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evil bastard
 
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Quote:
Originally posted by markjenn
All IC engines lose most of their energy to the heat cycle involved. I can't remember the exact numbers, but with typical combustion and heat sink temperatures, the absolutely theoretical efficiency (no losses whatsoever) of an IC engine OF ANY TYPE is around 30%. Friction may cost you a couple percent on top of this, but is very small in comparison. So whenever you see anyone talking about truly dramatic changes in fuel efficiency for a new engine, your bull**** meter should immediately peg.
I wasn't disputing the theoretical efficiency limits of Carnot cycle at all. I was merely trying to say that, at least in theory, the piston engine design is inferior to rotary design because for every turn of the crank the piston accelerates and deccelerates twice, which is a complete loss of energy to inertia and consecvently to the friction as well. That's why, idealy, the pistons should weigh as little as possible.

Old 06-30-2007, 04:10 PM
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Quote:
Originally posted by Moybin
To John Lyon's concerns about the govm't getting into this: the number of small startup companies developing bio-diesel is unknown, but looking at the activity in Iowa and Illinois what's going to happen is this: private enterprise and investors are going to create a ground-swell of new production so much faster than Congress can keep up that by the time they stick their Luddite fingers into the pie they won't know where to start.

Bio-diesel by its very source is "low sulfur". I've talked to a few truckers already running it and they say it's cleaner, the engines run better and cleaner, longer runs between major overhauls, all those pie-in-the-sky things.

Personally my next cage will most likely be a diesel. In three or four years I expect all the major manufacturers to be building small (less than 2 liter) 3 and 4 cylinder diesel powerplants.

Longterm question... If farmland becomes a source for transportation energy..... What will we do for groceries??? Seems like that could drive the price of food way up. Could be an unpleasant consequence.
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Old 06-30-2007, 08:02 PM
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Then again, the internal combustion engines are not heat engines strictly speaking, right? They use an energy of an expanding gas (explosion), rather then the difference in temperatures. The heat is just a byproduct of the explosion.
I'm not quite sure about this, but at first glance it seems that way. My thermodynamics are rather rusty, so I'm not sure if Carnot cycle actually applies to IC engines.

Can anyone elaborate on this?
Old 07-01-2007, 05:32 AM
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Quote:
Originally posted by wswartzwel
Longterm question... If farmland becomes a source for transportation energy..... What will we do for groceries??? Seems like that could drive the price of food way up. Could be an unpleasant consequence.
Looks that way until you realize that the record for most corn acreage was set in 1944 to supply Europe with food! 63 years ago! Before there was the total mechanization of farming we have now!

Best part of this is the farm subsidy system is going to finally fail all on its own. It will be more profitable to plant than to ask the government for $$ because you didn't plant.

And I can't remember a time in the last 5 years that I've driven down through Indiana that I saw much of any farmland planted. The whole state seems to just sit fallow every year, more subsidy garbage.

No, this is a good thing. I might actually wake America up to the notion that food is a very powerful weapon, and there are only 4 (maybe 5 now) countries that can and do export food beyond their own needs. USA, Canada, Argentena, Australia. I think Brazil might now be on the list, too.
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Old 07-01-2007, 07:05 AM
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Actually, I'm more interested in what modification(s) would be required to run E85 or pure ethanol in my R11S. Certrainly a new chip. Shouldn't need new valve seats like the airheads did when leaded gas disappeared.

I know I won't get the same gas mileage, but that's not the point. The biggest local propane dealer just put up the first E85, gasahol and bio-diesel pump set close by. I think that's a trend just getting started.
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Old 07-01-2007, 07:09 AM
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Might need different injectors and fuel lines that can cope with alcohol. Some types of rubber quickly deteriorates when subjected to alcohol.
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Old 07-01-2007, 09:26 AM
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Quote:
Originally posted by Fenring
I wasn't disputing the theoretical efficiency limits of Carnot cycle at all. I was merely trying to say that, at least in theory, the piston engine design is inferior to rotary design because for every turn of the crank the piston accelerates and deccelerates twice, which is a complete loss of energy to inertia and consecvently to the friction as well. That's why, idealy, the pistons should weigh as little as possible.
Sorry, got to disagree yet again. Basic mechanical engineering stuff here.

There is no "complete loss of energy to inertia" when the pistons go up and down. The idea that there are huge loses here appeals intuitively, but in fact, the energy required to accelerate and decelerate the pistons is supplied by the entire rotating mass of the engine (mostly the crankshaft and counterweights) which are constantly storing and releasing the energy. When a piston declerates at the end of the stroke, the energy is saved in the rotating mass and/or used to accelerate a different piston.

