A/t in general and CVT in particular require proper power from the engine at all times. Since CNG can prove to be a bit slow, the tranny dont get enuff power at times hence it bears the load. especially for CVT, jerks and power loss are deadly and can ruin your transmission. engines are specifically designed for a fuel type. If you get a dedicated CNG engine with a/t tranny, there are least chances for a failure (cng buses).