It seems that in low loads the engine efficiency is very low, while in heavier loads the engine efficiency or fuel efficiency/economy is much better. What are the reasons for this?
I think the simplest answer is, generally, it's all about the design of the engine. An engine is generally designed with the load in mind first, and the torque and RPMs second, but the torque is the driving factor for efficiency. Since:
the torque and RPMS are integral variables in achieving your desired goal for the engine performance. It is important to realize, however, that maximum efficiency of the combustion cycle corresponds with the maximum torque, whereas the maximum power corresponds to the maximum product of torque and RPMs.
The maximum torque (and hence maximum efficiency) is designed around a specific rpm where the engine is likely to operate. Moving away from this point in either direction (i.e. faster or slower rpms) will allow you to change the power to meet requirements, but generally cause a lower overall efficiency.
Well, at zero load the efficiency is zero by definition. If the curve of efficiency versus load is continuous and smooth, it must turn toward the zero-zero point. I don't need to know anything about engines to say that.
But then again, power falls off below a peak located at high engine speed, because less fuel/air (energy source) is brought in and burned per second.
If that were the only factor, then power output would be proportional to angular speed
and torque would remain constant down to idle. This is based on the simple relation between torque, power and angular speed:
In fact passenger car engines have a fairly broad and flat torque curve. This can been seen in the curves at http://en.wikipedia.org/wiki/Power_band.
However engine efficiency drops at low speeds since combustion chamber shape, bore/stroke ratio, manifold runner shape and length, valve lift and intake/exhaust valve overlap, to name just a few factors, are tuned for best performance at higher engine speeds. Thus torque eventually falls. In racing cars, the tuning is "peakier," that is, they produce far more peak power but only over a narrow RPM range.
As you might expect, the torque curve isn't as flat in this case, and it falls off more rapidly. See Fig. 3 here http://www.corvetteactioncenter.com/tech/hp_torque.html.
Vehicles that are optimized for very high torque at very low vehicle speed either have no high end to speak of (road graders, bulldozers) or, if they need both, use different systems (diesel-electric locomotives).