I was at the local ice cream place that is adjacent to one of the local
The Amtrak pulled up to exchange passengers at the nearby station. A
bit after it stopped, I heard a very familiar and unexpected sound.
It was a large air compressor driven by a 60 Hz 3-phase motor. I'd
guess from the sound it very well could be a Quincy, and I'd guess maybe
15-25 Hp 4-cyl 2-stage, with pneumatic unloaders.
Anyway, it struck me as just a tiny bit odd they'd use an electric
compressor, as they have mechanical power available off the Diesel engine.
I also noticed the Diesel didn't seem to change speed when the train
accelerated, so I'm guessing they run a single engine at 60-Hz
synchronous speed all the time, so they can distribute 60-Hz power for
air conditioning, etc. to the coaches. So, no aux engine for the
Anybody know anything about this?
I personally think that it is very smart to run the train's compressor
from an electric motor. After all, the compressor needs to run only
very intermittently, to charge the braking system and such.
No idea on constant 60 Hz from the main motors, but if this is done, I
would think that it is fantastic for a passenger train that needs a
lot of auxiliary power.
I think the explanation is that the diesel engines aren't used for motive
power, instead, they drive generators (alternators?) that in turn power
electric motors for motion, by way of motor drive circuitry.
That's the way I've heard it explained anyway.
Dunno about the air supply.
"Jon Elson" < email@example.com> wrote in message
This 270,000-pound (122,470-kg) locomotive is designed to tow passenger-train
speeds of up to 110 miles per hour (177 kph). The diesel engine makes 3,200
horsepower, and the generator can turn this into almost 4,700 amps of electrical
current. The four drive motors use this electricity to generate over 64,000
thrust. There is a completely separate V-12 engine and generator to provide
electrical power for the rest of the train. This generator is called the head-end
power unit. The one on this train can make over 560 kilowatts (kW) of electrical
See also http://en.wikipedia.org/wiki/Diesel_locomotive
about 1/2 way down "Propulsion system operation"
I think technically they're called "diesel-electric" locomotives. 560 kW is
a lot of toasters!!
The hp of diesel-electrics varies from 330 hp (used for the "work trains" in
subways) to 6,600 hp behemoths, that are so large they can't navigate
certain areas of track. 3,200 hp is indeed a good-sized locomotive.
The diesel-electric strategy greatly simplifies the mechanicals, and gives
very sensitive control of the traction motors, AND allows the electric
motors to brake regeneratively (produce electrical power), altho this
regenerated power is not always utilized but is often bled through gigantic
banks of iron resistors, which can glow red hot under heavy braking.
The steel wheels alone of these locomotives are proly thousands of pounds
each, as the wheels on bitty subway cars (NYC) weigh 700-800 lbs each -- no
axle, no bearings, just the wheel..
History channel had this guy, a railroad conductor in the NE, do a train
reality show, which was really perty inneresting, if you could get past his
own personal issues/drama.
They might have some shows archived, altho I'm sure they're more innerested
in $29.95 for each dvd.
My SWAG is that the compressor is for service air to hold the brakes
open. The diesels run the generators which power the massive
electric motors which power the train. I don't see any reason they
couldn't tap a bit of juice off that to run the compressor motor.
Yes, the compressed air is used mainly for braking. The system was
largely invented by Westinghouse. The locomotive pumps air through a
long "train-line" (string of pipes and hoses) to local reservoirs in
each car. This locally stored energy is used to APPLY the brakes when
desired. The brake application is started by REDUCING the pressure in
the train-line by opening a "hole" in the locomotive's brake control
stand (venting pressure). The reduced train-line pressure activates a
valve on each car that then applys the pressure in the car's reservoir
to the brake cylinder(s). Thus each car stores the energy necessary to
apply it's own brakes.
The degree and rapidity of the brake application is controlled by the
pressure drop in the train-line. The locomotive's brake control stand
has various vents and a pressure gauge to control and monitor these
The system, while complicated, is relatively fail-safe. since any
brake-in-two of the train separates and vents the train-line, applying
each cars brakes. Things can go wrong, both mechanically and due to
human error, but it mostly works.
To release the brakes, the engineer closes the vent in the train-line,
which increases pressure, again activating the individual car control
valves so as to release pressure in the brake cylinders. The valves also
allow train-line air to recharge the main reservoirs in each car. This
recharging is why it takes a train a considerable time to "pump-off" the
brakes after a full brake application. You can hear the locomotive's
compressor working to do this.
