In the thread “4-axle units” traisessive1 mentions those types of units could be called “four-axle” power. Which got me to wondering…
Why would any railroad bother with such an arrangement? Yes, I know part of the reason is to save a few $ by having less traction motors, but if that is the case, why not just buy a new (and presumably cheaper) 4-axle unit?
If not getting to use all the HP you’ve paid for is what you’re after, then the units make sense; otherwise–what a stupid concept!
Buying such a unit would be something I could see a railroad like NS or CSX doing, but not a supposedly ‘smart’ railroad like BNSF.
It should be noted that BNSF uses the four-motor power primarily in expedited service and FEC has no real grades. The horsepower is still getting used in either case.
The main reason for not using B-B type locomotives, but rather using A1A-A1A or on EMDs B1-1B powered axle arrangements is weight. Either you sacrifice fuel to hold the weight to allowable limits or engine size, or perhaps both. Both the GP60M and B40-8W locomotives were right at the maximum allowable weights, and both had reduced fuel capacity, and remember these locomotives were built to Tier 0 emissions standards. So imagine what enlarged cooling capacity carrying more water with more weight is going to do.
I think the original theory was that BNSF’s intermodal traffic was always powered highly enough that the trains wouldn’t drag up grades at speeds low enough to require the tractive effort of 6 AC motors, so eliminating 2 reduced cost.
Of course they show up with some regularity on everything from coal trains to heavy manifests, but…
A GEVO with A-1-A trucks is putting 4400 hp into 4 traction motors - 1100 HP each. That much HP at a traction motor tends to make the engine ‘slippery’ in drag tonnage situations. With ‘suitable’ tonnage in a intermodal situation the train should easily maintain track speed.
To my mind, if the ‘idler’ axle is allowed to carry it’s weight on rail, the engine would be even more ‘slippery’ in drag tonnage situations. Never having had any first hand dealings with these locomotives, I don’t have the real answers.
What new 4 axle unit? I’m guessing (but have no clue if right) cheaper to do what they did and strip down an existing 6-axle than to engineer a whole new 4-axle.
An ES44C4 costs about the same as an ES44DC. About $1,000,000 less than an ES44AC. That is a chunk of change per locomotive. I have read that BNSF engineers say that they are slippery, but he did not state in what service the locomotive was being used.
Don’t the ES44C4 units have, for lack of a better term, ”levers” that are supposed to raise the idler axles slightly so that more weight is on the powered axles to improve tractive effort when needed?
They are not leavers, but pistons. If you look closely at a picture of one of the trucks, you will see cylinder on the outside of the center axle. That is the piston that raises and lowers the center wheels a little.
It’s not a million dollars difference these days between a C4 (Or ES44DC when they were still making them) and an ES44AC. Back around 1995, Trains would regularly quote a half million dollars difference between something like the SD70M and SD70MAC (Or Dash 9 and AC4400CW).
And that gap has closed a lot which is one reason why most of the Class 1’s just buy all six motor AC road power these days (And why a C4 with just two less AC traction motors and two less inverters can be sold for approximetely what an ES44DC would cost).
I bet we’re talking about ~$250,000 or less these days for what BNSF saves by buying a C4 instead of a six motor unit.
I wonder how long that system will continue to function. And when it fails…?
“Tractive effort when needed”–when is tractive effort NOT needed? Granted, not as much is needed at higher speeds, but those higher speeds still need to be maintained.
Having locomotives with unpowered axles seems akin to having a “B” unit–sure, it saved a few bucks at purchase, but the operational limitations soon doomed them to obscurity. And BNSF hardly needed to penny-pinch.
Somebody correct me if I’m wrong but I remember reading years ago that in a fast freight/intermodal context, a 4 motor locomotive will go faster than a 6 motor of the same horsepower since more current is fed to each motor so that it can rotate at a higher speed.
People have been known to say that, but no reason to believe it. If cutting out two traction motors gave a C-C more tractive effort at 70 mph, it would be easy enough to include that feature on the engine. Then if you wanted even more TE, you could cut out two more motors. Think a 1A1-1A1 would be a champ at 70 mph?
