Greetings. To my understanding, when 4-axle and 6-axle locomotives of similar HP are M.U.ed together, the 6-axle unit will operate at full horse at minimum speed causing the 4-axle unit to derate to protect itself. At the other end of the spectrum, when the 4-axle unit is operating at full horsepower in the higher speed range, the 6-axle locomotive (even with identical horsepower) becomes a liability at speed. I want to use a SD40-2 and GP40-2 because of identical HP and because data is readily available for these units.
Question 1: How is it determined that the horsepower for a GP40-2 (or any high powered 4-axle locomotive for that matter) begins to derate at a particular speed. I believe for the GP40-2 it is below 20 MPH? How is that calculated?
Question 2: Aside from fuel consumption, will a 6-axle unit such as the SD40-2 be that big of a hinderance in high speed operations if horsepower is available?
(By the way, both units are standard geared and ballasted locomotives).
Thank you for any input,
MLG4'8.5"
There are others here that are much better than me at this, but I'll give it a shot...
MLG4812Question 1: How is it determined that the horsepower for a GP40-2 (or any high powered 4-axle locomotive for that matter) begins to derate at a particular speed. I believe for the GP40-2 it is below 20 MPH? How is that calculated?
SD40-2s have a minimum continuous speed of about 9 MPH, and GP40-2s about 13. Minimum continuous speed is the slowest that a DC locomotive can operate at full power without suffering traction motor damage. Newer locomotives do have backup circuitry to prevent overheating traction motors, before this, engineers had to make sure the ammeter wasn't in the red zone for too long. Power is calculated in the amount of amps into the traction motors. I'm not sure how this was calculated originally.
MLG4812Question 2: Aside from fuel consumption, will a 6-axle unit such as the SD40-2 be that big of a hinderance in high speed operations if horsepower is available?
Above 12 MPH, the performance curves for SD40-2s and GP40-2s are the same. The extra traction motors allow for a slower minimum continuous speed, and greater tractive effort when starting. Otherwise, the other two motors are just extra maintenance and weight. It will still perform the same.
Each locomotive's tractive effort vs speed curve looks the same in terms of shape. Taking Northwest's figures for MCS as accurate, above 13 MPH each loco will pull equally hard at each throttle notch. In this range TE is a function of fixed horsepower divided by speed. In simple terms TE falls by half from 13 to 26 MPH and half again from 26 to 52.
Below the MCS the TE curve is flat at a value based on weight on powered axle divided by coefficient of friction which is something in the range of 20 to 25 % given DC motors and wheel slip control system of the era. For example assume both have axle loading of 65,000 pounds and say 23% adhesion. TE per axle will be 14,950, say 15,000#, at or below MCS.
The SD puts out 90,000# and the GP 60,000# at MCS. On any given grade tonnage rating is a function of TE at MCS. The GP cuts back engine horsepower, and thus amps to its traction motors. This protects the GP from overload and wheel slip and lets it put its full rating to the train AND enables the SD to put out its full rating.
If the GP is not derated in an MU consist with one or more SDs, then the entire consist must be operated at 13 MPH to protect the higher MCS units. That is why mountain railroads soon figured out that adding an F9 or 7 or GP 9 and 7 to an otherwise six axle consist in drag service was a very bad idea, since the tonnage rating of that combination was often less than the three SDs by themselves. Note that some carriers used F/GP 9/7 as a separate helper consist. Here all was well since the 4 axle units were controlled separately from the six axle units.
Using all SD meant a heavier loco consist for a given HP, which was desirable in drag service and if entire fleet was SD it saved dispatcher's and roundhouse foreman from a dose of higher mathematicis. If the railroad had a lot of hot traffic, and the hot trains were assigned sufficient HP to keep speed above MCS on the upgrade, then 4 axle power was fine. The implication of that policy was two road loco fleets. Most carriers decided that was more trouble than it was worth which is why the vast majority of road units today are 6 powered axles. Standardization won the day.
Mac
MLG4812when the 4-axle unit is operating at full horsepower in the higher speed range, the 6-axle locomotive (even with identical horsepower) becomes a liability at speed.
A 62:15 GP40-2 with the usual PF21 module is supposed to put out full power down to 23 mph. (No way to "calculate" that-- that's just the way EMD set up the control system.) At 12 mph the SD40-2 will be producing around 75000 lb TE and the GP will be around 50000 lb.
Why did EMD set up the GP40-2 that way? So it could MU with F7s and GP9s and have the same minimum continuous speed as the older units (about 11 mph).
"Above 12 MPH, the performance curves for SD40-2s and GP40-2s are the same."
Above 23 mph, he meant (with 62:15 gearing).
Yes, that's a typo. Thanks! You guys have provided a lot of great information.
These are all great answers guys for anyone who has never worked in the rail industry.
Timz wrote:
(Quote) A 62:15 GP40-2 with the usual PF21 module is supposed to put out full power down to 23 mph. (No way to "calculate" that-- that's just the way EMD set up the control system.) At 12 mph the SD40-2 will be producing around 75000 lb TE and the GP will be around 50000 lb.
Why did EMD set up the GP40-2 that way? So it could MU with F7s and GP9s and have the same minimum continuous speed as the older units (about 11 mph). (/Quote)
I guess that was the thing that was confusing me. Exactly what speed the higher horsepower geeps began their derating process down to minimum continuous TE. I came across the PF21 module but wasn't sure how in the world it properly operated. As long as I know what the beginning derating speed is, I can plot a rough traction effort graph/curve down from 22-23 mph to 11-12 mph lowering the horsepower increments at a time if it is a linear decrease. It is hard to believe that the GP40-2 is operating around 2000 HP at 11mph. That's a GP38 for goodness sake! I assume the GE units and ALCO's work in a similar fashion when derating their high HP units (???).
Thank You,
MLG4812It is hard to believe that the GP40-2 is operating around 2000 HP at 11mph.
The GP40-2 TE-vs-speed curve is about a straight line from 23 to 11.3 mph.
Think the RR could set a U30B however it wanted-- full power down to a min cont speed of 17ish mph, or derate like a GP40-2. But supposedly if you set your GE's to derate, they maintained constant power down to a chosen speed, then constant TE down to some lower chosen speed, then constant derated power below that speed. In other words their TE-vs-speed curve would have a notch in it, unlike the GP40-2.
Our community is FREE to join. To participate you must either login or register for an account.