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Comment by rdl | Hacker News Reader

I'm curious if I'd want a Model S (top model) or a BMW 535d or other mid-luxury diesel sedan in Germany in a couple years.

Around the city, sure, big advantage to the Tesla. For Autobahn trips at 150-250 km/h, stopping every 1-2h would get old fast, even if every service stop had a supercharger.

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If you're going that fast, you'll have to stop every 1-2h to refuel in just about any car, ICE or EV. On the one hand, yeah you might have to stop for a little longer in a Tesla, but on the other hand you're going to save shitloads of money: driving that fast is massively inefficient, so you're going to spend a lot of cash on fuel in an ICE, whereas the superchargers are free.

Heh. The BMW 535d mentioned has a 70L tank, and even on worst-case drives (sporty behaviour, not highway), it's quoted (by users) around 15L/100. On standard highway (not unlimited-speed autobahn, euro highway 120~130km/h) it's quoted (again by users) around 7~8L/100.

edit: and do note: your 5~10mn refuelling break on a normal car brings you back to full range, on a Model S it's 170 miles not the original 260-ish.

[0] http://www.bmwblog.com/2011/03/16/the-diesel-driver-reviews-...

At 160-180 km/h in a rental 116d with winter tires I was actually getting about 6L/100km. I was able to do Berlin to Auschwitz (Poland) on a single tank, with 0.2 tank left over. (Poland technically has speed limits I think, but they weren't enforced) For a combination of high-speed-autobahn and city/etc driving, I was basically refilling for EUR 65 or so every 600-700km.

The rental 116d couldn't do >200 km/h, so I'd probably want something a bit bigger, but even a 123d would be a reasonable option. I was thinking 535d just in case I had to have passengers.

Regrettably the 328d is the smallest diesel BMW bothers to bring to the USA.

Part of that now might be that we went from dirtier diesel than Europe to cleaner (at least, lower sulfur, but I think otherwise cleaner) in the past few years, and have more strict emissions standards. But that shouldn't necessarily hurt the 2.0 diesel.

Sporty behavior not on a highway is still going to get better consumption than extremely high speed driving, because wind resistance rises as the square of the velocity. If you double your velocity, then air drag increases by 4x, so if users are quoting 8L/100 on 120-130km/h then you can expect to get about 32L/100 going 250km/h.

With a 70L tank at 250km/h you may well exhaust the tank in under an hour. These are worst case numbers though, so if you drive a bit slower, then 1-2 hours between refueling seems like a reasonable estimate.

Actually it doesn't look that bad.

http://www.automobile-catalog.com/economy/2014/1900655/bmw_5...

Fuel consumption:

in liter / 100 km

Estimated fuel consumption by constant speeds on top gear, steady ride without acceleration or braking, flat concrete or tarmac surface, no wind

by 50 km/h (31 mph): 8 l/100 km

by 60 km/h (37 mph): 6.8 l/100 km

by 70 km/h (44 mph): 6.5 l/100 km

by 80 km/h (50 mph): 4.9 l/100 km

by 90 km/h (56 mph): 4.9 l/100 km

by 100 km/h (62 mph): 4.9 l/100 km

by 120 km/h (75 mph): 5.2 l/100 km

by 140 km/h (87 mph): 5.6 l/100 km

by 160 km/h (100 mph):6 l/100 km

by 180 km/h (112 mph):6.5 l/100 km

by 200 km/h (124 mph):7 l/100 km

by 220 km/h (137 mph):7.6 l/100 km

by 240 km/h (149 mph):8.4 l/100 km

by 260 km/h (161 mph):9.3 l/100 km

Clearly around 200 km/h is where the wind resistance (and gearing) is starting to become a factor, but I think you could probably hit 280 or so without being over 12 l/100km.

The other factor is that when I've driven >250 km/h I've gotten tired much faster, as you need to be hyperaware. (I did almost 200mph in a "very nice" Corvette Dubai-Abu Dhabi for about 20 seconds, and could see the fuel dial dropping)

200-250 seems like a reasonable high cruise speed for parts of the Autobahn, though.

The smarter option is probably Berlin to Munich/Frankfurt/Prague/etc. by train, of course, but I like having a car.

Your link says that those are "estimated fuel consumptions", but I'm somewhat inclined to doubt the veracity of these estimates: between 120 km/h and 240 km/h wind drag is going to quadruple, and ICEs get lower efficiency at higher RPM. So if the efficiency falls by less than 50% when you double the speed, then that implies that tire drag and internal friction represent the majority of the force you're overcoming at highway speeds, which is definitely incorrect. Something doesn't add up.

Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h.

The engine is sized for peak acceleration (and marketing). It has a turbo (in the 535d; the 550d has 3....), which helps somewhat without adding more fuel consumption, but still, a 300hp engine is pretty big.

Basically, to cruise along at 120kph in a reasonable car with reasonable Cd probably is about 20-30hp (yay for mixing metric and imperial units...). Wind is probably 20hp of that. The engine is less efficient at low output (specific output), so you won't get fuel consumption at 10% power which is only 10% of peak fuel consumption.

Wind resistance does go up, but your engine also gets to operate more within its power band. With the right gearing, you can stay within the optimal RPMs and be close to the design power level of the engine at a reasonable cruise speed.

Empirically, I've driven big heavy cars fast for long distances, and gotten reasonable fuel economy; especially big German cars with big diesel engines. What kills them is stop and go, or when you have a smaller car with, say, 5 gears, and you are running it at redline to keep your desired speed.

What is awesome about electric cars is you could get much more efficiency in the motor across a range of power levels, plus regen. What is not at all awesome is the power density of lithium ion batteries.

   ...between 120 km/h and 240 km/h wind drag is going to quadruple...

Your figures here are an idealization. Real results are not going to match this because air resistance and fuel consumption are not a smooth process. (Some versions of the Tesla S even have active suspension, which really makes things interesting.) I expect you'd need some pretty serious finite element modeling, and engine modeling, to really understand the linkage.

> Sporty behavior not on a highway is still going to get better consumption than extremely high speed driving, because wind resistance rises as the square of the velocity.

It's a heavy car, and one built for highway, ramping up and slowing down all the time is extremely expensive.

> With a 70L tank at 250km/h you may well exhaust the tank in under an hour.

No. That would be a fuel consumption of above 28L/100, that's twice the worst case I've seen from any user of the car.

Hell, you can run long-haul trucks with a better efficiency than that: http://newsroom.scania.com/en-group/2011/05/27/scania-fuel-e...

> These are worst case numbers though

They're closer to lunacy than worst-case numbers (and the autobahn is not free-speed throughout). Although we'd need an actual 535d with an actual driver to know for sure.

Calling my numbers "lunacy" will, unfortunately, not change the laws of physics. If you drive twice as fast, air drag multiplies by four, and the energy to overcome that air drag has to come from somewhere. At highway speeds air drag is the only appreciable force at work. Tire drag makes a little bit of difference too, but that will be more than canceled out by the fact that ICE efficiency drops at higher RPMs.

Your "long-haul truck" citation is irrelevant to this discussion because those numbers are for drastically slower speeds than the ones you're talking about.

> Calling my numbers "lunacy" will, unfortunately, not change the laws of physics. If you drive twice as fast, air drag multiplies by four, and the energy to overcome that air drag has to come from somewhere. At highway speeds air drag is the only appreciable force at work.

As you will. Not that this changes a thing, the Model S's efficiency will also drop further, thus further lowering its range. The issue remains, even driving at inane speeds, the Tesla will have a significantly lower range than the 535d.

Heh.

250km/h for 2 hours on a 70 litre diesel tank in a BMW 535 would require > 73% efficiency of the engine, which car engines don't reach.

Calculations assume: Coefficient of rolling resistanc) = 0.01 Mass = 1700kg Coefficient of drag = 0.25 Frontal area = 2.2 m^2

...and those numbers are on the kind side to the car.

On the other hand, 150km/h would require < 20% efficiency to last 2 hours.

Writing calculations is a bit tiresome, so I'm not going to worry about it unless/until someone provides some calculations producing significantly different numbers.

>If you're going that fast, you'll have to stop every 1-2h to refuel in just about any car

Yeah, no. No way in hell, actually. You think driving 150-200 for 2 hours would drain a 70-80 Liter gas tank? I drive 50 minutes to work each day going that fast, I have an 80 Liter tank and I have to stop for gas once every 5 business days. You can calculate yourself that this equals to a "little" more than 2 hours of net driving time.

