2 stroke petrol engine - practical PDF

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Experiment No : 9 Objective: To study cut model of Two Stroke Petrol Engine.

Apparatus: Cut Model of Two Stroke Petrol Engine.

Engine: The word Engine comes from which means (ability). Engine is combustion engine is defined as power by the combustion of

the Latin word ingenious machinery. Generally a a machine that produces water or a fuel.

Heat Engine: In thermodynamics a heat engine is defined as, a “system that converts heat, thermal and chemical energy to mechanical energy, which can then be used to do mechanical work”. Engines are classified into various categories based on the type of the cycle they use , the layout ,the energy source used, the cooling mechanism or its use. Based on the layout, there are two types of Engines.  Internal Combustion Engines  External Combustion Engines  External Combustion Engines: An External Combustion engine is that in which energy is externally applied on a non combustible fluid like pressurized hot water ,liquid sodium etc. These fluids are heated here in the external chamber like boiler and steam is used to dive engine.

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Internal Combustion Engines: If the combustion of a fuel like petrol takes place inside , in a chamber .Then the engine is called an Internal Combustion Engine. The oxidizer used in it is usually air.

The gases produced in combustion are of higher temperature and pressure .These gases exert forces on piston which moves and produces energy.

Based on the source of energy, internal combustion engines are classified as  Petrol Engines  Diesel Engine Petrol Engine is again divided into  Two Stroke Petrol Engine

 Four Stroke Petrol Engine Here, we discuss about Two Stroke Petrol Engine and its Working

Parts of Engine: 

Piston: The piston is moved up and down by a crankshaft, which is connected to it via the connecting rod. When the piston reaches the top the spark plug then lights the air mixture burning it and sending the piston back down .It is free to move. It compresses the air fuel mixture and converts the fuel energy into the mechanical energy. It transmit the power to the crankshaft.

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Crankshaft: It is located in the bottom end of the cylinder block. It transmits the reciprocating motion of the piston into rotary motion. This rotary motion used to rotate wheels of the vehicle.

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Connecting Rod: It connect piston to the crankshaft and transmit the motion of the piston to crankshaft. The lower end is connected to piston and the upper end to crankshaft.

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Cylinder Head: The top end of the cylinder is closed by means of a removable cylinder head. The inlet valve, exhaust valve, spark plug and injector are bolted on the cylinder head. The main function of the cylinder head is to seal the cylinder block and not permit entry and exist of gases on the cover of head.

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Cylinder block: The main function of the cylinder is to guide the piston. It is in direct contact with the products of the combustion so it must be cooled. For cooling of cylinder a water jacket or fin are situated at the outer side of the cylinder. So at the upper end of it cylinder head and at the bottom crankcase is bolted.

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Carburetor: A device for mixing vaporized fuel with air to produce a combustible or explosive mixture, as for an internal combustion engine.

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Spark plug: It is used in petrol engine. It is fitted on the cylinder head. It is used to ignite the air fuel mixture inside the cylinder at the end of each compression stroke.

 Piston Rings: Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

 They prevent the air mixture in the combustion chamber from leaking into the sump during compression and combustion.  They keep all oil in the sump from leaking into the area, where it would be burned and lost.

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Piston Pin: It is the parallel spindles fitted through the piston and connecting rod small end. It connects the piston to the connecting rod. In the internal combustion engines, the gudgeon pin connects the piston to the connecting rod and provides a bearing for the connecting rod to pivot upon as the piston moves.

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Fly wheel: A flywheel is a rotating mechanical device that is used to store rotational energy. The amount of energy stored in a flywheel is proportional to the square of its rotational speed. To keep the crankshaft. The flywheel is a disk that is about 30 to 40 cms in diameter.

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Dead Centre: In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead center while the latter is known as bottom dead centre.

 Stroke: Distance covered by piston from top dead centre to bottom dead centre or bottom dead centre to top dead centre is called stroke. The length of stroke is called stroke length. 

