BASIC ENGINE CYCLES (4-STROKES AND 2-STROKES): A Simple Guide To The Basic

BASIC ENGINE CYCLES (4-STROKES AND 2-STROKES): A Simple Guide To The Basic :

Most internal combustion engines, both spark ignition and compression ignition operate on either a four-stroke cycle or a two-stroke cycle. These basic cycles are fairly standard for all engines, with only slight variations found in individual designs.

4-Stroke Engine:

Have you heard the term ‘four-stroke engine' and wondered what on earth it means? Well, wonder no more, because we're on a mission to educate. In this article, we will try and break down the four-stroke combustion cycle for you as simply as we can.

The majority of petrol and diesel engines today are four-stroke engines, that power everything from mopeds to high-powered buses. In the past, two-stroke engines were popular, but we'll get into why they are practically extinct today a little later.

Four-Stroke SI Engine Cycle (Petrol Cycle):

1. First Stroke: Intake Stroke or Induction: The piston travels from TDC to BDC with the intake valve open and exhaust valve closed. This creates an increasing volume in the combustion chamber, which in turn creates a vacuum. The resulting pressure differential through the intake system from atmospheric pressure on the outside to the vacuum on the inside causes air to be pushed into the cylinder. As the air passes through the intake system, fuel is added to it in the desired amount by means of fuel injectors or a carburetor.

key Points
Inlet Valve -Open
Outlet Valve-Closed
Crankshaft Rotation-180 degrees.

2. Second Stroke: Compression Stroke: When the piston reaches BDC, the intake valve closes and the piston travels back to TDC with all valves closed. This compresses the air-fuel mixture, raising both the pressure and temperature in the cylinder. The finite time required to close the intake valve means that actual compression doesn't start until sometime a BDC. Near the end of the compression stroke, the spark plug is fired and combustion is initiated.

Key Points.
Inlet Valve- Closed
Outlet Valve -Closed
Crankshaft Rotation -180 degree (Total-360 degree)

3.Combustion: Combustion of the air-fuel mixture occurs in a very short but finite length of time with the piston near TDC (i.e., nearly constant-volume combustion). It starts near the end of the compression stroke slightly before TDC and lasts into the power stroke slightly after TDC. Combustion changes the composition of the gas mixture to that of exhaust products and increases the temperature in the cylinder to a very high peak value. This, in turn, raises the pressure in the cylinder to a very high peak value.

4.Third Stroke: Expansion Stroke or Power Stroke: With all valves closed, the high pressure created by the combustion process pushes the piston away from TDC. This is the stroke that produces the work output of the engine cycle. As the piston travels from TDC to BDC, cylinder volume is increased, causing pressure and temperature to drop.

Key Points
Inlet Valve -Closed
Outlet Valve -Closed
Crankshaft Rotation -180 degree (Total 540 degree)

5.Exhaust Blowdown Late in the power stroke, the exhaust valve is opened and exhaust blowdown occurs. Pressure and temperature in the cylinder are still high relative to the surroundings at this point, and a pressure differential is created through the exhaust system which is open to atmospheric pressure. This pressure differential causes much of the hot exhaust gas to be pushed out of the cylinder and through the exhaust system when the piston is near BDC. This exhaust gas carries away a high amount of enthalpy, which lowers the cycle thermal efficiency. Opening the exhaust valve before BDC reduces the work obtained during the power stroke but is required because of the finite time needed for exhaust blowdown.

6.Fourth Stroke: Exhaust Stroke: By the time the piston reaches BDC, exhaust blowdown is complete, but the cylinder is still full of exhaust gases at approximately atmospheric pressure. With the exhaust valve remaining open, the piston now travels from BDC to TDC in the exhaust stroke. This pushes most of the remaining exhaust gases out of the cylinder into the exhaust system at about atmospheric pressure, leaving only that trapped in the clearance volume when the piston reaches TDC. Near the end of the exhaust stroke before TDC, the intake valve starts to open, so that it is fully open by TDC when the new intake stroke starts the next cycle. Near TDC the exhaust valve starts to close and finally is fully closed sometime after TDC. This period when both the intake valve and exhaust valve are open is called valve overlap.

