Homebrewed Electricity > Other
Engine / Flywheel mass discussion
Watt:
Peter, the bigger the flywheel the more mass and therefore the more btu required to sustain rpm.... On another note, keeping the rotating assembly as light as possible, improving efficiency and removing as much mass by way of moving components in the coupling will increase watts per power in.
Also, if that battery was so flat as to barely start the engine, my guess would be that it would not ever accept much of a charge once running and charging.
Because we have confirmed you have an electronic ignition and I assume electronic fired injectors, install a duty meter or pulse width meter accross the injector wires to one injector. Now start the engine and let it idle at normal operating temperature. Then, turn on the head lights as suggested. You'll notice the duty cycle increasing / pulse width increasing. Now, if you have say 50 watt head lamps x2, you'll find the power required ( fuel ) will be about 300 watts equivalent running off that car engine. Most is exiting the tail pipe and the rest is through the radiator and wiring....
--- Quote from: Peter- on May 17, 2010, 12:59:35 AM ---
--- Quote ---What kind of car do you have? Is it a 70's vintage or is it late 80's or 90's or newer with fuel injection and computer controls?
You imply idling that gas engine is nearly free and so is the wattage used by electrical accessories and such. First, is you alternator so new as it is field controlled by your ecm? Is the alternator driven by resistance from a load such as a bulb? Or, has your alternator fully charged you battery and now at float stage? Or, the more relevant the question, have you started your vehicle and swapped the battery for a very low charge state battery and then watched the tach? A WATT is a WATT. Once your engines mass has delivered in the way of energy, to the load, it's mass is no longer usable. meaning for every action, there is an equal but opposite reaction. That engine now has to deliver the power to sustain the new load.
If that is not a good enough explanation, put a vacuum gauge on your engine. Put a load on that engine and watch the vacuum gauge when it starts towards zero, that is your throttle delivering more air and therefor fuel. That car engine idling is very inefficiency, just to keep it idling takes a huge amount of energy.
Seems you need to remember HP is a product of BTU. Huge mass is great for getting a load moving or the initial start of huge energy requirements but only for very short burst but none can be more of a load than that available as energy.
--- End quote ---
Point taken. Mine is a 1998 4-banger with ecm and etc. The flattest battery I ever had in it, barely cranked it enough to start. The tach didn't show any difference. However, I was picturing a different scenario during this discussion: mainly, something that is basically running all the time without deep discharge. Much lke a stationary power plant. I take it that this isn't the case. I do wonder if enough flywheel couldn't overcome even that though; because I've never seen a car alt stay above a few amps for more than like 5 minutes. They put a bunch of power right away and then taper right off as the battery tops up.
--- End quote ---
bob g:
"Peter, the bigger the flywheel the more mass and therefore the more btu required to sustain rpm.... On another note, keeping the rotating assembly as light as possible, improving efficiency and removing as much mass by way of moving components in the coupling will increase watts per power in. "
i would strongly disagree with this statement in total,
some of the most efficient engines on the planet have enormous mass!
the only thing cutting flywheel mass will do, is reduce windage losses which are so small so as to be almost unmeasurable outside of a very
well equipped test cell., removing mass from a drive coupling will be unmeasurable outside of an incredibly precision test cell
if by cutting mass on a typical 4 banger you increase the efficiency of the alternators output by even 1/10% i would be shocked.
generally speaking reduction of mass for a genset is about the last thing you want to do, unless
you plan on making a portable unit that you can backpack into a remote site. even then it is very unlikely that it will be even close to
as efficient as a very heavy high mass stationary model.
once a heavy flywheel is up to speed it takes no more btu's to keep it spinning than does a light flywheel (not accounting for windage losses
which are infinitesimal)
further efficiency has nothing to do with the btu's value of the fuel or amount of fuel put into an engine, efficiency is the measure of
how much power you get out (in btu's) vs the fuel consumed in btu's,
engine type and design, along with size (generally favoring larger high mass versions) alternator type and design (again generally favoring
larger high mass versions) are the primary factors dictating the efficiency of a genset as a system and is expressed as overall efficiency.
bottom line, there is no replacement for displacement and that goes for both engine and alternator when it comes to getting maximum efficiency.
certainly one can build up a very light weight aluminum block engine, driving a small lightweight car alternator to charge a battery bank, and
work on making it as light as possible, but
it will burn twice the fuel per kwatt/hr produced of a heavy high mass genset that might weigh over a half ton or twenty times the weight of the little fellow.
nothing wrong with small compact and lightweight, but it is not the direction to go for high efficiency.
bob g
Watt:
So then why don't we have huge heavy gasoline engines? Huge diesel engines exist due to the large amounts of energy in fuel oils. Are you saying a lawn mower engine spinning a thousand pound flywheel can do the same work the same lawnmower engine does spinning just enough mass to complete its cycles? How long does it take to use a 5hp lawn mower engine to spin up that 1,000 pounds to 3,000 rpm and then how much fuel will be required to sustain that load? First of all, I hope you have a huge starter to get the flywheel spinning just to get the engine to start. Try it by hand, spin that thousand pounds for an hour and tell me if you'd rather be spinning one weighing 1 pound.
Cut that 4 banger down to 1 cylinder, run that same alternator under a constant load. At the least the energy saved will be that required to rotate the other mass in the three cylinders.
What about friction in your no more mass statement? Why will 1hp keep a motorized bicycle moving at 30 mph but, only move a train a inches per hour?
