How to send a research proposal to find potential PhD supervisor?

In most branches of engineering, sending a research proposal to a prospective PhD advisor is not particularly helpful, unless you are providing your own funding.

The reason for this is that most PhD students are funded through grants obtained by the professor, either from traditional research funding agencies (NSF/DOE/DOD/NIH and their counterparts in other countries), or through industrial contracts. In such cases, the projects that are to be worked on are well-defined, so the possibility of deviating from those projects and using the money to fund other projects—such as the one you propose—are remote.

On the other hand, if you can provide your own funding, things change significantly. Under such circumstances, you could propose a project of your own design, so long as it fits within the general scheme of the professor's existing research (or moves slightly outside of it). However, proposing a project that lies too far outside of the mainstream typically is unlikely to be well-received, since the professor won't be able to provide much useful support.

However, even under such circumstances, the initial proposal should be more like a "white paper"—a faculty member, if he or she reads such a document at all, is not going to spend more than a few minutes reading it. So the initial idea will need to be compellingly presented in a short amount of space—two or three paragraphs at most.

The best way to show your interest in a professor's group is to explain how you could contribute to ongoing research in the group—that is, do you homework before writing to the faculty member. Your email should make it clear that you are writing the specific faculty member, and addressing the concerns and interests of their group. Anything that has the whiff of being a form letter will likely be ignored, unless your CV looks like a perfect fit for the research group!