Hello there. I've lurked about these forums for a few years, but have been inspired to register and contribute recently due to the technical content of a few different discussions. Please forgive and correct any newbie mistakes.
I'd like to offer some input on the MARC motor gearing vs. acceleration discussion.
It was stated in different variants that double-heading these engines doesn't have much of an payoff for acceleration:
I believe that the fundamental oversight here is the relationship of the locomotives to the entire train consist, and I'll try not to be too long winded.
Basically, adding a loco to a longish train effectively doubles the available power and torque while only adding a small percentage to the overall mass of the consist. So, there is a benefit in performance and acceleration, and it is not insignificant. The argument quoted above makes some sense if we are discussing a consist of locomotives only (train mass and power increasing at the same rate as units are added) but that is not the practical condition.
Detail:
The specific MARC gear ratio is relatively unusual in the industry, but the concept is quite common. I believe their ratio is 2.5 with a more common "standard" being around 3.5. The inherent limitations of a DC motor plus the desire to operate around 100 mph necessitate the change. Since the wheel rpm is proportional to the motor rpm * the inverse of the gear ratio, and a DC motor has a pretty touchy max rpm before it turns itself into a bird nest, there are only so many options to achieve the higher speed. We've heard above about one of the major trade-offs: low-end acceleration is affected. Another is that the infinite minimum speed is bumped up - no biggie for commuter service. However, as we approach the maximum rpm of the motor, the payoff begins as you gain around 30 mph+ at motor rpm over 2100 (max assumed around 25-2600).
Scenario:
Making some assumptions for simplicity and argument here, so please don't get too fussy with the digits.
Locomotive = 125 tons (pretty generous for our subject, but we must never ask a girl about her weight)
Trailing Coach = 125 also (probably not too far off for a bi-level around an AW2 load)
So, one loco and 6 coaches = 875 tons. The locomotive is about 14% of the train mass.
Two locos and 6 coaches = 1000 tons (WOOH! A KILOTON!). The locomotives are 25% of the train mass.
We have an 11% increase in train mass, but a 100% increase in available torque. Since torque and current are proportional on a DC motor, you have effectively decreased the time that your motor current will be pegging the ammeter. Once the initial moment of inertia is overcome, the voltage rises and motor rpm increases. The increase in acceleration isn't going to win any drag races, and we're throwing out some real-world considerations like adhesion, grade, various rolling stock resistances, but it is a real increase and I bet the calibrated posteriometers of the actual train crews on these forums would agree.
BTW - the real offender causing poor acceleration in these units...... is the slower engine ramp rate necessary to achieve the emission targets.
Have a good weekend, and you Easterners be safe in the "snowmageddon" or whatever it is called this year.
I'd like to offer some input on the MARC motor gearing vs. acceleration discussion.
It was stated in different variants that double-heading these engines doesn't have much of an payoff for acceleration:
It's not that generic loco can't accelerate faster MU'd. It's that MARC's specific roster of MP36's aren't geared to provide that capability. They can spend money to re-gear them...at a penalty of lower top speed...or they can buy a make that's a bit more flexible with fewer limitations. They're choosing to buy something a bit more flexible with fewer limitations.,etc.
I believe that the fundamental oversight here is the relationship of the locomotives to the entire train consist, and I'll try not to be too long winded.
Basically, adding a loco to a longish train effectively doubles the available power and torque while only adding a small percentage to the overall mass of the consist. So, there is a benefit in performance and acceleration, and it is not insignificant. The argument quoted above makes some sense if we are discussing a consist of locomotives only (train mass and power increasing at the same rate as units are added) but that is not the practical condition.
Detail:
The specific MARC gear ratio is relatively unusual in the industry, but the concept is quite common. I believe their ratio is 2.5 with a more common "standard" being around 3.5. The inherent limitations of a DC motor plus the desire to operate around 100 mph necessitate the change. Since the wheel rpm is proportional to the motor rpm * the inverse of the gear ratio, and a DC motor has a pretty touchy max rpm before it turns itself into a bird nest, there are only so many options to achieve the higher speed. We've heard above about one of the major trade-offs: low-end acceleration is affected. Another is that the infinite minimum speed is bumped up - no biggie for commuter service. However, as we approach the maximum rpm of the motor, the payoff begins as you gain around 30 mph+ at motor rpm over 2100 (max assumed around 25-2600).
Scenario:
Making some assumptions for simplicity and argument here, so please don't get too fussy with the digits.
Locomotive = 125 tons (pretty generous for our subject, but we must never ask a girl about her weight)
Trailing Coach = 125 also (probably not too far off for a bi-level around an AW2 load)
So, one loco and 6 coaches = 875 tons. The locomotive is about 14% of the train mass.
Two locos and 6 coaches = 1000 tons (WOOH! A KILOTON!). The locomotives are 25% of the train mass.
We have an 11% increase in train mass, but a 100% increase in available torque. Since torque and current are proportional on a DC motor, you have effectively decreased the time that your motor current will be pegging the ammeter. Once the initial moment of inertia is overcome, the voltage rises and motor rpm increases. The increase in acceleration isn't going to win any drag races, and we're throwing out some real-world considerations like adhesion, grade, various rolling stock resistances, but it is a real increase and I bet the calibrated posteriometers of the actual train crews on these forums would agree.
BTW - the real offender causing poor acceleration in these units...... is the slower engine ramp rate necessary to achieve the emission targets.
Have a good weekend, and you Easterners be safe in the "snowmageddon" or whatever it is called this year.
