Published on December 19th, 2015 | by Michael Barnard83
Why Are Teslas Quicker Than Gas Cars?
December 19th, 2015 by Michael Barnard
According to Motor Trend’s recent test, the Tesla P90D with Ludicrous Mode hits 60 miles per hour in a shattering 2.6 seconds. This is absurdly quick, and there are only three currently produced street-legal cars which are quicker than a Tesla. Those cars are by Porsche, Lamborghini, and Ferrari and would set you back $2.5 million USD to buy all three. It’s worth pointing out that only one of those cars doesn’t have electric drive train components as well.
So how does a heavy passenger sedan capable of seating 7 people beat all but the most exotic of modern sports cars?
There are 11 separate factors I’ve identified which contribute to the Tesla P90D being incredibly quick compared to internal combustion vehicles. Other Teslas share most of the characteristics, which is what makes them so blisteringly quick as well, just not quite as ludicrously so.
It’s important to note that we are talking quick, not fast. Teslas get to 60 mph faster than any comparable car on the planet, but are limited to 155 miles per hour. That’s fast enough for me, but is exceeded by quite a large number of production cars.
Let’s step through the reasons for Tesla’s quickness one-by-one.
Teslas use electric motors which create maximum torque — the tendency of force to rotate an object about an axis — at zero rotations per minute and continue to generate the same amount of torque at pretty much all RPM levels. Internal combustion engines by contrast have very low torque at the beginning and end of their torque curves and high torque at the centre of their torque curve. That’s why they need to be started out in a low gear and rapidly shifted upward as speed increases, to match the torque output to the speed of the car.
Teslas use electricity in batteries instead of physical fuel. Getting electrons from a battery to an electric motor is much faster than getting fuel from a gas tank to a piston. Electrons travel much faster along a wire than fuel does along a fuel line, and the electrons basically go straight to the place where they are needed, while the fuel goes through a fuel pump, then to a fuel injector, then is sprayed into a piston, and then is ignited, turning into force which drives the piston to finally create torque. At idle in an internal combustion car, there is already fuel traveling through the fuel line, gas pump, fuel injector, and igniting in the piston, but a lot more fuel is needed to accelerate.
Tesla P90Ds use two electric motors which run at different gear ratios, one for the rear wheels and one for the front wheels. Outside of exotic sports cars, internal combustion cars have a single motor. Two motors mean that they get more torque to the wheels, which increases acceleration, and the different gear ratio allows better mixes to the wheels at different speeds, kind of like different gears in internal combustion cars. This gets into the fast vs quick point again. While the torque remains constant, more horsepower is required at higher speeds to maintain acceleration due to air resistance. Air resistance increases with the cube of velocity, so it gets harder rapidly to push the air aside to keep accelerating. Two motors enable the Tesla to keep pushing the air aside aggressively at higher speeds. For more on torque and horsepower, here’s a brief primer: Horsepower vs Torque.
Speaking of horsepower, Tesla’s have a lot of it. The two motors in the Tesla P90D could pump out 762 horsepower between them. High horsepower keeps them moving forward as air resistance increases. And, like torque, the max power in a Tesla is available immediately, no waiting around as RPMs build, unlike internal combustion cars which have to build up the revs to get to a peak, then shift. Similar luxury sedans are running well under 650 horsepower. Poor internal combustion cars — they just can’t catch a break. The graph on the right is for one of those obsolete things. (Editor’s Note: Admittedly, there are limiting factors that keep the P90D from achieving the motors’ combined max horsepower, but the higher horsepower potential is still helpful.)
Teslas have all-wheel drive. This means that the available torque is spread among four sticky contact patches with the ground instead of just two. All else being equal, this means that they can deliver double the force to the ground without the tires spinning. That’s why muscle cars typically have very large rear wheels, to increase the size of the contact patch to achieve the same effect, and why top fuel dragsters have enormous wheels on the back and tiny wheels on the front.
The quickest Teslas have 21″ wheels instead of the standard 19″ wheels. By definition, a 21″ wheel has more rubber on the ground than a 19″ wheel. As per the all-wheel drive point, this increases the contact patch with the ground, allowing greater force to be applied without spinning the tires.
