Is steam engine a proper noun?

No, the compound noun "steam engine" is a common noun, a general Word for a device used to generate power by the use of steam; a Word for any steam engine of any kind.

A proper noun is the name or title of a specific person, place, or thing; for example, Steam Engine USA (company) in Providence, RI or "A Short History of the Steam Engine" by Henry Winram Dickinson.