Why Tesla’s Model S Is So Incredibly Fast
Blink and you’ll miss it: The Tesla Model S was just rated the third-fastest accelerating production car in the world, beating out cars such as the Lamborghini Aventador and the Bugatti Veyron.
The head-snapping acceleration of the new supercharged Model S raises a question: Just how did engineers at Tesla get the electric, seven-seat family sedan to go so fast?
It turns out, one part of the car largely determines the Tesla’s impressive performance.
“They’re adding a bigger battery, and adding a bigger battery makes it more powerful,” said Mike Duoba, a mechanical engineer at Argonne National Laboratory in Illinois, who develops standards for hybrid plug-in vehicles. [Hyperloop, Jetpack & More: 9 Futuristic Transit Ideas]
The Tesla Model S, used in what the company calls “Ludicrous mode,” can go from 0 to 60 mph (96 km/h) in 2.5 seconds, the company said in a statement. The only commercial cars on the planet that can beat the Tesla Model S, 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. (The fastest custom-built race car, the Grimsel, can reach the same speed as the Tesla in about 1.5 seconds.)
Tesla confirms that the secret sauce behind this record-breaking time is the new battery. (Those who want to upgrade their existing Model S can purchase a bigger battery for $10,000.)
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, said Jordi Cabana, a chemist at the University of Illinois at Chicago, who studies battery chemistry.
The new Tesla battery helps quickly achieve these lightning-fast speeds by increasing the latter, Cabana said.
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), Cabana said. 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 likely has thousands of these cells, Cabana said. (Tesla’s home battery uses similar technology.)
The new battery may have crammed more cells into the same space of the older Tesla S battery pack, Cabana said.
“It looks like they changed the internal design of the battery pack,” Cabana told Live Science. “They reduced the amount of packaging that they put in the battery to make it safe.”
Historically, batteries that could produce enough power for fast acceleration or enough energy for long-range driving were typically quite expensive. That’s in part why battery-powered electric cars had a reputation for being less peppy than an equivalent gasoline car, Duoba said. However, a 2014 study in the journal Nature found that the cost of electric batteries has been plummeting, paving the way for cheaper, speedier, longer-range vehicles.
Take the battery out of the equation, and electric cars have an edge in speed tests.
For one, engines have myriad tiny parts that must spin, push, open and close to produce internal combustion at the right times, Duoba said.
“An 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.”
Electric motors can achieve their maximum torque, or the rotational force that is transmitted from the motor to turn the wheels, anywhere from 0 to 4,000 revolutions per minute (rpm), which roughly corresponds to vehicle speeds between 0 and 30 mph (48 km/h), said Paul Chambon, a controls engineer who is an expert on powertrains at Oak Ridge National Laboratory in Tennessee.
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, Chambon said.
So at zero speed, gas-powered engines are not at their peak.
“They don’t have that peak torque right away, you have to accelerate to middle speed to gain enough torque,” Chambon said.
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 speed to that needed to rotate the wheels at a certain torque, Chambon said. Gear shifting creates lulls in the car’s acceleration.
But because electric vehicles can operate at peak torque anywhere from 0 to 4,000 rpm and can spin faster than engines, they often have no gearbox.
“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.
Original article on Live Science.
March 13, 2018
February 20, 2018