Here's some ideas from a guy in Germany: http://www.treefinder.de/ideas.html

Question: Is it impossible to exceed the limits of the efficiency grade as per the heat-to-mechanical-energy Carnot cycle?

As energy can't be created nor destroyed, only converted from one form into another, theoretically then it should be possible to convert heat into mechanical energy with very few losses, mainly friction I'd imagine, which of course produces more heat, which of course also could be recycled in the process to further boost performance.

It would be nice to start off with some device which offers say 50% efficiency to start with, after that then experiment ways to further enhance it, perhaps trying to include some form of pulsing somewhere in order to be able to impose mechanical or heat (or whatever) resonance along the way, etc etc, just brainstorming here.

Or are we forever bound to the about 40%-ish maximum COP as per Carnot?

One idea that springs to mind while writing this is not to rush things in the process, I.E if the process is performing at peak performance from the start only to slowly decrease to zero, which would be when the heat source has the same temperature as the surrounding temperature, then it could be of interest to store the mechanical energy in say batteries from the very start, so the end user payload could vary over time, that way (nearly) all the heat would be used in the process from start to finish.

hy Lynx,

IMHO this is a very fascinating but complex subject. So maybe we should start with the basics first, I hope you agree to this.
How is the carnot process defined?
Basically it only has to do how heat is transfered to pressure in a reciprocating engine (no matter if steam engine or ICE). The pressure then drives the piston. The theoretical factor of ~40% says, that a lot of energy during the energy transformation from heat to kinetical energy is lost. At this point of view (40%) there is no other losses descriped. Like friction of the piston and bearings,transmission.....

So let us concentrate on the other 60%. Where are they wasted? So the point is that the ignition of the chemical energy (e.g. gasoline) gives us with an optimized ignition system 100% (ideal combustion) from the chemical energy out of the gasoline in heat energy. This 100% heat energy is transfered in different "directions".
The first is the -for us- most usable pressure or kinetic energy with around 40%. The second direction where the energy flows are the walls of the combustion chamber. Because they are cooler than the explosion´s heat, they are able to take out heat energy of the system. The third is the hot exhaust gases. The gases are very hot and the most of the energy is lost there.
So carnot says (simplified) that only 40% max. can be used of an heat source to convert it to mechanical energy.
Today we use turbo chargers to reclaim some of the exhaust heat. But again, also the turbo charger can only work with the rules of carnot with max. 40% efficiency.

So the major points will be to inhibit the heat losses by isolating the system. So more energy stays in the chamber to drive the piston.  And to take use of the exhaust heat with higher efficency. 
I had some ideas in the past:
Instead of a turbo charger we can use the hot gases to warm up water. Because of that, electrolyses with efficency of 70-80% can be done easier. When we rise the internal energy of water by warming it up less energy is needed for electrolyses. The bonds of the water molecul become weaker.
Also we can use a stock cooling system of an ICE to heat this water. 
So the most of the heat energy will be saved in water which we can electrolise with less energy.

An important note is also. As better the cooling of the exhaust gases the better is the efficiency of the carnot cycle, because of the high delta T. That is the reason why big power plants always have so big cooling towers (air cooled condenser)!

Keep in mind: The steam engine, invented by Thomas Newcomen at first was very inefficient. After James Watt improved it, it was a acceptable engine to use. Gues what, he recognized that the low delta T was the reason because of the poor efficiency and added an condenser at the steam outlet. This generates a higher temperature drop between heat source (steam in the boiler) and exhaust.

So to rise this temperature drop between source of heat (temperature of burning) and exhaust is also an factor to improve the carnot efficiency.

Thanks Amsy, appreciate it :thumbsup:
That's quite a nut to crack the Carnot cycle to say the least.
But yes, recycling as much of the waste heat as possible would in fact increase the efficiency grade in a heat engine, be it a Stirling engine or what have you.
Insulating the whole machine/device, atleast the parts where the heat otherwise would have gone to waste, is more or less imperative for boosting it's efficiency.
Also, if the cold part used in the heat engine/carnot cycle is room temperature then by using the above argument it would be better if the room temperature wasn't heated (much) by the machine itself, thus making sure that the (waste) heat is used to the full in the machine.
All these heat leaks are contributing to making the heat engine less efficient, so by logic it should be possible to up the efficiency grade solely by recycling wahtever waste heat there may be.

Hi Lynx,

indeed the carnot cycle is a "hard nut to crack" :D
Carnot theory based on the fact that hot gases always flow to cool regions and generate a balance of the temperature. This flow can be used for mechanical energy. Unfotunately the chamber of the heat source (e.g. combustion chamber of an engine) is most of the time at a cooler temperature level than the explosion´s heat. After ignition a lot of heat energy "flows" to this chamber walls (from hot to cool). The wall itself first will "collect" this heat energy. But when the surrounding area is cooler than the chamber wall, the heat will flow in this surrounding area.

