Jon Elson wrote:

Robert Swinney wrote:

A form of regen braking is available on some "diesel" locos. It
consists of a huge bank of resistors absorbing generated current from
the DC traction motors. The resistors are housed in a pod somewhere
on the loco, usu. behind the cab near the top.

Actually it is most of the roof of main line locos. The Diesel
radiators are the front 1/3 or so, the back 2/3ds are the braking
resistor array. When you hear a loco pass and there is a loud humming
whir, that is mostly the fans on the braking resistor grid. Our house
is a mile from the track, and those fans are the first thing I hear
coming, and I can hear them for quite some time as the train passes.

Jon
While it varys a bit from make to make, for most USA Diesel-Electric
locomotives, you have the relative locations of the engine radiators and
dynamic brake rediators reversed. The "dynamics" are normally near the
middle of the long hood behind the cab (often in a "hump" on older
locomotives). The radiators are normally at the far end of the long
hood, away from the cab.

The engine radiators are commonly about twice the size of the dynamic
brake radiators. A typical modern D-E locomotive has one or two dynamic
brake fans on the roof, and from three to four radiator fans. This does
vary a bit with the relative sizes of the fans (typically 36" or 48"
diam.). The fans are electrically driven, and controlled as needed by
thermal switches.

The dynamic brake resistor system, the main generator of the loco, and
the traction motors must be sized to approximately match one another in
capacity. Thus the traction motors (four or six) can absorb (use) the
full output of the generator in "run" mode, and then the dynamics can
absorb the full OUTPUT of the traction motors (then used as generators)
during dynamic braking.

Around locomotive shops it is common to find a complete dynamic brake
unit removed from some scrapped locomotive in use as a "load box".
During loco testing, this is connected by cables to a locomotive's main
generator, and used to "load" the generator (and Diesel engine) during
the tests. The "load box" stands-in for the traction motors, allowing
full power testing without moving the locomotive.

In addition to the rooftop fans, the locomotives also have large
internal blowers to cool the traction motors in the trucks. This air is
delivered by ductwork, some flexible to allow the trucks to pivot as needed.

There is also a large air compressor used mainly to charge the train's
air-brake system. This acivates (clutches) automatically as needed to
maintain desired air pressure. The "chugging" noise of this compressor
is usually quite audible, especially after an air-brake application
(that releases air from the system). The pressure in the train's air
line must be restored before the brakes will release again.

Altold, there is a LOT of air moving through several systems.

Dan Mitchell
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