Open Rails Steam Parameters for Advanced Modellers

Aim - this section describes the "advanced" settings for steam related parameters in Open Rails (OR) ENG files. Example settings are based upon NSWGR standards.

NB: These parameters are intended for advanced modellers who have an understanding of the information required, and have access to appropriate information.

If you wish to provide any feedback on this page, please use the contact page. It would br great to have some feedback as this helps to ensure the accuracy of the information and models.

Index

Open Rails Implementation of Steam Cylinder Indicator Diagram

Boiler Evaporation

Cylinder Port Opening Size

Cylinder Exhaust Opening Point

Boiler Efficiency

Fuel Combustion

Initial Pressure Drop

Cylinder Back Pressure


Open Rails Implementation of Steam Cylinder Indicator Diagram

Open Rails has been uses a "typical" steam indicator diagram approach as described above to caculate the M.E.P.. Once calculated the M.E.P. is used for calculation of the tractive effort, and IHP. The M.E.P. is also instrumental in calculating the steam used during each cylinder cycle, and as a result the total steam (and by association the amount of coal) used by the locomotive.

The base model reasonably closely approximates actual test figures recorded during tests of some representative locomotives by various railway companies. For example, the test results on page 4 of the summarised E2A Atlantic test report for the Atlantic locomotive show the pressure values at various points on the indicator cycle, at different speeds and reverser settings. Whilst the exhaust port settings for these tests are somewhat different from those used inthe Open Rails model, they are still within a reasonable degree of tolerance.

Some adjustment parameters have been added to Open Rails to allow experienced modellers to adjust settings to more closely align a locomotive's performance with known test results. it should be noted that these parameters are not required by default, and should only be added where specific known outcomes are sought. The various "advanced" settings are described below.

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Boiler Evaporation

The amount of steam that a locomotive could produce on a consistent basis every hour is a critical value. The steam generation rate needs to be disigned to be greater then the usage rate of the locomotive, within practical bounds. If the steam generation rate is exceeded by the usage rate for long periods of time, then the boilker pressure would start dropping, until eventually the locomoive would be "powerless".

Typically the amount of steam that could be produced would be determined by a number of factors, including the type of fuel used in the locomotive, its grate area and draught through the firebox (determined the amount of coal that could be realistically burnt). The evaporation rate of a boiler could only be accurately determined by an appropriate steaming test. Evaporation rates were either measured in terms of the amount of steam actually produced, or when it was desired to compare performance of different locomotives "Equivalent Evaporation" was used as the measure. See the section below on Fuel Combustion for more detailed description.

From various tests, it was determined that the normal approximation for the amount of steam generated was in the order of 10-15 lbs per sq ft of grate area.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, and the defaults should suit most content modellers.

However, if a modeller wants to customise these values to specific values the following parameter statement should be added to the ENG file.

ORTSBoilerEvaporationRate ( x )

Where:
x = Evaporation factor in lbs/sq ft of grate area.

Any value between 10 and 15 may be entered for this parameter.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSBoilerEvaporationRate ( 15.0 )

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Cylinder Port Opening Size

As the speed of the engine increases, the cutoff pressure point decreases as the port opening on the cylinder is not sufficient for the increased steam to operate the increasing speed of the piston efficiently. To understand this better, the speed of the piston increases from zero at the dead point (start of stroke) to the middle of the stroke, while in the ordinary working cutoff notches, the port opening begins to reduce soon after passing the dead center, and closes when the piston is at its greatest speed. In general, the shorter the steam port, the greater will be the drop in cutoff pressure.

By studying the valve motions it appears that the length of steam port, in inches, varies from 0.05 to 0.12 of the area of the cylinder in square inches. In the case of the Allen or Wilson valves, which give a double opening, the effective port length is really twice the actual length, and it should be so considered when analyzing the cutoff pressure point on the steam line.

The ratio of cut-off pressure to initial cylinder pressure varies for various speeds of rotation and percentages of cut-off. As a general rule, lower cutoff pressures will occur at a given speed for early, or small cut-off values. By way of explanation, when the valve closes the port early in the stroke, the steam is much more wiredrawn, partly because the port is not fully opened at any time, and partly because the closing of the port begins almost as soon as the stroke has commenced and, in fact, closes very shortly after it has opened.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, and the defaults should suit most content modellers.

However, if a modeller wants to customise the size of the port opening to a different specific value, then the following parameter statement should be added to the ENG file.

