Get "The Drawing Textbook" by Bruce Mcintyre.
Pretty sure you can get a copy on Amazon.
When he did this book he was an animation illustrator for Disney.
He also had an educational show teaching the drawing basics in the book.
It is great for the basic principles of spacial representation and shading. This will get you what you need to illustrate your automotive ideas.
Once you get those basics, you just practice drawing or painting, whatever.
Mom's cousin, the senior art professor at a local University told me (when I was 4) to just draw and paint and DO NOT take any lessons.
Said if you take lessons you end up drawing or painting like whom ever you took lessons from.
Said to just get the stuff and make pictures. That way you will develop your own style.
This especially includes working in different mediums.
Ever notice Chip Foose doing illustrations of the cars they are building up. He uses pencils, charcoals, markers, and paint all on the same illustration.