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Tesla Reveals High-Profile Electric Roadster; Calculates EV is More Than 3x as Efficient (WTW) as Fuel Cell Vehicle

The Tesla Roadster.

Tesla Motors unveiled its much-anticipated all-electric two-seater roadster. The lithium-ion battery powered sportscar features a 248hp (185 kW) electric motor that accelerates the car from 0 to 60 in four seconds.

Built by Lotus for Tesla, the Roadster has a range of about 250 miles and a top speed of 130 mph. The price for the Roadster will be around $100,000.

The custom-designed battery system (Energy Storage System) weighs in at close to 1,000 pounds and uses commodity lithium-ion cells. The system addresses thermal balancing with a liquid cooling circuit.

The 3-phase, four-pole motor uses a low resistance “squirrel cage” with large copper end rings. This allows the rotor to develop high current flows and torque, with low resistance losses. The use of a small air gap allows tight inductive coupling which, combined with low loss magnetic materials, enables the development of high torque at high rpm. Together, these factors allow the induction of large currents, even at high rpm, producing much flatter power and efficiency curves from approximately 2,000 rpm to 12,000 rpm. The motor redlines at 13,500 rpm.

(Devising a cost-effective method for the production of copper motor rotors has been under investigation for years. Siemens introduced three motors with die-cast copper rotors to the US market in April.)

Comparing Well-to-Wheel Efficiency and GHG emissions. Click to enlarge.

In a white paper (The 21st Century Electric Car) published on the Tesla Motors website, the company calculates the tank-to-wheel (actually, the “electrical outlet to wheel”) energy efficiency of the Roadster to be 2.18 km/MJ.

Assuming electricity supplied from a combined-cycle natural-gas-fired generator, and accounting for transmissions losses over the grid leads them to calculate the “well-to-wheel” efficiency of the Roadster to be 1.14 km/MJ—double the efficiency of the Toyota Prius.

Tesla then tackles the question of hydrogen fuel-cell vehicles (FCV) fuel cars, deriving a theoretical efficiency for an FCV fueled with hydrogen produced by steam methane reforming of 0.85 km/MJ.

Theoretical efficiency of battery-electric and fuel-cell vehicles. Click to enlarge.

This is impressive when compared to a gasoline car, though it is 32% worse than our electric car. But real fuel-cell cars do not perform nearly this well.

...The best fuel-cell demonstration car measured by the EPA is the Honda FCX, which gets about 49 miles per kilogram of hydrogen, equal to 80.5 kilometers per kilogram. We know that the energy content of hydrogen is 141.9 MJ/kg, so we can calculate the vehicle efficiency to be 80.5 km/kg / 141.9 MJ/kg = 0.57 km/MJ.

...When we calculate the well-to-wheel energy efficiency of this Honda experimental car, we get 0.57 km/MJ x 61% = 0.35 km/MJ, not even as good as the ordinary diesel Volkswagen Jetta, let alone the gasoline-powered Honda Civic VX or the Honda Insight hybrid car.

However, some proponents of hydrogen fuel cells argue that it would be better to produce hydrogen through electrolysis of water. The well-to-tank efficiency of hydrogen made through electrolysis is only about 22%, and the well-to-wheel energy efficiency of our theoretical fuel-cell car would be 2.78 km/MJ x 50% x 22% = 0.30 km/MJ, and the well-to-wheel energy efficiency of the Honda FCX would be 0.57 km/MJ x 22% = 0.12 km/MJ, even less efficient than a Porsche Turbo.




If the up front cost of the battery is a problem (doubt it will be in 5 years), and charge time on long trips is a problem then the cars should be made in such a way that the batteries can be easily swapped (and standardized as well). Energy companies could own the batteries and operate as a service. (Like a diaper service) You pay a monthly fee for the batteries (which are owned by the energy company) and then just pay for charge and swap fees. They can amortize the cost of the battery. You pay a much smaller up front price for the car. On long trips you don't have to wait for hours for a recharge.



I agree with your comments above until you complain about limited range. I agree this is the way to go for a performance EV that will produce the distance everyone seems to demand. It is a classic skimming price strategy as used by many technology firms. However, the fact that you (like others in this discussion) would not buy a car with limited range does not mean that there is not a market for it. I think this is a great attention-getter and it proves that EV's can be performance cars. Now, the public, like the folks on this site, will be demanding more, cheaper (perhaps lower performance) cars.

