Patents can be an important part of engineering research. In exchange for public disclosure of their invention, inventors are granted a temporary right - usually 20 years in the United States - "to exclude others from making, using, offering for sale, or selling the invention throughout the United States or importing the invention into the United States," (United States Patent and Trademark Office). Patent laws vary around the world, so it's important to research the specifics in countries in which you intend to operate or seek patent protection.
In the U.S., there are:
Because the invention must be "adequately described or enabled (for one of ordinary skill in the art to make and use the invention)" in the patent, research into historical patents can be extremely informative. In addition, "a search of all previous public disclosures (prior art) including, but not limited to previously patented inventions in the U.S." is a critical part of applying for a new patent - if your invention is not new, then you can't patent it (USPTO).
Visit the USPTO "Getting Started with Patents" page for more information and resources.
Google Patents provides keyword searching and advanced search functionality for patent documentation. Patent records connect to citing and cited documents, patent-family documents, and explanatory materials.
USPTO Patent Full-Text and Image Database provides full-text search functionality for patents issued since 1976; search by patent number for PDF images of patents issued before 1976.
USPTO Patent Application Full-Text and Image Database searches published patent applications.
PatentsView is a platform designed to facilitate data visualization and analysis of U.S. patent data.
Espacenet (from the European Patent Office) provides indexing and full text patents and applications from patent offices worldwide (focus on European countries).