In terms of energy input/output ratio?
Either man-made or biological machines (or other categories ,if you can think of one), but not things like weather systems or neutron stars please.
Well, technically all machines put out the same amount of energy they take in. It’s called conservation of energy or something, and it’s pretty solid. If you mean useful energy, and all you want is a high useful out to paid-for input ratio, I would go with an industrial heat pump, which could provide useful cooling and heating at the same time with a COP of 4.5 or so, meaning its 450% efficient. I doubt this is what you are looking for though. Can you be more specific? What do you mean by machine, and what do you mean by efficient?
Bicycles would be on the short list. I think bikes are about 90%+ efficient re energy in and work produced.
I don’t know, of course I’m aware of the conservation of energy thing, although I hadn’t thought of it in this connection as I’m definitely talking about output of useful energy; it just struck me the other day, while looking at issues related to dieting, how efficient the human body seems (like it takes an hour of brisk walking to burn off the calories from a chocolate bar) - this strikes me as quite good value, but appearances can be deceptive.
I suppose I’m really talkling about the amount of useful work obtained from a machine in relation to the energy value of the fuel required to run it, does that help?
Actually the most efficient machines, in terms of work performed for energy applied, are the simple machines: the lever, the inclined plane, and the wheel. All other machines are combinations of simple machine manifestations, and the more complex combinations you make, the less efficient (in general) you get.
It depends, though. The human body, and most animal bodies for that matter, produce a very large amount of usable work and beneficial heat per unit of energy consumed. And they are incredibly complex systems.
astro has an excellent choice:
The most efficient animal on earth in terms of weight transported over distance for energy expended is a human on
a bicycle.
Actually, no machine puts out as much energy as it takes in. If it did, you could make a perpetual motion machine (type II, I believe).
Some energy always goes to increasing the entropy of the universe (which sounds so much cooler than it actually is). I believe reversible work is an exception, but that takes an infinite amount of time, so it’s something of a moot point.
Anthracite: It depends, though. The human body, and most animal bodies for that matter, produce a very large amount of usable work and beneficial heat per unit of energy consumed.
Just one minute! If you add the mechanical work plus the heat, that pretty much covers all energy and all machines come out the same! The energy goes into mechanical work or into heat. Add the two an you get 100%! Yes?
What I thin he means is highest rate of conversion into mechanical energy and lowest losses in the form of heat. In that regard I believe muscles qre quite inefficient but I do not have figures. maybe someone fill in
Handy says: The most efficient animal on earth in terms of weight transported over distance for energy expended is a human on a bicycle
I think that can be disputed. I think a gliding bird would probably be more efficient, or a fish going with the current in a river. You say that’s cheating? Well, you should then count the energy it took to buid the roa and the bicycle… I do not think you can settle this, it is a quetion of what you consider.
At any rate I believe muscles to be quite inefficient. I’ll throw in that a Diesel motor can convert about 1/3 of the energy into mechanical energy at the shaft and the rest is lost as heat one way or another. And that is pretty efficient.
Probably huge installations can do better although by the time you convert it into energy you have further losses. Anthracite, what are the numbers for a large power plant?
Ahhh…but that is why I said “beneficial” heat. A body must lose a large amount of heat, or else the biological processes are in danger. But it also must keep some of it’s heat as well, or else the organism is in danger too. Total heat plus total work will be equal to 100%, but not the total work plus the beneficial heat.
BUT…as for the actual efficiency…well, I am always ready to admit I am wrong. I found some interesting websites that seem to say I am full of shit about the human body being so efficient:
High School Level 1 Energy Audit (Bonneville Power)
These pages at MIT seem to have some interesting information on the actual level of body heat produced by humans:
The assumptions used in this acrobat file are a “15%” conversion efficiency of food to usable work. Eek!
This link gives some human power output and efficiency numbers, giving “25%” for a human-bicycle combination.
[
[URL=“http://www.ruf.rice.edu/~kines/laboratory5.htm”]This link from Rice University gives the energy required on an ergonometer in terms of oxygen uptake.](http://members.theglobe.com/alexweir/thresher/Thr060.htm) By doing some quick math, it seems that for an energy output of 250 W, the human body needs 3.5 liter/min of oxygen, which at 5 kcal/liter means that it uses 1221 W to produce 250 W, or an efficiency of 20.4%.
This link gives info on calculating basal metabolic rates.
And this link gives some interesting info on how different metabolic process contribute to the heat production.
A top-of-the line real (non-experimental) coal plant can reach 34-38% on a gross basis. A simple cycle gas turbine can be from 35-44% on a gross basis, and a combined cycle plant even higher. I have seen Tokyo General Electric claim that some of its combined cycle plants reach 60% efficiency on a net basis, about 54% gross. But I think near 50% is more typical for a good combined cycle plant.
If muscle converts 15% into mechanical energy and 85% into heat, that’s not too good at all. Especially if you consider the cost of “fuel”. It is probably the worst efficiency in terms of cost.
