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Can Your Car Get 1,700 MPG?
Xaroth writes "Given all the hubbub over EPA mileage ratings, I'm a little surprised that this one hasn't come up earlier. SAE apparently holds a contest each year to encourage students to design single-person, fuel-efficient vehicles. This year's winner achieved 1,747.4 MPG, with the press release that tipped me off pointing out that third got a 'measly' 1,194. There are more details on the competition over at SAE's site about the competition. Now, if only they could make these street-legal..." However, even the winner has nothing on top entries we mentioned in Shell's competition a few years back.
When I was working on a solar powered car in college there was one of those SAE cars next to our bay. I don't think they're all that plausible because they are little more than go carts. I think we should work toward some of the technologies they use, like superatomizing and mixing the fuel, and trying to get engines above their pathetic 30% efficiency, but 1500 mpg is a bit out of reach. Of course, I guess I should never say never.
These cars typically achieve their best mileage using a 'coast and burn' strategy. They run the engine full throttle until they reach ~20mph, then cut the engine. This way the engine is always operating at peak efficiency (no throttling losses). This driving technique could be a little impractical in stop-and-go traffic...
Answer: To the end of the universe.
Unless you're one of those people who figures in friction from air resistance, rolling resistance, etc.
"Prefiero morir de pie que vivir siempre arrodillado!"
You can get 125 miles per gallon all ready with one of these little mopeds, And they're cheap and street legal too.
gasoline has energy content of 114,000 btu/gallon
With a Carnot engine, that means you get half of that, max, assuming perfect cooling (which doesn't really exist).
By the way, 114,000 btu == 33.41 kWh
"I assumed blithely that there were no elves out there in the darkness"
Except that's not combusting.
Gasoline that has not been oxygenated with MTBE or ethanol holds about 115,000 BTUs of energy per gallon, give or take depending on environmental conditions and a few things about the blend. Oxygenates typically reduce that value by about 3% or so.
You can never go home again... but I guess you can shop there.
Checking some classnotes there is approx 686 kcal/mol in glucose which gets converted to 219 kcal/mol of potential energy in ATP. Which means only 32% efficiency for converting glucose to the energy form used by about every cell operation inside a cell.
Now there is also inefficiency in getting glucose to those cells (and not all of it of course gets to them). Plus ATP isn't completely itself efficiently used so 10% is prolly around the correct figure for the amount of energy used of digested food.
The misquoted Simpsons line is "40 rods to the hogshead", which is about 0.0019 miles per gallon. Presumably, though, that horrendously low fuel economy is made up for by traveling more than 5 furlongs per fortnight.
See subject line. I accept your rant, and raise you a hear-hear, in general.
However, you seemed to have invoked shades of a strawman - the grandparant did _not_ make any reference to a hydrogen fuel cell. It is, in principle possible to make a fuel cell that will convert fuels other than pure hydrogen into electricty (+ wastes).
That's not to say that they exist - most 'methonal' fuel cells are reformation style, where the carbon -> CO2 converstion is not used to produce power, but just to free up the hydrogen.
In principle, however, there is no theoretical barrier to a gasoline fuel cell, with high efficency (just a huge, _huge_, long list of practical ones). There _is_ a theoretical barrier to raising the efficency of an internal combustion engine.
The PDF on the web site says the engine in question is a Briggs & Stratton Corporation (Model 091202 Type1016E1A1001). The engine is air cooled, four cycle, with a 2.61 kw (3.5 horsepower) rating at 3600 rpm.
It's a tiny 1 cylinder engine.
-ted
If you look at a torque vs RPM graph, you'll see that torque rises then usually falls off, but on a naturally aspirated engine, it tends (not in all cases, due to VTEC, VVT-i, cam timings, etc) to plateau for a time. In that plateau is where your fuel efficiency is greatest. Get in gear and get to the point where your torque band starts, you win. This is the idea behind constantly variable transmissions. Keep the engine in its powerband and change the gearing constantly. Only problem is you can't put too much torque to them or they fall apart.
Also, you almost made a full connection there: horsepower will almost always rise with engine RPM as HP = (tq*RPM)/5252. The almost meaning if the torque band falls off dramatically, the HP may go down. Looking again at a Dynamometer readout, you will always see torque and HP cross at 5252. This is why even though a Honda may have 240HP and my car has a paltry 225HP, my 310ft/lbs+ of torque will "own" most any Honda (except the S2000, because that car weighs almost half of mine, but it still would be a pretty good race).
Slashdot is proof that Sturgeon's Law applies to mankind.
For freight there's no doubt that diesel locomotives are the winner. Diesel locomotives are hybrid vehicles: a 2-stroke diesel generator, but electric motors. They are very efficient at moving large loads, not so good at light loads due to the weight of the loco itself (something like 135 tons). That's why passenger trains tend to be purely electric - to keep the huge weight of the generator off the train.
Here are some links:
HowStuffWorks article on diesel locomotives.
A CN Railroad page claiming a diesel locomotive can travel 3.5 times further than a truck on a gallon of fuel (presumably pulling equivalent loads).
A BNSF Railroad page claiming fuel efficiency of approx. 750 GTM (gross ton miles) per gallon. Most high efficiency cars would probably weight a ton or less so a 50 MPG Prius would be about 50 GTM per gallon.
My recollection is that maximal efficiency is roughly at torque peak (ignoring such things as aerodynamics and gearing), and that underpowering a car kills the mileage.
That's exactly what I said - though actually, efficiency is pretty constant in the power band, so maximum fuel efficiency is at the lowest point in the power band.
(Except for the last part, but that's addressed later...)
Case in point: a particular truck is offered in an economy V6 and a V8 trim. The V8 got better mileage because the V6 was always running full throttle (above the powerband).
Woah, woah - you're talking about two different situations here. Most cars are way overpowered for going at the speed where aerodynamic losses equal total residual losses - this is about 35 or 40 mph for most cars. So when I said underpower the engine, I meant underpower it compared to most cars, not underpower it compared to its needs.
You're exactly correct that a car that's running full throttle will have crap efficiency, but that's because it's past it's torque peak. You want to be at the torque peak, not above it (full throttle) or below it (going slow).
In your case, the aerodynamics and rolling resistance are so high because the weight is so high that the car is now not overpowered to go the speed that's efficient for aerodynamics. The V6 would get better gas mileage than the V8 if it went slower.
Your Geo may get good mileage, but it's crap, and I won't drive one. I have an MR2 that gets 30 MPG and handles nicely, so I don't have to.
I don't own a Geo. It is however a good example of a car that uses standard design principles to get high gas mileage. Small engine, light weight.