Road trips w/ electric vehicles

Factual Questions
261

There was an article a few years ago about a Model S that was being used in a taxi service between LA and Vegas. This would be the absolute worse treatment a battery pack could get - multiple fast charges a day in a hot environment. The first pack lasted about 170,000 miles, and the vehicle was at 300,000 with the second (so 130,000 on the pack) and still going strong. The first pack did not so much as “degrade” together, as that one or more cells failed completely so the pack was not as useful. The battery pack consists of collections of batteries a bit bigger than AA cells, with a computer managing their charge and monitoring capabilities to avoid overworking one group.

The other thing is, if there’s a business where the trucks are being charged overnight, then they are slow-charged, which also helps with battery life. Another useful application for chargers in that case would be the ability to evaluate all the vehicles plugged in, and detemine what level of power is needed to finish charging them all by the target time, instead of charging everything starting full power at 8PM, say, and finishing at 3AM. (Since many businesses are charged for peak use too.) The smarts to adjust to demand billing - charge when it’s cheapest - would also help.

Also I see a lot of semi’s on the road with two trailers; I recall some article about “semi-trains” that mentioned that for the route across Australia, they sometimes have 3 trailers. I wonder if the Tesla Semi could be adapted so that multiple semi’s could connect and be pulling a larger number of trailers like freight trains do, in some cases. It seems like an ideal application for computer control, one or two people in the front cab “driving” multiple engines and trailers on long hauls. Certainly that would help minimize overall wind resistance per unit.

262

Tesla is claiming a Cd of 0.36. Their cars are close to best-in-class when it comes to drag, so it doesn’t seem too unreasonable. The overall shape is way sleeker than a typical truck, plus they employ a number of tricks on top of that–pulling the trailer close in to reduce gaps, having flaps on the side that nearly reach the ground, etc.

The 500 mi/800 km range was claimed for 60 mph (96 kph). Which sounds about right given the improved Cd. Of course, that’s still only under optimal conditions–a small headwind could cut the range by a lot.

263

I’d be very impressed with 0.36. That gets them down to 1570N of drag at 96km/h. So yeah, just within doable. Assume they get the absolute best tire design for rolling resistance and they might claw everything in. Just.

That 0.5 number came from studies that already close in the gaps between elements and the ground. Totally enclosing the sides - wheels included - can pull it a little lower. The length of the rig also makes a huge difference. 0.36, will, I bet, be with a short single trailer. A B-Double could be twice as long. That folds back into the effective C_D. I would suspect most of the low hanging fruit on aerodynamics has already been done.

The Wikipedea page on the Tesla Semi notes that Pepsi claim that they can get 684km if the truck is hauling packets of Frito-Lays. If it is hauling cans of soda, they limit runs to 160km. I suspect in both cases they are limiting wide battery cycling to avoid degradation.

264

US-ian here. Is there a difference between a “B-Double” and the US version called “tandem trailers”?

The term is new to me.

265

It is subtle. A B-Double has a fifth wheel connecting the two trailers. I’m not sure if a “tandem trailer” does this or has a drawbar between the two trailers. The B stands for “bridge” referring to the coupling between trailers.
The difference is that a B-Double is more manoeuvrable because of the articulation - in particular you can reverse one. This means they can be sensibly allowed through cities (at least on designated routes) whereas simple doubles - without the second fifth wheel are generally not allowed. My daily commute has me share a significant part of the journey with semis, and a goodly number are B-Doubles. (It isn’t great.)

266

Acceleration/deceleration shouldn’t be relevant to mostly-freeway routes, and mass won’t affect air resistance. If mass is making that much difference to range, it looks like rolling resistance on the tires is the limiting factor. It seems like there ought to be some way to custom-make tires for high-weight loads, that don’t deform as much.

267

In a US freight company, a double or rarely a triple uses the 5th wheel attachment method. Here (US) it’s called a dolley. There are tandem trailers (no 5th wheel), mostly fuel and farm usage. I worked at Red Star Express for a few years and due to a bridge restriction, we couldn’t pull doubles/triples to our dock from the interstate. ONE of my jobs was to drag a trailer or dolly over to the interstate parking area and set up for the drivers to attach. Union rules! I’d also drag back a single that was being dropped off a double in the winter (Union rules be damned - too cold to go out for them).

268

I was curious what the differences were and found this image helpful.

269

It seems to me that if we’re serious about making long-haul trucking electric, then the solution isn’t gargantuan batteries but taking a page from railways and adding overhead supply lines and put retractable electric pickups on the tops of the trucks.

Or just move a lot more traffic onto railways, which are much more efficient to begin with. But trucks are preferred for lots of stuff because trucks are more flexible in terms of start/end points of trips. If you could cruise along the freeway beings supplied with electricity the whole time, and just needed batteries for the beginning and end of the trip, then a fairly modest battery would be sufficient.

Obviously there’d be some significant infrastructure investments required. Australia seems like a good case, though, given the relatively few major routes and the seemingly higher reliance on trucks vs rail. Outback road trains probably diesel for the foreseeable future.

270

Yes, but very little rail in the US is electrified. If we’re going to electrify a mode of transport with overhead lines, rail should be done first. It would have lots fewer technical issues than semi-trucks.

271

I would suggest that unlike Europe, North America outside the eastern seaboard and north midwest (Ohio to Minneapolis) population is not dense enough for electrifying rail lines. Perhaps they too could learn from the EV industry and create battery-electric locomotives. This is even more suitable because the nexus for swapping and charging locomotives is more limited, a few large rail centers, and the lines are less rolling resistance than tires on asphalt. Then there could be the equivalent container facilities to redistribute where the rails don’t reach.