Key Points
Inlet Valve - Closed
Outlet Valve - Open
Crankshaft Rotation -180 degree ( Total 720 degree)

  

Four-Stroke CI Engine Cycle (Diesel Cycle):

1. First Stroke: Intake Stroke The same as the intake stroke in an SI engine with one major difference: no fuel is added to the incoming air. 

key Points
Inlet Valve -Open
Outlet Valve-Closed
Crankshaft Rotation-180 degrees.

2. Second Stroke: Compression Stroke The same as in an SI engine except that only air is compressed and compression is to higher pressures and temperature. Late in the compression stroke fuel is injected directly into the combustion chamber, where it mixes with the very hot air. This causes the fuel to evaporate and self-ignite, causing combustion to start. 

Key Points.
Inlet Valve- Closed
Outlet Valve -Closed
Crankshaft Rotation -180 degree (Total-360 degree)

3. Combustion: Combustion is fully developed by TDC and continues at about constant pressure until fuel injection is complete and the piston has started towards BDC. 

4. Third Stroke: Power Stroke The power stroke continues as combustion ends and the piston travels towards BDC. 

Key Points
Inlet Valve -Closed
Outlet Valve -Closed
Crankshaft Rotation -180 degree (Total 540 degree)

5. Exhaust Blowdown Same as with an SI engine. 

6. Fourth Stroke: Exhaust Stroke Same as with an SI engine. 

Key Points
Inlet Valve - Closed
Outlet Valve - Open
Crankshaft Rotation -180 degree ( Total 720 degree)

Two-Stroke SI Engine (Petrol Cycle):

1. Combustion: With the piston at TDC combustion occurs very quickly, raising the temperature and pressure to peak values, almost at constant volume.

2. First Stroke: Expansion Stroke or Power Stroke: Very high pressure created by the combustion process forces the piston down in the power stroke. The expanding volume of the combustion chamber causes pressure and temperature to decrease as the piston travels towards BDC.

3. Exhaust Blowdown: At about 75° before BDC, the exhaust valve opens and blowdown occurs. The exhaust valve may be a poppet valve in the cylinder head, or it may be a slot in the side of the cylinder that is uncovered as the piston approaches BDC. After blowdown, the cylinder remains filled with exhaust gas at lower pressure.

4. Intake and Scavenging: When blowdown is nearly complete, at about 50° before BDC, the intake slot on the side of the cylinder is uncovered, and intake air-fuel enters under pressure. Fuel is added to the air with either a carburetor or fuel injection. This incoming mixture pushes much of the remaining exhaust gases out the open exhaust valve and fills the cylinder with a combustible air-fuel mixture, a process called scavenging. The piston passes BDC and very quickly covers the intake port and then the exhaust port (or the exhaust valve closes). The higher pressure at which the air enters the cylinder is established in one of two ways. Large two-stroke cycle engines generally have a supercharger, while small engines will intake the air through the crankcase. On these engines, the crankcase is designed to serve as a compressor in addition to serving its normal function.

5. Second Stroke: Compression Stroke: With all valves (or ports) closed, the piston travels towards TDC and compresses the air-fuel mixture to a higher pressure and temperature. Near the end of the compression stroke, the spark plug is fired; by the time the piston gets to IDC, combustion occurs and the next engine cycle begins. 

Two-Stroke CI Engine Cycle (Diesel Cycle):

The two-stroke cycle for a CI engine is similar to that of the SI engine, except for two changes. No fuel is added to the incoming air so that compression is done on air only. Instead of a spark plug, a fuel injector is located in the cylinder. 

Near the end of the compression stroke, fuel is injected into the hot compressed air, and combustion is initiated by self-ignition.