I was at the local ice cream place that is adjacent to one of the local train tracks. 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 non-traction 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.
This 270,000-pound (122,470-kg) locomotive is designed to tow passenger-train cars at 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 pounds of 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 power.
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.
On Sun, 13 Jun 2010 20:35:02 -0500, Jon Elson wrote the following:
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.
If you're generating a 60Hz three-phase rail anyway, a switch to a motor is a heck of a lot easier to design, manufacture and maintain than a shaft to distribute power and a bunch of clutches to turn it on and off.
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 settings.
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 higher reliability.
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.
These locos seem to be quite new, in fact I hadn't seen this type until last week, but I have also seen several of them while driving up to Chicago last week. They have a BIG round "nose" on the front.
One thing I noticed is that the engine RPM seemed to stay the same as the loco arrived and then later pulled away from the station. I'm used to the typical Diesel-electric locos increasing RPM as they need power for acceleration.
I did notice very familiar Carrier V4 semi-hermetic Air Cond. compressors on each of the coaches, so there's one of the places a lot of the AC power goes to.
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. i
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 trucking companies?
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 Westinghouse.
Well, except maybe for those pesky maintenance issues.
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 and sliding.
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 configured.
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 rather common.
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.
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.