Air conditioning on public buses are normally categorized as the
'direct-drive air conditioning', meaning that the main engine
drives the air conditioning unit. How it actually drives, will be
explained below...
For double decks like the LO, VO, VSO and B9TL, the air-con unit
is mounted above the engine at the rear compartment. Cooled air is
directed through the ducts towards the front of the vehicle. Air is
recirculated by two intake ducts, one on the lower level, all the
way to the rear (LO recirculation ducts are placed at the cabin
midship section).
For underfloor air-con units like the O405, the recirculation
ducts are the two netting sections along the duct in the passenger
cabin above the rear axle. Air is directed up and down through
pipings in the bodywork structure (window frames).
Direct-drive air-con units have drawbacks like lower cooling
capacity when the engine is idling, especially in traffic jams. Why
do some buses still have strong air-con when the bus is not in
motion? Likely that the compressor is taking more power from the
engine. This affects the performance of the vehicle.
In the engine, the engine drive shaft powers the main drive line
plus the alternators, compressors and radiator fan. More mechanical
resistance from the compressors create more resistance the engine
have to drive, hence affecting performance. Low speed operations
this will not be obvious, once it speeds up it will be evident.
Compressors not only provide air-con pressure, but also the
brake pressure systems if the bus is using a full air brake system.
Then again, more work for the engine to do.
This is also why some coaches have an auxiliary engine that
solely powers the air-con unit. The main engine can concentrate on
doing its own job plus providing electric power through the
alternators, which does not have as much mechanical resistance as
compressors. Auxiliary engine however consumes fuel with the main
engine and is not a fuel efficient method.
