What defines a CNG system that fuels an engine designed to move the vehicle?

A CNG system that fuels an engine specifically designed to move a vehicle is referred to as a motor fuel system. This terminology is used because it encompasses all components and processes involved in storing, delivering, and using compressed natural gas as fuel for the vehicle's engine.

The motor fuel system is integral to the vehicle’s operation as it directly affects performance, efficiency, and emissions. This system includes the fuel storage tanks, lines, regulators, and injectors that are critical for the transformation of stored CNG into usable energy for propulsion.

In contrast, the other options do not adequately encompass the full functionality or purpose of a system specifically designed for moving vehicles. While a fuel delivery system might refer to the process of moving fuel from one point to another, it is not explicitly focused on the use of that fuel for vehicle movement. Likewise, a mobile fuel system could refer to fuel systems designed for transportable or temporary fueling setups, rather than permanent systems integrated into the vehicle itself. An auxiliary fuel system usually indicates a secondary fuel system that supports the primary fuel source rather than serving as the main engine fuel source.