Posted on August 28, 2016 by Matt Pressman
Gearheads everywhere are mystified by the latest news from Tesla Motors [TSLA]. They've grown up with gas flowing through their veins, loudly (and proudly) revving the internal combustion engines of their fossil fuel powered cars. Yet they're scratching their heads, how could an electric car leave them in the rear view mirror? This question was posed by the folks at Live Science*, after Tesla's P100D became the quickest production car in the world: "The only commercial cars on the planet that can beat the Tesla Model S [P100D], the LaFerrari and the Porsche 918 Spyder, each cost about $1 million and are 'tiny' two-seater roadsters. Neither was built for the masses and neither is currently being produced."
Above: The Tesla Model S (Image: Car Magazine UK)
This begs the question: "Just how did engineers at Tesla get the electric, seven-seat family sedan to go so fast?" The easy answer — they've gone from their top-of-the-line 90 kWh battery to an (even bigger) 100 kWh battery: "They're adding a bigger battery, and adding a bigger battery makes it more powerful," explains Mike Duoba, a mechanical engineer at Argonne National Laboratory in Illinois, who develops standards for hybrid plug-in vehicles. But it goes deeper than that, electric vehicles also have inherent, built-in advantages.
Case in point — for a gas-powered car: "An [internal combustion] engine is a sort of a breathing animal: It has to take air in and squeeze it," Duoba told Live Science. "Those processes are not instantaneous." (In a gas-powered engine, a piston compresses a mixture of air and fuel, causing combustion, which turns the motor). Electric motors, meanwhile, don't have all those tiny moving parts. "The electronics in an electric motor are almost instantaneous," Duoba said. "There's no delay in power, no waiting for throttles to close. All those little effects add up."
Above: Torque generation in an internal combustion engine inside a gas-powered vehicle (Source: Car Throttle)
In addition, it's key to look at the torque mismatch between gas-powered cars and electric cars: "Electric vehicles can achieve their maximum torque, or the rotational force that is transmitted from the engine to turn the wheels, anywhere from 0 to 8,000 or 10,000 revolutions per minute (rpm), which roughly corresponds to speeds between 0 and 75 mph (120 km/h), said Paul Chambon, a controls engineer who is an expert on powertrains at Oak Ridge National Laboratory in Tennessee."
Above: Electric vehicle torque delivery (Source: Car Throttle)
Looking at internal combustion engines: "In contrast, gasoline-powered cars cannot achieve peak torque at either a very low or very high rpm. Engines are optimized to run best with certain combinations of air flow, temperature and rotational speed. That means the torque in gas-powered engines peaks around 4,500 rpm, and that a graph of torque versus rpm looks like a domed hat... So at zero speed, gas-powered engines are not at their peak." Chambon explains, "They don't have that peak torque right away, you have to accelerate to middle speed to gain enough torque."
Above: Internal combustion engine torque delivery in a gas-powered vehicle (Source: Car Throttle)
Furthermore, "The dome-shaped torque graph also has another implication: At low speeds, the torque needed to propel the car doesn't match the torque produced by the engine. As a result, manufacturers place a gearbox between the engine and the wheels, which matches engine torque to that needed to rotate the wheels at a certain rpm, Chambon said. Gear shifting creates lulls in the car's acceleration. But because electric vehicles can operate at peak torque anywhere from 0 to 10,000 rpm, they often have no gearbox like gas-powered cars. 'There's no gear shifting, that alone is probably worth half a second or maybe a third of a second,' in the 0-to-60 test, Duoba said."
Above: The new front end redesign of the Tesla Model S (Source: Motor Trend)
Of course, it's key to also look at Tesla's superior electric vehicle battery tech. And, according to Jordi Cabana, a chemist at the University of Illinois at Chicago, who studies battery chemistry: "In general, a battery's energy density predicts how much energy it can release (meaning how far the car drives) before recharging, while the power density (the energy density delivered per second) determines how fast energy can go in and out of the battery. That, in turn, governs how fast a car can accelerate... [the] Tesla battery helps quickly achieve these lightning-fast speeds by increasing the latter."
Above: The Tesla Model S 18650 battery cell made in partnership with Panasonic (Source: Electric Vehicle News)
Cabana also speculates: "Though exact details haven't been released, the Model S likely uses a lithium-ion battery where one layer, called the cathode, is made of a blend of nickel, manganese and cobalt oxide (NMC)... When charged, lithium ions from the cathode are driven through an electrolyte solution into the anode, which is made of stacks of graphite. Lithium-ion batteries that overheat can sometimes produce a runaway chain reaction and catch fire; to prevent that, manufacturers encase individual cells containing both a cathode and anode in protective shells. The Tesla Model S battery [pack]... has thousands of these cells."
Above: Over 7,000 individual battery cells are inside a Tesla Model S battery pack (Source: Copper Motor via Ricardo Strategic Consulting)
Battery pack architecture
Electrek reported on this week's Tesla conference call with media. Tesla CEO Elon Musk clarified some changes made to the P100D battery pack, "People often think that a battery and a battery pack is the same thing, but the technical complexity once you get to do a large number of cells in a pack is very much on the module/pack level. You can think of the cell level as being a chemical engineering problem and the module/pack level as being a mechanical, electrical and software engineering problem... The cell is the same, but the module and pack architecture is changed significantly in order to achieve adequate cooling of the cells in a more energy dense pack and to make sure we don’t have cell to cell combustion propagation."
Above: Fig. 1 Current Tesla battery cooling configuration conceptualized uses a cooling ribbon that snakes through the cells. Glycol coolant is circulated in the cooling ribbon; Fig. 2 Conceptual P100D Battery cooling configuration (Source: George S. Bower)
And Tesla CTO JB Straubel added, "It is a pretty big change on the battery module and pack technology. It’s a complete redo of the cooling system, which is quite unique to Tesla and that we have been improving on for many years. This new pack is the next version of that... [and] some of the key improvements that enabled the new pack are directly on the roadmap for the technologies that make Model 3 possible." That said, we look forward to seeing the battery improvements coming to the much-anticipated Tesla Model 3. With this sneak peek at the P100D, it's evident that the forthcoming Model 3's third generation battery technology will further showcase Tesla's battery dominance in the auto industry.
*Source: Live Science