Diesel Hydraulic Locomotives

In Sri Lanka only few diesel hydraulic locomotives can be seen when compared to diesel electrics. They are Class W locomotives & Class S DMUs up to S8. Specially W class locomotives does an important role on up country tracks. The significant feature of diesel hydraulic locomotives is the high acceleration.

Torque is a major factor for diesel hydraulic transmission & when the torque increases the speed get decreased. The solution for this is using an engine with a higher RPM value. The RPM of the Sri Lankan diesel hydraulic locomotives are around 1500. In these method following are the main parts of the locomotive. Fuel tank, diesel engine, radiator, torque converter, gear box & final drive which is not included in the diagram. We’ll examine the procedure here.

Diesel from the fuel tank travels to the combustion chambers of the diesel engine & the drive shaft starts its rotation. There’s a connection between this shaft & the driven shaft through a special component called Torque Convertor. It’s a modified fluid coupling & the structure & function of it will be discussed later. The other end of the driven shaft is connected to a gear box. The throttle of a diesel locomotive has 8 notches. When the driver changes the notch, gearbox acts accordingly & rotates the wheels of the locomotive by a shaft. The dynamic brakes of these locomotives are known as Hydro Dynamic Brakes. Here a special chamber gets filled with oil which creates a resistive force for the rotation of the wheels.

Now let’s skip to the torque converter. As stated above torque converter is a modified fluid coupling. Therefore we should identify the features of fluid coupling. A fluid coupler consist of 2 main parts.

1. An Impeller or a Pump connected to the engine which rotates with the engine
2. A turbine connected to the gear box

These two parts consist of 2 rotors fixed very close to each other & the space between these fans are filled with a high dense oil. Therefore when the engine rotates impeller also get rotated. But there is a limit for power transmission by this method. Torque doesn’t increase above a certain value & you cannot transmit more power than that level. Therefore power output get decreased & the highest torque is equal to the input power.

Torque converter was introduced to fulfill these errors. A torque converter has a small part called Stator between turbine & pump (Impeller). It changes the direction of the oil which comes after hitting the turbine. The stator doesn’t rotate & it increases the torque. Therefore the power output is greater than the input. So when the engine rotates fly wheel, torque convertor housing & pump starts their rotation & when they rotate the oil inside the converter also gets rotated. As a result of it the turbine & the turbine output shaft start their rotation & power gets transmitted to the gear box. There are two torque converters in a normal diesel hydraulic locomotive.

The advantages of this locomotive type is the high acceleration, high power efficiency, less wheels slips & ability to run on flooded conditions. Disadvantages are high maintenance cost, lack of spare parts & high wear & tear.

Based on a Sinhala article by Subhash Dhanasekara.