DIESEL ENGINE - 2 STROKE & 4 STROKE

Diesel Engine - Strokes

A Diesel engine is a machine which produces power by burning the oil or fuel in a body of air which has been squeezed to a high pressure by a moving piston.

2 Stroke Diesel engine

2-Stroke Diesel engine has the following strokes: -

1. Power stroke
2. Scavenging/Compression stroke   

The 2 stroke cycle is completed in two strokes of the piston or in one revolution of the crankshaft.
The fresh air must be forced in under pressure and is used to clean out or scavenge the exhaust gases and is then filled with fresh air.

Pic: 2 Stroke Cycle

The 2 strokes are explained as follows: -

Power Stroke: -

• The piston is at the top of the cylinder i.e, TDC* where fuel injection and combustion has taken place.
• Due to combustion, the piston is pushed down to the BDC** thereby producing power.
• Due to this movement, exhaust ports are opened where burnt gases escapes out and continues to move downward till scavenge ports are opened. 

Scavenging/Compression: -

• Pressurized fresh air then enters the cylinder through scavenge port and thereby driving out the remaining exhaust gases.
• The piston then moves from BDC to TDC thereby closing all the ports.
• The air is then compressed as the piston moves from BDC to TDC thereby completing the cycle. 

4 Stroke Diesel engine

4-Stroke Diesel engine has the following four strokes: -

1. Suction stroke
2. Compression stroke
3. Power or expansion stroke
4. Exhaust stroke

Pic: 4 Stroke cycle

Suction Stroke: -

• At starting the piston will be at top dead center, the inlet valve is opened from where fresh charge of air comes in.
• The crankshaft rotates towards its right and pulls down the piston from TDC to BDC.
• Due to this movement, fresh air is drawn inside the cylinder through the inlet valve.
• As the piston reaches BDC, the inlet valve closes and the intake or suction stroke is completed.

Compression Stroke: -

• As the crank turns to its left and thereby pushing the piston towards TDC.
• Since all valves are closed, it is forced into a smaller space.
• This increase its pressure and also temperature, when piston reaches TDC air occupies 1/16th of its original space and rises its temperature to nearly 540 degree celsius or more.

Power or Expansion Stroke: -

• At this point, fuel is injected to the cylinder where it instantly ignites due to the presence of hot compressed air.
• The oil burns quickly as it is mixed well with the hot air and also due to the burning of fuel, the mixture becomes more hotter.
• Due to this high temperature and pressure being created due to combustion pushes the piston from TDC to BDC thereby expanding it.
• This push of the piston from TDC to BDC produces the power thereby this stroke is also known as power stroke.
• As the piston reaches BDC, exhaust valve opens and the burnt gases escapes to the atmosphere till its pressure becomes as same as that of atmosphere.

Exhaust Stroke: -

• At this stroke, exhaust valve is opened and the exhaust gas are released or escapes until its pressure falls to near to the atmosphere pressure.
• The piston moves from BDC to TDC thereby expelling the burnt gases through the exhaust valve.
• As the piston reaches TDC, all the exhaust gases are expelled and exhaust stroke is then completed.

* TDC - Top Dead Center

** BDC - Bottom Dead Center