the cylinder resulting in sudden push to the piston in downward direction. During this stroke both the valves are in closed condition. Of all four strokes, this stroke produces power. Both pressure and temperature decreased during expansion.

iv.             EXHAUST STROKE

                              Fig: EXHAUST

At the end of expansion stroke the exhaust valve opens instantaneously and the inlet valve remains closed. The burnt gases the produced in the expansion stoke is pushed away by the piston when it moves upward to the top dead center. As soon as it reached top dead center the exhaust valve closed instantaneously and the fresh charge is ready to drawn to repeat the suction stroke.

2.   FOUR-STROKE COMPRESSION IGNITION ENGINE OR ‘DIESEL ENGINE’

These types of engines run by diesel, so it is often called as diesel engine. It also involves 4 strokes but here instead of spark plug we are using fuel injector at the end of compression stroke for ignition. The compression ratio {clearance volume/cylinder volume } is higher as compared to SI engines and it ranges from 16 to 20 whereas SI engine  compression ratio ranges from 6 to 10. Due to high compression ratio of the engine the temperature inside the cylinder after the compression stroke is high enough to self-ignite the fuel injected into the combustion chamber.

i.                  SUCTION STROKE

Due to the downward motion of the piston from TDC to BDC air alone is drawn into the cylinder during the suction stroke. During this stroke the inlet valve opens and the exhaust valve is closed.

ii.               COMPRESSION STROKE

The inducted air gets compressed by the return stroke of the piston to clearance volume. During this stoke both the valves are in closed condition.

iii.            EXPANSION OR POWER STROKE

At the end of the compression stroke fuel injection starts. Due to the high temperature inside the combustion chamber the fuel get self-ignited and expanded inside the cylinder. Due to the sudden expansion the piston is pushed downwards from TDC to BDC and we get a power stroke. In this stroke both the valves are in closed condition.

iv.             EXHAUST STROKE

After the expansion stroke the exhaust valve opens instantaneously and the inlet valve remain closed. The piston moves from TDC to BDC pushing away the product of combustion {burnt gases}.

  

FIG: CYCLE OF OPERATION OF 4-STROKE DIESEL ENGINE

                                                                                        by: Radhe