Published on February 27th, 2018 | by Christopher Arcus0
Does Tesla Semi Break the Laws of Physics?
February 27th, 2018 by Christopher Arcus
Any sufficiently advanced technology is indistinguishable from magic
— Arthur C. Clarke
A while before the anticipated unveiling of the Tesla Semi, Elon Musk tweeted, “It can transform into a robot, fight aliens and make one hell of a latte.”
It can transform into a robot, fight aliens and make one hell of a latte pic.twitter.com/8h9vvWu4f5
— Elon Musk (@elonmusk) November 15, 2017
Those claims remain unconfirmed, but let’s take a moment to see if the Tesla Semi does indeed break the laws of physics.
Range, Battery Size, & Weight
Musk himself stated at the unveiling that the Tesla Semi highway range was due to extremely low aerodynamic drag.
One of the major surprises in the unveiling was the long range of the pair of vehicles, 300 miles and 500 miles. Experts were surprised and in fact predictions had been made that it would be impossible to achieve such range and the battery sizes and weights would be enormous. (I wrote a couple of articles later in response to the various false claims.)
The Tesla Semi consumes less than 2 kWh/mile. It can be shown by a number of different means that this energy consumption is certainly possible and valid. (Also see this for a deep dive on electric trucking.)
Electrical efficiency is high and regeneration reduces energy in short-haul use.
The evidence that aerodynamics reduces required energy is clear. For example, Daimlers aerodynamic Freightliner Super truck doubled mileage.
Unimproved electric truck efficiency combined with data on the effects of aero improvements underscore Tesla’s sub-2 kWh/mile efficiency. Battery pack size can be estimated at 500 kWh for 300 miles of range and 800 kWh for 500 miles of range. The difference between the diesel and electric semi can be compared to find the weight difference. The electric truck chiefly adds motors and battery, but has no transmission, just a simple reduction gear. The Tesla cab is carbon fiber, so there is some reduced weight there by comparison.
The Department of Energy gives a breakdown of Class 8 semi tractor component weights. The Diesel Powertrain components for a class 8 semi weighing 18,000 lbs weigh about 7,000 lbs.
What is removed when we go electric? Engine — 2500 lbs, transmission — 1000 lbs, 300 gallons of diesel — 2000 lbs, and 1000 lbs of driveshaft, empty fuel tanks, exhaust, particulate trap with urea tank, and injection. The electric truck also dispenses with the solid axles and tandem transfer shaft. The electric motors and reduction gear are countered by the weight savings in carbon fiber cab. For comparison, a Bolt motor is 76 kg. Tesla Semi has 4 motors.
The cabinet and cooling are not optimized for lowest weight. In the truck, the cooling systems would not be duplicated and would be more efficient due to moving air under operation. The physical cabinet would be built into the chassis, thus saving weight as well. We can estimate the battery weight per 210 kWh at about 3,300 lbs. 500 kWh would require about 2.4 times the weight of one Powerpack, or about 7,900 lbs, a penalty of only about 900 lbs for the 300 mile range vehicle. For the 800 kWh pack, the weight would be about 3.8 x 3,300, or about 12,500 lbs, but a long-range diesel sleeper cab should be compared and that is an extra 4,000 lbs, or a total drivetrain weight of 11,000 lbs, so the Tesla Semi comes out to about 1,000 lbs more for the 300 mile range Semi (~19,000 lbs), or 1,500 lbs more than a sleeper (at ~23,000 lbs). Those weights are competitive with existing diesel Semis. The area under the cab is sufficient to house the batteries.
|Semi truck / drivetrain type||lbs|
|Diesel class 8 day cab drivetrain||7,000|
|Diesel class 8 day cab minus drivetrain||11,000|
|Diesel class 8 day cab total||18,000|
|Tesla Semi 300 mile drivetrain||7,900|
|Tesla Semi 300 mile total||18,900|
|Tesla Semi day cab vs diesel day cab||+900|
|Semi truck / drivetrain type||lbs|
|Tesla Semi 500 mile drivetrain||12,500|
|Tesla Semi 500 mile total||23,500|
|Tesla Semi 500 mile vs diesel sleeper||+1000|
Here are some DOE bullet points on what’s in a semi truck as well as the weights of these components:
- Powertrain includes engine and cooling system, transmission, and accessories.
- Truck body structure includes cab-in-white, sleeper unit, hood and fairings, interior and glass.
- Miscellaneous accessories/systems includes batteries, fuel system, and exhaust hardware.
- Drivetrain and suspension includes drive axles, steer axle, and suspension system.
- Chassis/frame includes frame rails and crossmembers, fifth wheel and brackets. Wheels and tires include a set of 10 aluminum wheels, plus tires.
Weight of Truck Components on a Typical Class 8 Truck Tractor
|POUNDS||SHARE OF TOTAL|
|Wheels and Tires||1,700||10%|
|Drivetrain & Suspension||2,890||17%|
|Truck Body Structure||3,230||19%|
|Source: National Academy of Sciences, Technologies and Approaches to Reducing the Fuel Consumption of Medium and Heavy-Duty Vehicles, prepublication copy, March 2010, p. 5-42.|
For more interesting facts about 18 wheelers, see this.
