Knowing how your engine performs in terms of fuel efficiency over its operating cycle is key for its optimization. A convenient way of doing it is to plot the engine operating points (BMEP/torque and engine speed) on the BSFC (Brake Specific Fuel Consumption) map.
Thus, you can see if your engine is operating in its most efficient range or if you should change something to the powertrain or the control system.
The video below shows how to quickly build such a plot in Simcenter Amesim:
Hope this helps!