* No throttle body: Unlike gasoline engines, diesel engines lack a throttle body that restricts airflow. The air intake is primarily controlled by the engine's speed and fuel injection. A vacuum is created by the restriction of airflow, and without a throttle body, this restriction is minimal and highly variable. This would produce an extremely weak and inconsistent vacuum signal.
* High compression ratio: Diesel engines operate with much higher compression ratios than gasoline engines. This high compression already reduces the pressure in the intake manifold, making it even more difficult to create significant vacuum.
* Turbocharging/Supercharging: Many modern diesel engines are turbocharged or supercharged. These forced induction systems pressurize the intake air, further eliminating the possibility of generating a useful vacuum. In fact, they generate positive pressure, the opposite of vacuum.
* Reliability and Consistency: A power brake system, for example, requires a consistent and reliable source of vacuum to function safely. The inconsistent and low vacuum generated by a diesel engine would make this unreliable and potentially dangerous.
While a weak vacuum *might* be present in some circumstances, it wouldn't be sufficient or consistent enough to power common vacuum-dependent systems. Therefore, diesel vehicles typically rely on alternative systems such as:
* Electric vacuum pumps: These are separate pumps that generate vacuum electrically. They provide a more consistent and reliable vacuum supply for power brakes and other systems.
* Hydraulic power steering: Diesel vehicles often use hydraulic power steering systems rather than vacuum-assisted systems. Hydraulic systems are less dependent on engine vacuum.
In short, the design and operating characteristics of diesel engines make the generation of a sufficient and consistent vacuum impractical and unreliable for powering auxiliary systems. Electric or hydraulic alternatives are more effective and safer.
