Miles of Progress
They call me “the range guy” at Tesla Motors, which is fitting since it’s my job to characterize and improve the driving range of the Tesla Roadster. This means I get to:
- Conduct official range testing according to EPA/CARB procedures,
- Work with our development teams to extract the most miles out of the car,
...and best of all...
- Spend lots of time driving the Engineering Prototypes (EPs) and Validation Prototypes (VPs) to collect data to calibrate our simulation models and understand real-world range.
It’s a dream job, but it also has its challenges.
Tesla Motors CEO Martin Eberhard recently announced that we have officially revised our range expectations for the Tesla Roadster to more than 200 miles per charge, rather than our original goal of 250 miles. However, it’s still a fact that the Tesla Roadster will have the highest driving range of any production EV in history (see chart below), not to mention its blistering performance, adhesive handling, and to-die-for looks. So while we’re focused on a range of more than 200 miles, I wanted to share an engineer’s appreciation for design features of the Tesla Roadster that contribute to this historic result.
(Source for GM, Nissan, Honda, and Toyota range numbers: Department of Energy Advanced Vehicle Testing.)
But before we talk engineering, we need some theory, and the factors that influence driving range in an EV are best explained by this simple formula:
Range (mi) = battery useable energy (kWh) x vehicle energy efficiency (mi per kWh)
So if your EV has useable battery energy of 25kWh and efficiency of 4mi per kWh, the range will be 100mi (just for example, not actual numbers for the Tesla Roadster). Note that the useable energy and vehicle efficiency must be measured at the same point – the terminals of the battery pack. The formula tells us that, to get a high range, you must equip an EV with a lot of useable battery energy and design the EV to use this energy efficiently. It’s a simple formula, but it also explains our range improvement strategy for the Tesla Roadster. So now let’s talk about the car...
Our battery pack, or Energy Storage System (ESS), is the heart of the Tesla Roadster. Since the Tesla Roadster is a relatively small and lightweight vehicle platform, the bottom line for us is energy packaging density: How many useable kWh can we fit in the given space, for the least weight?
To maximize our useable battery energy we:
- Opted for small-format, commodity lithium ion (Li-Ion) cells with extremely high specific energy and energy density but without compromising our goals for battery cost, life, and safety. As noted by Martin, we deliberately chose cells with slightly lower capacity than the highest available due to cell durability and abuse tolerance.
- Developed proprietary methods for cell interconnection and fusing to minimize parasitic electrical resistance (which consumes energy even before it gets to the ESS terminals).
- Developed proprietary thermal management and charge balancing systems, so that cells operate under best conditions to provide maximum useable energy over their life.
- Developed a custom vehicle chassis to accommodate the ESS in the Tesla Roadster.
For more information on our battery technology, see our whitepaper, The Tesla Roadster Battery System.
The result is an awful lot of energy in a very small, lightweight package – arguably one of the most advanced automotive battery systems ever developed. In fact, the specific energy of the Tesla Roadster (the entire car) will be almost as high as the NiMH battery pack (alone) installed in the General Motors EV1. However, as the above range formula shows, a high-energy battery on its own does not guarantee a high range – you also have to use the energy wisely.
|General Motors EV1: VRLA battery|| |
|General Motors EV1: NiMH battery|| |
|Telsa Roadster: Li-Ion battery only (est.)|| |
|Telsa Roadster: entire car (est.)|| |
Excess weight is detrimental to performance and handling, but it’s also the enemy of efficiency. Energy losses due to acceleration/braking, hill climbing and tire friction are directly proportional to weight and together these account for a major fraction of a vehicle’s energy use.
The Tesla Roadster was designed to be lightweight from the beginning. Evidence of this philosophy can be seen throughout the whole car, but most notably in our:
- Bonded/extruded aluminum chassis built using techniques pioneered by Lotus
- Carbon-fiber body panels that reduce part weight
- Energy Storage System (see above)
- High-speed induction motor drive with its awesome power-to-weight ratio
- Lightweight seats, steering wheel, dashboard, centre console and interior trim
We originally targeted a curb weight of 2,500lbs. But, as explained by Martin, we made numerous design changes to make the car more durable and safe that increased weight by a few hundred pounds. Even so, at approximately 2,690lbs the Tesla Roadster will still be one of the lightest vehicles on the road, even compared against other lightweight, high-performance roadsters.
|Roadster Make/Model|| |
Curb Weight (lbs)
0-60 mph (sec)
|2007 Lotus Exige S||2,077||4.2|
|2007 Tesla Roadster (est.)|| |
|2006 Corvette Z06|| |
|2006 BMW M Roadster|| |
|2006 Porsche Carrera Cabriolet|| |
|2005 Ferrari F430 Spider||3,351||4.6|
|2007 Mercedes SL65 AMG||4,555||4.6|
The ideal shape for low aerodynamic drag is a teardrop. But this isn’t practical for vehicle design (aren’t wheels a nuisance! :) ), nor is it appealing to the mass market, let alone high-end sports car enthusiasts. Therefore, automotive stylists are constantly torn between increasing efficiency and increasing market appeal. Good examples can be seen in the Honda Insight and General Motors EV1. (As two of the most-efficient production cars in history, I think both are engineering marvels, but I can see why some people didn’t like their looks.) Fortunately, our Body Engineering team and our friends at Lotus Engineering struck a nice balance between looks and efficiency. Not only is the Tesla Roadster drop-dead gorgeous (seeing is believing), but it also has surprisingly low aerodynamic drag. The car has about 10 percent less aero drag than a Lotus Elise. But it also has less aero drag than a Toyota Prius or Honda Civic hybrid and only 20 percent more than a Honda Insight. So the Tesla Roadster’s aerodynamics fall in a similar class to these fuel-efficient hybrid vehicles and this certainly helps our overall vehicle efficiency.
