The speed drops when the engine is strained under load — how do I prevent this?
Speed falling under load is called droop and is normal with a mechanical governor. A kit driving throttle position does not measure speed, so cannot remove it.
The speed falling somewhat when a load comes on is called droop and it is normal on engines with a mechanical governor. A kit that drives throttle position does not remove it, because it does not measure how many revolutions the engine is turning at. To bring droop close to zero you need feedback that reads the speed, that is, a governor working isochronously.
Normal droop, or a fault?
| Symptom | Likely cause | What to do |
|---|---|---|
| The speed falls somewhat when a load comes on and settles there | Normal droop | Do nothing; choose the operating point accordingly |
| The speed falls far more than expected | Blocked fuel filter, air being drawn in, an injector problem | Have the fuel path serviced |
| The speed falls and then swings up and down | Governor setting or slack in the linkage | Have the governor setting checked |
| The engine cannot take the load at all | An oversized load | Reduce the load or increase the engine |
A mechanical governor works on a balance of spring and weights and cannot produce zero droop. That is not a fault; on generators running in parallel it is this very behaviour that shares the load out. When you assess the complaint your question should not be "did the speed drop" but "did it drop more than expected, and does it stay there".
Why the kit does not close droop off
The GS740 and the GS740-3P are not governors; they drive the position of the throttle lever, and the mechanical governor stays on the engine. If the load rises while the position is fixed, the speed falls all the same. That is the basic difference between open-loop throttle control and closed-loop speed control: in open loop the controller does not measure the result, it only sets the input.
You see the difference in the field like this: put the potentiometer at one position, note the speed with no load, then bring the load on. Although the potentiometer has not moved at all, the speed falls. The kit is carrying out the instruction exactly; the instruction is not "turn at this speed" but "hold the lever at this angle". Setting the expectation on that basis stops the kit being blamed unfairly.
What can be done in practice
- Eliminate the fuel path first: filter, air being drawn in, injectors. Struggling with settings before these are put right is a waste of time.
- Size the load correctly against the capacity of the engine; do not run continuously very close to full load.
- Move the operating point up a little, so that after the drop when the load comes on the speed stays within an acceptable range.
- If the speed really has to stay constant, consider the optional active feedback speed holding module for the GS740, that is, the sensor that reads engine speed. The control then comes closer to isochronous behaviour.
On a generator the margin is narrower
On a generator, speed means frequency directly, so the acceptable droop range is narrower. Whether throttle position control is enough for frequency stability is a separate question and the answer depends on the load: on a plant with a steady load, position control can do the job, and on a plant whose load changes often and sharply, feedback is needed.
Measure before you decide
Read the unloaded and the loaded speed with a tachometer and write down the difference between them. Repeat the measurement after the engine has reached working temperature, with the real load and at the same throttle position; a single reading is not enough to decide on. If the difference is within the range the engine manufacturer expects there is nothing to be done, so build the system accordingly. If it is outside that range the problem is not droop; it is in the fuel path or on the governor side, and it is not solved by changing the kit.
The full topic: Electronic speed and throttle control on diesel engines