petrol engines

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Post on 16-Jul-2015




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BY- PERCY (B120020188) SOURABH RANA (B120020282) NAVNEET ARORA (B120020323)

PETROL ENGINESPetrol engines come under the category of Internal Combustion engines.Petrol engines can be four stroke or two stroke.4-stroke petrol engines work on Otto Cycle.As the ignition in engine is due to spark, therefore petrol engine is known as spark ignition engine. FOUR DIFFERENT STROKES Four strokes of petrol engines is as follows:Suction strokeCompression strokeExpansion or Working strokeExhaust stroke P-V Diagram for Otto cycle


During this stroke (also known as induction stroke) the piston moves from top dead centre (T.D.C) to bottom dead centre.The inlet valve opens and proportionate fuel air mixture is sucked in the engine cylinder.This operation is represented by the line 5-1 (fig 5.32).The exhaust valve remain closed throughout the stroke. COMPRESSION STROKE

In this stroke, the piston moves (1-2) towards (T.D.C) and compresses the enclosed fuel air mixture.The pressure of the mixture rises in the cylinder to a value of about 8 bar.Just before the end of this stroke the operating plug initiates a spark which ignites the mixture and combustion takes place at constant volume (line 2-3) (fig 5.32). Both the inlet and outlet valve remain closed during this stroke.EXPANSION OR WORKING STROKE

When the mixture is ignited by the spark plug the hot gases are produced which drive or throw the piston from T.D.C to B.D.C and thus work is obtained.The flywheel mounted on the engine shaft stores energy during this stroke and supplies it during other three strokes.The expansion of gases is shown by 3-4 (fig 5.32). Both the valves remain closed during the start of this stroke but when the piston reaches the B.D.C the exhaust valve opens.EXHAUST STROKE

Removal of gases are accomplished during this stroke.The piston moves from B.D.C to T.D.C.The exhaust gases are driven out of the engine cylinder ; this is also known as scavenging.This operation is represented by the line (1-5) (fig 5.32). Actual P-V diagram for Otto cycle

NOTE-the line 5-1 is below the atmospheric line.(this is due to the fact that owing to the restricted area of the inlet passages the entering fuel air mixture cannot cope with the speed of the piston.) NOTE-the exhaust line 4-5 is slightly above the atmospheric pressure line.(this is due to restricted exhaust passages which do not allow the exhaust gases to leave engine cylinder quickly).The loop which has area 4-5-1 is called negative loop; it gives the pumping loss due to the admission of fuel air mixture and removal of exhaust gases.The area 1-2-3-4 is the total or gross work obtained from piston and net work can be obtained by subtracting the area 4-5-1 from area 1-2-3-4. DERIVATION of otto cycle


Fig shows a two stroke engine.The cylinder L is connected to a closed crank chamber C.C.During the upward stroke of the piston M , the gases in L are compressed and the same time fresh air and fuel(petrol) mixture enter the crank chamber through the valve V.When the piston moves downwards , V closes and mixture in C.C is compressed.

The piston is moving upwards and is compressing an explosive change which has been supplied to L.Ignition takes place at the end of this stroke.The piston then travels downwards due to the expansion of the gases and near the end of this stroke the piston uncovers the exhaust port (E.P) and burnt gases escape through this port.(refer fig 3)

The transfer port (T.P) then is uncovered immediately.The compressed charge from the crank chamber flow into the cylinder and is deflected upwards by the hump provided on the head of the piston.The piston then again start moving from B.D.C to T.D.C and the charge gets compressed when E.P and T.P is covered by the piston ; hence cycle is repeated. P-V Diagram for 2 stroke engine


ASPECTS FOUR STROKE CYCLE ENGINETWO STROKE CYCLE ENGINE1.Completion of cycleThe cycle is completed in 4 strokes of the piston or in the two revolution of crankshaft.The cycle is completed in 2 stroke of the piston or in the one revolution of the crankshaft. 2.Flywheel required Because turning movement is not so uniform ,therefore heavier flywheel is required.More uniform movement hence lighter flywheel is required.3.Valve and valve mechanism4 stroke engine contains valve and valve mechanism.Instead of valves it has ports.

4.Volumetric efficiencyVolumetric efficiency is more due to more time of induction. Volumetric efficiency is less due to lesser time for induction.5. Applications Cars , buses, tractors etc.Scooter ,motorcycles etc.6.Power produced for same size of engineBecause of one power stroke for two revolutions ,power produced for same size of engine is small.Because of one power stroke for one revolution, power produced for same size of engine is small.7. Lubrication requiredLesser rate of wear and tear hence lesser requirement of lubricants . Higher rate of wear and tear and consequently lesser requirement of lubricants.8.Initial cost Because of the heavy weight and complications of valve mechanism , higher is the initial cost.Because of light weight and simplicity due to valve mechanism absence , lower is the initial cost.


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