Why diesel engine does not take intake vacuum manifold as a source of braking?

Diesel engines don't use intake manifold vacuum for braking (like some gasoline engines do with vacuum-assisted power brakes) primarily because they don't generate significant intake manifold vacuum.

Here's why:

* Diesel Combustion: Diesel engines rely on compression ignition. There's no throttle plate restricting airflow into the cylinder like in a gasoline engine. The air is drawn in throughout the intake stroke, and the amount is largely determined by the engine's displacement and speed. This means there's relatively little pressure drop in the intake manifold, resulting in minimal vacuum.

* Turbocharging/Supercharging: Many modern diesel engines are turbocharged or supercharged. These forced induction systems increase intake air pressure, completely eliminating any possibility of intake manifold vacuum.

In essence, the conditions necessary for significant intake manifold vacuum—a restricted airflow path—simply don't exist in most diesel engines. Therefore, alternative braking assist systems, typically hydraulically powered, are used.