Archive for the 'Vehicle Engineering' Category

Tesla engineers have been keenly focused for some time on the 1.5 powertrain, which we have been testing extensively both on roads and at closed facilities. We have our own data collection kit, but we wanted to see what sort of performance figures we would get from a public track. Read more…

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.

Motor Update

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.

Gearbox Update

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.

Range

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!

Thermal Performance

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.

Peak Power

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.

In the beginning of January, Tesla Motors Vice President of Vehicle Integration, Mac Powell, sat down with Dave Leggett of just-auto for a Q&A session which recently appeared in the 24th issue of the Lotus Engineering Newsletter, proActive. We wanted to share with you this excerpt from the Newsletter as well as include some comments on recent updates and additional information.

DL: When will those Tesla customers who have already put money down receive their cars?

MP: We’re still on plan for the first customer to get his car in the first quarter of this year.

[ed: We have since delivered the first car and the rest are on their way soon.]

DL: So when does series production start then?

MP: It depends what you call production. We’re doing a very controlled ramp rate, a prudent and sensible thing to do. We want to make absolutely sure that the production cars are working as anticipated and that the whole process and supply chain is working before we ramp up production further. There is a danger when you ramp that you keep an eye on the big balls, but it needs all the balls to build the car; it’s no good having just 99% of the parts because you can’t build it. So we will control our ramp rate carefully. We will start producing the cars this quarter and we will monitor and ramp as quickly as we can, but under control.

DL: And what sort of volume are we looking at for the first year of production?

MP: It will be some hundreds of cars this calendar year – we should be running at around 600 for the model year.

DL: And building up to what level?

MP: We have always targeted the capacity to be able to build 2,000 cars a year. That’s because we have to have targets to choose appropriate tooling investment for the components. If the demand is absolutely extraordinary then we could choose to make more. It’s very difficult to predict exactly where demand will be on a new product such as this. We had to set a level that was achievable to fix our investment – and that was 2,000 a year.

DL: I understand the Tesla Roadster’s transmission has been a major source of delays to the programme schedule. What has been behind the problems in that area and how are the problems being overcome?

MP: You might think that a transmission is a known technology, particularly when viewed against all the new technology being incorporated into the car, but it’s not that simple.

As a new company it was difficult, initially, to get major manufacturers to even talk to us, never mind commit any resource to actually help design, engineer and build systems. We started off going down a particular route with a transmissions supplier and we got to the point where we realised that the way that particular programme was headed, it wasn’t really going to meet our performance targets for the car.

We chose, pretty late in the programme, to change course and go with an alternative supplier with a different approach.

The transmission sounds simple: two-speed transmission and therefore you depopulate a regular transmission, but if you think about it, our gear shift between first and second is a factor of two and the motor spins up to 13,000rpm, so first gear takes you to over 60mph and second takes you to over 125mph so that is a huge ratio step against which we don’t have a conventional clutch.

There’s a rotor with a very high inertia and we have to change the speed of that rotor very, very quickly to give us a performance shift. There are also issues about electrical isolation because of the way we use the charging system and the motor.

So, it isn’t straightforward. It isn’t ‘just another transmission’ and some issues have been thrown up that weren’t anticipated and have taken longer than we had hoped to resolve. With change of supplier, change of design concept and with some of these unforeseen Tesla aims to deliver supercar acceleration and technical issues to overcome, delays have been the result.

[ed: We have since decided to use a single reduction gear in place of the two-speed transmission. With increased energy flow to the motor we can achieve 4.0 second 0-60 mph acceleration with a single gear ratio. This solution is also more efficient than the two-speed transmission.]

DL: Just following on from that, the first Tesla Roadsters are being fitted with an ‘interim transmission’ that can be upgraded later on. What are the performance implications of that? Will the first cars off the line be capable of doing 0-60mph in under four seconds?

