lean conditions

just a query , why if a mixture is lean do we generate heat , say causing melting etc in the engine. I thought you need fuel to generate heat and lack of fuel would not increase temps.

A lean condition is only a few percent less fuel. You still have plenty of fuel for combustion that causes heat. What you don't have is excess fuel that can cool the fuel/air mixture before the combustion occurs, so the combustion itself is actually a bit hotter.

thanks David appreciate your time