In addition to the train brakes, the locomotive's brakes are normally
controlled separately. These are called the "independent" brake. Careful
use of both train brakes and locomotive brakes allow the accumulated
'slack" in the train's couplers to be controlled as desired ... either
stretched out or compressed.
And Yes, most locomotives today are properly called "Diesel Electric".
As to the electrical system used ... there are a whole bunch.
Traditionally, USA locomotives used DC generators and DC traction
motors. Many years back (1960's) AC generators began to supplant DC
units, with rectifiers then providing the DC to the traction motors.
Many such locomotives are still in use, and some still being made, today.
More recently, the use of AC traction motors has been on the rise. This
is mainly due to computerized and all solid-state control circuitry.
Many such locos are now in service. They provide better control, thus
higher traction, and are more efficient. The downside is very
complicated, and often trouble-prone, control circuitry. Railroads are
divided on the merits of each type propulsion. Some who've tried
straight AC have gone back to the AC-DC units for lower maintenance and
At the moment, all AC units are common, but the technology just doesn't
seem to be quite perfected yet. Like all computerized equipment, when it
works, it's wonderful ... and when it doesn't, its a nightmare.
Amtrak's locos now provide high-power 60 Hz AC to the entire train. This
is locally available in each car. It runs local lighting, air
conditioning, heating, cooking, refrigeration, etc. it's also available
(sometimes at each seat) as 115VAC for personal appliances (laptop
computers, TVs and such).
This is called "Head End Power" since it all comes directly from the
locomotive. In olden days, each car had it's own low voltage (often 32
volt) DC generator and batteries.
With lots of electrical power available it's no wonder that electric air
compressors are now in use. In older units, the compressors were
mechanically clutched to the engines, and engaged as needed.
To maintain the 60 Hz power, older Amtrak locomotives needed to maintain
engine RPM even when the train is not moving. They did this by cutting
out cylinders in the main engine reducing power and fuel consumption,
while maintaining RPM. This was complicated. Newer units use large
inverters to produce the 60 Hz power, and thus allow the engines to
reduce RPM while still generating 60 Hz power (assuming higher speeds
are not needed to move the train). They can't drop to full idle, but can
slow revolutions to save fuel.
All this is just an overview ... there's lots more to it, an no one way
each manufacturer / railroad / country goes about it. And, LOTS of older
technology is still in service.
You are quite welcome.
Train air brakes are a very complicated affair. Due to the long length
of the train-line, a delay exists between the action of operatign of the
valves in the locomotive, and the actuation of the braking action at
each car. The longer the train, the greater the delay toward the back of
the train. Hence the engineer must anticipate actions ahead of time.
This reqires experience, and a prior knowledge of the "profile" (grades)
of the track being operated over. Incorrect brake operatin can either
break the train in two (breaking couplers or pulling drawbars), or
result in a loss of control of the whole train.
Some work is being done on "electric" brakes for trains (probably still
air actuated, but electrically controlled) ... these would have faster
response to commands, but the technology is yet unproved. Air brakes,
while complicated and smoetimes troublesome, are a proven technology.
So, there's some resistance to change.
Dan, write something else about trains. I love reading this stuff. I
am a somewhat of a train fan and one of my kids (4 year old) is an
avid train fan, so we go see trains a lot.
As for brakes, I thought that the brake system is a lot simpler, that
air holds brakes away from the wheels, and when air comes out, the
brake pads touch wheels. Now I know that it is more complicated.
"OK", here's some more.
Depending on how you look at it, the air both applies, and releases the
brakes. It's all controlled by ONE air supply, the "train line", pumped
up by the locomotive's air compressor. The compressed air in the train
line both charges the local reservoirs on each car, AND controls the
brake application and release.
The important safety features of the Westinghouse brake system are that
1) each car has it's own reservoir, and THAT air is used to apply that
car's brakes, independently of any other car. and 2) that a DROP in air
pressure in the train line applies the brakes. Hence, in the event of a
separation of the train line (a break-in-two of the train for example),
the pressure loss applies the brakes.
All this is actuated by a complicated pressure sensitive valve on each
car. This is called a "triple valve". Depending on the pressure changes
in the train line, this valve routes air either from the train line to
the car's reservoir (charging), or routes air from the reservoir to the
brake cylinders on that car applying the brakes, or vents air from the
brake cylinders, releasing the brakes.
