Category Archives: Tesla Roadster

Tesla’s Refrigerator Sized Cooling Problem

Electric cars require cooling systems. Not nearly to the extent that a combustion engine vehicle does, but lithium ion batteries get hot. Put your laptop on your lap for an hour and you might start cooking your own thighs. So you can imagine, thousands of lithium ion batteries will require a significant amount of cooling. There are 6,831 lithium-ion batteries in the Tesla Roadster, each about a third bigger than a typical AA battery. They’re linked together in a unique package that incorporates liquid cooling, safety fuses, and sensors that prevent the cells from experiencing what battery engineers like to call “thermal events.” The batteries feed 410 volts to the Roadster’s air-cooled AC induction motor. Here is the AC cooling system, for the cabin and battery pack:

Telsa Roadster AC Cooling System

Martin Eberhard, former CEO of Tesla Motors, is making the claim that the current cooling pump is working overtime, a lot of overtime. In, fact Eberhard stated that the pump for his Tesla Roadster seemed to be on all of the time, which is completely unnecessary after the car has been parked for an hour or so. Now the ESS cooling system is crucial to the life of the battery, but this is ridiculous. So, Martin Eberhard, being the electrical engineer that he is, installed one of those electric meters you see on the sides of houses ahead of his car’s charging station. Then he charged the car up fully, unplugged it for four days, then measured how much power it took to recharge the battery after the car sat, fully charged and cool, for four days. Doing some math, he found the car consumed 1,278 kWh per year, from just sitting there. That is enough to power two big refrigerators. Not only is this significant on your electrical bill, but this drastically reduces the lifespan of the pump.  Eberhard explains in his blog post:

The second question is the life expectancy of the pump. I expect that Tesla used an automotive-grade pump from a good supplier. I am also sure that no other car leaves a pump running 24/7. Consider a typical car designed to run for 200,000 miles at an average speed of 30 mph. Such a car is designed to run for 200,000 / 30 = 6,666 hours. Let’s say the designers want some room for error, and design the water pump for that car to operate for 10,000 hours without failure. 10,000 hours life expectancy would be a good-quality automotive pump.

Now, let’s run that same pump 24/7 instead of the couple of hours per day it would run in our typical car. Running 24/7, that pump will pass 10,000 hours in only 13 months. That’s all – end of life. Just to make it through Tesla’s 3-year warranty, that pump would need to last 26,280 hours without failure. To last just 5 years, the pump would need to run 43,800 hours. Hopefully, Tesla installed a pump rated for at least 50,000 hours of operation without failure, implying an MTBF of at least 70,000 hours, assuming an exponential failure distribution. Does any automotive parts manufacturer even make such a pump?

Tesla also told Eberhard that the new Drivetrain 1.5, that they guarantee to retrofit all the purchased Tesla Roadsters with, will not make any changes to the pump. However, in a recent article from TIME, Elon Musk responded to Eberhard’s cooling pump complaints and also shed some light on the nature of their tumultuous business relationship:

Eberhard, the ousted cofounder, says Musk interfered with the design of the roadster, demanding changes that were costly and led to delays. These included installing electronic door latches, building a lightweight carbon-fiber body and lowering the doorsill by two inches. “It cost us $1.5 million to lower that doorsill,” Eberhard says. “We would have been better off to have a simpler car shipping a year earlier.” Musk says his design changes were not the cause of delays. Eberhard says that despite Tesla’s green-tech credentials, the roadster has a coolant pump that operates even when the car is parked, wasting as much electricity as two refrigerators. Musk says that will be fixed next month. Eberhard also gripes that Musk controls the board of directors, whose members include his brother Kimbal Musk. “I’m very unhappy about what’s happened to my company” under Elon, says Eberhard, who still owns about 3 percent of Tesla. “I think he’s a terrible CEO.” Elon Musk responds that “Martin is the worst individual I’ve ever had the displeasure of working with.” –TIME, An Electric Car Loses Its Juice

While Eberhard does have a legitimate complaint, keep in mind he probably takes some pleasure in publicly criticizing Elon Musk’s Roadster. But in the end, both sides just want to see the Roadster improved, and hopefully the issue can be fixed fairly easily and cheaply.

What is the Tesla Roadster 1.5, exactly?

The second generation of Tesla Roadster will look exactly the same, but sport a new “Powertrain 1.5”.  This powertrain is an improved motor, inverter and gearbox designed to replace their previous two-speed transmission that had many durability, efficiency and cost challenges.  The faulty 2-speed transmission effectively forced Tesla to sell its Roadster locked into second gear, dramatically hampering acceleration. The one speed transmission “improves motor torque by a bit more than 30% beyond what was already great and the ¼ mile time for the car is now in the 12.9 second range. The top speed of the vehicle remains over 120 mph,” according to Tesla CTO JB Straubel.

