2 & 4 Stroke Engines Rating  :      

Burning of fuel to run an engine in a controlled environment is called combustion and since the combustion happens inside the cylinder, these engines are called internal combustion engines. The most common internal combustion engine type is gasoline / petrol powered. Others include those fueled by diesel, hydrogen, methane, propane, etc. Engines typically can only run on one type of fuel and require adaptations to adjust the air/fuel ratio or mix to use other fuels. Burning fuel in a controlled way inside the engine was a problem that was solved in the late 19th century with the advent of the 4-stroke or Otto cycle (named after the inventor of this concept).  


The two-stroke engine is simple in construction. The cylinder has no intake or exhaust valves. Intake and exhaust are accomplished by means of ports - special holes cut into the cylinder wall which allow fuel-air mixture to enter and exhaust to exit the engine. Air-fuel mixture is drawn into the crankcase from the carburetor.

When the piston is forced down (One Stroke), the exhaust port is uncovered first for the hot exhaust gases to leave the cylinder, with the piston in it's downward most position the crankcase becomes pressurized the intake port into the cylinder is uncovered allowing pressurized air-fuel mixture to enter the chamber.  As the piston begins to move up (Stroke Two), the ports are closed off, and air-fuel mixture gets compressed by the rising piston in the cylinder and the mixture is ignited.

Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes the two stroke engine popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes. Unfortunately, most two stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.

 4 Stroke Engines 

 Combustion Cycle 

Intake stroke

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine draw in a cylinder-full of a mixture of air and petrol. The lowest most position of the piston is know as BOTTOM DEAD CENTRE (BDC). This stroke is known as intake stroke.

Compression stroke

The piston moves back up to compress the air and petrol mixture to about one-tenth of its original volume.  Compression makes the explosion more powerful. The topmost position of teh piston is known as TOP DEAD CENTRE (TDC). 

Combustion stroke 
When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the compressed fuel & air mixture
Exhaust stroke
The burn mixture expand he gasses and pushes the psiton down to its BDC. Exhaust valve opens and the piston returns to TDC pushing the burned exhaust gas from the explosion out of the cylinder. And then the process repeats itself by taking in another charge of air and fuel.
The rotational motion of an internal combustion engine is converted to linear (straight line) motion crankshaft.

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