The author's main reason for writing "Lean, Green, Eco Machines" is to promote the reliability and potentiality of eco-cars. The author wrote this as a persuasive essay. A persuasive essay is one that tries to convince the audience to adopt a particular point of view. In this case, the point of view that the author wants to express is that eco-cars are a real, convenient, ecological option for all people to consider. He wants people to know that the possibility exists, and that it is a very advisable one.