Operation of 4 – Stroke Engine A four stroke engine completes it's cyclic operation into four strokes of piston or two revolution of crankshaft. These strokes are suction stroke, compression stroke, power or expansion stroke and exhaust stroke. Both SI and CI engines follow these four strokes to complete one cycle. Working 4-stroke engine 1) Suction Stroke  Piston travels from TDC to BDC  Intake valve kept open  Exhaust valve kept close  Fresh air fuel mixture inducted inside the chamber  Total crank shaft rotation at the end of the stroke is 180 degrees 2) Compression Stroke  Piston travels from BDC to TDC  Intake valve kept close  Exhaust valve kept close  Pressure inside combustion chamber increases  Total crank shaft rotation at the end of the stroke is 360 degrees 3) Power Stroke  Piston travels from TDC to BDC  Intake valve kept close  Exhaust valve kept close  Ignition takes place in combustion chamber  Total crank shaft rotation at the end of the stroke is 540 degrees 4) Exhaust Stroke  Piston travels from TDC to BDC  Intake valve kept close  Exhaust valve kept open  Burnt exhaust gases pushed away  Total crank shaft rotation at the end of the stroke is 720 degrees ADVANTAGES OF 4 STROKE ENGINE :- More torque :- In general, 4 stroke engines always make extra torque than 2 stroke engine at low RPM. Although 2 stroked ones give higher torque at higher RPM but it has a lot to do with fuel efficiency. More fuel efficiency :- 4 stroke engines have greater fuel efficiency than 2 stroke ones because fuel is consumed once every 4 strokes. Less pollution :- As power is generated once every 4 strokes & also as no oil or lubricant is added to the fuel; 4 stroke engine produces less pollution. More durability :- We all know that more the engine runs, quicker it wears out. 2 stroke engines are designed for high RPM. If an engine can go for 10000 rpm’s before it wears out; a 4 stroke engine with 100 rpm will run for 100 minutes than the other 2 stroke engine which has a higher rpm of 500 & will run for only 20 minutes. No extra addition of oil :- Only the moving parts need lubrication intermediately. No extra oil or lubricant is added to fuel. DISADVANTAGES OF 4 STROKE ENGINE :- Complicated design :- A 4 stroke engine has complex valve mechanisms operated & controlled by gears & chain. Also there are many parts to worry about which makes it harder to troubleshoot. Less powerful :- As power gets delivered once every 2 rotations of crankshaft(4 strokes), hence 4 stroke is less powerful. Expensive :- A four stroke engine has much more parts than 2 stroke engine. So they often require repairs which leads to greater expense.

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