I have many questions regarding the flow regulation of the engine pump; I’d like to ask those who are experts!

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My pump is currently driven by a four-stroke diesel engine. What I want to know now is what are the differences between using an engine as a power source and using a motor as a power source? For example, I can change the head and flow rate of the pump by adjusting the engine speed (assuming the engine is electronically controlled). Is there anything wrong with this statement? Hehe, the question is still quite abstract. Let’s put it this way: 1. Do pumps driven by engines have a smoother curve compared to those driven by motors? Because I can set the engine’s speed to a constant value; thus, it remains at the same speed whether there is no flow or at 100% of the rated flow. This is different from motors, where as the flow increases and the motor’s output power rises, its speed decreases. On the pump’s curve, it will appear to be more gentle. Is my understanding correct? 2. For the same pump, if different rated speeds are used to operate it from zero flow to a higher flow rate, then at lower speeds, regarding the length of the performance curve (flow rate versus head), does a higher speed result in a longer curve as well? There is another issue that has been bothering me. Previously, I used a mechanical throttle; in my opinion, it’s simply something used to adjust the engine speed. Used on diesel fire pumps, I noticed that when the flow rate increases (i.e., the pump’s output power rises), the engine’s speed also decreases. This principle seems similar to how a car’s throttle controls speed. However, in the case of an engine mounted on a generator, since its power generation frequency is fixed, it requires that the engine’s rotational speed remain constant regardless of the output power. In this case, the engine’s speed should not be controlled by the throttle; then, how is its speed controlled? There is another question: why does the engine speed increase when the throttle of a car’s engine is increased? An increase in the throttle only increases the amount of diesel injected into each cylinder; why then does the engine speed rise? Thank you!