Now, maybe Tesla can produce one that is a little less stylish, with interchangable battery packs. I would drive one that has only 30 mile range most of the time. If I could then rent a battery pack (rather than a car)for longer trips once a quarter, or upgrade the battery pack when I change jobs and have to drive further -- that would provide value. It would be particularly attractive to those of us who assume the price of oil will go up, and the price of replacement batteries will go down. I could start out with lead-acid for my needs right now (drive my wife's car for longer trips), and upgrade the batteries later as the price/performance ratio improves.


Andrey if RPM and torque are independant variables in your formula (I assume RPS = RPM?) why is it BS to say that at high RPM they can get high torque? It gives them higher peak power no?

I'm not an engineer so I'm not trying to criticise, merely to understand.


But what is the curb weight?



RPS is rotations per second, not per minute. The formula just bond torque and power output at any moment. Say if motor develop 50nm torque at 3600RPM=60RPS, than exactly at that moment engine power output is : 50*60*6.28 = 18840 W = 18.84 kW.

And vice versa, you can calculate engine torque knowing power and rotational speed. Just play with different numbers, and you will see my point.


However, the fact that you (like others in this discussion) would not buy a car with limited range does not mean that there is not a market for it.

I never claimed there wasn't a market for EVs. The question is: How big is that market? And can Tesla exploit it to its full potential? The company is working on a sedan-style vehicle, to be produced in 2008. It will have a shorter range, since the aerodynamics won't be as good as the sports car's.


Andrey, the author's comment that their engineering has resulted in the ability to generate high torque at high RPM (or RPS). If they weren't able to do this then they presumably wouldn't have gotten up to 185 KW peak power (according to your formula).
I presume that in most engines torque decreases as RPM increases (for some reason)? For their motor they get a flatter (I presume they mean horizontal) power vs RPM graph meaning that torque is not decreasing as RPM increases as much as for other types of engines. At 12000 RPM (top of their quoted range) they presumably are able to obtain a torque of 147.3 nm (according to your formula). Is this higher than most motors at 12000 RPM? If so then what they are saying seems fair enough to me.


I guess my calculation could be wrong if torque peaked before maximum RPM meaning that peak power occurs not at peak RPM. I repeat, I am no engineer!


For those interested and who are as ignorant as me there is a graph showing power, torque and RPM for a typical motor here:

Torque typically rises and then peaks before declining over the RPM range. Peak power therefore usually occurs sometime after peak torque but before peak RPM (as far as I gather...).

All this goes to show that that the comments on torque and RPM regarding the Tesla motor are not BS as far as I can see.


"285 hp is not needed. They can reduce the 285 hp and the cost."

"And how about efficiency? That’s a honking powerful motor we put in the Roadster. Everyone knows that the bigger and badder your gasoline engine, the more gas it sucks – no matter how gently you choose to drive it.

For gasoline, bigger equals less efficient – there’s no way around it. Amazingly, the opposite is true for electric motors: the bigger the electric motor the more efficient it is. So by making a powerful electric car, we are also making a highly-efficient car. It seems almost like cheating.
-- Martin Eberhard. CEO."

Harvey D.

Neil: Dont worry, many low cost Chinese PHEVs and EVs will be around soon.

Those of us with only one car may be better served with a PHEV for the next 5-6 years.

However, if you can afford two cars, one may be a relatively cheap EV with limited range (100-150 Km) or enough to go to work and back.

An ideal situation would have standardized battery packs switchable between the PHEV and EV to suit particular applications.


Switchable batter packs will never work. Unlike propane tanks, batteries are expensive bulky items. While Blue Rhino may not care if they ocassionally get a bad propane tank turned in at one of their bottle swap out stations, I can assure you that exxon would be reluctant to risk giving you a new $10,000 battery pack, without knowing that the one you were turning in had the same number of cycles remaining. Also, such a system would require that all vehicles be standardized to one or two battery configurations. And then there is cost, did you ever notice that it costs twice as much to swap out the tank on your gas grill as it does to actually refill your tank.

However, let not your heart be troubled. Toshiba, Altair and A123 all claim to have batteries now that could recharge in minutes with a high enough current. Instead of piling up tons of batteries at every "gas station" those same stations could simply install a line of high voltage high current chargers. You go inside, have a cup of coffee 5-10 minutes later you unplug and you're on your way.


