Complex sentence: Compound engines (engines with more than one stage of expansion) had already been invented, and there was even a crude form of internal-combustion engine, which Carnot was familiar with and which he described in some detail in his book.
Split into simpler sentences: Compound engines (engines with more than one stage of expansion) had already been invented. There was even a crude form of internal-combustion engine, with which Carnot was familiar and which he described in some detail in his book.