At high altitudes, the air density decreases with the decrease in the atmospheric pressure. For this reason, if the fuel is injected under the same conditions as at low altitudes, the air-fuel mixture becomes too rich, making the engine susceptible to generating black smoke. Therefore, the HAC automatically reduces the maximum fuel injection volume in accordance with the vehicle’s present altitude.
The HAC, which is installed above the pump governor, consists of vacuum bellows, a push rod, connecting pin and control arm.
(1) Low altitude
As the bellows are contracted when the atmospheric pressure is high, the push rod is pulled upward by the spring. The spill ring is kept at the normal position.
(2) High altitude
When the atmospheric pressure is low, the bellows expand to push the push rod downward. This movement is transmitted via the connecting pin, the control arm and the tension lever, and serves to move the spill ring to the left. In this way, the maximum fuel injection volume is decreased.