Teslas have much better traction control than internal combustion cars because electric motors have extremely simple torque characteristics and can be controlled in much finer increments and much more quickly than internal combustion motors. Internal combustion cars are constantly changing how much torque is getting to the tires with every change in speed and have dramatic changes with every gear shift. By comparison, electric motors, as discussed earlier, give the same torque at every speed and don’t have gear boxes so avoid the dramatic surge which comes with shifting. And for the same reason that the acceleration is instant instead of delayed due to electrons getting to the motor much faster, changing the amount of electricity creating force is faster and more accurate as well.
To change the amount of force generated by an internal combustion motor, you have to tell the gas pump to change its physical pumping speed and orchestrate that with the fuel injectors so that they are delivering the right air fuel mixture into the piston, and orchestrate that with the timing to get the ignition exactly right. That all takes eons compared to adjusting the flow of electrons. It’s all still well under a second, but traction is a millisecond thing. The sensors which detect wheel slip are the same for Teslas vs internal combustion cars as far as I know, but the ability to respond to what they are saying is much quicker, kind of like reflexes vs conscious thinking.
Turn off the traction control on a Tesla and this is what happens:
Teslas have no gears and don’t need to shift. Internal combustion cars have to shift multiple times on the way to 60 mph. Each shift has a brief period of time when the gears are not engaged and accelerating. Shifting is necessary to try to keep the engine in the peak portion of the power band. Teslas and most sanely built electric vehicles don’t bother with gearing because it’s unnecessary. As such, the car doesn’t have any periods when it isn’t accelerating as fast as it can.
Teslas are heavy. This is paradoxical, but specifically, they are heavy in the right way with a very low centre of gravity, with the very heavy battery pack spread equally from the front axle to the rear axle of the car and slightly below the level of the axle. This means that the car pushes down on the contact patches equally and that the force on the front vs rear wheels changes less under acceleration under cars where the centre of gravity is higher. The electric motors are also much smaller than gas motors and mounted close to the level of the axles.
Internal combustion engines and gas tanks, by comparison, are much bulkier and much higher above the axles, which causes the vectors of force to put a lot more pressure on the rear wheels during acceleration than for a Tesla. This is true for deceleration and cornering as well, which is why the Tesla performs exceptionally well for a car of its weight in those tests too. “The Physics of a Front-Wheel Drive Muscle Car” post from Wired includes a graphic showing the effect of a higher centre of gravity on a car, while the picture of the Tesla battery back shows how much lower the CoG of a Tesla will be.
Then there’s the Tesla hardware secret sauce. Tesla’s Insane Mode was one thing. They tweaked the power controller and added some neat micro-fuses to the power transfer from the battery to the motor so that they could push a lot more electricity from the battery to the motor. It’s kind of like putting a thicker gas hose between the gas tank and the engine of an internal combustion car; they get more fuel to the place it’s needed faster.
The Tesla Ludicrous Mode is something else again. First off, the P90D uses a more powerful rear motor than the P85D, which has merely Insane Mode. Second, Tesla upgraded the main pack contactor — a large switch controlled by electromagnets operating under software control — from steel to inconel, a high-tech alloy which resists heat from high amperage better. Basically, they took a limiting component to pushing lots of electrons faster and upgraded it to a component which wouldn’t melt as quickly.
Internal combustion cars have an awful lot more hardware secret sauce because getting enormous amounts of power safely out of exploding gasoline in metal cylinders is actually a lot harder.
Finally, there’s the Tesla software secret sauce. A little-known fact about Teslas is that they have been getting quicker via software releases as the engineers at Tesla figure out how to get more out of the power controller just by tweaking parameters, and existing customers have been benefiting by getting these tweaks in downloads. To quote a Tesla owner from the forums,
When I bought my 85D it was rated over 5 seconds. Software upgrade took it to 4.4s. More recent software upgrade apparently takes it to 4.2s. I’m amazed. What more do you want?
Like the hardware secret sauce, internal combustion engineers have been doing quite amazing things with software to get more out of cars. But they are starting from an inferior technology that was much more mature, so they aren’t getting as much or as quickly as Tesla does.
So, there you have it. Eleven reasons why the Tesla P90D is so absurdly quick compared to internal combustion cars, most of which aren’t available to the older technology. And some of these eleven reasons will be amplified in the upcoming Tesla Roadster, leading to my belief that it will hit 60 mph in 2 seconds or less.
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