Diesel locomotive special Electric diesel locomotives
These special locomotives can operate as an electric locomotive or as a Diesel locomotive. The Long island and Metro-North Railroad operate dual-mode diesel-electric/third-rail locomotives between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels.
For the same reason, Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives.
This allowed railway yards to remain un-electrified, as the third rail power system is extremely hazardous in a yard area.
Diesel locomotive hydraulics
The diesel locomotive hydraulics use a torque converter or hydraulic drive system to convey the power from the diesel engine to the wheels.
Hydrokinetic transmission also called hydrodynamic transmission uses a torque converter. A torque converter consists of three main parts, two of which rotate, and one that has a lock preventing backwards rotation and adding output torque by redirecting the oil flow at low output RPM.
All three main parts are sealed in an oil-filled housing. To match engine speed to load speed over the entire speed range of the diesel locomotive, some additional method is required to give sufficient range.
One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission on a car.
Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others.
The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power.
Diesel-hydraulic multiple units, a less arduous duty, often use a simplification of this system, with a torque converter for the lower speed ranges and a fluid coupling for the high speed range.
A fluid coupling is similar to a torque converter but it lacks the stator. The output torque is equal to the input torque regardless of the ratio of input to output speed; loading the output shaft results not in torque multiplication and constant power, but in reduction of the input speed with consequent lower power throughput.
In car terms, the fluid coupling provides top gear and the torque converter provides all the lower gears. The result is that the power available at the rail is reduced when operating in the lower speed part of the fluid coupling range, but the less arduous duty of a passenger multiple units compared to the locomotive which makes this an acceptable trade-off for reduced mechanical complexity.
The locomotives hydraulics are slightly more efficient than diesel-electrics, but initial versions were found in many countries to be mechanically more complicated and more likely to break down.
Hydraulic transmission for diesel engines was developed in Germany. The bad reputation of diesel-hydraulic principle was caused by the poor durability and reliability of the Maybach Mekydro hydraulic transmission. The Mekydro consisted of a hydraulic torque converter followed by a four speed automatic mechanical gearbox.
Voith developed a different solution using several torque converters, and it has proven to be extremely durable and very well-suited for the purpose.
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