Ceramic
In the early 1980s, Toyota researched an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C).  Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency.  Fuel efficiency of the hotter engine is also higher by Carnot's theorem. In a metallic engine, much of the energy released from the fuel must be dissipated as waste heat so it won't melt the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult.  Imperfection in the ceramic leads to cracks, which can wreck the engine, possibly by explosion.  Mass-production is not feasible with current technology.