Wicked Fast Electric Vehicles on (Pareto) Curves

By now most of you have probably heard about the Tesla Roadster, Fisker Karma, and other high performance electric cars that demonstrate we can make spectacular gains in energy efficiency AND enjoy amazing performance by designing cars in a new way. Improving efficiency and performance simultaneously is an impressive feat. These are competing objectives, that is, improving one objective normally involves degrading the other. We can design a car that is either high performance or highly efficient, but not both. We can visualize this kind of design tradeoff using a tradeoff curve usually called a Pareto curve or efficient frontier. The drawing below is a conceptual illustration of a Pareto curve in automotive design, showing the tradeoff between performance and efficiency.

We would like to maximize both performance (one aspect of performance is acceleration) and energy/fuel efficiency. Ideally we would like a design that is in the upper right corner of the plot above. Unfortunately, when a design tradeoff exists, this is not physically possible. We can’t focus on both performance and efficiency because they are competing objectives. If we focus on performance in vehicle design, we might end up with something like a Porsche 911 Turbo, which has a blistering fast 0-60 mph time as low as 3.2 seconds. Unfortunately this car doesn’t get great fuel economy. If we want to improve fuel economy we will need to sacrifice performance, that is, we will need to trade some performance for fuel efficiency (perhaps by reducing engine size, using smaller tires, reducing mass, etc.). If we focus on just fuel efficiency we might end up with something like a Geo Metro. The Pareto curve connecting these two points on the plot above represents what designs in between the Porsche and Geo are physically realizable. It’s not possible to create a vehicle to the upper right of the curve. Something on the interior of the curve is not Pareto optimal, meaning that it’s possible to improve both objectives simultaneously. Designs on the interior of this curve are to be avoided. Advanced design techniques, such as simulation and design optimization, can help engineers ensure that their designs are on the Pareto curve. It is up to the engineers and market analysts to determine where on the curve their product should be.

What if we changes the rules of vehicle design? What if instead of assuming powertrains had to include a conventional gasoline engine linked to a manual or automatic transmission, we allowed battery electric powertrains? The previously impenetrable Pareto curve shifts to the upper right if we can escape the inefficiencies of gasoline engines. New technology, and new ways of designing things, can push the Pareto curve to a new and better location, as shown in the diagram below. We can improve both performance and efficiency by introducing new technology. This is what’s going on with the Tesla and Fisker. The Aptera 2e places more emphasis on energy efficiency than performance, and solar cars are the ultimate in energy efficiency. The Prius uses a power split hybrid electric powertrain. It’s an improvement in efficiency over conventional powertrains, but it can’t compare in efficiency to pure electrics like the Aptera (that’s why it is a design on the interior of the Pareto curve). In fact, although my crude diagram doesn’t really depict this, the powerful Tesla gets better energy efficiency than the Prius.

The Tesla might be a fast EV, but have a look at the X1 Wrightspeed. It’s wicked fast. See where it’s positioned on the Pareto curve? The X1 is an Ariel Atom retrofitted with an all-electric powertrain created by AC Propulsion, makers of the eBox. Here is the X1 smoking both a Ferrari and a Porsche:

Now that’s what pushing out the Pareto curve looks like! Here is another race between the X1 and a Lamborghini, and then with a NASCAR racer:

The above analysis is admittedly simplified. The diagrams are conceptual and do not represent actual performance and efficiency numbers (if they did the solar car point would be way off to the right of your computer screen). In addition, there are many other competing objectives that need to be considered in vehicle design, such as range, safety, durability, utility, cost, and total lifecycle environmental impact. Nevertheless, Pareto curves are a helpful tool for visualizing and understanding design tradeoffs.

What emerging technologies do you think will expand the current Pareto curve for vehicle design (or other products)? Can you think of some additional tradeoffs important to vehicle design that I haven’t listed here? If we want to look at three, four, or more competing objectives, how do you think we can visualize the tradeoff relationships between them?