ORTSCylinderPortOpening ( x )

Where:
x = Size of the port opening on the cylinder.

Any value between 0.05 and 0.12 may be entered for this parameter.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSCylinderPortOpening ( 0.085 )

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Cylinder Exhaust Opening Point

This is an input which can be inserted into the ENG file to indicate when the exhaust port will open. Typically this value should be adjusted with care as setting an incorrect value could "crash" Open Rails. The value should typically be greater then the maximum cutoff value defined in the cutoff controller. The x value is typically between 0.8 and 0.95, as shown in the example below.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, and the defaults should suit most content modellers.

However, if a modeller wants to customise the point at which the cylinder exhaust port opens to a different specific value, then the following parameter statement should be added to the ENG file.

ORTSCylinderExhaustOpen ( x )

Where:
x = Size of the port opening on the cylinder.

Any value between 0.8 and 0.95 may be entered for this parameter.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSCylinderExhaustOpen ( 0.9 )

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Boiler Efficiency

Boiler efficiency is a measure of the efficiency of the boiler. It represents the ratio of the heat absorbed by the steam generated, to the heat contained in the coal as it was fired. Typically it is depicted in graphical form as shown below, where x-axis is Dry coal fired per Square Foot of Grate per hour, and the y-axis is efficiency.

Boiler efficiency

On the above diagrams a number of boiler efficiency lines for different locomotive types have been ploted for the sake of comparison. The curves shown represent the actual curves of some of the test locomotives available from the test rolling stock page. The default curve used by Open Rails is shown in grey.

An example graph plot of Boiler Efficiency vs Dry Coal Fired per Square Foot of Grate per Hour can be found in this publication, see Fig 32 on pg 47.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, and the defaults should suit most content modellers.

However, if a modeller wants to customise these values to specific values the following parameter statement should be added to the ENG file.

ORTSBoilerEfficiency( x1 y1 x2 y2 .................. xn yn)

Where:
x = Dry Coal Fired per Square Foot of Grate per Hour in lbs/sq ft/hr
y = Boiler efficiency

Any number of value pairs can be used, as long as there are more than two pairs, and the info is always in pairs.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSBoiler Efficiency( 0.0 0.875   20.0 0.85   40.0 0.775   60.0 0.70   80.0 0.625   100.0 0.55   120.0 0.475   140.0 0.40   160.0 0.325 )

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Fuel Combustion

The amount of fuel used by a locomotive was an important element as it determined the economic efficiency of the locomotive. Therefore an important design consideration of a locomotive was the amount of coal used by the locomotive to generate its designed steam output. Typically the following type of graph plot would be developed to demonstrate this locomotive performance parameter.

Steam evaporation diagram

Often for the sake of comparison in test reports, only the red (or Equivalent evaporation From and at 212 deg F) curve was shown on the graph plot, and it may be necessary to do calculations to convert it. See the following section for more information.

An example graph plot of Equivalent Evaporation vs Dry Coal Fired can be found in this publication, see Fig 6 on pg 19.

Interpreting Steam Locomotive Boiler Test Reports

When a steam boiler heats up water, bubbles breaking through the water surface will pull tiny water droplets in with the steam. Unless a superheater is used, this will cause the steam supply to become partially wet (wet steam) from the added liquid.

Steam dryness is important because it has a direct effect on the total amount of transferable energy contained within the steam (usually just latent heat), which affects heating efficiency and quality.

For example, saturated steam (100% dry) contains 100% of the latent heat available at that pressure. Saturated water, which has no latent heat and therefore 0% dryness, will only contain sensible heat. Naturally superheated steam had more heat in it, and therefore wet steam was not as likely to be present in superheated locomotives. When wet steam was injected into the cylinders some of the steam would tend to condense due to coming into contact with the cooler walls of the cylinders.

To calculate the amount of "dry steam" used by the locomotive a correction factor was used to convert from "wet steam".

Steam locomotive boiler test reports were empirically generated, ie the locomotive was run under certain circumstances and key pieces of information were measured. Using these measured values other information was calculated. For example, the amount of steam generated was determined by measuring the amount of water delivered into the boiler. It was then assumed that all of this water was converted to steam. Thus a value for the amount of "wet steam" was able to be measured.

Dry Steam (lbs/hr) = Correction Factor x Wet Steam (lbs/hr)

Typically this correction factor might be around 0.99, thus a value of 1.0 could be used without significant loss of accuracy.