Is that within speed limit there? I didn't know that

Many stretches of the federal roads in Germany merely have an 'Advisory limit' with no legal maximum in good weather.

http://en.wikipedia.org/wiki/Autobahn

If this sounds crazy, keep in mind that getting a drivers license in Germany isn't a one-day affair with minimal testing standards like it is in the US. Getting a German driving license is somewhat laborious but the upside is that German drivers are pretty darn safe and have amazing discipline on the road.

http://www.amcham.de/services/drivers-license/us-citizens-in...

The nice thing is most US states have reciprocity with Germany, so you don't actually have to take the tests.

I noticed a fair number of "bad" (by German standards) drivers on the southern Autobahnen over the past few weeks, but they were mainly non-Germans; I'm 99% sure the drunk guys in the 15 year old Honda were US military, and by the plates, most of the other bad drivers were also not German. So it's not foolproof, but still the standard is far higher, and the roads are well maintained.

What I noticed most acutely was the lack of objects-in-road, vs. California. Not sure if that's due to the general lack of pickup trucks, people hauling junk, etc., or due to laws/training/maintenance.

It's also because people in Germany have manners. In California, or the rest of US, people just throw garbage on the side of the road. It's so infuriating! I once saw a guy throw a cigaret pack on the ground literally, and I kid you not, literally 3 feet away from a garbage can. I had to "kindly" insist that he picks it up and disposes of it properly. It's like some people here were raised by fucking wolves or something. I see people throw fast food bags, half way full drink cups, and plastic bags out of their cars on the freeway all the time.

Pretty sure he meant debris, not trash. Stuff that falls off trucks because it hasn't been properly secured. I've had a number of close calls due to large items blocking entire lanes on the freeways.

Yeah, I mean like a sofa, bumper, etc. I leave a larger-than-usual amount of space between me and the car in front of me, but in heavy traffic, you can't always evade, particularly if the car in front is a high-clearance truck, and the object clears that car, but not necessarily my normal-height sedan. Within hours of being back in SFBA I'd run over a boot, then another shoe (possibly the boot or another shoe), in separate directions of 101. It struck the front bumper and didn't seem to catch on anything underneath, but I was worried.

Roadside trash sucks, too, for different reasons.

(I call 911/CHP to report this stuff frequently, on average once a week or more. They send a CHP car or two out to road-break and remove it, or CHP+Caltrans if it's big.)

Worth noting that US military that are stationed in Germany must take a test before receiving a US Army Europe license (they run the program for all US military in Germany) in order to drive in Germany. Based on my experience (got there in 2004), the test was not that easy and did require a bit of studying, especially for road signs and the rules at intersections, which are a little bit different than US rules, from what I can remember.

The US military is semi-enlightened with regard to traffic safety in that they often insist on extra training even for domestically stationed personnel. For example some branches and some bases require all enlisted personnel to undertake basic motorcycle safety training before they are permitted to ride motorcycles, even though most states do not require such training. There are also branches and/or bases where servicemen returning from overseas deployments are required to take refresher driving courses before they are permitted to drive their cars again. On any installation the local regulations are up to the post commander; some of them are more strict than others.

It is amusing that the US military is in some ways the most highly regulated and the most European-like in their regulations, due to the fact that commanders have broad authority and the services provide universal health care and housing for enlisted men and their families. They have good incentives to establish standards of vehicle safety.

A lot of that is that losing your forces due to non-combat accidents can hurt your mission. In addition to broad authority to regulate lives, they're given responsibility for accomplishing a mission; that doesn't really exist in the wider world (what is the "mission" of the USA, or really any country, w.r.t. its citizens? This becomes a philosophical debate rapidly.)

IIRC, more junior enlisted Marines were dying from motorcycle accidents, per capita, back at their home bases (it might have been specific to San Diego), than in combat in Fallujah.

(OTOH, they went overboard with things like the PT belts...)

The USAFE/USAREUR driving test is way more difficult than any test in the states. But still far easier than the tests the Germans themselves have to pass.

The priority road concept, with unmarked intersections, is one that wouldn't work all that well in the US.

I believe they also require a regular safety check of all vehicles that would determine your car was capable of safely driving at speed on the autobahn.

There is no "speed limit" per se on the Autobahn.

Within specified areas.

Inside cities/towns, the limits are often both strictly enforced and dynamic. That is: they change with traffic conditions and are signaled via electronic signboards.

Germany also has some horrific auto accidents, though the death rate per 100k population/yr. is less than half that of the US, at 4.4 vs. 10.4.

https://en.wikipedia.org/wiki/List_of_countries_by_traffic-r...

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