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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.
What kind of gas mileage will they get when they are loaded up with 1000+ pounds of DOT required safety equipment?
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How about the most fuel efficient 4 door seating for 4 w/ trunk space, radio, air conditioning, that meets federal safety and crash tests?
Than watch those MPG numbers plummet. Add to that must have respectable performance numbers (ie it must not be so slow accelerating as to cause a hazard on public roads)
That's a real contest.
When I can buy a car with that kind of effencieny I'll look into it, but until then, a walkin' I a' go.
Must be a bitch to take that shiny new 21 inch monitor home from the store.
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It's important to note that MPG has a lot to do with driving style. While my car cannot get 1700 MPG, a bit of predictive driving (i.e. know when to start slowing down, when to build up momentum) will greatly increase the MPG.
It depends on where you are, but in the US the NTHSA and the DOT would strongly disagree with you. Vehicles sold in the US have to meet certain standards of crashworthiness in order to be allowed to be sold here for street use. This has kept a lot of cars from being imported here, because they would require significant modification. It is a result of all the big old cars (and big new cars) we have on our roads, of course. In Japan, where large vehicles are relatively rare (you have delivery vehicles, and tiny vehicles, and not much in between) you can have lots of little beer can vehicles because they can't do nearly as much damage to one another, whereas here in the US you have scads of two-ton-plus vehicles, even passenger cars with that kind of weight.
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There's one catch. Nitrogen is very stable. Almost any chemical reaction will take more energy than it releases. When it comes to engine efficiency, this is Not Good.
Ideally, what you'd want to do is separate the oxygen and nitrogen, so that the oxygen ratio in the engine is much higher. Since you're losing less energy through the nitrogen, you would (by implication) get more useful energy out.
Ok, so how to do this, without reducing the energy you're getting from the oxygen at the same time?
That's tough. However, it may be possible. Nitrogen, as mentioned, doesn't react easily. The electrons in the outer shell are tough to displace. With oxygen, the reverse is true. Oxygen reacts very easily, and electrons are displaced with considerably less effort.
You can certainly use this to separate oxygen and nitrogen. Just set up an electrically charged grid, such that the charge will convert O2 into O2+, but leave nitrogen (N2) electrically neutral. Set up a second grid, with the reverse charge. The oxygen will be attracted towards it, the nitrogen won't.
If you picture the first grid at the entrance to a y-shaped tube, and the second grid at the fork splitting off of the long section of tube, you can see how the nitrogen will travel straight on, whilst the oxygen will be diverted.
Now, here's the tricky bit. The oxygen is one electron short (it's charged), and you've got to put quite a bit of energy into a device like this to charge the grids up enough. Will you get a net gain in efficiency?
That part, I can't answer.
Would it be worth doing anyway? Maybe. Well, it'll cut out a major air pollutant. The oxides of nitrogen that you get off will react with water to produce nitric acid. Not really something I want to be breathing in, if I don't have to.
Are there better solutions? Not using a conventional piston engine. We're almost at the limits for those, given a standard air mix. A rotary engine might get you a better theoretical limit (you don't have to keep reversing mechanical devices), but they're costly to make (they develop far higher pressures) and you have to develop one that's large enough that the increased surface area to volume is no longer a factor.
For ultimate fuel efficiency, I suggest a small fusion reactor. Though you may need to wait a while for them to be approved for use in cars.
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It's not really a fair question, as aircraft are so very different from cars. Their handling and common behaviors are different, as are the tasks to which they are put- An airliner may be less efficient than a car on paper, but if you try to move 300 people across the US with both of them, the plane may still come out on top.
Also, planes can use propulsion systems much more exotic than a reciprocating mechanical engine.
The SAE competition in the link requires a four cylinder engine. This kind of rules out other types of power such as steam, fuel cell, and stirling engine. Although, I suppose with enough modification, the provided Briggs and Stratton engine could be converted into a steam engine (not that this is necessarily more efficient). Let's see, new camshaft, a means to adjust the valve cutoff, maybe one of those cool looking fly-ball governors... Since a steam engine can apply power in each cylinder on every revolution, this makes it equivalent to a V-8. If you seal off the crankcase into a separate compartment for each cylinder, you can use both sides of the piston and make the equivalent of a V-16. Of course, details like, how to water from condensing in the oil will have to be addressed.
Also, since the peak horsepower of a car is rarely needed except in rapid acceleration, I would think that the key to reducing engine size, and thus, improving efficiency would be to use a small engine with some kind of storage system. Since batteries are bad for the environment, maybe two flywheels rotating in opposite directions (to cancel out precession) under the floor can be used, along with an electric motor/generator to transfer power to/from them. Extra power generated by the engines, as well as from braking, can be used to accelerate the flywheels. This would also improve handling because the gyroscopic effects would keep the car perfectly level on fast turns.
Also, I would think that the car would be cheaper to engineer and produce if you could eliminate most of the mechanical parts. How about a gasoline fired generator, a flywheel battery, and an electric motor on each axle?
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Some students do something cool in a contest and all most people are saying is "yeah, call me when it's really a car." Criminy. Articles on case mods get friendlier comments than this, and this is something that I would have thought geeks would have found interesting. Or nerds. Or whatever we are.
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The Carnot engine is the basis of the 3rd law of thermodynamics. I think you can put that in the theoretically sound category.
The actual efficiency is 1-Tc/Th. From wikipedia.
In this equation, Tc is the temperature of the heat sink, and Th is the temperature of the engine's heat source. For a 40% efficient engine, your hot engine gases have to be about 1.75 times hotter than the atmosphere that you discharge your exhaust into.
That doesn't sound like much - but remember you have to use absolute temperature. Room temperature is about 300K. So you need a 500K heat source - which is 230C, which is actually fairly hot. And of course you have all kinds of non-idealities in a real engine.
The only way to get anywhere near 100% efficiency is to get the hot part really hot and the cold part really cold. That is why metal-cooled reactors are fairly efficient - you have liquid sodium metal (very hot) coupled with river water (reliably cold).
If you run the math backwards it tells you what the maximum efficiency of an air conditioner is as well. As the temperature difference between hot and cold grows the efficiency drops accordingly. Of course, in real life you also have to deal with the fact that as delta-T grows your walls also start leaking heat like a sieve...
I'm a machinist, and I've dealt with automotive engine blocks before. I think the big problem is going to be manufacturing costs. When machining a ceramic, it tends to chip very easily, which could raise costs due to high waste, and special manufacturing procedures that hamper productivity. However, since it's non-ferrous, you can use diamond tooling instead of the traditional carbide tooling, which will save a fortune on tooling costs
Ceramics are also very abrasive, which might drive up maintenance costs due to the need to frequently replace piston rings. The engine block itself should wear much more slowly than a normal cast iron block, however.
Ceramics can be pretty resilient even when faced with temperature stresses, but I don't know how well a car that needs to be running one moment, and parked the next would fare. I doubt people would put up with the need for a 5 minute warm up period, especially if failure to do so would destroy their car.
Another issue is that a ceramic block would be impossible to repair, and would probably be a good deal larger than a regular cast iron engine to provide strength at every location on the block that feels stresses. But, if it's possible to build ceramic handguns, I'm sure it's possible to build a durable ceramic engine block.
I doubt there's very many manufacturing experts who read slashdot, but I would be very curious to see solid numbers on the costs of ceramics manufacturing compared to traditional cast iron. I haven't done much work with ceramics, so much of the above is just educated speculation. Treat it as such.