And again, Power out vs power in. MASS Ok so you say no more power to keep a flywheel moving once up to speed, so this wouldn't burden efficiency? What happens once the power is added to the engine with the huge flywheel? Not only does the flywheel have to be brought back up to speed but the load has to be compensated at the same time. Huge mass stores energy but once overcome....... Again, as stated earlier, Mass is great for starting loads.
Another so why then.... Why not have huge heavy blades on wind turbines maybe steel or hollow fiberglass filled with lead? Forgetting fatigue and of course hoisting, the required wind speed to get heavy blades moving would be huge. Once moving, heavy blades would store energy and none the less require a larger amount of wind than that of wood blades to keep moving. How much energy is in wind and what's the limit...
As far as the engine block, you must be reading more into the post. Short of reducing friction from piston and ring interference and well lighter for driven car...... Again, not part of my post so I'll quit...
Reducing heat rejection into the block / air, what ever cooling media, would most definitely increase efficiency allowing more fuel to be converted to mechanical energy.
No replacement for displacement is an old term used when naturally aspirated engines of all types were most common and charging atmospheres into an engine was brutal. Ole turbocharged 1400cc 85ci 4 bangers in reference are making, commonly, 1000hp ( at 8,000 rpm ) where a big ole bbc at 10289cc 632 ci v8 of equal hp rating is required for equal btu rating. work over time = hp, work = btu... now which one is more efficient? the 4 banger, less friction and so on. Which one will work better as a power source for generating electricity? the bbc... which one will pump water better? The 4 banger. Which one will last longer, lol the bbc....
One more how come in close.
Why are we moving away from v8's and huge SUV transportation in the U.S. and possibly else where? It's not because we are trying to save from red light to red light, as far as getting the mass up to speed, it's because driving down the road with all the mass requires added energy.
Single cylinder engines are at a disadvantage of sorts. Extra mass is required to not only store energy but required to complete its cycles. If there were no mass, all stored energy from pulling the start rope would be absorbed by the compression stroke. So we pull harder. Mass is required to compress the air up until the power stroke. Once in the power stroke, too small a mass would increase rpm abruptly and cause erratic rpm fluctuation between power and compression cycles. Even the exhaust stroke requires energy requiring the small amount of mass to over come that stroke then the intake stroke and further the compression stroke. Not to mention torsional dampening, or the lack thereof. So hence minimizing the mass.
Again, what is the most efficient engines running as fuel and at what rpm?
Diesel engines, now there we go.... another topic....
--- Quote from: bob g on May 27, 2010, 01:41:54 AM ---"Peter, the bigger the flywheel the more mass and therefore the more btu required to sustain rpm.... On another note, keeping the rotating assembly as light as possible, improving efficiency and removing as much mass by way of moving components in the coupling will increase watts per power in. "
i would strongly disagree with this statement in total,
some of the most efficient engines on the planet have enormous mass!
the only thing cutting flywheel mass will do, is reduce windage losses which are so small so as to be almost unmeasurable outside of a very
well equipped test cell., removing mass from a drive coupling will be unmeasurable outside of an incredibly precision test cell
if by cutting mass on a typical 4 banger you increase the efficiency of the alternators output by even 1/10% i would be shocked.
generally speaking reduction of mass for a genset is about the last thing you want to do, unless
you plan on making a portable unit that you can backpack into a remote site. even then it is very unlikely that it will be even close to
as efficient as a very heavy high mass stationary model.
once a heavy flywheel is up to speed it takes no more btu's to keep it spinning than does a light flywheel (not accounting for windage losses
which are infinitesimal)
further efficiency has nothing to do with the btu's value of the fuel or amount of fuel put into an engine, efficiency is the measure of
how much power you get out (in btu's) vs the fuel consumed in btu's,
engine type and design, along with size (generally favoring larger high mass versions) alternator type and design (again generally favoring
larger high mass versions) are the primary factors dictating the efficiency of a genset as a system and is expressed as overall efficiency.
bottom line, there is no replacement for displacement and that goes for both engine and alternator when it comes to getting maximum efficiency.
certainly one can build up a very light weight aluminum block engine, driving a small lightweight car alternator to charge a battery bank, and
work on making it as light as possible, but
it will burn twice the fuel per kwatt/hr produced of a heavy high mass genset that might weigh over a half ton or twenty times the weight of the little fellow.
nothing wrong with small compact and lightweight, but it is not the direction to go for high efficiency.
bob g
--- End quote ---
Watt:
http://www.physicsforums.com/showthread.php?t=201254
Just for giggles on power required to keep a given mass spinning, lol in a vacuum chamber if you wish.
Wonder what power it would take to spin a standard unsharpened pencil? Bet it's not free...
TomW:
I am very interested in this flywheel discussion.
Not interested in getting involved as I likely suffer from some old skewl misconceptions.
Things I know from experience are:
Take the blade off many vertical shaft lawnmowers and the motors don't seem to run well. Missing flywheel??
Several chain saws I replaced centrifugal clutches on would not even start without the clutch on.
I always figured the fltwheel stored power albeit small that helps the IC cycle operate smoothly and then mostly at low speeds?
Bob;
Several times you mention "not accounting for windage losses" Maybe I know this by another term? What do you mean here?
Life has been my teacher and laboratory on most stuff but I have been drawn through the keyhole of several college level physics courses at a time I was mostly just overwhelmed by the math.
As an aside I would mention modern devices are built to a cost with many corners cut and compromises made. What makes a profit may not jibe with what is mechanically "best" or "most efficient".
Thanks.
Carry on the discussion and I will leave it at that.
Tom
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