MP: The point about the interim transmissions is that they will effectively be locked in second gear. A lot of the prototypes have been running like that for some time and the vast majority of customer drives have been done like that and the feedback has actually been that the car hits performance expectations in that state – it still does 0-60 in something like six seconds.

So you’ve now got a rather interesting car that you can drive very rapidly from zero up to its top speed in one gear. I don’t think customers will be too disappointed.

The reason for the two-speed set-up was because the torque capacity of the motor and the power system was ‘X’ and to be able to say to the world ‘electric cars aren’t just golf buggies’ we needed the ratio to give us the four seconds and the top speed. In a single gear to begin with it would have done 0-60 in say 5.5 seconds with a top speed of 100mph.

That would have been fine, but we wanted to make it clear that we could deliver this sort of supercar acceleration and an acceptable top speed. That means two-speed shift – which is doable but it’s just a question of time and money.

It’s important to stress that we have had very positive feedback from customers about the second gear and that’s the only reason we’re doing this: many customers have said they would rather have the car now with the single gear and current level of performance than wait any longer. This is about satisfying customer demands to deliver before we get the full performance of the vehicle.

[ed: It is also important that we retain the supercar acceleration of the Tesla Roadster. The new powertrain with a single reduction gear and increased power will achieve 0-60 mph in 4.0 seconds and have a top speed around 125 mph without a gear shift. Cars delivered with the interim transmission will receive a free upgrade as soon as the new powertrain is complete.]

DL: Can Tesla’s lithium-ion battery fully meet acceptable performance parameters and will the battery last 100,000 miles – does it degrade over the lifecycle?

MP: Sure, they do degrade and it’s all about charge cycles. The battery manufacturers do the testing to determine what the battery capacity is. They fully charge, then fully discharge and keep doing that – it’s actually pretty abusive to a lithium-ion battery.

In the case of lithium-ion, keeping it charged up is actually a good thing to do – you don’t need to fully discharge them the way you need to with some other batteries.

The other thing is temperature control. That charge-discharge cycle that the battery manufacturers have to do is pretty abusive to the battery because of the temperature the battery gets to.

We are aware of all these things and we pay very detailed attention to the management of the batteries because that is so critical to what we’re trying to achieve.

How we charge them and what capacity we charge them to, and how we maintain temperature – because they are actively cooled within the car – is all about preserving battery life.

Most people don’t drive many miles on average each day – and we know this is not a road trip car – meaning they won’t be using a huge percentage of the battery pack each day. So, we say, when you get home just put the battery on charge and that will keep it in good condition.

If we take the standard fairly abusive battery charge cycles the battery manufacturers carry out, that implies driving say 200 miles in one stretch before recharging each time and that works out at 100,000 miles of life. And that’s with that rather unrealistic implied usage pattern. In reality, the battery should be good for more than 100,000 miles.

Also, the battery makers say that the battery life is over when it only achieves 80% of its original performance capacity.

So, there will be some degradation of range over the life of the battery. When new you will get over 200 miles range on a full charge but over time you will notice some degradation on that. But it will be a slow process and even under the extreme battery testing regime of full charge-discharge, that yields 100,000 miles with end of life deemed at 80% of original capability. Performance will also tail off slightly but when you start with 0-60 mph in 4 seconds, you’ll still have a pretty amazing car.

[ed: It is important to note here that the battery cell chemistry that is used in the Roadster also has a calendar life. The expected calendar life is five years.]

DL: In terms of readying the Tesla Roadster for production, what has been your experience in meeting US federal regulations in areas such as safety?

MP: It’s frightening and scary in terms of all the things you have to do, but it is also fairly straightforward. I’ve been in the industry a long time now and it’s a case of knowing what needs to be done and getting on with it. From the outset we knew we were designing for the federal market so things like headlamp design, crash testing and so on, were known factors. We just had to go through the process.

We have had to run certain rig tests on certain parts of the car because it does carry more weight than the Lotus Elise – the Tesla is significantly heavier. The uninitiated look at the car and say it’s the same as the Elise, where is all that additional weight?

