What materials stronger than steel could be used to build engines?
That depends on the forces in play: tension, compression, and shear. Some materials, like stone, are stronger than steel against compression. Steel also loses strength when exposed to high heat. This is why steel structural members in buildings are encased in concrete: for additional compression strength of the entire supporting column, and as protection against fire. In an engine, assuming you mean an internal combustion engine, the material has to withstand high temperature. Ceramics withstand high heat better than metals, but are very weak against tension and shear forces, so are not good candidates for structural components of engines. The components in a modern gasoline engine are made out of many different classes of materials, cheifly: metals, ceramics, and plastics.
