This thesis describes the development of a tank insert for diesel cars, from idea to a prototype. The idea is to design an insert which makes it impossible to refuel petrol in a diesel car. The insert shall be installed in the existing tank opening on diesel cars. The tank insert is designed for the Swedish company SverigeGrepen AB located in Vänge, just outside Uppsala. Their main product is a pitchfork that is ergonomically designed and durable. A pre-study were performed in which information relating to fuel caps, tank openings, petrol- and diesel nozzles were collected. The information comprised mainly dimensions and standards. The pre- study also included information retrieval about suitable materials that might be used in the design. Some research and surveys on existing solutions for this problem were made. After the pre-study, a user requirement specification was written to define measurable requirements and to facilitate the concept design process. After the pilot study followed a concept design process that began by sketching ideas for different concepts for the two main functions of the product, identified through a product function analysis. Simple CAD models were also created from the sketches. The best of the concepts of the respective main function were selected and combined into a new concept. The new concept was further developed. The construction and the individual parts were adapted for manufacturing and it resulted in a 3D-prototype model. Based on the 3D-prototype model a 3D-printed prototype were created and evaluated. The prototype solves the problem with misfuelling and makes it impossible to refuel petrol in a diesel car. However the prototype is not optimized but small adjustments can make it more reliable and make it operate much better.