VASIMR

[Diogenes]

In an IC engine, if the exhaust was the same temperature as the air and the fuel, then wouldn't it be 100% efficient in a Carnot viewpoint?

Edit: Bah, Exhaust plus heat output to the engine structure and other heat sinks.

Wasn't that what Diesel was trying to do?

Not exactly sure what you are referring to. Diesel was attempting to make an engine that would initiate combustion from the heat of compression. This makes the Air/Fuel ratio irrelevant, which is a significant advantage for a diesel, in terms of efficiency.

In a standard Gasoline engine, the stoichiometric ratio is about 14.7. If the fuel is too rich, it doesn't burn. If it's too lean, it doesn't burn.

For this reason, a carburetor (or fuel injector system) must add exactly the correct amount of fuel to the air stream. Since the Air flow is directly proportional to how much power the engine produces, controlling the air flow regulates the engine's output power. At idle, or partial throttles, the airflow is restricted by use of a "butterfly valve" and the fuel is added proportionally.

The "butterfly valve" results in something called "pumping losses" which are the result of the engine trying to suck air in through the restriction. This produces a mostly vacuum in the intake manifold. Because so much energy is expended in pumping a vacuum, for no other reason than to maintain the air/fuel ratio at low engine speed, the efficiency suffers.

Diesels do not have this problem. Their intake manifolds are at full atmospheric pressure, and each diesel piston sucks in an entire cylinder full of air on each intake stroke. As the air is compressed to the point where fuel will ignite spontaneously, the amount of force produced by the downward stroke can be regulated by how much fuel is injected into the extremely hot compressed air.

As a result of this design, diesels do not have "pumping loses" like those of ordinary gasoline engines.

Diesels are also more efficient because of their higher compression, and the manner in which they are optimized for the fuel they use, but the most significant difference between diesels and gasoline engines is the absence of "pumping loses" .

[MSimon]

But, it is the introduction of a high temperature gas to the container(cylinder)

Just like a steam engine. Except the hot gas is BURNED in the cylinder and not a boiler.

Ever read a college level thermo book?

[MSimon]

Diesels are also more efficient because of their higher compression, and the manner in which they are optimized for the fuel they use, but the most significant difference between diesels and gasoline engines is the absence of "pumping loses" .

You still have pumping losses. The biggest difference is compression ratio. That determines how much energy you can extract out of the amount available.

i.e. you are rejecting heat at a lower temperature with a higher ratio.

PV = nRT

[Helius]

Interesting. I though Diesel's original attempts were to apply Carnot's perspective into the exhaust gasses, giving up heat through expansion and driving a piston. It's all closely related. Carnot that established the thermodynamic value of compression, and it's expansion. The cooler the exhaust gasses, by extracting energy (not by actively cooling the gasses) would imply that more energy available to do work.

[Diogenes]

How does an Internal Combustion engine use heat to do work?

I think a return to the books is in order for you.

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BTW the wiki on Carnot is WRONG. Dang. The process going on inside the Carnot engine is unspecified. The Wiki got this one right:

devices that convert the fuel's energy directly into work without burning it, such as fuel cells, can exceed the Carnot efficiency

Carnot: You have a hot reservoir, a cold reservoir, and "some process" in between that extracts work or pumps heat.

Maybe my wording was bad, but the heat generated in an IC is not the reason the piston moves, if you twist the words maybe you can think it is. But, it is the introduction of a high temperature gas to the container(cylinder) by a chemical reaction, not by heat transfer, which looks to expand to static equilibrium, and finds that the container will expand to accomodate the pressure. Simply, the piston moves as gas expansion from chemical combustion of the fuel mix occurs. There is no need for heat exchange or flow in an IC to move energy from a reservior to do work. There was no heat cycle.
I just do not see how you can call IC a Carnot Cycle. It does not fit. Your quotes cited above even agree to this point. Where is the "hot reservior" in an IC? Where is the "cold"? You can not count stored chemical energy in the fuel as a "hot reservior".
Please define the how you see the heat cycle for an IC engine according to the Carnot Model.

The "Hot reservoir" is created on the spot by the burning of the fuel.

It doesn't have to exist in a perpetual state of Hot. It need only exist when it is needed. When the "Hot reservoir" is triggered into existence, The Carnot equation applies.