Voltage, not current, determines the rotational speed of one of these motors. Think about why transition is used. And about why field-weakening to reduce back EMF (which is voltage) works to increase speed.
Now, the losses in six motors will be higher than in four in most cases, and of course the capital costs of the added motors, cases and gears are non-trivial, so if you have relatively high horsepower and can control slip adequately there are cost advantages to 4 over 6 since the limiting factor over most required road operation is imposed by fixed horsepower. Fuel and lower emissions, as said, make longer chassis with weight distribution essential… the question then becoming why we don’t see intermediate “BoCo” designs for that sort of reason, following the same logic as the FL9s and 5-axle FMs when weight went up and carrier idler axle was added.
A big piece of this is improvements in C truck design, notably the HTCR radial and the ‘rollerblade’ wing-box primary suspension on GEs, in a world where really good track at only moderate speed has become a ‘norm’. It might be interesting to see how effective the Alco Hi-Ad C truck would be once no longer subject to dreadful harmonic-rock effects… but we have better ones cheaper now.
They want that jumbo sized fuel tank you get with a 6 axle frame! If you could jam all the equipment, crash-worthy cab, emissions controls/cooling and that jumbo fuel tank into a four axle package with the same axle weight and without making the locomototive out something other than cheap, easy to deal with steel, the RRs would be on it.
The real difference between a six motor six axle and a four motor six axle is between roughly 12 and 18 mph. If, at full HP, you aren’t going to drop below 18 mph, you don’t need those other two motors for anything. Might as well not have them along for the trip.
Again, no, you’re not right. Let me try plainer or better English.
“Current” is not the same thing as horsepower; it’s just a measure of the number of electrons being moved. It is measured in amps, and a take-home message most places in power engineering is that you want to minimize this in conductors because its square shows up in heat losses.
Voltage equates to pressure, and for reasons I’ll let Erik express in English, it determines how fast a typical DC traction motor will turn … which in turn determines the thing you were trying to ask, how fast the motors and gearing will move the locomotive.
Grossly simplifying for a moment, power in watts (which is how much ‘twist’ the motor exerts on a load) is amps x volts, and dividing kilowatts by ¾ roughly gives you horsepower. Now in the sense that more torque will overcome more resistance, more current will give you more power, and if there are fewer losses in 4 motors vs. 6, there will be more power available for torque, hence more acceleration and perhaps a higher balancing speed shy of the voltage-determined speed. But the actual power the locomotive as a whole exerts is already limited by the effective prime-mover power.
Note that if you had said ‘makes the locomotive move faster’ you’d have been correct, and perhaps that is what you meant. But this is not because current makes the motors ‘turn faster’ but because it makes them turn ‘stronger’.
To put it simply, for DC current times voltage equals power in the same way that torque times rotation speed equals power.
In a DC electric motor the torque varies with the product of armature current and field, while the voltage across the terminals (often referred to as back EMF (ElectroMotive Force) is proportional to the field time the rotational speed. In a typical DC series traction motor, the field is proportional to armature current at low current and approaches a limit at high current due to saturation of the magnet pathways of the motor (it is possible to use more iron in the motor frame to avoid saturation, but the motor would be too heavy to use). The onset of saturation is usually about the continuous current rating of the motor.
A simple rule to keep in mind is that for a given torque (drawbar pull), the terminal voltage of a motor will be proportional to the speed, while current remains roughly constant. Another thing to keep in mind is that at high speeds, the field needs to be weakened to keep the commutator from flashing.
Back in the days of DC traction generators, there were limits on how much voltage could be generated before the commutator started flashing (really bad news) and how much current could be generated before the windings overheated. Similar issues exist with the motors, where too much voltage will cause the commutator to flash. For low speeds and high tractive effort where the motors are drawing a lot of current, the solution is to connect the motors in series and for a given traction generator current, 6 motors in series will give 50% more drawbar pull than 4 in series, albeit at 2/3rds the speed. Conversely at high speeds, the motors are connected in parallel to limit the voltage required from the traction generator. (This is what transition was all about).
An additional aside about traction generator (and traction alternators for DC motors). The windings for these beasts were set up so that voltage would decrease with increasing voltage in a w