welcome
an engine is a mechanical device with which we convert the energy stored in one form to another.. this is the most important concept.. so we can extract the engergy from diffent sources ... and our desire is to convert this energy to do a work that is rotary work..
the process by which we convert the energy in fuel such as petrol is to use a 14 ratio of air and 1 ratio of petrol .. we first compress this mixture with a piston in a closed chamber called cylinder then burn it burn and as it burns it releases energy which forces the piston down.. so u see the piston only travells linear motion. or straight line motion.. yani sirf uper/neecha ya aga/peecha.. now this is a linear motion but we want our wheels to rotate so a cleaver gearings techniques are used to convert this linear motion into a circular.. coming back to the fuel combustion.. there is the whole trick .. feeding the cylinder the air fuel in proper ration of petrol and air.. and feeding the proper amount of this mixture... and feeding this at the exact time it is needed.. so there are two methods of doing this.. and these form the basis of the type of engnie..
one is a 2 stroke and anothe is 4 stroke.. 2 stroke is old and not ideal techniqe and is vastlyr replaced by 4 storke .. but there are certain needs that require 2 stroke.. and it is still there..
all the 70cc bikes in pakistan are 4 stroke same is the honda/chonda 70cc ,100cc,110cc,125cc,150cc bikes..
now how a 4 stroke works..
first it feeds the proper amount of mixture at the right time in the cylinder this is feed by the piston moving down a cylinder.. this sucks air.fuel mixture through carburetor.. a carb is the device which only mixes the petrol with proper amount of air..
now when the piston moves downward and it sucks the mixture through one hole called intake port.. when the piston reaches the absolotute lowest point the port closes.. and the piston now travells up and compresses the mixture this compression is the second stroke of 4 stroke engine
when the piston reaches the top the fuel mixture is very compressed and a sprak plug the ignites the mixture which makes explosion and releases the energy.. this energy has to go somewhere most of it is transfered in pushing the piston down...this pushing of piston down is the third stroke
after the piston reaches the lowest point .. it moves up and now as it moves up there is another valve that is called the exhaust valve ....and as u know valves control flow.. so that opens and the burnt gases now leave the cylinder through that port...this is the fourth stroke or step...
again this cycle starts to repeat it self..
this is an ideal description of a 4 stroke engine.. in reality the valves doesnt open instantaneously... bas itna hi kafi ha.
advantages of 4 stroke.. are many ..
also remember that the phrase " 4 stroke" is one thing and the word stroke is another thing...
remember when the piston was at the top of cylinder ? also remember that the piston was at the absolute lowest point ? the distance between those two points is called a stroke and it is measured in milli meter or "thousandths of a meter" and the stroke of 70cc ,100cc,125cc bikes is 49.5mm
also anther term used in conjunction with stroke is a bore . it is the diameter of the cylinder.. it is also measured in mm and the bore of different bikes is different.. honda cd 70 has 47mm,china 78cc has also 47mm..
the bore and stroke values of a cylinder gives the capacity of the engine.. they determine what an engine capacity is.. the engine capacity is measured in cc's...
bigger the cylinder bigger the air fuel mixture it needs to fill it up so greater the fuel consumption...
hope this helps.