WHY THE internal combustion engine is not 100 percent efficient?

The internal combustion engine is not 100 percent efficient due to several inherent limitations. A significant portion of the energy from fuel is lost as heat during combustion and through friction between moving parts, which does not contribute to useful work. Additionally, incomplete combustion and energy losses in exhaust gases further reduce overall efficiency. These factors, combined with mechanical and thermodynamic constraints, prevent the engine from achieving perfect efficiency.