SBB-CFF-FFS Am 4/6 1101
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1942 publicity photo of Am 4/6 number 1101
The Am 4/6 1101 was a gas turbine-electric locomotive (one of the first of its kind) of the Swiss Federal Railways (SBB-CFF-FFS). It was built by Brown, Boveri & Cie (BBC) in 1938 and delivered to SBB-CFF-FFS for experimental services on non-electrified lines.
BBC first built a gas turbine with generator for the production of electric energy in 1938. An emergency power supply for the city Neuchâtel, providing 4000 kW of electricity, was also built based on the same technology. Thus BBC evaluated whether it would make sense to use a gas turbine to power a locomotive and filed a proposal to the Swiss Federal Railways for a gas turbine-electric locomotive with a power of 1,620 kW (2,170 hp)
The SBB-CFF-FFS were ready to accept the proposal under some conditions and thus offered the possibility to try applying the gas turbine technology to railways. The concept of a six-axle locomotive did not permit the application of more than 1620 kW of traction power. Top speed was set to 110 km/h (68 mph). The weight (including fuel) must have not exceeded 92 tonnes, otherwise the SBB-CFF-FFS would have denied even trial runs. On the other hand, the SBB-CFF-FFS were obligated to take the locomotive into service upon its completion.
The project was led and financed by BBC, while the Swiss Locomotive and Machine Works (SLM) built the mechanical parts.
The locomotive was based on existing technology wherever possible to avoid failures of components not directly associated with the gas turbine from endangering the project. The electric power transmission was chosen because it showed its reliability in conjunction with conventional diesel engines and because it allowed as many axles as desired to be driven, which was an important aspect because the weight per power output was much lower compared to diesel and steam engines of the time. A possible alternative would have been a hydraulic transmission, but this technology was not yet considered ready for powers above about 300 kW (400 hp).
The turbine consisted of an air compressor, a combustion chamber and the turbine itself. The air compressor needed about 4,500 kW (6,000 hp) to push the air into the combustion chamber (air pressures of 1 to 3 kgf/cm2 (0.098 to 0.294 MPa; 14 to 43 psi), depending on the turbine rotation speed), where the fuel was injected and burnt, leading to an expansion of the gases, which, with a temperature of 500 °C (932 °F) to 600 °C (1,112 °F), hit the turbine and produced about 6,000 kW (8,000 hp). The exhaust gases flowed through the heat exchanger, where they preheated the incoming air, and were ejected via the roof. The remaining power of about 1,500 kW (2,000 hp) were used to drive the locomotive.
Measurements showed the efficiency of the turbine to rise steadily from idle (0% efficiency) to medium load (15% at 1,000 hp or 746 kW), reaching its top at high load (18% at 1,700 hp or 1,268 kW) and going down again towards maximum power output (16% at 2,200 hp or 1,641 kW); note that these numbers do not include losses of the electrical power transmission. This is low compared to conventional diesel engines and is one of the main reasons that prevented the wide adoption of this technology.
Starting the locomotive
First, an auxiliary diesel engine was started by the help of batteries. This engine was coupled to a generator which, in turn, provided electricity to start the turbine. The turbine was brought up to speed by the attached generator, which was now used as a motor. This process took about 4 minutes; then the turbine could be ignited and would then run by itself. While the turbine continued to speed up, the electricity produced by the auxiliary diesel engine could now be used to shunt the locomotive in front of its train at low speeds (10 km/h or 6 mph). After another four minutes, the turbine had reached its idle speed (100 rpm at the generator). The locomotive was now operational.
Increasing the power output
To increase the power output, the engineer turned his power controller, which had the following effects:
- More fuel was injected into the combustion chamber
- The speed governor was adjusted to achieve a higher rotation speed
- The overload protector noticed an overload situation (rotation speed lower than the target speed) and lowered (!) the load on the turbine
Because of the lower load and the more fuel being injected, the rotation speed increased (up to 300 rpm at the generator under full load) and at some point the turbine reached its target speed, where the load was increased again up to the desired level to reach a new equilibrium between the turbine's power output and the power needed by the traction motors.
To decrease the load, the same processes happened in the opposite direction.
To avoid using the air brakes during long descends (they wear out and tend to overheat), an alternative braking system is desirable. Since the compressor of the turbine needed up to 4,500 kW, it was planned to use it to convert electricity generated by the traction motors into heat, by shutting down the oil supply to the combustion chamber such that the turbine generated no more power. It is unclear whether the necessary installations were ever made.
If the engineer had increased the power output of the turbine too late (say within a grade instead before it), then the rotation speed of the turbine might not have risen fast enough, too much fuel might have burnt and the turbine might have overheated. A temperature too high was shown to the driver by a warning lamp; if he did not decrease the load, the fuel supply was cut after another rise of the temperature by 30 °C (54 °F).
It might also have happened that a broken cable or fuse suddenly cut off the load from the turbine, leading to a rapid rise of its rotation speed that could not be compensated by the speed governor. In this case a safety device would have decreased the air supply to the turbine, leading to a rise of temperature in the combustion chamber due to the lack of cold air which, in turn, would have led to the shutdown of the turbine due to over-temperature.
The combustion chamber was also monitored. If the temperature became too low (the oil did not burn any more), the controller would have tried to ignite the oil again and shut down the oil supply if this failed for 5 seconds.
The logic to control the turbine was implemented using oil. All inputs (speed governor, power controller, ...) were valves or pumps attached to an oil cycle and influenced the oil flow such that actuators (pistons) made the necessary regulations.
The locomotive was in use until 1958, when it was rebuilt for experiments with new electrical systems. It never made it into series production because of its relative inefficiency and, by 1958, the lack of non-electrified lines in the SBB-CFF-FFS network. The locomotive was tested in Germany as a potential replacement for the class 01 steam engines (which it outperformed especially on grades). Similar tests were made in France.
- "Schweizerische Bauzeitung, Band 119 (1942), Heft 20 (Teil 1)". retro.seals.ch.
- Schneeberger, Hans (1995). Die elektrischen und Dieseltriebfahrzeuge der SBB, Band I: Baujahre 1904-1955 (in German). Luzern: Minirex AG. ISBN 3-907014-07-3.[page needed]
- Schweizerische Bauzeitung (in German): 229–233. May 16, 1942. Missing or empty
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- This article incorporates text from the equivalent German Wikipedia article
- Jeanmaire, Claude. Die elektrischen und Diesel-Triebfahrzeuge schweizerischer Eisenbahnen, Die Lokomotiven der Schweizerischen Bundesbahnen (SBB) (in German).
- Description by Thomas Brian (German)