A snack bar costs $1 and has 200 Kcal, of which I get 30 Kcal of useful energy at the shaft. 
A gallon of diesel costs $1.30 and has 33000 Kcal of which I get 11000 to the prop of my boat.
Ratio of useful energy costs: 282 (and that’s before you start adding social security and other taxes)
Also, this is using cheap fuel such as snack bars. If you consider the cost of a meal at a restaurant then hiring people to row your galley is not cost efficient 
>> A top-of-the line real (non-experimental) coal plant can reach 34-38% on a gross basis
I imagine by gross you mean gross electrical output and then you have to deduct the electrical power consumed by the plant itself. That sounds pretty good because it means the shaft (mechanical) output is even higher.
Small generators are grossly inefficient compared to these figures.
Also, the OP is not very clear. What kind of machine? a chemical to mechanical? Chemical to electrical? mechanical to mechanical?
A simple lever is a machine and probably has over 99% efficiency. Pulleys also (called blocks on a boat)
Another point: Heat pumps do not have, strictly speaking, an efficiency greater than 1. I put in 1Kwh of electrical energy and the machine gives me that 1Kw in the form of heat plus 1.4 Kwh worth of heat it took from the evaporator.
Another aspect is that while a calorie is a calorie is a calorie, all energy is not equally valuable. Heat is low grade energy much less useful than electrical or mechanical energy. The energy contained in a steak is much more valuable than the energy contained in a chunk of wood.
Too many variables. Everything is relative and depends on your point of view and your needs.
reminds me of a couple of things:
From the point of view of a worm, animals can be divided into two groups. Those non agressive like the lion and the tiger and those which are aggressive and attack savagely, like the chicken.
Also reminds me of a friend who when asked “how’s your wife?” liked to respond “Compared to what?”
Well, these cogitations are interesting, but technically, an organism isn’t a machine, it’s a bunch of chemical reactions.
Some organisms are complex enough to employ mechanical advantages, from a cilium or flagellum (a lever), to a tooth (a wedge, or more correctly, a type of inclined plane), or a limb (another lever). As for wheels, hmm… I’m trying to think of a real-life organism that employs the wheel.
Tumbleweed? Somehow that’s not gratifying enough. (Anyone here read Piers Anthony or David Brin?) Actually, that would make a good GQ.
Anyway, technically speaking, a machine transforms one type of “work” (i.e. a force exerted through a distance) into another, either by changing the vector of the force or by amplifying either the force or the distance.
Machines lose efficiency due to friction, and theoretically, no simple machine (lever, wheel, inclined plane) is more efficient than another. But in practice, the lever is probably most often encountered with the highest efficiency, since it’s pretty easy to make a low-friction fulcrum.
Technically speaking machines don’t perform the kinds of conversions that sailor is seeking clarification for, although we’ve taken to calling devices that perform those conversions “machines” since they typically utilize mechanical advantages to make the output energy into useful work.
According to you it isn’t, but I think most people would agree that that is exactly what an organism is. Including Merriam-Webster
The most efficient machine in the world is the electric heater. They are designed to convert electricity into heat, and does that at a 100% efficiency - that is, 100% of the input energy is used to do the intended task.
Actually an electric heater converts some of the energy into sound waves, or at least most of the heaters I’ve seen(heard?) make a gentle humming sound because of the alternating current, although it could be argued that even the sound waves end up being converted to heat as they dissipate.
I’ve been thinking about this and I’d like to make the original question more specific:
I think what I was actually meaning to ask is What is the most efficient engine in terms of the amount of mechanical work (or kinetic energy) out in relation to the amount of chemical energy available from the fuel. Animal/human bodies are still valid in this as ‘biological engines’.
Bughunter:
What about the dung beetle? - OK a ball of dung is not strictly a wheel as there is no axle involved, but with your username, i’m surprised that you plumped for tumbleweed instead!
I remember an article on kangaroos being very effiecient. Can anyone back me up?
I’m with bughunter, and so is the Oxford Dictionary.
machine:
-
apparatus using or applying mechanical power…
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…a vehicle, a piece of electrical or electronic apparatus
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instrument that transmits a force…
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controlling system…(party machine)
-
a person who acts mechanically and with apparent lack of emotion
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coin-operated dispenser
origin greek ‘contrivance’
Although definition 5 mentions us carbon-based life-forms, it clearly means that if you act in a certain way, you can be compared to a mechanical contrivance.
I’d like to see what Merriam-Webster defines a robot as?!
No, for the coal plants that was Net Heat Rate. Gross is somewhere between 5 to 12% higher (relative scale), with an average of about 7-8%. So about 36-41% gross.
The steam turbine-generator assembly can be as efficient as 42-48% on a net basis. On a gross basis…ugh. I don’t normally see numbers for steam turbine-generators on a gross basis, since it is impractical to assume that the boiler feed pump is not present. Maybe about (quick calculations) 43-50% gross?