The degree of brake application is also controlled by the pressure drop
in the train line ... the greater the pressure drop, the more strongly
the brakes are applied.
The train line pressure is controlled from the locomotive by selectively
venting pressure from the train line. a series of vents ("holes") can be
selected to obtain various rates of pressure loss. An "emergency"
(maximum) brake application is obtained by selecting the largest vent
... hence the railroad slang term for an emergency brake application is
the "Big Hole". This is also what results from a break in the train line.
Such emergency applications are frowned upon unless really needed, since
they often result in equipment damage, derailments, or wrecking the
entire train. At best, one can expect flat-spotted wheels from locking
Sometimes an individual wheelset (axle) will improperly lock in even an
ordinary brake applcion, resulting in a flat-spotted wheel. Hence the
loud "Clunk-Clunk-Clunk" noise one sometimes hears in a passing train.
This intermittent "hammering" is hard on both the car and the track
(aside from making an awful noise).
Also: Each brake application depletes the car's air reservoir a bit.
Normally this is soon recharged by the loco's air compressor once the
brakes are released. However, one has to be careful to not apply the
train brakes too hard and too frequently, without allowing adequate time
for the loco's air compressor time to properly recharge the cars' air
reservoirs. Insufficient air in the reservoirs can lead to inadequate
braking being available when needed, and a train run-away. Again, proper
brake usage requires both experience with the equiupment, and a prior
knowledge of what lies ahead on the route. A train may need to "rest"
briefly to recharge the reservoirs before beginning a long descent.
The above problem can be reduced by using another appliance on each car
called a "retainer" valve. Actuating this device (manually on each car)
retains at least some air in the reservoir. It is used mainly when
descending long grades, where the brakes need to be "dragging" but not
fully applied. Thus some "air" is held in reserve. "Setting retainers"
must be done manually by crew "walking the train", and done BEFORE
descending a long "Often only some subset of cars in the train are so
Also, when a car is parked, with the train line open, the brakes are
applied ... but there is always some leakage, and the car's reservoir
will bleed down, eventually releasing the brakes. To prevent this
situation, each car has a mechanically applied "parking brake" actuated
by either a large wheel or lever mechanism with some form of ratchet to
lock it. This mechanically pulls on the end of the brake cylinder's
piston rod, extending it, and applying the brakes the same as if the air
cylinder had done so.
The above is much the same system as used in olden days to mechanically
apply the brakes before air-brakes were invented. The crew had to walk
the length of the moving train, usually along the roof, setting or
releasing the brakes car by car as needed. It was a VERY dangerous
business, especially in bad weather. Also, imagine the time delay
involved in getting most of a train's brakes applied! This made even
early air brakes a wonder of responsiveness.
And "brake inspections" are needed periodically when running any train.
Normally they are inspected BEFORE the train is allowed to leave a yard,
and after any really hard brake applcation. Crew must "walk the train",
on each side, and inspect each car's brakes to be sure they are either
applied or released as may be required. "Stuck" (dragging) brakes are
The brake shoes (cast iron or composition) wear, and need to be replaced
perodically, or as may be needed. The wheels also wear (due to both
braking and track contact), and need to be periodically re-machined to
the proper contour (turned on a large lathe), or replaced, as needed.
Air pressure above a certain point releases the
The engineer pumps up the train brake line to
release the brakes and fill the small reservoirs
on each car. Then when he "sets some air" by
blowing down the pressure on the brake line, the
three-way valve on the car channels air pressure
from the reservoir to the wheel brake cylinders.
The more he sets in one application, the higher
the pressure applied to the wheel brakes.
Breaking the airline or putting the brake system
into "emergency" applies the full reservoir pressure
to the wheel brakes.
Now a clever listener might ask "Then how can the
engineer start a train that's stopped on a grade?"
That's more-or-less what I said, but it's more complicated than you
imply. Essentially the air both APPLIES and RELEASES the brakes.
The locally STORED air in the main reservoir is what provides the force
needed to APPLY the brakes. A DROP in pressure in the train-line is what
triggers the brake application (via the triple-valve as previously
described). Once the pressure in the train-line is restored to normal,
the pressure in the brake cylinders is vented (again via the
triple-valve), and the brakes are released.
What's important to understand is the pressure in the train-line is used
to CONTROL the brakes, NOT to provide the force to apply them.
