Fires of Invention by J Scott Savage

Trenton Colman is a creative thirteen-year-old boy with a knack for all things mechanical. But his talents are viewed with suspicion in Cove, a steam-powered city built inside a mountain. In Cove, creativity is a crime and "invention" is a curse word.
Kallista Babbage is a repair technician and daughter of the notorious Leo Babbage, whose father died in an explosion-an event the leaders of Cove point to as an example of the danger of creativity.
Working together, Trenton and Kallista learn that Leo Babbage was developing a secret project before he perished. Following clues he left behind, they begin to assemble a strange machine that is unlike anything they've ever seen before. They soon discover that what they are building may threaten every truth their city is founded on-and quite possibly their very lives.

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