Given that this car won't be out for a year. What are the chances that better batteries will be out by then anyway. Altair's 9000 cycles (earlier post on this site) sounds a lot better than 500.


Cervus - I agree with JM about the range issue.

The distance that MOST (average) people demand in everyday driving is about 20 or 30km. We are buying dreams of long cross country trips that most of us seldom take. The cars with 600km range end up doing 5 km trips to the shops and schools 99% of the time.

We really need to get over the range issue and concentrate on the fact that electric cars do not waste 85% of the energy provided to them and do not use a resource that is limited to what we can extract from the ground in ever decreasing and more difficult amounts.

We are nearing the end of times where it is viable for you or anyone to drive 1000km on a whim. The cost of this is enormous and we are not paying this price yet.

Joseph Willemssen

We are nearing the end of times where it is viable for you or anyone to drive 1000km on a whim.

Try selling that one to America. Good luck.



As long as you ignore the range and charging time issue, you will never sell many EVs. You have to sell a product that people will buy. EVs have not been able to get a toehold in the market because they do not have similar capabilities to gasoline cars.

When the economics work, EVs will sell in large numbers. Until then, they'll be a niche market.


Cervus, Markus:
You got it right. Knowing max power at specific RPM, you could calculate torque at this point. Nothing variable here – it the law of physics. There is no such thing as high-RPM torque.
Guys just playing backward common knowledge then low-RPM torque is good . Low-RPM torque is perfectly viable term, it is good thing to have, and usually it is one of advantages of diesel, turbocharged, or high displacement engines.



Again, charging time is or will be a non-issue in short time. Like already said before, three independent battery developers have announced 5-minute recharge capable li-ion batteries. A123 is on the market already too, although not in fast-charge configuration yet.

Fast recharge equals long range once the infrastructure is there. Btw, ICE-powered Elise does not get much more than 250miles on a tank of gas ...



I've been paying close attention to the battery developments listed here on GCC. They show a lot of promise. Especially if they can bring battery costs down. I believe there was also a bipolar lead-acid battery under development that has nearly the energy density of low-end Ni-Cd batteries for less than half the cost.

PHEVs will be a bridge, if they can make the EV economics work in the longer term.


BTW, converting this car into a series hybrid means just bying or renting a range extender generator trailer ( diesel, flex-fuel, gas ) for those rare occassions when you need to go more than 250 miles.

Why not build it in in the first place ? Well, why add that extra space,weight and cost for just 1% of the drives and compromise the simple car design ?


I checked for die-cast copper motors at:

Nothing revolutionary. 2-3% less losses (93% efficiency instead of previous generation 91% typical)



This is the torque curve Tesla is referring to:

I think they are just trying to show that with an electric motor you could leave it in 2nd gear all day and never notice. The GCC reference to "high torque" should probably interpreted as "higher torque than normal electric motors"


For the people who say that manufacturers should develop low range electric vehicles, in order to make them less expensive, I point out this: Most battery chemistries work best when discharged at only a fraction of their maximum momentary current rating. if you were to remove 1/2 of the batteries on this car,you would end up with only about 1/3 of the range. As the current drawn off of a battery pack is increased, the heat that the battery pack produces also increases. This in turn increases battery internal resistance and produces even more heat. This is a quickly degenerating cycle known as the peukert effect. Traditional lithium ion batteries also have a nasty habit of overheating and evploding when you try to discharge them over the 5C rate. The high range of the lithium ion cars that have been built so far is mostly just a side effect of the battery pack size required for the acceleration desired.

allen Z

Perhaps one of the big automakers could get in on this. With Lotus already onboard, Fiat, Renault, or GM could get a partenrship/stake in the 20-30% range. Then sic some of their best engineers, lots of money and resources, and people on it.


"There is no such thing as high-RPM torque."

Torque occurs at all speeds. By it's very nature. The power generated is the product of torque and shaft speed. Therefore torque is available at all speeds.

If there is no such thing as torque at high speeds then power is zero.

I'm not sure what your meaning is here. It think the article's meaning is that through clever motor and design eg field weakening etc, they have managed to extend the const power region of the motor over a wider speed range ie increasing the avalailable torque at higher speeds compared with existing designs.


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