Tires produce their own form of rolling drag. Yokohama has provided us with a high-traction yet low-friction tire – ideal for an electric sports car – and continue to investigate lower-friction compounds and tread patterns for our use. Meanwhile, Lotus Engineering continues to optimize tire inflation pressures and toe/camber/castor and suspension settings for best efficiency, traction, safety, and ride quality.
Braking Systems (Regenerative and Friction)
The Tesla Roadster is equipped with regenerative braking (regen) to capture kinetic energy during deceleration so it can be re-used next time the car accelerates. For best efficiency, we have maximized our regen settings within the safety limits imposed by our traction control and ABS systems (See our recent blog on traction control and ABS testing). This provides a boost to our real-world range and, conveniently, allows us to complete the EPA city and highway cycles without needing to use our (friction) disc brakes. We estimate that regen improves our vehicle efficiency by up to 40 percent in the city, but only 5 percent on the highway. Stay tuned for more details of our regen braking system in a future blog...
Speaking of friction brakes, we’re also trialing a “low-drag” disc-braking system that has less rub between disc and calipers when brake pressure isn’t applied.
Drivetrain: High-Speed Induction Motor with Two-Speed Transmission
If the ESS is the heart of the Tesla Roadster, then the drivetrain is the muscle that helps us rocket from 0-60 mph in about 4 seconds. But our drivetrain is also highly efficient, consisting of the following:
- A 250hp peak high-speed induction motor that spins to more than 13,000rpm. This provides tremendous power-to-weight ratio, but also produces high-efficiency across a broad range of speeds and torques, which is exactly what you need for high vehicle efficiency. For more info, see our blog on Induction Versus DC Brushless Motors.
- Our power electronics module (PEM), which we designed to control motor torque, regenerative braking, and charging. It utilizes sophisticated digital methods to precisely control voltage, current, and frequency at every motor operating point – again leading to optimal efficiency.
- Our custom two-speed transmission, which adopts a unique architecture including novel bearing design to reduce spinning losses. The dual ratios also allow a driver to optimize efficiency under various driving conditions. For example, it’s more-efficient (and quicker) to do 0-60mph runs in first gear, whereas second gear is best for cruising efficiently at 65mph.
“Ancillary systems” translates to parasitic loads on the ESS that don’t directly move the car along the road. These losses can be quite draining, so we’ve targeted them too, for example:
- We use an electric A/C compressor, like in some hybrids, to efficiently keep our batteries and driver/passenger cool (see our blog on climate control). Also, all our powertrain cooling systems, components and algorithms are optimized for best efficiency.
- We’ve developed low-standby-load infotainment systems, e.g. the computers onboard the Tesla Roadster draw roughly the same power as the laptop I’m using to write this blog.
Things We Have Not Done
For some ideas we did NOT choose to increase range, e.g. onboard solar panels, wheel motors, exotic batteries, perpetual motion machines, etc., see Martin’s blog, Balance.
Dynamometer Range Testing
No discussion of vehicle range is complete without explaining how we test, measure, and report it. Tesla Motors tests its vehicles according to strict EPA procedures based on recommended practice set forth by the Society of Automotive Engineers (SAE). These procedures use a chassis dynamometer (or dyno) to simulate on-road conditions. Here are some fast facts about EPA dynamometer testing of the Tesla Roadster:
- The front wheels are clamped to keep the vehicle stationary.
- The rear wheels rest and turn on a 48-inch diameter steel roller.
- The steel roller connects to an electric generator that provides an opposing force to simulate both the weight (inertia) and friction (drag) of the vehicle. During regen braking events, the generator operates as a motor to “drive” the rear wheels.
- The dyno’s simulated inertia is calibrated based on the measured curb weight of a real Tesla Roadster, plus 300 lbs of passenger/cargo weight.
- The dyno’s simulated drag is calibrated using a total drag versus speed curve measured via “coast-down testing” of a real Tesla Roadster on a real road.
- A/C load is accounted for in the test.
- A professional driver operates the vehicle to control the accelerator and brake pedals and shift the transmission during the test.
- The powertrain otherwise operates just the same as it would on the road.
- The EPA procedures include several standard driving cycles (for examples, see here). We normally test on the EPA city and highway cycles, as required by official procedures.
- To measure range, we fully-charge the Tesla Roadster and then drive the test cycle repeatedly until the vehicle self-limits its performance due to a low battery. The range we achieve on the dyno is the range we report – plain and simple.
To get a better idea of how the dyno works, watch our video of the Tesla Roadster during testing. (Video requires free QuickTime player to view. Note: The dyno is quite loud, so you won't be able to hear the more muted sound of the Tesla Roadster's motor.)
We will announce our certified range figures very soon. In the meantime, Tesla Motors remains committed to delivering the Tesla Roadster with the highest possible range while retaining the performance, handling, looks, and safety of a world-class electric sports car. We expect more than 200 miles and, since I have personally driven 200 miles on the streets of San Carlos, Calif., in an early EP Roadster, I can assure you this expectation is very real!