I hear diesel has a high energy density compared with the batteries in electric vehicles. Will technology advance enough that electric becomes an attractive alternative? Or is there a better alternative than electric to replace diesel vehicles?
On an unrelated note, do you think there will ever be an electric, or other non-chemical powered vehicle, that could beat a top fuel dragster?
First of all, it’s not an either-or. Most large diesel vehicles are already electric: The diesel engine turns a generator, and the power from the generator turns small electric motors at the wheels. They’ve been doing it this way since long before small cars started using hybrid systems.
Second, are you assuming that the energy source will be aboard the vehicle? Most diesel trains can very easily draw power from an overhead wire, if one is available, and large portions of the eastern US do have such wires. So a “diesel train” passing through Philadelphia, for instance, is completely electric.
all common liquid fuels have much higher energy density than the best feasible battery tech available today. The biggest battery in a Tesla Model S stores the equivalent of 3 gallons of gas, yet weighs 1,200 lbs.
now, all else is not equal. the conversion efficiency (% of energy used to actually propel the car down the road) of an EV is on the order of 85%. For a gas car, it’s about 15%. diesel maybe 18%. So that 3 gallon equivalent battery goes a lot longer way than it would seem on the surface. but barring a significant breakthrough, it’ll be very difficult for battery energy storage to approach that of petroleum.
Not likely, at least for the foreseeable future. Top Fuel dragsters do what they do by
being relatively light, they’re little more than an engine, tube frame, and wheels
running on nitromethane, which while it only has about 1/4 the energy density of gasoline it brings its own oxygen (in the form of a nitro group which is extremely eager to give up that oxygen) so you can burn nine times the amount of nitro for a given volume of air
when Top Fuel teams are racing, they don’ t care if the engine is junk at the end of a run so long as the car makes it down the track in one piece.
with what we have right now, an electric car with enough horsepower to compete with Top Fuelers (about 11,000 hp) would need a lot of (heavy) motors and an incredibly heavy battery.
Electric motors have the advantage that they produce full torque right from the start and continue to produce torque all through their operating range, where internal combustion engines have a much narrower power range. Performance electric cars like the Tesla have wicked crazy acceleration off the line.
It’s hard to beat a top fuel dragster, though. They accelerate so hard off of the line that they actually deform the tires rotationally. Any more torque off the line and all you are going to do is shred the tires. Here’s a good slow-motion video that shows what I am talking about:
Top fuel dragsters also use automatic transmissions these days that can outperform a human shifting. The power curve isn't quite as linear as that of an electric engine, but they are pretty close to producing full power all the way down the track.
Electric dragsters aren’t there yet, but they are starting to play in the same playground. Here’s a video of an electric dragster.
Here’s an interesting article about an electric dragster that is currently setting a lot of records for electric vehicles. It’s not on par with top fuel yet, but the performance is still rather amazing.
Electric dragsters are very quiet compared to top fuel, and this has led to a lot of complaints that they are boring to watch. You don’t get that loud and exciting roar of the engine.
For conventional electric vehicles, energy density is a key goal, and there is a huge amount of research going into batteries. Currently the game changer is expected to be lithium-air batteries, which achieve an energy density pretty much on par with ordinary gasoline. Coupled with the much lower mass of an electric engine, and this makes for some wild possibilities. Including electric powered aircraft with significant range.
Comparing with diesel explicitly is an odd question. Diesel’s big advantage isn’t in vehicles, but rather in very large marine and stationary engines, where it achieves some of the best fuel efficiencies. Over 50%. In vehicles it has suddenly become enemy number one, due to particulate and NOx emissions.
Beating a top fuel dragster? I can see it being possible. Adopting much the same ethos in building an electric dragster could end up with similar performance. Batteries cease to be a limiting problem; if you only need to travel the length of the strip you could probably manage with a supercapacitor. Which could probably be designed to be happy delivering the silly current needed. The motor could be designed for purpose, and one could do such amusing things as dousing it in liquid nitrogen before each run, to improve conductivity and give it a fighting chance of not melting before it reached the end of the strip. Working the numbers on a viable design would be fun.
It’s not that odd, given current events in Europe. Diesel cars are much more popular in Europe than in the US, partly because of the way that diesel and gasoline have been taxed historically in Europe, and partly because early US diesels were a bit obnoxious with respect to sound and particulate emissions, giving the US a distaste for diesels that Europeans generally do not have.
Britain just announced last month that they are phasing out diesels, with a mandate that no new diesel or gasoline vehicles will be permitted to be sold after the year 2040. The popularity of diesels being what it is over there, it’s not surprising that a lot of folks are suddenly interested in comparing electric and diesel vehicles.
The UK 2040 ban has been all over the news, even here in the US, so I’m not surprised to see questions about anything even tangentially related to it.
Can you (or anyone) explain why they have to change engines in Albany on the Montreal-NY run and why they used to have to change engines in New Haven on the Boston-NY run if all (or most) diesel trains can use overhead catenaries?
For Europe, and perhaps especially the UK, electric is already attractive compared to diesel, assuming that one has a convenient place to charge overnight. The Tesla 3 and the Chevy Bolt both have ranges over 350 km. Britain is a compact country. Driving distances between cities are able to be done within that range there, and daily average mileage is only about 40 km (pg 63). Fuel costs obviously vastly lean to electrics’ favor and performance of similarly priced electric and diesel family cars favors the EVs as well.
Diesel sales have plummeted in the last couple of years: the main reason for their relative popularity was cheaper fuel, and it’s not cheaper any more. And for small vehicles it was never as popular as gasoline. Also, the bans multiple countries have announced won’t apply only to diesel.
So yeah, asking about diesel specifically seems strange.
