An Engineering Update on Powertrain 1.5.
by JB Straubel
Tesla CTO. Published Tuesday, May 27th, 2008 – source link
It has been several months since we have shared any progress updates on our powertrain development work and in particular how the “powertrain 1.5” program is progressing for the Roadster.
For those of you not thinking about this every day like we are the powertrain 1.5 is an improved motor, inverter and gearbox designed to replace our previous two-speed transmission that had many durability, efficiency and cost challenges. We officially kicked off this program just last fall but it is something that Elon and I had discussed several years ago. A very low-cost and efficient single-speed gearbox mated with a continually improving motor, inverter and battery is the core competency of Tesla’s powertrain team and it is also our roadmap for future vehicles.
Many people are now working very hard on this project and I’m happy to report that we are holding our initial schedule for production deployment around vehicle #41 later this year. Several significant milestones have been passed in each of the key components that I will discuss in more detail below.
We also have a Roadster with a prototype 1.5 powertrain that we are now driving regularly. The higher torque is really phenomenal. I have many hours behind the wheel of the 1.0 powertrain and this is simply much better. The motor torque is improved 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.
Here is a quick refresher on what the powertrain 1.5 is and is not:
- 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
Power Electronics Module (PEM) Update
It is counterintuitive that one of the most diminutive parts in the powertrain is actually responsible for most of the performance improvement between 1.0 and 1.5. The IGBT (Insulated Gate Bipolar Transistor) inside of the PEM is what converts and regulates power from the battery. These small parts are improving in both efficiency and power handling capability and by integrating the latest generation of parts, we have been able to boost the PEM output current by about 33% from 640A rms to 850A rms with the same number of IGBTs.
We could have increased the current and torque by just using more of the older IGBTs but this would have required a much more extensive redesign of the entire PEM including the mechanical packaging and cooling systems.
Since the new IGBTs have improved efficiency they also end up giving the PEM better overall efficiency and improve the range of the vehicle slightly. At most operating points the PEM is already very efficient (95-98%) but every little bit helps. Other than this change to the IBGTs and a few improved internal cables the PEM is identical to a 1.0 PEM; without the external serial number labels you can not tell them apart.
The 1.5 motor has slightly more substantial changes to deal with the higher current. We have modified the castings on both ends of the motor called “endbells.” These were modified to allow for a different interface to the new gearbox and also to improve the durability of the fastening between the motor and transmission. We also modified the motor shaft slightly with a larger and stronger output spline to handle the higher torque that the motor can generate at 850A. The bearings remain the same and the internal electromagnetic design of the motor is identical. The same number of turns and lamination geometry are used.
One additional improvement was made to the motor terminal lugs in order to significantly reduce their resistance yielding better efficiency and much less temperature rise at very high currents. Connected to these lugs is the motor cable that attaches the PEM to the motor. We have also reduced the resistance of this motor cable by changing wire material from copper clad aluminum to pure copper. This increases the mass slightly but also improves the efficiency and reduces temperature rise.
Below on the right is a prototype 1.5 motor that is now being tested on the dynamometer at our shop in San Carlos. On the left is a plastic SLA model that we made of the new endbells before the metal parts were ready.
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.
These are all of the gears and shafts assembled in one case half. The blue automatic transmission fluid (ATF) is not normal and we were just using a blue dye in the ATF to test lubrication distribution in this unit.
This is one of the machined aluminum case halves. This part started life as a solid block of metal! Machining the cases is a quick process for prototyping but it is far too wasteful and expensive for production. The second picture below shows a newer cast version of same part (from the other side) after post-machining. This is how we will build the production gearbox cases.
This is a completed and assembled gearbox prototype ready to go on the dyno. The motor attaches to the top in the picture on the far right and the output shafts to the wheels are in the bottom center of the two pictures on the right. The outputs holes are plugged with aluminum covers to keep oil in and dirt out during assembly.
And finally here are two pictures of a prototype gearbox being tested on the dyno. We run many different tests to thoroughly beat up the gearbox in a more aggressive and controlled environment than possible in a vehicle. One of these tests called the “Wide Open Throttle” or WOT test repeatedly simulates vehicle accelerations from stopped to high speed at maximum torque and then back to a stop under maximum regen. This test is repeated hundreds of times back to back.
