Working cycle of CNG engine??

It works as normal four stroke engine.
Compression ratio can be reached up to 10.5:1
Consider it as a petrol engine but instead of petrol, CNG is used, just like in the engines that uses LPG as fuel.

I just want ask whether CNG engine works on OTTO cycle,diesel cycle or any other cycle?? :

As I said, it works same as the petrol engine, and petrol engine works on Otto cycle, so this must follow the same.
But CNG can be used in Otto-cycle (gasoline) and modified Diesel cycle engines both. Lean-burn Otto-cycle engines can achieve higher thermal efficiencies when compared with stoichiometric Otto-cycle engines at the expense of higher NOx and hydrocarbon emissions. Electronically-controlled stoichiometric engines offer the lowest emissions across the board and the highest possible power output, especially when combined with exhaust gas recirculation, turbocharging and intercooling, and three-way catalytic converters, but suffer in terms of heat rejection and fuel consumption. A suitably designed natural gas engine may have a higher output compared with a petrol engine because the octane number of natural gas is higher than that of petrol as this would allow for an engine design with a higher compression ratio.

I think you are right,, but I'm not pritty sure about that,,
But anyway thanks-a-lot,,
Would you like 2 telll me what is the modification is done on petrol and diesel engine so that it can work with CNG,,

It is same as it is done for LPG fuel in petrol engine.