So guy’s today we describe about “4-STROKE SPARK IGNITION ENGINE” so stay connected.
●4-STROKE SPARK IGNITION ENGINE.
Is the name of implies it works on 4-stroke principal and charge is ignited with the help of a spark plug plug ignition engines are used in all the cars and light commercial vehicle usually referred to aS 4-Stroke Petrol engine.
When we revolve the Engine crankshaft through cranking handle or self starter the inlet valve open and the Piston being tied up with crankshaft with help of connecting rod travels toward bottom dead centre (B.D.C.).
when the Piston moves towards b.d.c. partial vacuum is created on top of piston in the combustion chamber due to this partial vacuum air fuel mixture is drawn into the combustion chamber through open inlet valve.
The volume of air fuel mixture getting into the combustion chamber depends upon the speed of piston as well as on throttle opening.
We have started earlier the inlet valve open as the Piston starts moving down but as a matter of fact in present day engine the inlet valve open 10° to 12° before TDC (top dead Centre) so that sufficient charge air fuel mixture could be filled in the cylinder and combustion chamber.
When the Piston reach b.d.c. the inlet valve close various in actual practice this valve remains open up to 50° after b.d.c you must be wondering as to why it is kept open so late even in compression stroke, second stroke.
The reason is that incoming charges which enter the cylinder at approximate speed of 3 to 9 cu.m/second goes on entering the cylinder even in second stroke at the same time we want enough mixture to get into the cylinder and combustion chamber so that enough power could be obtained.
After the Piston has come down on induction stroke it has to go up to word T.D.C. due to rotary motion of crankshaft as soon as Piston starts going up in compression stroke inlet valve which was kept open in induction stroke closes.
Exhaust valve is already closed the mixture or charge start compressing we know it that if we press the charge its temperature and pressure rises. the rise in temperature and pressure depends upon the opening of throttle the speed of engine and the compression ratio.
In petrol engine the compression ratio is usually 6 to 12:1 and pressure reaches (90 to 120 lb/sq”) 6.33 kg/cm² to 8.4 kg/cm² on full throttle.this stroke finishes when piston from B.D.C. to T.D.C.
In this stroke both valves remain closed before the Piston reaches T.D.C. in compression stroke the charge is ignited with help of spark plug as soon as charges ignited spontaneous burning of charge take place due to burning of charge gases expand in the cylinder.
Piston is the only moving part as such this expanded gases but pressure on Crown of piston making it to travel down with force since the piston is connected with crankshaft with the help of connecting rod the crankshaft revolves.
in powerstroke at full throttle the temperature of burning charge reach up to 2000°C and the pressure up to (500 lb/sq”) 35 kg/cm². this pressure decreases as the Piston approaches b.d.c. where by brunt gases get more space.
In this stroke exhaust valve open and Piston starts moves up from B.D.C. to T.D.C. due to the momentum of flywheel. as the Piston moves up it was the brunt gases out through which exhaust valve.
To relieve piston of exhaust gases pressure the exhaust valve in actual practise opens about 50° before b.d.c. in power stroke more ever this opening of valve before b.d.c. also gives the exhaust gases enough time to clear out from the combustion chamber.
we have seen that in four stroke spark engine ignition engine there is only one power stroke rest are all ideal strokes.