Correct fuel oil viscosity is essential for proper atomisation of the fuel in the fuel injector. Efficient atomisation is necessary for the proper mixing of fuel with the heated air, which is required for reliable ignition and combustion.

If the fuel viscosity is too high, the fuel will not atomise correctly. This results in poor fuel-air mixing and can lead to incomplete or inefficient combustion.

Therefore, the fuel oil viscosity must be maintained within the specified range to ensure proper atomisation and efficient combustion.

1. Capillary tube method

A viscosity regulator consists of a small gear pump, rotating at a constant low speed of approximately 40 rpm. The pump supplies a regulated flow of fuel oil through a specially designed capillary tube.

As the fuel flows through the capillary tube, a pressure difference is produced between the inlet and outlet. This pressure drop is related to the viscosity of the oil flowing through the tube.

The pressure difference is measured by a differential pressure (DP) cell. The DP cell sends a signal to the controller, which controls the fuel oil heater. By varying the oil temperature, the heater changes and maintains the fuel oil viscosity at the required value.

2. Rotational / electronic resonance-type viscometer

A rotational viscometer operates on the principle that the torque required to rotate an object in a fluid is a function of the fluid's viscosity.

A disc, bob or similar sensing element is rotated in the fuel oil at a known speed. The torque required to maintain the rotation is measured, and the viscosity is calculated from the measured speed and torque.

Modern engine rooms may use electronic or resonance-based sensors, such as torsional vibration or oscillating-rod types, instead of older capillary systems.

• Working principle: A small sensing element, such as a rod, pendulum or cylinder, is immersed directly in the flowing fuel oil and made to undergo torsional or rotational vibration.
• Process: The surrounding fuel oil dampens the vibration of the sensing element. The amount of damping depends directly on the dynamic viscosity of the heavy fuel oil.
• Control signal: Electronic circuitry converts the damping effect into an instantaneous digital or analogue signal, typically 4–20 mA, which can be sent to the control system for precise, real-time automatic temperature control. This system does not require a mechanical gear pump or capillary tube.

A P + I (Proportional + Integral) controller would be suitable.

The proportional action provides a control response according to the difference between the measured and desired viscosity, while the integral action eliminates the offset that would remain with proportional-only control.

The controller therefore keeps the fuel oil temperature, and consequently its viscosity, within close limits.

A more complex control system would not normally be necessary because the fuel oil viscosity system has a relatively slow response time.