A Top Fuel dragster travels 305 m in 3.7 s. Per the link, we have:
d = sqrt(8Pt[sup]3[/sup] / (9m))
Solving for P and assuming m=1 kg, we get P=2066 W. Therefore we need a specific power of at least 2.066 kW/kg.
The best lithium-polymer batteries available have a specific power of about 7.5 kW/kg. So with perfect efficiency and all battery, it could be done. In reality, you are going to be traction and/or current limited for the early part of the track. I’d ballpark this as doubling the requirements to 4 kW/kg. Then, of course, we have the fact that there’s no frame, motor, tires, etc. Could we dedicate half the mass to the battery? Maybe… this is tough, since we’re talking about several-G accelerations here. But it might just be possible.
That puts us at 8 kW/kg, is approximately what we have. So I’d say we’re in the ballpark at least, although there’s some serious engineering to be done. I like the idea of precooling the vehicle with liquid nitrogen; it only has to go for 3.7 s.
Supercapacitors are interesting but they have a tradeoff: as energy density goes up, power density goes down. At their worst, they’re not even as good as lithium-polymer. And at their best, they may not have enough energy to reach the top speed. So I dunno.
OK, so being somewhat silly, and thinking aloud here.
Top fuel dragster does the strip in about 4.5 seconds. Call it 5 for a bit of slop and a energy for a burnout. Probably power comes in at about 8 megawatts, call it 10MW for a bit of slack. So we are looking at about 50MJ. Supercapacitors - OK, 15kg for 166F at 50v. We can probably get half of that out of the cap before the voltage is too low for use. 2kA for one second seems the basic capability. That is 100kJ. So we need 500 of them. 7500kg. :eek: These are ruggedised units, and if we look at the bare supercap that goes into them, the mass per unit energy halves. So we are down to 3250kg. Using these maybe 3 tons. Even these cells are rugged production cells, and it isn’t too big a jump to imagine a further halving of mass if one pushed the idea hard. 1.5 tons. :dubious: Maybe. A top fuel dragster is limited to a minimum mass of almost exactly on metric ton.
So, supercap power is not totally out of the question. In the mid-term. Not doable right now, but we are not wildly out of the ballpark. A reasonable improvement in the technology will see us over the hump soonish. If we could get the mass down to say 500kg we would be in.
Motor is another issue. Basically you need to stop it melting in the first few fractions of a second. Say we build a 1000V motor. That still needs 10,000A. For a full five seconds. Copper melts at about 1300K. In reality we are only going to make things work if we can keep the motor very cold. Some sort of total loss liquid nitrogen pressure feed cooling maybe. Keeping the motor that cool means the resistance of the copper drops significantly, well over ten times. So we can drive more current through it, and the motor is proportionality more powerful. You might be able to get a motor of roughly the mass of a 1000kW conventional design to deliver the needed 10,000kW, for maybe the needed 5 seconds. Managing all the materials issues that come with cryogenic temperatures is left as an exercise for the reader. :rolleyes:
Heroic engineering. Again, not totally out of the park, just really really difficult, and not just yet. Now if we had liquid nitrogen superconductors with sufficient mechanical strength, it might be a different call.
So, given enough money, and about ten years, I suspect a viable top fuel dragster competitive car could be built. It would probably be an order of magnitude more complex and expensive than the nitromethane fuelled beasts.
ETA, ninja’ed, but complementary by Dr. Strangelove
I based my power on the claimed power of the engines used in the dragsters.
Those are quite a bit better than ones I found a while back. They have basically the opposite problem as the li-poly cells: whereas those had plenty of energy density but were on the ragged edge of power density, these supercaps have plenty of power density but barely enough energy density. The dragster is going about 150 m/s at the end of its run–that’s 11.25 kJ/kg of KE. Those supercaps have an energy density of 24.5 kJ/kg. It’s not totally insane that you could make the vehicle half out of caps, but it’s certainly a challenge. Plus, as you note, you have to add extra for the voltage drop (batteries don’t really have that problem).
Wait…surely you didn’t mean to say that catenary wires “aren’t suitable” for powering a modern Diesel-electric locomotive because can’t carry enough current to power Modern Diesel-electric locomotive. They can and do, albeit at much higher voltage than third rail systems.
The General Electric Genesis P32AC-DM is a Diesel-electric locomotive used in the northeast corridor, and it’s one of those that can switch between getting its power from the built-in Diesel engine and electric power via a third rail.
Your post implies that while a third rail can provide enough power for this locomotive, catenary cables somehow cannot. The Genesis cranks out 2400 kW from 650V DC third-rail power, while over in New Jersey, the Bombardier Alp-46 makes 5600 kW—more than twice as much—from catenary wires at 12.5 kV AC. And some of the French TVG trains produce 12,240 kW from the same 12.5 kV. (That’s over 16,000 horsepower!)
That third rail is carrying more amps than the catenary wires are, sure. But insisting that catenary wires aren’t suitable for powering trains because they carry fewer amps at higher voltage makes as much sense as arguing that overhead wires are “unsuitable” because they’re overhead, and the Genesis engine can only get power from a third rail, which is underneath.
Both overhead wires and third rails are perfectly capable of powering trains (supplying enough volts * amps) as long as the engines are designed (electrically and mechanically) for the tracks they’ll run on.
There’s nothing about overhead wires that makes them inherently unsuitable for powering trains. In fact, high-voltage AC (via catenary wires) is generally more efficient for long-distance power transmission than low-voltage DC (via a third rail). Since skin depth issues make high-voltage AC via a third rail inefficient, you could argue that catenary wires are much better suited to powering trains than are third rails simply because the catenary system’s transmission losses are lower.