An engine can be designed theoretically that has a perpetual hot reservoir, (I have actually seen designs like this) but it is usually not convenient to produce such an engine because attempting to maintain a hot reservoir around 1644 degrees Kelvin is hard on the materials necessary to serve as valves to control the airflow. Even typical engine exhaust valves are barely able to function at half that temperature.

Tungsten might work, but who could afford tungsten engine parts, and then how would you lubricate them at that temperature?

No, it's far easier to keep everything below the failure temperature of the engine components and TRIGGER the Hot reservoir when it's needed.

Make no mistake, the piston is indeed pushed by the heat generated from combustion. The Air expands in accordance with this famous formula. PV=TK. (Pressure * Volume = Temperature * K Constant.)

In a typical engine, once combustion commences, the Temperature goes up to around 1644 Kelvin, and the pressure goes up to about 55 bars. Needless to say, this results in an enormous force pushing down on the piston. (about 10,000 lbs for a 4" piston. Only momentary, the pressure goes down as the chamber expands.)


edit: fix typographical error.

[Diogenes]

Diesels are also more efficient because of their higher compression, and the manner in which they are optimized for the fuel they use, but the most significant difference between diesels and gasoline engines is the absence of "pumping loses" .

You still have pumping losses. The biggest difference is compression ratio. That determines how much energy you can extract out of the amount available.

i.e. you are rejecting heat at a lower temperature with a higher ratio.

PV = nRT

What pumping losses? Diesel intake ports are at full atmospheric pressure, (minus the small loss from the air filter restriction.)

And yeah, you are getting a greater temperature differential because of the compression ratio. (usually around 22 to 1.)

[Diogenes]

Interesting. I though Diesel's original attempts were to apply Carnot's perspective into the exhaust gasses, giving up heat through expansion and driving a piston. It's all closely related. Carnot that established the thermodynamic value of compression, and it's expansion. The cooler the exhaust gasses, by extracting energy (not by actively cooling the gasses) would imply that more energy available to do work.

It is axiomatic that the lower the exhaust temperature, the more energy has been extracted from the heat. It is also axiomatic that once the expansion inside the cylinder has reached the same pressure as that on the other side of the piston, no further energy can be extracted.

Most engines do not release their exhaust gases at or near atmospheric pressure. Often they release their exhaust at something like 6 bar or so, and at a temperature around 800 degrees kelvin, basically throwing away the last 500 degrees of usable heat.

Diesel WAS attempting to improve on the gasoline engine design using Carnot's principles, and one of the most significant improvements of the diesel engine is eliminating the parasitic pumping losses.

[Diogenes]

Diesels are also more efficient because of their higher compression, and the manner in which they are optimized for the fuel they use, but the most significant difference between diesels and gasoline engines is the absence of "pumping loses" .

You still have pumping losses. The biggest difference is compression ratio. That determines how much energy you can extract out of the amount available.

i.e. you are rejecting heat at a lower temperature with a higher ratio.

PV = nRT

Are you referring to the Energy required to compress the Gas as pumping losses? If so, I understand what you mean, but the term "Pumping losses" normally only refers to the losses from pumping a vacuum on the intake manifold. Some people use the term "Vacuum pumping losses."

In any case, the energy required to compress the gas is mostly returned on the power stroke. (with the additional energy from the combustion.)

Compressing and expanding a gas, is mostly a wash. In theoretically perfect terms, the energy required to compress the gas, is returned by the gas when expanded. In reality, some of it is lost because the pressure isn't expanded till it reaches atmospheric, but instead is released early.

[MSimon]

What pumping losses?

Ah, you mean no pumping losses from carburation. True. But if they ever get manifold injection that works (an ultrasonic atomizer?)......

[WizWom]

I'm really sorry I started this digression.

*pulls out first year physics book*

The step of the Carnot cycle which is NOT used in an open system is the third step, compression at constant temperature, and the fourth step, compression adiabatically. In fact, the cycle for an engine which has a cold draw and hot exhaust is quite different.

If I was going to calculate the efficiency of my car, I'd need to know the rate at which it burned fuel, the specific energy of the fuel (giving me the maximum heat input available) and then the actual distance traveled, speed and mass of the car, and coefficient of drag (giving me the work done). In other words, I would determine it experimentally. Whether it matched the Carnot Efficiency of the cylinder gas temperature and the exhaust temperature would be immaterial.

