Equipment Acceleration.

[railfan365]

I shoould preface my question here with 2 bits of background:

1. I've seen assertions in other threads that an MU train will accelerate faster than a locomotive driven train - without explanation of exactly why.

2. I'm familiar with car wheels spinning when the driver applies too much power to quickly.

My question is: Do six axles locomotives accelerate faster than four axle models, and do MU trains accelerate better than locomotive hauled trains because the power is distributed to more axles, with less power to any given axle while the same or more total power is applied to make the train accelerate without wheel spinning?

Thanks for any answers on this.

[timz]

"do MU trains accelerate better than locomotive hauled trains because the power is distributed to more axles"

Correct. No mystery there.

A six-axle locomotive won't accelerate any faster than a four-axle (running by itself, that is, with no train) but if each of them is pulling the same train weight then as you'd expect the six-axle will accelerate away from a standstill a bit faster than the four-axle. Once speed reaches, say, 20 mph the train with the four-axle will accelerate slightly better since it's producing the same tractive effort and weighs a bit less.

This all assumes the six-axle weighs 50% more than the four-axle, which it usually would.

[Allen Hazen]

Railfan 365--
What Timz said.
Further details:
"Wheel slip" -- the railroad analogue of a car's wheels spinning uselessly when the driver applies too much power too fast (or is trying to start from an icy patch in the parking lot) is a very real concern in railroad operation: a major part of the technological advance in locomotive design since dieselization has been electronic (since about the early 1980s involving onboard microcomputers) improvements to the control system to keep the power on any axle from getting too great: on a modern locomotive, the motors driving different axles can have their power changed automatically to prevent slip (or end it once it starts). The tractive FORCE (power divided by speed) that can be given by a powered axle before the steel wheel "loses its grip" on the steel rail and starts to spin uselessly is a certain fraction of the weight(*) on that axle: with diesel locomotives before the late 1970s the "adhesion factor" assumed in assigning locomotives to trains was about 18%: on the hardest part of the run (say, climbing the longest steep grade on a freight train's route), at the speed the train would be going at that point, the power on the locomotive's driving axles should not convert to a force more than about 18% of the weight on that axle. (Under ideal conditions you could do better: the nominal tractive effort quoted for locomotives is often based on an assumption that they can pull 25% of their weight. In dispatching freight trains, however, you have to allow for the effects of rain and sleet, and fallen leaves on the track, all of which can make things slipperier.) With modern control systems it can be over 30%.

So. Depending on the weight each axle carries, it may very well be a good idea to distribute the power through a larger number of axles. Which is why, for example, in the 1960s, the New York Central ("the Water Level Route") bought high-power 4-axle locomotives (U30B, GP40, C430) even though railroads with main lines going over mountains had converted to six-axle power. And -- acceleration from frequent stops being an issue when the stations are only a few blocks apart -- the New York Subway system used multiple-unit cars and not locomotive-hauled trains!

(*) Weight is just the force with which an object is pulled down by gravity. Tractive force is the force with which the locomotive pulls the train. In the U.S., both are conventionally stated in pounds. Power amounts to force times speed: one horsepower is defined as 33,000 foot-pounds per minute: the power equivalent of a 33,000 pound force at a speed of one foot per minute. Now go play with a calculator-- a lot of things about railroad operations begin to make sense fairly quickly!

[Jtgshu]

I shoould preface my question here with 2 bits of background:

1. I've seen assertions in other threads that an MU train will accelerate faster than a locomotive driven train - without explanation of exactly why.

2. I'm familiar with car wheels spinning when the driver applies too much power to quickly.

My question is: Do six axles locomotives accelerate faster than four axle models, and do MU trains accelerate better than locomotive hauled trains because the power is distributed to more axles, with less power to any given axle while the same or more total power is applied to make the train accelerate without wheel spinning?

Thanks for any answers on this.

Interesting question and Allen and timz have given very good reasons why - im going to give you an actual example of why, and why sometimes that ISN'T the case and locos can be faster.

I run both MUs, diesel and electric locos in commuter service. MUs tend to be faster acceleration wise than a loco hauled set. But not always. Obviously wheel condition is a MAJOR player, bad rail (wet, greasy, oily, comtanimated, etc) will obviously have an effect on things, and in that instance, MUs will almost always be faster, simply because the first few sets of wheels will "clean" the railhead, and clear the way for other axles to gain better traction. A loco PULLING the train will have bad acceleration because the first few wheels to come in contact with the bad rail are the powered ones. A loco PUSHING will have pretty normal acceleration because the wheels of the coaches have cleaned the rail, and given a pretty clear path for the powered axles to gain traction, especially if sand was laid down from the cab car and crushed by the coaches, that really helps the loco with traction.

