Performance criteria

When I read reviews of the various HO engine models, invariably, the performance is generalized in some form or another. In some cases the min current and maximum draw bar pull force is noted, but not always. Recently, there have been two sets documented, a DC version and a DCC version. My response is compared to what?

I know that sounds a little callous. However, it seems like a general criteria does need to be used to define the performance relationships. The advantage of a criterion is many fold. Firstly, it will relate the units performance to a standard that reflects how the trains should work.
Additionally, the criteria can be used to understand the normal variation that will exist between units of the same model. These are mechanical devices and will have variation, like any other mechanical system.
If constructed properly, the criteria can also be used to indicate when a unit needs a tune-up and when the units motor is sick and needs to be replaced.
While any criteria developed will likely be used to judge between different manufacturers models, if constructed properly it will indicate primarily how the various models meet the mission of powering a train with the least impact to the layout power needs. The criteria will indicate when the model is a very good to excellent fit for the mission, a range where it is adequate, but could be better and where it is a poor fit or suspect.

Walters Amtrak GP9M

In construction a potential criteria, a list of critical measures is required.

1- the engine needs to be able pull a realistic length train up a rational grade.

Full sized trains are said to be able to pull 2 loaded cars for every drive axle. So for a B-B unit, it should be able to pull 8 cars.
The rub here is that it should be able to pull these cars up a typical modelers grade, say 2 1/2 percent. It should be able to do this with some margin. Typically when the performance indicates a maximum number of cars a unit can pull, the speed is slightly greater than 0. A ten percent margin will allow the train to run at a finite speed and handle additional resistance from curves or less than perfect track work.

So, a term of the criteria should be the excess draw bar force the unit produces over the force required for the above description. Because the typical unit will be a candidate for use in a DCC system, this maximum force should be defined at 12 volts or less, where ever it occurs.

2- the engine needs to operate smoothly at low-speed.

While some of this can be accomplished with a pulse wave power signature, the unit should be able to achieve this low initial velocity with a standard DC power signal. Introducing pulse from a power supply or decoder is actually also bringing those devices into the assessment. This actually masks the actual engine capabilities and brings in the varying performance of these other devices.

BLI UP Vista dome

3- the engine needs to operate at low electrical power.

Thus the VI at minimum sustained velocity is important. Because the minimum current draw is important in many situations, including DCC, the minimum current draw and voltage are defined separately.

This set of considerations has been used to define the initial performance criteria.
The equation is as follows:

PC1 = ( Fdb – Freq ) / ( V * I * vel )min

Where: Fdb is the maximum measured draw bar force in grams
Freq is the required draw bar force in grams
V is the minimum voltage
I is the minimum current in Amps
vel is the minimum velocity in scale miles per hour

While in this form it is just a number. An efficiency relationship would have velocity in the numerator. However, in this case, the excess force is modulated by excessive values in the other three parameters. While not perfect, it does point to some interesting perspectives.

A chart showing the results for the tests to date is shown below.


Engines with a value of 5 or less are considered suspect or sick. If the value is between 5 and forty, the engine is an adequate match. Over forty it is considered to be very goo to excellent.

A troublesome observation from these results is that some engines that show to be in the very good zone run excessively slow over the power range. These tests are run without any additional load. As indicated in the basic physics chart, shown below, at any voltage setting the velocity is the highest for the no external load condition.


While if more draw bar force can be achieved by adding engines, the engine speed will only be changed by the average of the velocity of the two engines with half the load. It will not exceed the no load condition.

Bachmann CN 0-6-0

A common complaint is that a given engine model runs very slowly. Again the question, relative to what? Full sized freight engines are said to be capable of 70 – 80 mph. Passenger engines were geared to run over 100 mph. Wether they actually ran those speeds was dependent on several things like track condition and traffic. Usually not limited by the capability of the engine.

It seems that the model should have a speed capacity to allow a reasonable speed at a power setting that is less than the maximum for the system. This is a situation where using a DCC system is going to require more capacity in the engine than using a DC system. The maximum voltage that the DCC systems will fed the motor is just under 12 volts. Where the standard power supply on a DC system will provide 16 volts. Because of this, a criteria that includes a representative speed at 12 volts is required. In this case a speed of 80 SMPH seems like a good choice. Again, one does not expect that the loaded train will operate at this speed, but the capacity is there to pull a train at an acceptable speed. It is easy to slow a train down by reducing the input power. It is impossible to speed it up when operating at the maximum power.

For this reason, the criteria should be penalized for running slow, but get no impact for operating faster than a reference. Initially a process to modify the PC1 defined above is proposed. A dividend in the form of:

Maximum( 1, ( Vel. ref – Vel ))

Where- Vel. ref = 80 SMPH
Vel is the measured velocity in SMPH at 12 volts

At first blush, this looks reasonable, but it may be a little severe. The dividend factors increase rapidly with every SMPH short fall. A reasonable technique would be to take the square root of this factor. Thus, the dividend is 2 when the difference is 4, 3 when the difference is 9, 4 when the difference is 16, etc. this may be to soft, but it is a reasonable place to start. The impact of this proposed factor compared with the PC1 is show in the following figure.

PC comparison

The grey bars are the first version of the criteria, and the green bars are the second criteria. A negative value in the first criteria is not reduced by the speed short fall. As shown in the figure, a number of engines performance assessment stay the same between the two criteria. Others would fall from the very good to excellent zone to very close to the suspect zone. This brings the train speed into the assessment.

Bachmann Thomas Arry

As indicated previously these are just numbers. They should be used only as a guide in the described above. When considering final selections, one should examine the values for each model. Understand how they pertain to the way you want your models to perform.

The desire to go faster than a reference at 12 volts, but very slow at low voltage is clearly contradictory. The desire for sound may out weigh these parameter arguments. These are reasons why the details need to be part of any decision process.

Leave a Reply