http://en.wikipedia.org/wiki/Diesel_cycle

   During the intake stroke, the intake valve opens as the piston moves down to allow air into the cylinder. The air is pushed by atmospheric pressure into the cylinder through the intake valve port.

    The next stage of the combustion cycle is the compression stroke.  After the piston reaches its lower limit, it begins to move upward and as the piston moves up, the intake valve closes.  The exhaust valve is also closed, so that at this point in the cycle, the cylinder is sealed.  As the piston moves upward, the air is compressed and the compression of the air causes the pressure and the temperature of the cylinder to increase.  At the end of the compression stroke, the pressure in the cylinder can reach 500 psi or 34 atm and the air can be heated up to 1000° F or 530° C.   

    Then during the power stroke, as the piston reaches maximum compression of the air, diesel fuel is injected to the combustion chamber filled with compressed air.  The heat of the compressed air ignites the fuel spontaneously at the self-ignition temperature of the fuel.  The mixture of fuel and air burns quickly as the cylinder pressure increases, forcing the piston down in the cylinder.  The power impulse is transmitted down through the piston, through the connecting rod, and to the crankshaft, which is rotated due to the force.   

    The final stage of the combustion cycle is the exhaust stroke.  As the piston reaches the bottom of its stroke, the exhaust valve opens.  As the piston moves up on the exhaust stroke, it forces the burned gases out of the cylinder through the exhaust port.  Then as the piston reaches the top of its stroke, the exhaust valve closes, and the intake valve opens. The cycle repeats again with the intake stroke.