Interface - The Journal of Wheel/Rail Interaction

CURVING RESISTANCE

Assessing the Effects of Coupler Force and Train Speed on Freight Car Curving Resistance (continued)

Additionally, individual wheel L/V ratios were calculated. This was not to help determine potential curving resistance reduction, but to insure that any simulated scenarios did not have a high risk of derailment. The highest predicted L/V ratio was 0.60 for both the loaded and empty cars on a 10-degree curve at the maximum buff force. Although 0.60 is elevated for simulated “perfect” track, it did not pose a major derailment risk.

Simulation Results

To help manage the immense amount of data that the simulations produced, the maximum, minimum, and nominal contact patch energy and lateral wheel force values were evaluated for each curvature. “Nominal” values indicate instances in which the vehicles were simulated with zero coupler force while traveling at balance curve speed. “Extreme” and “nominal” differences were calculated to evaluate the potential reduction(s) in curving resistance. The “nominal” difference was the difference between the nominal and minimum values. Figure 4 depicts the tabular results.

Results indicated that the energy loss at the contact patch and reductions in lateral wheel loads were similarly proportional. Additionally, there was no consistent difference between new and moderately worn wheel/rail profiles. Overall, the results indicated a best-case potential reduction in curving resistance of 30% - 40%; the more realistic “nominal” potential reduction would be 15% - 20%.

Contour plots of contact-patch energy at various coupler forces and train speeds are shown in Figure 5. (Note that blue indicates low curving resistance and red indicates high resistance. Also note that the contour plots do not have the same legend scale because of the need to maintain descriptive contour plots.) Generally, the highest curving resistance occurs at high draft coupler force and low train speeds. The lowest curving resistance generally occurs in buff coupler force and high train speeds.

These results beg the question: Why do buff force and overbalance speeds promote lowered curving resistance? Although further analysis is required to fully understand this phenomenon, the following represents an immediate explanation. The wheelsets on a perfectly curving railcar would have zero angle of attack for each wheelset, with each wheelset laterally shifted to obtain the correct rolling radii (also known as coning). To curve correctly, a wheelset must shift laterally toward the outside of the curve to obtain the correct coning action. However, draft forces and underbalance speeds would inhibit this, as they would tend to pull the wheelsets toward the inside of the curve. Buff forces and overbalance speeds would tend to shift the wheelset laterally, in the correct direction.

Overall Possible Savings

Computer simulations indicated a best-case possible reduction in curving resistance of 30% - 40%, and realistic possible reduction of 15% - 20%. However, as previously mentioned, curving resistance is just one of many factors that figure into total train resistance. To help understand the bigger picture, a full-train longitudinal simulation was analyzed to help determine the effect of curving resistance on an actual train and an actual route. TOS™ software was implemented for the study. The route was approximately 200 miles with maximum grades of +/- 1.2% and 8-degree maximum curvature. Target speeds of 30, 40, and 50 mph were applied; throttle and braking commands were determined accordingly. The main focus of the simulation was to model with and without the curves, then to compare the difference in fuel consumption. Plots of the in-train force simulations’ input and output values can be seen in Figure 6. Results indicated that removing all curves from the actual route would reduce fuel consumption by 5.1% - 8.4%.

Combining the results of the TOS™ and VAMPIRE™ simulations indicates that a best-case possible reduction of total train resistance would be 3.3%; a more realistic possible reduction of total train resistance would be 0.9%. Results also indicated that reduced curving resistance generally occurred with buff coupler forces and higher speeds.

Fuel Savings at What Cost?
Results of the study indicated that there is a potential fuel savings from modifying in-train forces and train speed. They were minimal, however: 3.3% in the best case; 0.9% in the more realistic case. In order to obtain these reductions, increases in speed, which may require additional locomotives (especially on uphill grades) are necessary. Additionally, increased speeds create increased aerodynamic resistance. Theses factors may negate any reductions in fuel consumption.

Overall, the results of this study indicate that modifying operational practices to obtain an ideal coupler force and train speed in order to reduce curving resistance may not be effective. Further work is needed to fully understand how creepage and creep forces are influenced by coupler force and train speed. Currently, there is only speculation about why buff force and overbalance speeds promote decreased curving resistance.

Another major point is that only one-point wheel/rail contact geometries were considered in this study. Two-point wheel/rail contact should also be investigated, as it would most likely yield different results. Additionally, the worn rail profiles used in the study were generic in nature. Actual rail profiles from 2-, 4-, 6-, 8-, and 10-degree curves should be obtained and simulated to better replicate real-world conditions.

Additional curving-resistance simulation work is needed to understand the effects of various vertical and lateral wheel loads, and track conditions on energy loss. Finally, the industry needs the ability to import curving resistance results directly into track/train simulation software such as TOS and TOES in order to determine curving resistance on specific routes, train make-up, locomotive configuration, and train handling scenarios.

Matthew Dick is Assistant Vice President - Engineering and Dynamic Studies; Gary Wolf is President; Jack Chislett is Director - Engineering Application of Rail Sciences, Inc.

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