The brakes are NOT "spring loaded", and held "off" by the air in the
train line. It's only PREVIOUSLY stored air in the reservoir that can a
provide the force to apply the brakes (I suppose that could be thought
of as an air-spring). However, once that reservor air is used up, you're
in BIG trouble.
Once the main reservoir is depleted, as from too many brake applactions
in a short period of time, the brakes become inoperative regardless of
train-line pressure. Then you have a run-away train. NOT good!
It's the engineer's job to manage his available air supply to assure
adequate braking at all times.
I had a guy working under me who's previous work had been for his dad's
company, building one of those electronic train brake controllers.
_He_ felt the technology was quite well proven! His father had been
involved in discussions with the railroads to incorporate the
technology, and it wasn't falling to performance concerns, it was
falling to compatibility, logistics and economic issues.
Even with a technology that's known to work, there's a huge barrier to
adoption -- For an electronic system to work you'd have to equip every
damn rail car with it, which means a huge retrofit and scrap expense.
the Westinghouse air brake is technically inferior* to the
electro/pneumatic systems, but it is standard on damn near every rail
car in the US. Try to put a train together with an electric brake, and
the first time you slot a car in there with a pneumatic-only brake
you've just broken the system.
Imagine the difficulty you'd have with trying to pull a rail car from
the pool when the first question you have to ask is "does it have the
new brakes?". Every time -- if you were putting a train together with
electric brakes, you'd need _all_ electric-capable cars; if you were
putting a train together with pneumatic brakes you'd want to save the
electric-capable cars for the electrically braked trains. And what
happens if you have a rail car sitting there, full of lucrative cargo,
waiting for a train that it can be slotted into? "I'm sorry sir, your
car is the wrong flavor for the trains coming through." -- how many
times do you tell a customer _that_ before he starts calling the
I suspect that there was some of this resistance when the train
companies went from the old mechanical brakes to the Westinghouse
system, although in that case the rail car pool was not as developed (if
it existed yet at all), and the Westinghouse system would have held more
technical and economic advantages over the mechanical brakes than even a
perfect electro-pneumatic or fully electronic system would hold over the
* Well, except maybe for those pesky maintenance issues.
Control system and signal processing consulting
Very likely true. Any large organization or infrastructure is saddled
with both "institutional inertia" (a reluctance to change), and the
overwhelming cost of a change that must affect all or most of their
operation in a short time.
There are attempts at electric brake "improvement", but these almost
HAVE to be compatible with existing pneumatic systems to have much
chance of acceptance ... even if they ARE better.
Similar problems accompanied adoption of "standard" coupler types. This
too remains an ongoing issue, with speciallized couplers mandated for
passenger cars and hazardous material cars.
And a similar problem is now ongoing with the government mandating
acceptance of automatic train control systems, following several bad
train wrecks. The technology exists, but the cost of adapting it is
astronomical. In a time of bad economy, the railroads are understandably
balking, while largely admitting that its a good idea. It's inevitable,
but may take a while to implement.
From my brother, recently retired engineer for CN.
=============================Yes electric air compressors have been used on diesel electric
locomotives for at least 20 years. Couple of reasons, one is there is
no drive shaft clutch assembly needed etc to run the compressor off of
the diesel motor, ie less part to maintain, easier to install ie no
critical alignment with the drive shaft. Also easier to replace the
electric compressor set, rather than the motor driven one. Also does
not "vampire" fuel and horse power from the prime mover diesel. Also
if the electric compressor fails, the locomotive can still be used (
generally trailing) for moving freight etc. Generally if the old style
shaft driven compressors failed (catastrophic) the whole locomotive
had to be taken out of service. As a side note, most locomotives now
have electric fan for the radiator cooling systems. General Motors
have long had the electric fan, ALCO and GE still used the shaft /
clutch off of the diesel crankshaft to power the rad fans. Now use
electric motors, variable speed, basically for the same reason for the
electric compressors. Hope this helps.
takes to run the air compressor, it HAS to be taken from the Diesel
engine, by whatever means ... there's nowhere ELSE to get it from.
That's the same issue as electric applicances on automobiles.
The next question is the relative efficiency of the power transfer ...
whether electrical or mechnical. I suspect that's not a whole lot different.
That leaves reliability and maintence as the big issues, and, as you
point out, electric compressors likely win out in those areas. since
they're the common type on newer locomotives, that pretty much answers
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