In my work leading up to the Pennsylvania turbine locomotive project recently described on this website, an effort was undertaken to improve on blade design and to try out new manufacturing methods to increase efficiency and power. To this end, the existing wheel with blades having a 90° steam deflection was to be replaced by wheels with fewer blades featuring a deflection of 140°. Thus, the steam impulse to the wheel would be bigger! Efficiency therefore had to be higher - so I figured.

The new wheels should be able to drive a generator or an all-mechanical drive. So, they would have to have two rings of blades - an outer one for forward drive and an inner one for reverse. In case of a turbine-electric arrangement, reversal of direction would be done electrically. Then, the inner ring would not be used.

Machining an inner ring

The conventional way would make it necessary to assemble the wheel out of at least two pieces. Using the new 3-D technology the wheels could be printed!

With external help and SolidWorks program I managed to generate the necessary .step files.

Two Swiss companies undertook to produce the required pieces: Brogioli Casting Edelmetallguss AG: http://www.brogioli-casting.ch at Schaffhausen, with a printed wax model plus bronze casting and Ecoparts AG  (http://www.ecoparts.ch ) at Rüti, with a direct print in corrosion resistant steel.

The wheels were machined and shrouded for performance tests in the original turbine casing. The steam nozzle was a section of a 0.8 mm ID, 1.2 mm OD stainless steel syringe needle, directed sideways at a 15° angle to the wheel.

Three wheel candidates that were evaluated.

Produced on a milling machine. Material: bronze, diameter = 30mm + 2 x 0.3mm stainless steel shrouds, width 3mm, 48 blades, blade depth 1.2mm, steam deflection angle 90°. Balancing marks can be seen in the photograph.

Produced by 3-D printing a lost wax model and subsequent casting. Material: bronze, tensile strength 1000 N/mm2, diameter = 30mm + 2 x 0.3mm stainless shroud, width 4mm, 24 outer blades, 19 inner blades, blade depth 1.2mm, steam deflection angle 140 deg., balancing marks can be seen.

Produced by direct 3-D printing. Material: corrosion resistant steel (Corrax), tensile strength 1000 N/mm2, diameter = 30mm + 2 x 0.3mm stainless steel shroud, width 4mm, 24 outer blades, 19 inner blades, blade depth 1.2mm, steam deflection angle 140°. Balancing marks can be seen.

To generate the necessary steam for the tests, the boiler of the Mathematiker (an already existing live steam turbo-electric train) was commissioned. A three phase servomotor (Faulhaber, type 2036012B) served as a brushless generator and was screwed onto the turbine in test, containing Hall sensors to monitor the turbine rpm. Its output is rectified with a double rectifier bridge to get DC. A load resistor box and instrumentation for pressure, temperature and electrical readings were also connected.

Test results

To my dismay, all three wheels showed more or less the same maintainable maximum power output! At a boiler pressure of 4 Bar, electrical power generated was in the region of 6 Watt (22 VDC, 270 mA) at 37,000 rpm. With a generator/rectifier efficiency taken as 0.85, the power available at the turbine shaft for direct mechanical drives would then be 7 Watt. At a boiler pressure of 3 Bar, electrical power output dropped to 4.2 Watt (22 VDC, 190 mA) at 37,000 rpm with a corresponding mechanical shaft power output of 5 Watt.

Advantages / disadvantages of the three wheel types

Old wheel

Can be machined with a regular milling machine. Suitable for turbo-electric drive (unidirectional). To make the turbine bidirectional an inner row of primitive ‘blades’ could be added.

Cast wheel

Expensive solution - at present Fr. 440 (£350) / piece, may go down in the future. Requires 3D design capability (.step file). Easy machining. Blade surfaces fairly smooth.

Printed wheel

Moderately expensive - at present Fr. 140 (£110) / piece. More difficult to machine. Rough  blade surface due to printing process. Manual polishing deemed to be impractical and nearly impossible for the enclosed inner ring of blades. This fact does not reduce the power output, but causes the pressure inside the turbine to rise, resulting in increased steam leakage through the bearings.

Conclusion

The target of improved turbine efficiency clearly has not been reached - and I don't know why! (even steam should follow human logic). Should anybody have a reasonable explanation - I'm all ears. On the other hand, the power available from the turbine is sufficiently high to operate a gauge-one loco - provided the boiler of the model is capable of maintaining at least 3 Bar while discharging steam through a 0.8 mm diameter nozzle. With friction losses inside the loco kept to a minimum by the use of ball bearings throughout, a reasonable performance is possible.

My experience tells me that 40 to 50% of the turbine shaft power will be available as tractive effort at the locomotive coupling. With 3 Bar pressure in the boiler, this would make 2 Watt available. Independent of speed and track curvature, this is sufficient power to pull a train consisting of five coaches, requiring a pulling force of 1.5 Newton (approx. 150 g weight) or less, on level track.

From this value, the reduction gear ratio required for a mechanical driven model such as the LMS Turbomotive  can be calculated. At a target speed of 160 km/h the scale speed (1/32) would be 1.39 m/s. Driving wheels in 1:32 scale have a diameter of 62 mm. One turn will cover 0.195 m. To reach the required speed, the axles must turn at 428 rpm. The reduction gear ratio required then works out to 37,000 ÷ 428 = 86:1. For more information on this subject search in YouTube under Turbomotive, Jeggli .

The only way to increase the power output of my turbines is therefore to increase their power inputs. This means pointing two 0.8 mm dia. nozzles at the wheel, requiring a corresponding increase in steam generation. Another application could be their use as an auxiliary power turbo-generator unit on larger scale locomotives. There, insufficient boiler pressure should definitely not be a problem! I'd love to hear of any activity in these fields!

References

1-3. Jeggli, W. (2011). Dampfsprinter Gauge 1 Steam Turbine-Electric Locomotive.

part 1. Model Engineer  207 (4409): 153.

part 2. Model Engineer  207 (4410): 227

part 3. Model Engineer  207 (4412): 379

4. Jeggli, W. (2015) LMS  Turbomotive in gauge 1 . Model Engineer  215 (4514): 252.

5. Gauge 1 Model Railway Association NL&J 212, Winter 2006-7).