Vacuum-operated systems use the plentiful manifold vacuum available with throttled internal combustion engines like gasoline 4-cycle engines. Manifold vacuum is created by piston movement on the intake stroke and restriction of airflow through the throttle opening. Vacuum is highest when the throttle is nearly closed such as at idle or at low speeds and drops to zero when the throttle is opened during wide-open acceleration. However, diesel engines don't have throttle valves and don't produce vacuum; the power output is controlled by varying the amount of fuel injected into the cylinders.
Many HVAC systems use vacuum motors to control temperature and airflow. The vacuum source is connected to a vacuum reservoir, which is in turn connected to the dashboard control panel. When the driver selects the desired temperature and airflow direction at the control panel, the vacuum is directed to the appropriate vacuum motors and the heater control valve. The heater control valve controls the flow of hot coolant through the heater core. The vacuum motors operate the blend door, which controls the amount of air flowing through the heater core, and the mode door, which directs airflow to the floor, through the dash vents, and to the defroster.
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