2 Stroke VS 4 Stroke IC Engines

As we know an engine is a mechanical device which convert one form of energy into usable mechanical energy.

According to its operating principle an engine is classified as-

• Two Stroke or Two Cycle Engine

• Four Stroke or Four Cycle Engine

Before i explain what is two or four stroke engine? first we will concern about what is stroke? How many types of cycle is used by an engine?

Question: What Is Stroke?

Answer: Stroke refers to the movement of the piston in the engine.

Question: How many types of cycle used by an engine. name them?
Answer:   Four types of cycles used by an engine which are-

1. Intake Cycle/Stroke
2. Compression Cycle/Stroke
3. Power Cycle/Stroke
4. Exhaust Cycle/Stroke

1. Two Stroke or Two Cycle Engine-  As a name suggest two-stroke or two-cycle, engine is a type of internal combustion engine, which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaft revolution. In a two-stroke engine, the end of the power stroke and the beginning of the compression stroke happens simultaneously, with the intake and exhaust functions occurring at the same time. Fig shows the illustration of this type of engine-

Fig. Two Stroke Engine(gasoline engine)

1.Intake & Exhaust Stroke

2.Compression & Power Stroke

As clearly shows in this illustration the end of combustion stroke and beginning of the compression stroke happen simultaneously which result some proportion of the fuel/air mixture entering the cylinder goes directly out, unburned, through the exhaust port, which is a main drawback of this type of engine. As shown in fig, some proportion of fuel/air mixture goes directly out without burn because Intake and exhaust stroke occurs simultaneously (here green colour shows furl/air mixture). however several designs of this type of engine is made to overcome this drawback but still this types of engines does not burn fuel/air mixture efficiently.

Advantages of two-stroke engine-

1.   A two-stroke cycle engine gives twice the number of power stroke of the four stroke cycle      engine at the same engine speed.
2.               The mechanism of a two-stroke cycle, engine is much simpler.
3.              For the same power developed, a two-stroke cycle engine is lighter, less bulky and occupies      less floor area.
4.  High power-to-weight ratio.
5.  Two stroke cycle engine is easier to start.
6.  Initial  cost of two cycle engine is considerably less.
7.  Easier to maintain.
8.  For the same configurations, it produce more power.
9.  Two-stroke engines do not have valves which simplifying their construction. 
10.  Two-stroke engines can work in any orientation, which can be important in something like a  chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright,  and solving this problem can add complexity to the engine. 

Disadvantages of two-stroke engine-

1. Two-stroke engines don't last nearly as long as four-stroke engines. The lack of a dedicated  lubrication system means that the parts of a two-stroke engine wear a lot.
2. Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gasoline. You would  burn about a gallon of oil every 1,000 miles if you used a two-stroke engine in a car. 
3. Two-stroke engines do not use fuel efficiently, so you would get fewer KM per litre.
4. Two-stroke engines produce a lot of pollution. 
5. Two-stroke engines does not burn air/fuel mixture efficiently because some unburned mixture entering the cylinder goes directly out from exhaust port.
6. 
   

2. Four Stroke or Four Cycle Engine-  As a name suggest a four stroke engine or four-cycle, engine is a type of internal combustion engine, which completes a power cycle with four strokes. The four-stroke cycle engine is the most common type of small engine. Fig shows the illustration of this type of engine-

Fig. Four Stroke Engine(gasoline engine)
1.Intake Stroke
2.Compression Stroke
3.Power Stroke
4.Exhaust Stroke

 The four separate strokes are termed as:

1. Intake Stroke- This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture(in gasoline engine)/or only air(in diesel engine) into the cylinder by producing vacuum pressure into the cylinder through its downward motion.
2. Compression Stroke- This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture(in gasoline engine)/or only air(in diesel engine) in preparation for ignition during the power stroke. Both the intake and exhaust valves are closed during this stage.
3. Power Stroke- This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture(in gasoline engine)/or only air(in diesel engine) is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
4. Exhaust Stroke- During the exhaust stroke, the piston once again returns to T.D.C from B.D.C while the exhaust valve is open. This action expels the burned air-fuel mixture through the exhaust valve.

Advantages of two-stroke engine-

1. Four stroke engines last longer and use much less oil.
2. You don’t have to pre mix the oil in to the fuel.
3. Comparatively quieter.
4. An efficient burning of fuel makes it more efficient.
5. Less effect on environment.

Disadvantages of For-stroke engine-

1. It.s configuration & operating principle makes it more complicated.
2. It has low power to weight ratio.
3. It is more expensive than 2 stroke engine.
4. For the same configurations it produces half power then two stroke engine.