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?
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If this is only about combustion engined then it's a dupe of: http://engineering.stackexchange.com/questions/2349/how-can-mpg-be-high-under-low-engine-load?rq=1 – ratchet freak Mar 01 '17 at 10:01
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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 01 '17 at 10:42
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@ergon So when you say load do you mean output power or vehicle speed? – JMac Mar 01 '17 at 10:51
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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 01 '17 at 12:25
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This may help https://www.google.co.in/url?q=https://www.jstage.jst.go.jp/article/jime/49/1/49_100/_pdf&sa=U&ved=0ahUKEwiAsZWZmbXSAhWIipQKHdRsAEgQFggoMAg&usg=AFQjCNHCsfAaEqhtVSlyilRTFEZR9l5GKA – preet dhunna Mar 01 '17 at 15:54
4 Answers
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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).
Edwardo9
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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.
Brent Johnson
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It's been a while since you asked this question and nobody properly answered it. I'll take a shot at explaining why its low at lower load:
If we ignore intake and exhaust strokes which do not contribute much in energy production or consumption, when it comes to power production or usage, we have the expansion (after ignition of course) and compression strokes. An engine consumes energy to compress the air/fuel mixture from ambient temperature/pressure to a higher temperature/pressure. This is a combination of adiabatic and isothermal processes. Then after ignition it generates energy from the expanding gases.
The thermal efficiency of an engine is the ratio of: (energy produced in expansion - energy consumed in compression) / (energy produced in expansion)
Since the intake is mostly at ambient temperature and pressure, and compression ratio is fixed, the energy consumed in compression is more or less fixed regardless of the power output. So as per the formula for efficiency above, if the energy produced in expansion is lower, the efficiency is lower.
For example a 1 kW engine, that produces 3 Kw in expansion stroke but consumes 2 kW in compression stroke has an efficiency of (3-2)/3 = 33% . If the same engine is only producing 0.5 kW, then expansion stroke has to generate 2.5 Kw because compression still needs 2 kW of power. So the efficiency now is: (2.5-2)/2.5 = 20%
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The only variable in the formula for for engine efficiency is the compression ratio.
At low loads, low speeds, which means you're barely pushing the gas pedal...
The throttle plate is mostly closed, creating a massive restriction and therefore a vacuum in the intake manifold.
At idle, a gasoline engine typically only has 6 PSI absolute pressure in the intake manifold. At low throttle, there is still a decent vacuum... there's only about 9 PSI of absolute pressure. (Atmospheric pressure is 14.7 PSI.)
So the engine efficiency is directly tied to the throttle opening percentage (or you could say more precisely tied to the intake vacuum.
Peak efficiency only occurs when the throttle plate is fully open (or close to) and there is barely any vacuum. And it also only occurs around 3400 to 3600 RPM
Throttle position effectively controls the compression ratio. The more closed the throttle, the lower the effective compression ratio.
The reason why compression ratio governs efficiency, the more work you put into compressing the air gas mixture, the more energy you can extract.
Brandon
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