But exactly this law is also used to drive the piston. The exhaust is a kind of condenser. Which cool down the area after the engine to ensure the delta T between the hot region (combustion chamber) and the cool area (surrounding area/exhaust).

Yes you are right, so maybe we can not crack the cycle´s efficiency, but we can use the "wasted" heat for other apps.
Maybe you know about this, this is one idea how we can use the wasted heat.
http://en.wikipedia.org/wiki/Organic_Rankine_cycle#Waste_heat_recovery
or with sterling engines. But all this "heat to mechanical" apps working with the law of carnot. So the efficency of the apps is also 40% max.
IMHO the better idea is to use it chemical. Because the heat can be used to nearly 100%. Heat always rises the internal energy of an gas/fluid. So e.g. an electrolyzer with heated water is more efficient than a cold one. The energy we put in with heat then is not needed in voltage. So if we can reduce the voltage which is needed to electrolyze water we save this energy. This means e.g. instead of 2V we only need 1V or less. This would be an improvment of 100%.
This can be seen when an electrolyer becomes warm. The amps grow although the same voltage is applied. When the amps grow the gas output is higher (farday´s law). So in an warm electrolyzer we could reduce the voltage a little bit to get the same amperage than in a cold electrolyzer.

But maybe it is also possible to improve carnot´s process itself. E.g. it would be possilbe to reduce the heat flow to the chamber wall (like in a thermos jug) more heat energy would be existing for the mechanical part. It is like creating an resistor for heat so the energy has to "go" an other way.
This "energy flow" can be compared to electrotechnical applications. The most current (which is "working" operator) always flows where it has the lowest resistance. The same thing happens with heat. So if we block one way as good as possible (chamber wall), so more heat can be used out of the process in mechanical energy. Of course that is a very simplified idea, but under the laws of carnot, it should be working.

 :D

 

Thanks for the link Amsy, most interesting :thumbsup:
Regarding the chemical approach, it would be interesting to find out if there was a chemical that which say expands by heat and thus performs work by say pushing a piston in a cylinder, in a continous cycle.
That way the chemical would have to be completely cooled until the next stroke could take place.
(maybe I'm just explaining how a Stirling engine works :-P).
The thing with that would be that the heat would "only" be used to push the piston forward and while the cylinder cools down the excessive heat could be made to be "recycled" using say an heat exchanger in order to "push" the excessive heat back to the heat source, prefeably with as little losses as possible.

Keep it coming :-)

Quote from Lynx on September 16th, 2014, 08:46 AM

Regarding the chemical approach, it would be interesting to find out if there was a chemical that which say expands by heat and thus performs work by say pushing a piston in a cylinder, in a continous cycle.

Hi Lynx,
good idea. :idea: The ORC process uses a kind of gas which is used in a fridge (low boiling point). But other gases would work too. They have a low boiling point, but it is more interesting for low temperature application or heat exchanging applikations.
To do work in a piston based engine we have to look again in the past:
In the past the most common fluid was water (steam engine) to rise the pressure in the boiler, there was a good reason why they didn´t take air or something like that: If you look at the gas constant http://en.wikipedia.org/wiki/Gas_constant the pressure built up depends on R, which different for every gas/fluid.
Some examples [J/(KgxK)] :
-O2 ... 259,8
-N2 ... 296,8
-CH4 ... 518,3
-H2 ... 4124 (!!!)
-Steam/H2O .... 462

The calculation p.V = n.R_specific.T
p...pressure
V...Volume
n... amount of substance
R_specific... specific gas constant
T...Temperature
you see if V is contant you can influence the pressure by temperature (T) and the gas it self (R).
The H2 has an obviously very high specific gas constant value. This means the pressure in a clean H2 system will increase very quick (10x to water) when heating it up in a closed system when the same amount of substance is equal. This high pressure can be used to let the gas flow through a piston drive or a turbine to the cool side (condenser).

(Somewhere on youtube was a video about a theory how Stanley Meyer drove his car because of the dramatical expansion because of the transformation of H2O into H2 + O2 in the injector, indeed some kind of arc has a lot of heat inside and would heat up the H2 very high)

But again, the problem by turning heat energy in mechanical is the carnot factor of only 40%. Is not depending of the fluid itself.

A very smart approach IMHO was the http://en.wikipedia.org/wiki/Compound_steam_engine

I wonder if it would be possible to use a bunch of  "memory" metal rods that which all helps cranking an axis around while they're continuously being heated and cooled?
That would also vouch for a complete heating/cooling cycle, so perhaps the better part of the heat would then be put into good use as opposed to merely going to waste.....?
Just brainstorming :-)