Valves: The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

 Working of Two Stroke Petrol Engine: In two strokes the suction and the exhaust strokes are eliminated. There are only two remaining strokes these are the compression stroke and the power stroke and usually called as upward stroke and downward stroke. Instead of valves the two stroke engine

consists of inlet and exhaust ports. The fresh charge enters into the cylinder through the inlet ports and the burnt gases escape out through exhaust port. A Stroke is the movement of the piston from the top to the bottom of the cylinder. The two strokes are named as :  Upward Stroke  Downward stroke

Upward Stroke: During this stroke the piston moves and compresses the charge in the combustion chamber of the cylinder. Because of the upward movement of the piston a partial vacuum is created in the crankcase and this allow the entry of the fresh charge into the crankcase through uncovered inlet port. The exhaust port and the inlet port remain covered and the ignition of the fresh charge takes place through spark plug.

Downward Stroke: As the combustion of fresh charge takes place a large amount of hot gases is produced which exerts a high pressure force on the top of the piston. Due to the high pressure force the piston moves downward and rotates the crankshaft and does useful work. Further downward movement of the piston uncovers first the exhaust port and the transfer port and the exhaust start through the exhaust port. As the transfer port is open the charge through it is forced into the cylinder. The charge strike the deflector on the piston crown rises to the top of the cylinder. The cylinder is completely filled with the fresh charge but diluted with the exhaust gases finally the cycle event is then repeated.

Advantages of two stroke engine: 1) Two stroke engine gives a working stroke for each revolution of crankshaft. The four stroke engine gives a working stroke for each two revolutions of crankshaft. 2) It is simple in construction and mechanism. 3) It has high mechanical efficiency. 4) There is no value mechanism in it. The ports can be easily designed, covered and uncovered by the movement of the piston itself. 5) For the same power the two stroke engine is more compact light and requires less space then a four stroke engine. 6) Power developed by the two stroke engine is twice then the power developed by the four stroke engine for the same engine speed and volume.

Disadvantages of two stroke engine: 1) It has high fuel consumption. 2) Thermal efficiency is less than the four stroke engine. 3) It produces greater noise. 4) There is a greater wear and tear of moving parts. 5) It does more consumption of the lubricating oil.

Application: 1) It is used in the light weight vehicle like scooters, motorcycles etc which uses gasoline as a fuel.

2) It is also used in many diesel engines mainly industrial and marine engines. As well as in some heavy trucks and machine.

Difference Between Two storke petrol/Four storke petrol engine: S. No.

1. 2. 3.

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5. 6.

Two Stroke Engine

Four Stroke Engine

It has one revolution of crankshaft within one power stroke. It can generate high torque compare to 4 strokes engine. It used port to inlet and outlet of fuel. 2 stroke engines require lighter flywheel compare to other engines because it generates more balanced force due to one revolution for one power stroke. The charge is partially burn and mix with the burn gases during inlet. It is due to port mechanism. Easy lubrication due to lubrication oil mix with the fuel.

It has two revolution of crankshaft between one power strokes. It generates less torque due to 2 revolution of crankshaft between one power strokes. It used valve to inlet and outlet.

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More lubricating oil requires because some oil burns with fuel. These engines give less thermal efficiency. It has high power to weight ratio compare to others. It creates more noise. Two stroke engines are less efficient and generate more smoke.

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These engines are comparatively cheaper.

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These engines are easy to manufacture.

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These engines are generally lighter. These are mostly used in ships, scooters etc. Due to poor lubrication more wear and tear occurs

7. 8. 9. 10.

15. 16.

It requires heavy flywheel because it generates unbalance force due to two revolutions for one power stroke. In four stroke engine charge is fully burn and does not mix with burn charge in ideal condition. Comparatively complicated lubrication.

Comparatively less lubricating oil requires. These engines give more thermal efficiency. 4 stroke engines have less power to weight ratio. It is less noisy. Four stroke engines are more efficient and generate less smoke. These engines are expansive due to valve and lubrication mechanism. These engines are comparatively hard to manufacture. These engines are comparatively heavier than 2 strokes due to heavy flywheel and valve mechanism. These engines mostly used in car, truck, and other automobiles. Less wear and tear occurs....