I noticed a number of model cars which all appeared to have one thing in common and this was a wind-up mechanism. But the spring was wound with a key. Not only this but the spring was a separate element. However, looking at this I set about to simplify it, creating a keyless mechanism that can still be wound. The spring, rather than this being a separate element is integrate with a cog, the result being a integrated cog spring. As for winding the spring, a pull back mechanism which simply requires the user to pull the model back (with the wheels in contact with a surface), in the opposite direction to the direction of intended travel. The integrated cog spring (engine), would be 3D printed and the remaining elements will be made up from LEGO pieces. Full details of how this is integrated in to fully functioning pull back wind up car can be found on Instructables. https://www.instructables.com/Integrated-Coil-Spring-Car/