How Does Cng Decreases A Petrol Engines Efficiency?

CNG is 130 octane fuel so it burns slower, To take advantage of octain, engine timing or compression have to be increased. Not easy to do if you also want to run on gasoline. Natural gas also is introduced into the engine as a gas rather than a liquid, thus it displaces approximately 7% of the available air coming into the engine. Effectively making the engine 7% smaller in cu.in. Displacement. The higher octane will make up some of the difference, but normally you will see an approximate 3-4% loss in horsepower unless modifications are made to the engine or timing.

I have always been of the understanding that compressed natural gas increases a petrol engine's efficiency. It is much harder to transport and store but is also much more efficient cost wise when compared to petrol plus it is more environmentally friendly.