To enable easier comparisons of different locomotives, the graphs or tables often plotted "Equivalent evaporation From and at 212 deg F" against the amount of dry coal fired per hour. Equivalent evaporation From and at 212 deg F indicated the amount of water, usually in pounds per hour, evaporated from a temperature of 212 deg F (100 deg C) to saturated steam at the same temperature.

However the actual amount of steam evaporated would be dependent upon the actual temperatures occurring during the operation of the locomotive. Thus to convert the equivalent evaporation, a scaling factor called the Factor of Evaporation was used.

Thus

Steam evaporation = Factor of Evaporation x Equivalent evaporation From and at 212 deg F

In most instances it would appear that the factor of Evaporation is somewhere between 1.0 and 1.2, but can vary outside of this range.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, as shown in the comparison graph below. These defaults should suit most content modellers.

Combustion comparison diagram

However, if a modeller wants to customise these values to specific values the following parameter statement should be added to the ENG file.

ORTSBurnRate( x1 y1 x2 y2 ..................... xn yn)

Where:
x = Steam evaporation rate in lb/hr
y = Dry coal fired rate in lbs/hr

Any number of value pairs can be used, as long as there are more than two pairs, and the info is always in pairs.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSBurnRate( 0.0 10.0   9000.0 1000.0   14500.0 2000.0   21000.0 3000.0   26400.0 4000.0   30800.0 5000.0   34400.0 6000.0   37500.0 7000.0   40200.0 8000.0   42800.0 9000.0   45200.0 10000.0   47800.0 11000.0 )

Related Points

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Initial Pressure Drop

As described and shown in the indicator diagram, the steam will decrease in pressure as it is feed through the piping from the boiler, into the cylinder steam chest, and eventually into the cylinder itself. The pressure of the steam upon entery into the cylinder is called the "Initial Pressure". This loss of pressure can be attributed to frictional losses in the throttle valve, pipework between throttle and steam chest, leaks in steam chest, and losses due to the port openings into the cylinder. From empirical measurements it has been seen that the pressure drop increases as the speed of the locomotive increases.

The pressure drop between the working boiler pressure and the initial pressure is modelled within Open Rails by default, but can be changed if desired.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, and the defaults should suit most content modellers.

However, if a modeller wants to customise these values to specific values the following parameter statement can be added to the ENG file.

ORTSCylinderInitialPressureDrop( x1 y1 x2 y2 .............. xn yn)

Where:
x = Wheel revolutions per minute in rpm
y = Initial to boiler pressure drop factor

Any number of value pairs can be used, as long as there are a minimum of two pairs, and the info is always in pairs.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSCylinderInitialPressureDrop( 0.0 0.98   50.0 0.95   100.0 0.927   150.0 0.90   200.0 0.874   250.0 0.85   300.0 0.82   350.0 0.81 )

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Cylinder Back Pressure

As described and shown in the indicator diagram, as steam is exhausted from the cylinder, a certain amount remains, due to the construction of the cylinder passages and exhaust pipes, the speed of the engine, the elements of the valve gear and pressure of operation. The back pressure effectively reduces the amount of work done by the cylinder. The size of exhaust nozzle and the clearance of the valve probably control the back pressure more than any other factors. Locomotive designers ideally seek to reduce the back pressure in order to improve the work efficiency of the cylinder, but it needs to be understood that changing the back pressure will also effect the drafting capabilities of the locomotive.

Back pressure will naturally increase as the locomotive increases in speed and needs to exhaust more steam through the exhaust nozzle.

The change in back pressure as the locomotive speed increases is modelled within Open Rails by default, but can be changed if desired.

Open Rails Use

Open Rails uses a standard default set of values for all steam locomotives, and the defaults should suit most content modellers.

However, if a modeller wants to customise these values to specific values the following parameter statement can be added to the ENG file.

ORTSCylinderBackPressure( x1 y1 x2 y2 ................ xn yn)

Where:
x = Indicated Horsepower in hp
y = Back pressure in psi (atmospheric)

Any number of value pairs can be used, as long as there are minimu of two pairs, and the info is always in pairs.

As an example, the current Open Rails default values would look like the following statement, if entered through the ENG file:

ORTSCylinderBackPressure( 0.0 0.0   200.0 1.0   400.0 2.0   600.0 2.33   800.0 3.0   1000.0 5.3   1200.0 5.6   1400.0 8.0   1600.0 11.2   1800.0 14.25   2000.0 16.   2200.0 20.0   2400.0 24.0   2600.0 26.0   2800.0 28.0   3000.0 30.0 )

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