A typical diesel day cab could cost $120,000, but that’s usually a 12 liter or smaller engine. Costs can go up from there. The Peterbilt 389 costs from $136,000 to $156,000.
Cheaper diesel day cabs may have smaller engines and also may not have tandem axles.
Given NMC chemistry, 1 million miles is possible. A 300 mile range truck would require about 3,500 cycles, including capacity drop, while the 500 mile range truck would require about 2,400 cycles. Those are easily within reach of NMC cells used in Tesla Powerpacks, warranted for daily cycling for 10 years and 70% capacity retention, a figure of 3,650 cycles minimum.
Up until recently, NMC cells have been NMC 111. NCA has been 0.8-0.15-0.05. So up until recently, NMC has had more cobalt.
The evolution of NMC batteries is a big deal because for a small density reduction, longer cycle life can be obtained compared to NCA. That’s a big advantage for semis and taxis that put a lot of miles on during their service lives. An NMC pack cycle life can allow the electric semi to match a diesel powertrain lifetime.
As Tesla is offering of 7¢/kWh, there is still room for a 7¢/mile tax and still get $20,000 less in fuel costs compared to diesel.
Diesel at $2.80/gal x gal/6 mile = 46.7¢/mile
Tesla Semi at 2 kWh/mile x 7 c/kWh = 14¢/mile
Add 7¢/mile EV tax for the Tesla Semi and we get 24¢/mile
Diesel is still about 20¢/mile more than electricity in that latter scenario.
100,000 miles/year x 20¢/mile savings = $20,000/year fuel savings.
And that doesn’t count the maintenance and insurance savings.
Fleet managers and buyers are probably very eager to lower other operation and maintenance costs besides fuel. There is no way they would even order one Tesla Semi on a trial basis without seeing some pretty firm and convincing math to prove it was at least possible.
Indications so far are that other performance parameter claims are verified.
Battery Pack Cost
There is very little information on pack cost, but from what is known, we have this: A Tesla spokesperson stated that Tesla pack costs were below $190/kWh in early 2016. Approximately one year ago, Tesla stated in a promotional video that Gigafactory 1 cost reductions on the battery were 35%, but that video has since been removed. That would put pack cost at about $125/kWh. Cell costs are dropping at a rate of between 5% and 7.5% per year. Taking this into account, costs drop an additional 10% before product is shipped and continue falling during production. That means the costs at the time of production would be less than $111/kWh. For a 500 kWh pack, that is $55,575 and it’s $88,800 for a 800 kWh pack. The 300 mile range Tesla Semi is $150,000, and the 500 mile range Tesla Semi is $180,000. Those costs nearly match the estimated diesel drivetrain and accessory percent of total costs. (Related: Tesla’s Gigafactory May Hit $100/kWh Holy Grail Of EV Batteries, Report Predicts)
There is good reason to believe that aerodynamic drag has been reduced to the claimed levels. We know from other real-world examples that diesel mpg could be doubled by such efforts. Tesla has made an extraordinary effort to reduce drag, more so than any other previous attempts. Tesla has smooth door handles and covered tandem wheels. The cab sweeps all around and blends with the trailer with a shell that leaves no gaps. The shell articulates with the trailer when turning. The underbody is smooth.
Notably, two Tesla Semis were unveiled. One is the same height as a standard 40 foot trailer, the other is shorter and comes with a matching shorter height trailer. The taller one has no exterior mirrors. A government ruling on mirrors is due soon. The shorter version has external mirrors.
The taller version may be the 500 mile range vehicle and the shorter one may be the 300 mile range version. The shorter vehicle will have considerably less frontal area, more than making up for the extra area of the mirrors.
Diesel semi drag has been miserable, with a typical coefficient of drag (Cd) as high as 0.75. That’s almost a square plate. Tesla Semi is the first really serious attempt to lower semi drag. Other prior attempts using diesels did not go that far and the average semi air drag is much worse than the Tesla Semi.
There were two versions of the truck at the unveiling. One had an aero cap mated to a tall trailer, and no mirrors. The other had mirrors and a shorter aero cap mated to a short trailer. The shorter trailer would have a lower overall drag due to lower frontal area, so it’s extra mirror drag would not be of consequence. There is a new regulation coming up in 2018 regarding cameras. There’s some speculate that cameras may be allowed in the new regulation.
Where Did Tesla Skeptics Go Wrong?
1. Aerodynamic advances and electric efficiency were ignored. That led to unrealistically large battery estimates.
2. Battery costs have been lowered by large-scale Gigafactory production. Annual battery cost reductions amount to increases in regular energy density improvements. High-tech economics assumes costs are an average over production life and lower at the time of production.
3. Battery advances had quietly allowed NMC to attain effective energy densities and at the same time allow long cycle life.
Tesla Semi has not confirmed that it can fight aliens or make latte. However, it appears likely that detractors may have missed the significance of aerodynamic efficiency gains on range. A number of sources give credence to claims of less than 2 kWh/mile efficiency given excellent aerodynamics. Lowest prior estimates of aerodynamic Cd were as high as 0.45, badly missing Tesla’s claimed Cd of 0.36. Given the lower drag, battery pack sizes and weights are consistent with the data from the Powerpack, which Tesla claims has the same cell type. This cell type, NMC, is capable of 4,000 cycles with 70% capacity retention, enough to last a million miles, competitive with diesel rigs.