There is friction (rings, bearings, etc.) but that has nothing to do with inertia and rotary engines have no inherent advantages - in fact, they have disadvantages becasue the large rotor swept areas and big lateral seals required to seal the combustion chamber represent a longer length than the extended length of the piston rings. Cylilnders are round which is a remarkably efficient shape for enclosing a small volume with the least amount of sealing area.

Think about it this way: If you're trying to fence a given area with the least amount of fence, what's the best shape? A circle of course. Piston engines use circular fences to enclose their combution area, a rotary uses rectangular. Engine friction is directly proportional to the length of this "fence".

Generally rotarys have worse Specfic Fuel Consumptions (SFC) - commonly measured in units like lbs of fuel consumed per horsepower generated per hour - than piston engines. They have a bit more friction which hurts but the real killer is their larger "combustion chamber swept area". Rotarys have a square rather than cylindrical combustion chamber space and when the mixture is ignited, the expanding combustion chamber exposes a larger area of metal to the combustion gases as they expand. This larger area tends to conduct more heat away from the combustion and heat that is absorbed by the metal surrounding the combustion chamber is wasted heat that can't be used for useful work. Essentially, a rotary quenches its own fire a bit more during the critical phase of the engine where expanding combustion gases are doing useful work by pushing on the piston/rotor. This is also the reason rotarys produce higher emissions, require more cooling, and require more extensive exhaust cleanup in the cat.

Poor fuel mileage has plagued the Mazda RX7/RX8 since its inception. A Corvette making 75% more power and pushing a much-larger and heavier chassis gets 4 mpg better highway mileage than a RX8.

This is the theoretical side of rotarys. In practice things get a lot more complicated with lots more tradeoffs and nuances. Again, I really don't know much about the MTY engine, but that nobody does because it is snake oil. I'm just gving the basic fundamental principles that put rotarys at an inherent disadvantage in terms of fuel consumption. They historically DO have advantages in terms of power-to-weight, packaging, and smoothness. And they're constantly being reengineerred so the tradeoffs will change.

But rotarys don't have some big advantage in efficiency just because their rotors spin rather than the pistons going up/down.

- Mark
Old 07-01-2007, 10:22 AM
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Great post Mark. Thanks!


I still enjoyed my 83 RX7 though....
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Old 07-01-2007, 11:31 AM
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Quote:
Originally posted by wswartzwel
I still enjoyed my 83 RX7 though....
I had a 81. Great car and dead reliable for about 75K miles although I got the impression there were going to be some serious issues with the very complicated carb over time - this was a car that needed FI before FI was available.

I remember it as a very quick car, but I recall it was rated at something like 100 hp. My oh my, times have changed.

- Mark
Old 07-01-2007, 11:44 AM
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My College Roomate had an 81.. Boy did it smoke when he first started it in the morning....
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Old 07-01-2007, 11:47 AM
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evil bastard
 
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Quote:
Originally posted by markjenn

There is no "complete loss of energy to inertia" when the pistons go up and down.
You're right. Offcourse the inertia is a kinetic energy and therefore is not a loss in it self, so in a frictionless world that's all peachy.
Still, added motion of a piston's movement produces excess friction. But since I know almost nothing about rotary engines I can't say if that matters or not in comparison. Like you said, intuitively it seems like it would matter.
Old 07-01-2007, 12:57 PM
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Quote:
Originally posted by Fenring
Still, added motion of a piston's movement produces excess friction.
Pistons (and rotors) have friction with their cylinder walls (rotor housings). Beyond this, I don't know what you mean by "added motion" or "excess friction". Pistons have a different motion, but the alternating direction of a piston moving up/down is not inherently going to have more (or less) friction vs. a sweeping rotary seal moving at a constant speed in one direction.

An analogy might be whether a belt sander moving in one direction has more friction than an alternating sander moving back and forth. Assuming the total linear movement per unit if time is the same, it shouldn't make any difference.

There may be some tiny friction associated with the piston flexing due to alternating forces and the beaings being loaded first in one direction, then the other. This would definitely favor the rotary, but I don't this effect is significant at all. In fact, the majority of the friction in an IC engine is oil drag and gear meshing drag - I wouldn't think this would be different.

Interesting discussion.