The typical things which will increase the efficiency of a car have nothing to do with the Carnot cycle: making sure the combustion is of perfect ratio, making sure it combusts fully, assuring the least friction in the engine and drive train, having a body design with lowest coefficient of drag. Not a one of these corrections manipulates the temperatures involved. The only one which might would be putting a longer stroke on the cylinder, but running the engine with the same amount of fuel (which equates to running lean) - such a change would lower the exit temperature, essentially harvesting more work from the same amount of expansion energy.

Now, can we get back to the original topic, if anyone remembers it?

[ladajo]

But, it is the introduction of a high temperature gas to the container(cylinder)

Just like a steam engine. Except the hot gas is BURNED in the cylinder and not a boiler.

Ever read a college level thermo book?

Yes, and been graded, have you?

So please again, tell me how the gas fluid igniting in the cyclinder has a heat transfer across a boundary that is moving energy from one place to another with the intent to do work on the other side of the transfer boundary in an IC engine?

Tell me again how you would apply the Carnot equation and cycle to calculate correct efficiency in an IC engine?

[Diogenes]

I'm really sorry I started this digression.

*pulls out first year physics book*

The step of the Carnot cycle which is NOT used in an open system is the third step, compression at constant temperature, and the fourth step, compression adiabatically. In fact, the cycle for an engine which has a cold draw and hot exhaust is quite different.

If I was going to calculate the efficiency of my car, I'd need to know the rate at which it burned fuel, the specific energy of the fuel (giving me the maximum heat input available) and then the actual distance traveled, speed and mass of the car, and coefficient of drag (giving me the work done). In other words, I would determine it experimentally. Whether it matched the Carnot Efficiency of the cylinder gas temperature and the exhaust temperature would be immaterial.

The typical things which will increase the efficiency of a car have nothing to do with the Carnot cycle: making sure the combustion is of perfect ratio, making sure it combusts fully, assuring the least friction in the engine and drive train, having a body design with lowest coefficient of drag. Not a one of these corrections manipulates the temperatures involved. The only one which might would be putting a longer stroke on the cylinder, but running the engine with the same amount of fuel (which equates to running lean) - such a change would lower the exit temperature, essentially harvesting more work from the same amount of expansion energy.

Now, can we get back to the original topic, if anyone remembers it?

I don't think anyone is asserting that the Gasoline or Diesel engines are Carnot engines. What is being asserted is that the equations which govern them are the equations which were developed by Carnot.

Beyond that, there exists this engine design.


And this one too.





These two engine designs both have greater expansion volume than compression volume, and as a result, are more efficient than conventional gas or diesel designs.

[ladajo]

Although the Carnot cycle is theoretically the most efficient it is in no way a practical device. Also the energy transfers would be far too slow for any real benefits to be realised. Internal combustion engines work on non cyclic processes because the fuel-air mix enters the system and products of combustion exit the system. However theoretical cycles based on the hypothesis that air is the working fluid in a closed system receiving an rejecting energy to external sinks allows provide very crude estimations on the theoretical efficiencies possible internal combustion engines.

http://www.roymech.co.uk/Related/Thermo ... arnot.html

Note the part that says "very crude estimations".

I agree you can apply the basic concept of (T1-T2)/T1 to figure a "efficiency", but for IC this is at best an estimation. And it is not Carnot, it is Otto. Otto has that extra process leg reflecting constant P changing V across the bottom of the PV diagram that Carnot does not.

[MSimon]

Yes, and been graded, have you?

In the sense that my designs have to work. Yes.

And FWIW I was in the top of my class in Heat Transfer and Fluid Flow in Nuke school. I used to sit in back of the class and read motorcycle magazines. Which made the teacher really mad. So one day while I was deeply involved in my Bike mag he called on me for the next term in a problem covering 3 boards. I told him I couldn't give it to him. Well he had me. By the short hairs. Until I explained the reason, "You made a mistake 1 1/2 boards back and so I can't tell you the correct term because it would be in error." Sure enough. He never bothered me after that.

Any way: once the fuel is burned an IC engine is a heat engine. To get 75% of the energy out of the fuel requires (Carnot: eff. max = 1 - Tc/Th)

.75 = 1 - 300K/Th

.25 = 300K/Th

Th = 1200K = 1700 F

But of course that is without thermodynamic losses, friction, pump power, etc.

Now maybe with ceramic cylinders and pistons and sodium filled valves you can get the device to work as advertised. But maybe not.

[MSimon]

Carnot specifies no process. It is a notional engine.

Let me revise that. It specifies an ideal process. In a temp/entopy diagram all the changes are straight lines.