TIMING DIAGRAM- 4 STROKE AND 2 STROKE CYCLE

Engine timing refers to the relative time or position of the crank, at which each operation during the cycle is commenced and is completed. It is measured as the angle through which the crank has been rotated from a datum position such as top or bottom centre.


FOUR STROKE CYCLE

The majority of medium and high speed diesel engines for main or auxiliary drive operate on the four-stroke cycle, which takes place during four consecutive strokes, or two complete revolutions, of the engine. 

Four stroke are:

1. Induction stroke 

2. Compression stroke

3. Power or expansion stroke

4. Exhaust stroke

4 STROKE NATURALLY ASPIRATED ENGINE -: 

Naturally aspirated engine, is an internal combustion engine in which air intake depends solely on atmospheric pressure and does not have forced induction through a turbocharger or a supercharger.


1   Intake valve open 10-15°  before TDC.

2   Intake valve closes at BDC.

3.  Fuel injection start at TDC.

4.  Fuel injection end 30° after TDC.

5.  Exhaust valve open 20° before BDC.

6.  Exhaust valve closes 10°-15° after TDC.

     Valve overlap 20°-30°


4 STROKE TURBOCHARGED ENGINE-:

A turbocharger is used to force air into an IC engine. A turbocharger is a form of forced induction. It increases the amount of air entering the engine.



1. Intake valve open 65°-75° before TDC.

2. Intake valve closes 30°-35° after BDC.

3. Fuel injection start 10°-20° before TDC.

4. Fuel injection stop 15° after TDC.

5. Exhaust valve open 35°-45° before BDC.

6. Exhaust valve closes 55° after TDC.

    Valve overlap 130°




TWO STROKE CYCLE

Practically all large, slow-speed, direct drive marine diesel engines operate on the two-stroke cycle. As its name implies a two-stroke cycle takes place in two consecutive strokes of the engine piston, or one revolution of the crankshaft. 

Two strokes are:

1. Compression stroke.

2. Power or expansion stroke. 




Starting at TDC, combustion is already under way

1. Exhaust valve opens at 110°–120° ATDC .

2. Inlet opens at  40°-30° before BDC.

3. Inlet closes at 40°-30° after BDC.

4. Exhaust closes at 110°-150° before TDC.

5. Injection start at 10°-20° before TDC.

6. Injection closed at 15°-30° after TDC.

IMPORTANT TERMS

1.  SCAVENGING -: It is the process of replacing the exhaust gas in a cylinder of an internal combustion engine with the fresh air/fuel mixture for the next cycle. 

2. BLOWDOWN -: It is the escape of the combustion particles from the cylinder when the exhaust valve opens.

3. VALVE OVERLAP -: Valve overlap is the period during the valve timing where both the intake and exhaust valves are open.

Benefits-: 

 a) Valve overlapping is a secondary means of cooling the exhaust  valve by fresh air intake.

b) It ensure the complete opening of intake valve .

c) It help in scavenging of burned gases.



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