- Mark

Last edited by markjenn; 07-01-2007 at 02:39 PM..
Old 07-01-2007, 02:28 PM
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Quote:
Originally posted by markjenn
Pistons (and rotors) have friction with their cylinder walls (rotor housings). Beyond this, I don't know what you mean by "added motion" or "excess friction". Pistons have a different motion, but the alternating direction of a piston moving up/down is not inherently going to have more (or less) friction vs. a sweeping rotary seal moving at a constant speed in one direction.

An analogy might be whether a belt sander moving in one direction has more friction than an alternating sander moving back and forth. Assuming the total linear movement per unit if time is the same, it shouldn't make any difference.

There may be some tiny friction associated with the piston flexing due to alternating forces and the beaings being loaded first in one direction, then the other. This would definitely favor the rotary, but I don't this effect is significant at all. In fact, the majority of the friction in an IC engine is oil drag and gear meshing drag - I wouldn't think this would be different.

Interesting discussion.

- Mark
You answer your own questions better than I ever could. Truth be told, my knowledge of applied mechanics is very limited, so I'm going with the gut feeling. Anyway I looked up the wankel and it does constantly sweep with 3 sides over the inner surface of rotor housing, so friction again.

There is a small matter of extra friction on the bearings where piston attaches to crankshaft, but sufficently oiled (as they should be) that shouldn't matter much.

I did a little thought experiment and it turns out that as the mass of the crankshaft approaches zero, the movement of the piston becomes bouncing, as if it is striking a solid barrier. So that would mean, that the piston should have negligable mass compared to the crakshaft in order to operate smooth turns.

I learned some interesting things today.
Old 07-01-2007, 03:09 PM
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Quote:
Originally posted by Moybin
Looks that way until you realize that the record for most corn acreage was set in 1944 to supply Europe with food! 63 years ago! Before there was the total mechanization of farming we have now!
You forget that 63 years ago we were looking at a world population of less than 2 billion... now we are over six!

This planet only supports a certain amount of BioMass... I will go out on a limb, guessing that we are rapidly approaching (or may have even surpassed) that mark!

A lot of this acreage has been/will be used for development... population grows, farm land shrinks... and then there is a demand for BioFuel on top of that???

The math does not compute very well here...
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Old 07-01-2007, 04:29 PM
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Cannibalism maybe????
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Old 07-01-2007, 04:48 PM
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Quote:
Originally posted by wswartzwel
Cannibalism maybe????
Are you packing some food on our trip I hope?...I'll keep some Trail Mix in my Tank Bag for emergencies.

Seriously, the worlds current problems surely will be magnified by the exponential increase in population over the next 50 years...what exciting times we live in, eh? I can only hope in Jesus Christ and not myself or anyone else because all of mankind's devices will utterly fail him.
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Old 07-01-2007, 07:28 PM
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In response to the farmland discussion above, this months Popular Science discusses green algae as an alternative crop for producing vegetable oil (biodiesel).

Here are the relevant highlights:

1. It will take about 140 billion gallons of biodiesel every year to replace all petroleum-based transportation fuel in the U.S.

2. It would take nearly three billion acres of fertile land to produce that amount with soybeans.

3. It would take more than one billion acres to produce it with canola.

4. We only have 434 million acres of cropland in the United States.

5. Algae could supply current demands using just 95 million acres of land.

6. Because algae can be grown in ponds, or plastic bags, or tanks that can be in the desert, urban areas, or near a power plant it won't need fertile farmland to survive.

The next ten years ought to be interesting....

I would also run a diesel if Jeep would only put one in their Wranglers. Until then I'll drive my Cherokee into the ground and keep my new r1200r. All other gas powered toys are for sale...

...two vehicles ought to be enough for one man though, right?
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Old 07-01-2007, 08:08 PM
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Algae!!! cool, I have a heck of a time keeping that stuff out of my backyard Koi pond....
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Old 07-01-2007, 08:12 PM
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See?
The stuff is ubiquitous....you're sitting on a fortune out there.
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Old 07-01-2007, 09:21 PM
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Quote:
Originally posted by R111S
I can only hope in Jesus Christ and not myself or anyone else because all of mankind's devices will utterly fail him.

...and here is part of the problem... "if all else fails, rely on someone else to fix it"

That's not going to work... and that is why neither Jesus Christ nor anyone else will save this ship from sinking... our planet will be better off for it...

And yes, I AM including myself...

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Old 07-01-2007, 09:38 PM
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