Any way as the name implies it has one engine depend upon the requirement. And having four cylinder or may be any.
But the basic principle remains the same.
It consist of the cylinder in which there is one piston as the major component for your understanding!!
Now the cylinder has two valves one for the suction andthe other for the exhaust.
During the starting we providing the initial effort through our leg which is called as the cranking so as we do the same the piston moves down and the vaccume is created in the cylinder which is called as the suction stroke and the fuel or the mixture of the air -fuel enters the cylinder and due to the inertia the piston moves up and compresses the fuel and in the diesel it gets ignited it self as high temp. Is generated due to compression and in the petrol engine the mixture get the ignition from the sparkplug with the help of the spark and as the ignition start the exhaust gets pressurised and expands in the small space of the cylinder which produces tremendous pressure on the piston and the piston moves down produces the power stroke now the flywheel accumilate this high energy and it uses for the gases during the power stroke and this way the entire cycle is repeated.
The initial stroke called-suction