Ballari Jn - Look around and enjoy the station. A sacred place where Mahatma Gandhi once spent 8 hours. - Vishwanath Joshi

APU fitted WDM3A #16642R of Ludhiana (DLS) ready to take charge of 54626 Jalandhar City - Jaijon Doaba Passenger from Jalandhar City. Please watch the 5 mins video of this locomotive in youtube, I have captured idling, notching, shunting etc of this loco, here is the video link: click here
AUXILIARY POWER UNIT (APU) is actually an alternative source of power for a diesel locomotive. APU consist of a small naturally aspirated 3-cylinder 4 stroke diesel engine connected to an alternator and a small compressor. It is installed in the short hood of the locomotive.
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Power Unit (APU) is a unit consist of a three main components:
1. Naturally aspirated 3-cylinder 4-stroke diesel engine - to drive compressor and alternator simultaneously
2. Air compressor - to maintain Air Brake pressure
3. Alternator - to charge the main engine starting batteries
When a diesel locomotive idles, it actually performs two functions:
1. Prevents the batteries from getting discharged
2. Maintains the air brake's air pressure of the rake (if attached to some rake)
THIS IS WHY THE DIESEL LOCOMOTIVES ARE SELDOM TURNED OFF, but this results in massive wastage of fuel.
In order to get rid of this problem, APU is equipped in the diesel locomotives which performs these two functions, i.e. the main 16 cylinder diesel engine inside the locomotive is turned off and the small diesel engine of the APU is turned on.
Main function of APU are:
1. To maintain the Air brake pressure more than 1.5 Kg/Cm² (approx)
2. To keep the main engine starting battery in fully charged condition
This is how the whole system works.
Normally, diesel locomotives consumes about 25 L/hr diesel whereas APU consumes only 5 L/hr diesel.