According to Tesla’s site, Powertrain 1.5 makes the following improvements:

  • An improved inverter (PEM) to deliver higher motor current
  • An improved motor to handle higher current and torque
  • A new single-speed gearbox
  • A new motor to gearbox coupler and an improved motor cable
  • Upgraded vehicle firmware
  • NO changes to the battery pack

Tesla Roadster Inside

PEM-green, Battery-blue, Electric Motor-under the PEM

The Power Electronics Module (PEM) basically acts as an amplifier and guide that converts and regulates the power from the battery to the motor. When you put the pedal to the floor you tell the PEM to give the motor some juice. By using newer IGBTs (Insulated Gate Bipolar Transistors) that convert and regulate power from the battery, they have been able to boost the PEM output current by about 33% from 640A rms to 850A rms with the same number of IGBTs.  Fancy talk for using recent advances in electronic technology to make a more efficient PEM and a more powerful Roadster.

The motor has been improved by changing the casting in order to link with the new transmission. The motor terminal lugs have been changed, as well as the cable from the motor to the PEM to improve efficiency and reduce temperature rise.

The heart of the Powertrain 1.5 improvement is in the gearbox. CTO JB Straubel explains it best in his blog:

The new gearbox is the most significant change from powertrain 1.0 to 1.5. We have significantly reduced the complexity of this gearbox by getting rid of the need for shifting or speed matching between two gear sets. There is only one set of gears that is always engaged with a ratio of (8.2752:1). There are no clutches and we have also done away with the need for an electric oil pump and instead integrated a very efficient gear-driven oil pump into the gearbox. All of these simplifications have saved a great deal of mass and the new gearbox is approximately 45kg instead of 53kg for the old two-speed design.

This mass savings is even more impressive when you consider that we have designed this transmission to have a long life at a much higher input torque (400 Nm) and higher speed (14,000 rpm).

The engineering design of this new gearbox has been finished for over a month and we are now running prototypes through their paces on dynamometers. We built two initial gearboxes with machined aluminum housings so we could gain some early test results before finalizing the cast housing tooling and machining fixtures.

One of the most exciting features of this new gearbox (from an EV perspective) that we have been able to validate on the first prototypes is that it has extremely low spinning drag (less than 0.1 Nm of dry drag torque.) This is less than any other gearbox we have tested with the only possible exception being the EV1 gearbox. This low drag contributes to the 1.5 powertrain having a slightly improved range figure.

The two pictures below show some of the gears and shafts. On the left is the input shaft assembly…spinning up to 14,000 rpm bearing selection is very important. The spline on the right connects to the motor coupling. The picture on the right is the intermediate shaft assembly. Notice the transmission locking “gear” in the center that engages with a stationary pawl when the car is parked.

All these changes will dramatically improve performance without effecting the range or power draw from the battery.  Tesla has learned a lot from its inaugural launch, hopefully with this more efficient powertrain they will ramp up production.

Sources: [MotorAuthority, Tesla Motors]

Extraordinary Recession Affects Tesla Motors

Newly appointed CEO of Tesla Motors, Elon Musk, created a new post, Extraordinary Times Require Focus, in his blog today.

He announces Tesla is going to “consolidate operations”, which means “lay off” a few people. This was expected since every car company is getting hit by the economic crisis, even electric car startups. He also had some discouraging news on the Model S:

Tesla is absolutely committed to development of our next generation vehicle, to be unveiled early next year. However, we are going to reduce activity on detailed production engineering, tooling and commitments to suppliers until our Department of Energy loan guarantee becomes effective.

This will delay in start of production of the Model S by roughly six months to mid-2011.

Elon Musk is a very competent businessman, as exemplified by his Paypal success and his SpaceX gig. He also seems to have a genuine passion for electric cars, as he has spent about twice as much money as he expected in the project. So I don’t think a few bad quarters will end the ride.

On Jay Leno’s New Ride

The new video circulating the internet of Jay Leno’s Tesla Roadster is exciting and intriguing for many reasons.  See for yourself:

He really breaks down the car from a car guy’s perspective.  And he notes many factors that may make electric automobiles the heir apparent to the gasoline combustion car.  Net environmental advantage and price aside, Leno shows why the electric car is the more efficient, superior machine when compared to any combustion engine vehicle.

1) Because it has an electric motor, the car has peak torque from 0rpm to 13,000rpm.  Therefore, you receive maximum performance with minimal energy waste.
2) No gears and no engine compression means that you don’t lose momentum when you let off the gas.  So there is no wasteful high rpm when your car is struggling to go forward against the engine compression.
3) Jay Leno describes the sound.  Which is almost none at all!  Like him, I’d prefer a robust growl that you’d expect from a typical high end sports car; and I know a lot of other people that feel the same way.  But when the motor got going it sounded a lot like an aggressive turbo spooling up, which is a fair consolation.
4)  When they open up the car you really see how minimalistic an electric motor is.  A small reservoir for battery coolant is all that is necessary other than the battery and the motor.  No complicated, heavy exhaust system, no extensive engine cooling setup, no transmission as we know it, and no money gulping gas tank. With the amount of weight lost and simplicity gained by losing the traditional engine, surely the electric vehicle is the future of personal transportation. While bio diesel and hydrogen are viable alternatives to gasoline, I think we need to reconsider prolonging the already overextended life of the combustion engine.