272

I will point out that AFAIK a typical diesel/electric locomotive is in the 16000 HP range. Most of the train would be battery. A typical semi load is 60,000#. A single rail tanker is +/- 240,000#. Yes, I sit at the RR crossing often, and no, I’m not psyched about the rail companies pushing for mile long trains.

273

I think you mean aggregate for a long train, rather than a single loco. We regularly run interstate long interstate freight trains with of the order of 100 freight cars. They will typically be quad-headers, with up to 4 x 4000 hp locos. So 16000 in total. Actually I have seen 5 and even 6 for big bulk trains.

Diesel-electric locos run from about 1000hp for light duty to about 4500 for heavy freight. The most powerful I know of here is 4500hp. Locos go up to 6000hp for special uses, but they are rare beasts. It makes more sense to just add another loco for most uses. (I am reminded of the old Union Pacific gas turbine locos - claimed to be the worlds most powerful at 8500hp, and the steam locos they replaced - the fabulous Big Boys, at 6300hp. These were designed for a routes with significant grades.)

The rule of thumb for loco power is one horsepower per ton of freight. (Which is abusing units for just about every component.) Value varies around this, maybe 0.7 to 1.4, depending on load type, grade, and track. Compared to road freight the power requirements are very low. One 4000hp loco we run (the NR class) has about 500,000N of tractive effort. (Where I was sitting a little while ago, a favourite cafe, has a main rail line right out front, a double header NR loco freight train went. That was an unusually little one. Usually they are quad headers.) This number doesn’t directly get us energy use however.

One number I have seen is 0.4GJ per tonne per km for rail freight. This is gross mass. But I don’t see much support for that. A more nuanced study uses net load - so it is hard to find the gross load, but in many ways it is more useful. Energy for net load is however a very wobbly number, it varies very significantly by the type of freight, as volumetric efficiency matters. For instance car carriers are awful, and bulk loads particularly good.
For an electric train over moderate terrain the efficiency may get as good as 0.1GJ/tonne/km. A car carrier over mountainous terrain might be closer to 0.5. (Iron ore is probably fabulous, but since they generally only ever run from inland mine to a sea port, they don’t actually need to use any electricity at all.) General freight probably sits somewhere about 0.2. One sees a ratio of about 1:2 to 1:3 for car mass versus load.

A 20000 tonne gross (really big) freight train might therefore carry in a best case of the order of 15000 net tons of freight. At the nice end, using our notional 800km journey, is about 1.2PJ. At the unreasonable end, with poor volumetric efficiency, we could see five times that. However we won’t see trains that long, as they get length limited. But it provides a far end metric. So we get a range of about (rounding things furiously) 400,000 to 2,000,000 kWh of battery needed. Multiply by 5 (for a very efficient chemistry) to get kilograms and the range is 2,000 to 10,000 tons. One would want to use a more stable chemistry - so say lithium titanate which will easily double the mass, and send the price into the stratosphere. So a starting value of 20% of the train’s mass and it just gets worse.

So yes, I doubt anyone seriously thinks battery power is viable for a long route. And for shorter routes overhead electric will always win. Long distance electrification is possible, but it isn’t exactly cheap.

Very short route train freight is terrible, with efficiencies about 5 times worse than long haul. Which means trucks actually beat trains. One would only consider these runs as feeders to build up long through trains.

274

There are people working on battery storage for trains but it’s in the “let’s develop new chemistry” stage.

275

For someone like me with a poor grasp of trucking numbers - so what’s the comparison between trucks and trains, in terms of battery and tons hauled? Do electric semi’s work simply because they don’t haul anywhere near as much per hauler/battery pack as a train needs? (i.e. throw lots of trucks at the problem?) I can’t see tires on asphalt being as efficient for rolling resistance as steel on steel.

(and it occurs to me, those spectacular derailments in the news, or even a simple one, would be a lot more interesting if it then required a means of lifting huge heavy battery packs back on the rails. )

276

OK, I don’t know how long a time a train will go before it’s reasonable to swap out a locomotive, but you’re going to have to swap out the engineer every 8 hours or so, so let’s assume that. That means that the batteries need to store 8.6e10 joules. If we’re looking at a battery with an optimistic energy density of 1 MJ/kg, then that eight hours at 4000 HP would require 86 tons of battery. Which sounds like a lot, but I’m reading online that modern locomotives can be over 600 tons, so maybe that is reasonable.

277

I defer to @Francis_Vaughan obviously, but basically each train car=3-4 semis. Trains are far, far more efficient than semis, although you usually need some trucking for durable goods at each end.

278

I expect that the reason why battery trains haven’t caught on is that, in the places where there would be a push for them, there are already electrified rail lines and so they’re not necessary. I know the rails in much of the US eastern seaboard have overhead power lines, and I suspect the same is true for California. In the middle of the country, electrified lines will only be found on local transit systems, but those parts of the country mostly don’t object to burning diesel.

279

Not including some commuter lines,* the only electrified mainline railroad is from Boston to Washington DC (North East Corridor) with a branch from Philadelphia to Harrisburg. California High Speed Rail will be electric when it finally opens, but it’s still very much under construction. And there are no electric freight trains; freights run on the NEC but with diesel engines.

*Various routes in metro NYC ranging into Connecticut and New Jersey, the entire Philadelphia commuter system, one Baltimore-Washington line, a couple of routes in metro Chicago, a few lines in metro Denver, and the almost-completed electrification of the San Francisco-San Jose commuter line.

280

Huh, I thought it was more than that. That Philly-Harrisburg leg is the one I’m familiar with; I used to ride from Philly to Johnstown, PA on holidays, and we always had to stop a bit in Harrisburg for the changeover.

Still, Boston to DC covers a lot of passenger travel.