Actually it’s not the same car. For example, the crash structure at the front has new components in it that add load bearing paths and we have changed the aluminium chassis as well. Although the chassis is based on Elise technology it had to be changed in many respects, such as the fundamental siderails – these are a unique extrusion for the Tesla. The chassis is different. We have had to build significant reinforcement and strength in to those parts to control the mass of the battery pack as it is restrained during impact.

The pack weighs over 900lbs – it’s a significant lump of mass.

The motor and transmission are relatively light and we don’t have to carry a fuel system, but fundamentally the car is a lot heavier than an Elise.

The important thing is: we know all that and we have designed accordingly and have recently completed all the required safety and crash tests. We’re just getting all the final certificates in place that enable us to ship production cars.

[ed: All DOT, NHTSA, and EPA certificates are now in place and cars can be delivered to customers. Mac Powell wrote a blog about this milestone recently.]

DL: How does the supply chain work on the Tesla Roaster?

MP: Having chosen to work with Lotus as the contract manufacturer, we decided that there was no point in reinventing the wheel. We’re using the Elise structural concept, so anything we can carry over from the Elise makes a lot of sense. We at Tesla have enough to worry about with the new technology, without having to also worry about a lot of the other stuff that goes into making a car.

We have three categories of parts:
• Some parts are 100% Lotus carryover, things like the windscreen wiper. So we simply tap into the Lotus supply chain for them.

[ed: It is worth noting that this category constitutes less than 10% of the entire car.]

• Then there is a second category, which comprises new design or modified parts where it still makes sense for us to use the Lotus suppliers. So Tesla has design responsibility but we use the Lotus supply chain to procure those parts and deliver directly to the plant
in Hethel.

• And thirdly, there are the totally unique Tesla designed and Tesla procured parts. This category includes some simple parts that the supplier can ship to Hethel for final assembly, but also includes more complex elements such as the motor, which we make at our dedicated plant in Taiwan where we do all the manufacturing, testing and then put it in box to ship to Hethel.

DL: How difficult is coordinating the manufacturing and assembly process?

MP: The fundamental logistics is fairly straightforward in terms of things like shipping. But as a new company it’s been a hurdle to get our own systems in place so that we know exactly where we are. We’ve had to develop systems from scratch. Not only have we been designing a fantastic vehicle that uses new technology but at the same time we have been building a company, developing new systems, training people for those systems and making sure that the systems are working correctly – all from a clean sheet of paper. It has been a big challenge.

In terms of what we have done in the time available it has been a very rapid programme.

There’s still engineering change going on now to resolve some final issues and improve the product. Unfortunately it’s engineering change that actually interferes with the supply process. Yes, it’s a challenge.

DL: What’s the current thinking on successor models?

MP: Tesla’s intent has never been just to make the Roadster, but the Roadster acts as a technology pioneer. It has already provoked the industry to take more note of electric cars. Great.

But the Roadster is not our only game. Our game is to get as many electric cars on the road in the world and cut our dependency on oil.

So for us it’s also about making more vehicles – such as a sports sedan so that we can appeal to a broader market. We have got to get the volume up so that we can start getting the price down. We have learnt a lot and we know we can improve still further with future product.

The next car has to appeal to a wider market, as does the car beyond that.

[ed: Our next car will be a sport sedan. We will have more to say publicly in the next few months.]

We are also up to talk to OEMs to see where we can help anybody else to do this because we do not intend to be the only people in the world making electric cars because that doesn’t provide a sufficient solution.

DL: For you personally, how has the experience working for Tesla been?

MP: I’d been at Lotus a long time and achieved a lot – and I enjoyed it - but I’d got to a time in my life when I was needing a different sort of challenge.

I never perceived myself as being particularly green. I think, like most people, I cared but wasn’t doing anything proactively about it.