Now of course, mechanically, a train of MUs has a better chance of something being wrong with one of the pairs and hurting its acceleration. But USUALLY the worst that happens is the train has slower acceleration, but won't get stranded, unlike when the loco hauled train craps out, where there isn't usually another loco to "pick up the slack"

Basically, MUs will always be faster than diesel hauled loco sets. However, electric locomotives are given them a run for their money. Now, as a disclaimer, im comparing the newest electric locomotives with the latest technology (ALP46As) to 20 year old MUs (Arrow 3s) - yes, the cars are older than 20 years, and date from 1977-78, but they were TOTALLY rebuilt propulsion wise in 1992 time frame, and changed from DC to AC power, so propulsion wise they are 20 years old.

A married pair weighs about 290,000lbs (each car is about 145,000 lbs each IIRC) and has about 1200 HP. Thats a pretty good power to weight ratio, and they can accelerate about 2 to 2.25mph per second. Lets take an example of a 6 car MU train, with a 6 car ALP46A powered set. Lets just say 900,000 pounds of cars (300,000 times 3 pair for 6 cars) with 3600HP (1200HP x 3 pair = 3600HP) Pretty good numbers.

Now, lets do the '46A train - 7 Megawatts of power (ive seen lots of differnet numbers for horsepower equivalant, but about 7200HP seems to be a common one), with 6 coaches, which weigh lets say 130,000 lbs each. 780,000lbs. The loco weights 200,000lbs - so lets just make it 1,000,000 lbs for sake of argument.

The MU set has 3600HP to move 900,000, the loco hauled set has 7200HP to move 1,000,000lbs. The MU set has 18 powered axles (6 powered axles per pair with the Arrow 3s, 1 truck (2 axles) is not powered), the loco hauled set has 4 powered axles. Now, adhesion is going to be actually pretty close, as the '46As have incredible wheel slip control systems, and have to have the higher end of 30-35 percent adhesion, if not a lil more (I don't know the actual numbers, but im sure they could be found). The MUs are going to be slightly less, their wheelslip systems aren't as advanced but they have more wheels to make up for it.

So in this example, the numbers just work out, that the ALP46A loco hauled set is going to just as fast, if not faster than the MUs (arrow 3s) - and from my own personal experiences, I believe that to be the case. But it seems like 6 cars is the magical number - 8 car loco hauled sets are going to accelerate slower on the electric loco train, but are still pretty close, but MUs do win, and 10 and 12 car trains are obviously going to be WAY slower than the MUs. the acceloration of the MUs is pretty much the same no matter how many cars the train is, and that is a major advantage that they have over the locos. But there are times when loco hauled sets can and do have an advantage.

edit - one other thing..... wheelslip control systems are great, but a good engineer works with the wheelslip control system to get the best possible acceleration - Some eng just go straight to full power and let the WSS take over. Thats silly, and tehy accelerate slower and rougher than one who has a feel for what the loco (and MUs too) is doing and preventing the wheelslip system from totally taking over. The trick is to find that point of absolute limit of adhesion on your own, and the WSS will let you get a few more percentage points on top of that by controlling the wheelslip and power. A good engineer can feel it before the WSS even kicks in. If you can feel it kick in, you went too far. If you can SEE it kick in in the ammeters, but not feel it, you are at that absolute limit of adhesion and are getting the maximum acceloration and adhesion without the WSS having to kick in any more and reducing the power. If in the loco, you can hear it too, you can hear the wheel clawing at the rail, and then you hear and feel the slip.

[ex Budd man]

Another factor to consider is gear ratio. The higher gear ratio allows for faster acceleration but at a cost of lower top speed and vise versa. With everything there is compromise. I understand some European engines have two speed gear trains allowing for more flexability but I'm not sure if they can be shifted on the fly or they require a trip to the shop.
As was stated earlier a fixed consist loco hauled train will have a fixed rate of acceleration. Add additional power for higher rate of acceleration, MU trains will remain constant; more cars=more power+more weight. Plus acceleration rates are determined by electronics to lessen wheel slip and rough rideing (for the passengers).

[DutchRailnut]

Those European locomotives either have Hydraulic transmissions kind of like RDC or have AC propulsion with Alternator transition, the traction motors have one gear only

[mtuandrew]

There were some French locomotives that did have dual gear ratios (and a single motor per truck), but I don't know if they're in service anymore. That, and they couldn't be shifted on the fly - perhaps by the engineer, but only while stopped.