What are the reasons that engine efficiency drops in low loads?

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?

• If this is only about combustion engined then it's a dupe of: engineering.stackexchange.com/questions/2349/… – ratchet freak Mar 1 '17 at 10:01
• It is only about combustion engine, but it is not duplicate. It is actually the opposite question than the one you mention. Actually, combustion engines have better fuel efficiency at middle loads only. Going further on increased load, it reduces their fuel efficiency, which is the question you mention. However, I am asking about the other end of the curve, where lower loads result in worse fuel efficiency! – ergon Mar 1 '17 at 10:42
• @ergon So when you say load do you mean output power or vehicle speed? – JMac Mar 1 '17 at 10:51
• output power ofcourse, load has to do with power, not speed, you can be in low spead or high spead and have low load – ergon Mar 1 '17 at 12:25
• – preet dhunna Mar 1 '17 at 15:54

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:

$$P=\omega*\tau=rpm*torque$$

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.

• thanks but this question is regardless rpm. In both high and low rpm, when the load is low, there is worse fuel consumption. – ergon Mar 2 '17 at 13:36

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

$$P∼ω$$

and torque would remain constant down to idle. This is based on the simple relation between torque, power and angular speed:

$$τ=\frac{P}{ω}$$.

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).

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