When I got involved in Tesla and the philosophies behind it, I was attracted. So many firms produce electric cars that you just don’t want to drive – but Tesla is about turning that on its head and producing a car that delivers performance, that you want to drive and doesn’t burn oil, so it is also good for the environment. And the idea is to force real change.

Technically, it was interesting to me and morally I thought ‘yes, I could help to do something and why shouldn’t I get off my backside and actually make a difference?’

I was very lucky to be in a position where this opportunity came up. And I was in a position to help both Lotus and Tesla and act as a bridge between the two companies. Tesla needed somebody from the automotive industry – it was an ideal opportunity for me and a good fit for both companies.

For me it’s been a mixed bag. It’s been incredibly exciting and challenging and has meant working with a totally different group of people – so many of them not automotive, so they would challenge everything. Sometimes I would have to explain how the industry
works the way it does, but sometimes the challenges would make me think ‘why do we do it that way?’

Sometimes you need that different set of people with different thought processes to just challenge what you’re doing and look for better ways of doing things. That has been immensely rewarding. The other thing I would say is that the nature of my role is to find
problems, help teams resolve issues, keep the project on schedule as best we can. That’s my role and you don’t do that by looking at all the things that are going right – you look at the things that aren’t going as well as you’d hoped.

Continually looking at problems can be de-motivating at times and some days are worse than others. But if I sit in a prototype and go for a drive I think ‘this is why I am going through this pain’. It really is such an awesome car to drive and you realise the scale of the opportunity ahead for Tesla and everyone who wants to start thinking about electric drive.

We’ve just got to get there and we will get there – it just hasn’t been an easy road.

When I last posted to this blog in May, the news was a mixed bag. Martin (Founder Martin Eberhard) had just officially revised our range expectations down from 250 miles to greater than 200 miles, but we remained committed to holding the line on 4 second 0-60 mph acceleration and delivering the Tesla Roadster with the performance, handling, looks, and safety of a world-class electric sports car. Now it is September and in the last three weeks we have completed performance and range validation testing of Validation Prototype 1 (VP1 aka “the green car”) in order to verify our Tesla Roadster performance claims. I’m extremely pleased to say that the results are in and our hard work has really paid off! Read more…

We get some great questions and comments from the readers of our blogs, and this post takes its title from Brent, who on May 30 wrote:

“ The Roadster’s battery is arguably the most coddled automotive battery in history. It has its own climate control system, several monitoring computers, and perhaps some other mojo ‘they’ are not telling us. ” Read more…

In a battery-powered electric vehicle, regenerative braking (also called regen) is the conversion of the vehicle’s kinetic energy into chemical energy stored in the battery, where it can be used later to drive the vehicle. It is braking because it also serves to slow the vehicle. It is regenerative because the energy is recaptured in the battery where it can be used again. Read more…

They call me “the range guy” at Tesla Motors, which is fitting since it’s my job to characterize and improve the driving range of the Tesla Roadster. This means I get to:

1. Conduct official range testing according to EPA/CARB procedures,
2. Work with our development teams to extract the most miles out of the car,
   …and best of all…
3. Spend lots of time driving the Engineering Prototypes (EPs) and Validation Prototypes (VPs) to collect data to calibrate our simulation models and understand real-world range.

It’s a dream job, but it also has its challenges. Read more…

Richard Feynman is an icon for many employees at Tesla Motors. In 1959 the Nobel-Prize winning physicist delivered an inspiring lecture, Plenty of Room at the Bottom, that encouraged research into what would come to be known as nanotechnology.

The nano world plays a key role for many EV elements. It is what makes today’s state-of-the-art EVs very different from vehicles of the early twentieth century. The battery is probably the most central element. For decades, battery technology was developed using rather crude fabrication techniques. But in today’s lithium batteries, plate and separator thicknesses are controlled to within micron dimensions. The result is more powerful and efficient devices. The future of batteries may depend more on nano fabrication than on any other technological element.

Wally Rippel is one lucky man. Not only has he met Richard Feynman, but he has studied under him as well – as you’ll read below. Read more…