Rope brake dynamometer is a device used to measure the power output or torque of an engine. It works on the principle of friction between a rope and a drum. When the engine is connected to the dynamometer, the rope is wound around the drum, and as the engine rotates the drum, friction is generated between the rope and the drum. This friction generates a braking force, which can be measured to determine the power or torque being produced by the engine.

- Friction between rope and drum:
The friction between the rope and the drum is the key element in the operation of a rope brake dynamometer. It is this friction that generates the braking force, which is then measured to determine the power output or torque of the engine. The friction is necessary to provide resistance and absorb the energy produced by the engine.

- No lubricant:
The correct answer to the question is option 'D' - no lubricant. A rope brake dynamometer does not require any lubricant between the rope and the drum. This is because the friction between the rope and the drum is essential for the operation of the dynamometer. Introducing a lubricant would reduce the friction and affect the accuracy of the measurements. The absence of lubricant ensures a direct and consistent frictional contact between the rope and the drum.

- Importance of friction:
Friction plays a crucial role in the functioning of a rope brake dynamometer. It converts the kinetic energy of the rotating drum into thermal energy, dissipating it as heat. The amount of friction generated is directly proportional to the torque or power being produced by the engine. By measuring this frictional force, the power output or torque of the engine can be determined.

- Accuracy and reliability:
By eliminating the use of lubricant, a rope brake dynamometer ensures accurate and reliable measurements. Lubricants can introduce variables such as viscosity and temperature effects, which can affect the consistency of the frictional contact. Without lubricant, the frictional force remains constant, providing more reliable and consistent measurements.

In conclusion, a rope brake dynamometer does not use any lubricant between the rope and the drum. The absence of lubrication ensures direct frictional contact, allowing accurate and reliable measurements of power output or torque.