I have mentioned on several occasions that we are making parts more efficient in migrating from the 1.0 to 1.5 design. The PEM, motor cable, motor connections, and gearbox are all incrementally more efficient. When all of these are added up it amounts to a meaningful increase in overall vehicle range of around 10 miles.
One common question is why doesn’t the range drop since the motor current and torque are increasing? The answer is one of the beautiful characteristics of EVs. The efficiency of this new powertrain when compared with the 1.0 powertrain is actually BETTER at ALL of the operating points that they have in common. This is the exact opposite of how two gasoline engines would compare (an 8 cylinder engine versus a 6 cylinder engine for example.) With an internal combustion engine the efficiency of the larger engine is usually worse at all cruising power levels.
When the 1.5 powertrain is operating at torque levels that are higher than what is possible with the 1.0 powertrain a direct comparison is impossible but the efficiency levels are still very high. The efficiency remains relatively flat all the way up to maximum torque and power. Keep in mind also that very little time is actually spent in the vehicle at above 280Nm of motor torque (the previous limit to the 1.0 system) and on the drives where you do spend lots of time at full throttle you generally are not trying to maximize your range!
Along with improved efficiency the 1.5 powertrain will have improved thermal performance over the 1.0 powertrain at all common operating points. This is due to the efficiency of the PEM, motor and gearbox and also due to the slightly increased gear ratio. (Increased by about 12% from 7.4:1 to 8.27:1) This gear ratio change will reduce motor current by about the same ratio ~12% for a given vehicle operating point and this will reduce the thermal load on the motor and PEM.
When operated at torque levels beyond the 1.0 ceiling there is no baseline to compare against. One thought experiment is to imagine that the car is driven hard enough to limit motor performance due to temperature. Once in this condition the 1.5 powertrain will always have about 12% more torque to the wheels than the 1.0 (due to the gear ratio) for the same energy dissipation in the motor. Before thermal limit the 1.5 powertrain will have an extra ~33% from the motor plus ~12% from the gear ratio (45% total) better torque output to the wheels than 1.0.
Although this improved powertrain will have ~45% better torque at the wheels it will not have a significantly higher peak power output and it will not have a higher peak battery current draw. (The two are directly related by the efficiency of the PEM, motor and transmission) Our goal has actually been to keep the peak battery current at the same level (about 650A).
It is best to think about the PEM as an electronic transmission. The car with a two speed transmission didn’t have a higher peak power output either but it could achieve a faster 0-60 mph time because the gears multiplied the motor mechanical torque. We are using the PEM to multiply the battery current by stepping up the current to the motor while we step down the voltage.
What is coming next?
Testing, testing and then more testing! Over the coming months we will be continuing to exercise and push the new powertrain components to their design limits on various bench tests. The gearbox, motor and PEM will all spend many more hours running on dynamometers at high and low speeds and temperatures. This accelerated testing will be happening in parallel with upgrading the majority of our engineering fleet of vehicles and several marketing vehicles with the new powertrain 1.5 components and then testing them as fully-assembled vehicles. One car will be taken to Death Valley this summer for aggressive hot weather thermal limit testing and hill climbing tests. Another vehicle will be running a 40,000 km durability test around a track at high and low speeds, over rough cobblestone roads, through salt spray baths and potholes. Still other vehicles are slated to undergo transmission abuse testing and vehicle firmware testing. All of these tests are designed to find any problems before they have a chance to show up in production vehicles.
When all is said and done, this evolution of our powertrain system results in a vastly improved overall product for our customers. We have maintained the key performance targets while increasing efficiency and durability. The more powerful torque curve will make the overall driving experience even better than what was reported on in the major car reviews earlier this year.
Although we faced a significant setback last fall when we realized the previous 2-speed design was not sufficiently durable, the Tesla powertrain team is accomplishing an extraordinary feat in not only overcoming this setback but engineering a superior outcome for Tesla customers.