The subject of this thesis is based on the use of natural gas as alternative fuel for engines, especially for OTTO engines. The first chapter provides more detailed information on natural gas, its production, its use and its positive environmental impact. As a source of fossil energy, natural gas has multiple uses. In Croatia, it is used exclusively for the heating of industrial, residential and commercial buildings as well as for the propulsion of motor vehicles. This thesis also describes the design of the OTTO engine and its compatibility with compressed natural gas. Three types of gas engines are listed, namely clean gas engines, Bi-fuel engines and dual-fuel engines. The characteristics of these engines are to compress the mixture of fuel and air and then ignite this mixture with spark plugs. Devices required to run a gas vehicle are also listed and their functions described. The heading "Economic and Environmental Assessment" lists the advantages and disadvantages of natural gas as fuel for motor vehicles. If people were more familiar with natural gas and its benefits, it would certainly be applicable as an alternative fuel.