Under 621.4 is classed information about heat engines other than steam engines. Heat engines in general, and the generation and technical applications of steam, come under 621.1. Matters common to 621.1, 621.4 and 621.5 are classed under 621.1.
BS 1000[621.4]:1976 history
1976BS 1000[621.4]:1976 Universal Decimal Classification - English full edition - Heat engines (other than steam engines)