We often hear automotive engineers talk about the need for a better understanding of the components of an EV. Some have said that there is an inherent inefficiency in many systems because the parts – like the battery cells, motors and inverters – have historically been developed independently of each other and then integrated together. Others point out that the ICE has seen millions of man-hours spent on refinement – pushing efficiencies and maximizing power – while the study of advanced electric powertrains has only just begun.
The 2014 Cadillac ELR is a good example of exactly how a better knowledge of the system can lead to better vehicles.
The ELR comes standard with many upscale luxuries – including LED lighting, a Bose 10-speaker sound system with active noise cancellation, and a hand-cut-and-sewn interior with leather seating and microfiber suede headliners, just to name a few – and the plug-in media pundits have criticized GM for what seems to be an expensive repackaging of the Chevy Volt. But beneath the leather and before the hardware lies a very interesting innovation of the ELR, GM’s next-generation EV control techniques.
“We upgraded everything,” Tim Grewe, Cadillac ELR powertrain Chief Engineer, told us during an interview for Charged – Issue 11 (subscribe here). While all of the ELR’s hardware is essentially the same as the Chevrolet Volt’s, Grewe was referring to the control systems – the advanced mathematical algorithms that determine everything about how a vehicle operates.
The motor of GM’s Voltec powertrain is controlled by an inverter that operates at about 10 kHz and creates a fundamental frequency sine wave. To push the limits of the motor’s performance, the automaker embarked on a two-year development program that resulted in a unique pulse width modulation (PWM) control technique. The new algorithm builds upon a control theory developed in the 1990s that injects harmonics – through a complex series of math equations – into the original sine wave, boosting power.
“In the 90s, they used to call it third harmonic injection,” explained Grewe, “where you take the third harmonic of the primary frequency and inject it back in. That actually makes the motor have more capabilities. We went about ten levels beyond that, so it’s way past third harmonic injection.”
The result is a very complex PWM pattern that has been tuned to give the new Cadillac more performance without upgrading the hardware. Usually, when a motor’s performance is pushed to its limits, a lot of electrical noise is generated – which turns into audible noise. But Grewe says that GM’s new technique is capable of avoiding those pitfalls and offers power without the noise and vibration – which gives the ELR a more luxurious experience.
The team also developed new tricks to avoid generating more heat in the motor. “Fundamentally, there is more heat with more power because the efficiency is pretty level with these techniques,” said Grewe. “We started off saying that we’re going to go for more performance – full speed ahead and we’ll let the cooling system figure it out. But because this car is all about efficiency, we decided not to do that. Instead, we used an in-depth modeling of every grain boundary in the steel of the motor and found ways to control the magnets very precisely. In the end, we added more capabilities and more efficiency.”
The ELR also commands higher peak power from the battery pack. Through the use of Chevy’s opt-in data gathering campaign – in which Volt drivers allowed OnStar to collect their vehicles’ usage stats – GM was able to learn more about the load profile of batteries in the fields. With that knowledge, the ELR could push the technology envelope to control the battery pack more precisely.
Discharging cells to too low a level can permanently damage them, but with a more complete understanding of operating conditions and real-world demands, GM’s engineers are able to “push it right towards that minimum SOC a lot quicker and control a lot tighter around it,” said Grewe. Repeatedly running a battery to its minimum SOC is something that can easily be tested in a simulator, however on the road it’s very difficult to plan for all possible scenarios. For example, in a traction control event in the snow – with the tires spinning and then gripping the pavement hard – all that energy has to be absorbed by this system. If it’s not controlled accurately, the batteries could be overloaded and damaged.
“If you get a big tip in transient response where you want to pull 130 kW out of the battery,” explained Grewe, “we now have the dynamics in the real world about how that’s going to happen, so we can tune the controls to get 130 kW, which is the cell manufacturer’s recommended limit. We can do that with precision through all of these advanced controls. Before, we wanted to make sure we had a little bit of design margin, so the power limit in the Volt is about 110 kW.”
Field data from early Volt adopters also aided in the development of advanced driving modes. The ELR features a 32-bit RISC processor that, about 100 times per second, assesses where the vehicle is and where the driver will want it to go. Known as the real-time optimizer, the predictive system is trying to find the most efficient mode of operation while delivering the responsiveness desired by the driver.
“It’s technically a golden search algorithm,” said Grewe. “It’s sort of like you’re the pilot of the car commanding it where to go with this co-pilot getting you there when you want to be there in the most responsive and most efficient way.”
GM says that all the code for the ELR’s real-time optimizer has been rewritten and upgraded from the Volt’s system, to give the car the performance you would expect from a Cadillac. “You can imagine that when you’re getting closer and closer to the edge of stability, you have to be very precise,” explained Grewe. “So we actually built predictive models – strategic optimizers – to say here’s what we think is going to happen next and how we’re going to set ourselves up for it. That’s how you’re able to get this good range and have all this luxury feel. There is a huge amount of technology and optimization in the virtual world to make that happen.”
Grewe reports that it was not always clear that the components of the Voltec powertrain could be pushed beyond the performance limits of the Volt. “A lot of engineers were skeptical that the parts would not survive being pushed to the max. Well, I’m the powertrain chief, so it was my job to strike a balance between the skeptics saying ‘you can’t go any higher than the Volt’ and the hard data from the field saying ‘yes we can, if we just control it better.’ That was the development of this car.”
Read more about the 2014 Cadillac ELR in Charged – Issue 11. Subscribe now.