Engineering the Chevy Volt
5/13/2011 1:28 AM EDT
For engineers, development of the Chevy Volt is a ripping yarn which has been likened to a program of “moon shot” proportions. What’s great reading for an engineer? How about a monograph that details development of a product, program, or engineering project—and the bigger or more sweeping or innovative the subject the better.
That’s what you’re in for if you pickup (or download) a copy of “Chevrolet Volt, Development Story of the Pioneering Electrified Vehicle,” edited by Lindsay Brooke, and published by SAE International as part of its Vehicle Electrification Series.
The 219 page volume, just published, not only goes into the design of critical systems such as the battery pack and electrified transaxle, but discusses the reasoning behind the car, the experiences of the engineering team and their tools, and the battery charging infrastructure for the vehicle.
[Ed. Digression: Speaking of engineers and their reading habits—remember in the original Star Trek series where Scotty eschews beaming down to the “recreation planet” for some well-earned leave because he wants to catch up on some technical manuals?]
Most of the Volt book consists of 14 SAE papers about the car and its systems development. However, all but two of these were recently presented in April 2011 at the SAE Congress in Detroit.
They are thus the most up-to-date information GM has made available now that the company has finished the nearly four-year effort to go from concept to production first fruits. Some information was also published in the SAE’s monthly magazine, Automotive Engineering International.
While these “bricks” of detailed technology are the majority of information in the book, it’s the “mortar” of recent interviews of program principals, both GM and suppliers, connecting the papers that, in a way, provide the most technology insights as well as human interest.
These revelations include the generally conservative approach to the battery pack development—going with more efficient liquid cooling for stability and long life—while innovatively using the OnStar system’s connectivity as a telemetry system to record data from 24 test vehicles. The latter experience led to Volt telematics that can program and activate vehicle charging, climate control, and other functions via the Internet and smartphone apps.
And surprisingly for such a monograph, the book doesn’t hesitate to chastise retired GM Vice Chairman for Product Development Bob Lutz and his concept of “decoupled development.” That concept called for “disconnecting development of the most engineering-intense subsystems (i.e. a new powertrain) from a vehicle program’s critical path [for] overall efficiencies.” Had such a route been taken, the Volt would probably still be months or years from production.
The Li-ion battery is the heart of the Volt and its development and technology take up the largest section of the book. Goals were long life (thus using only 65% of its state-of-charge range), thermal stability (via liquid cooling), and energy density. Critical to realizing optimal battery operation was development of the Battery State Estimator (BSE), a proprietary algorithm that basically monitors charge and controls charging to keep the internal-combustion “onboard charging generator” from running when inappropriate, which would cut into driving range.
Obviously, battery cell quality was another key element because a single failure in any of the 288 cells could cause the entire battery pack to fail. Similarly, a small error in monitoring the cells with the BSE could create a performance problem.
“Volt is all about balanced operation—delivering an efficient point of running the engine, staying within the [charging state] bandwidth…and making the car pleasing to the customer,” says Vehicle Line Executive Tony Posawatz.
The next technology emphasized in the book is the electrified transaxle, which ties together the battery, internal combustion generator, and regenerative brakes.
Likewise, there was a tradeoff between battery capacity (larger for longer range) and the gasoline engine size (generally smaller if the battery range is higher). All these technologies are integrated, but not like a “traditional” hybrid in that the Volt delivers full performance on electric power alone, only requiring fueled power after the initial battery state of charge is depleted.
While engineers may first want to look at the technology in the Volt, the book rightly highlights the invaluable role of often overlooked development tools available to the team for parallel systems development that enabled the start of production within the tight four year deadline. Specifically, model-based design software allowed testing and verifying algorithms that were then proven by the time the first hardware was ready. Thus the vehicle testbed “mules” were running “earlier in a program than had ever been done before,” according to Greg Hubbard, drive controls senior manager. These could be equipped with systems that had gone through design iterations via simulations to validate design and verify performance.
Hand-in-hand with model-based design was automatic software code generation that gave a 30-35% efficiency gain compared to hand coding—significant when you realize the Volt has nearly 10 million lines of code (the Boeing 787 Dreamliner has 8 million lines).
The remainder of the book goes into the extensive effort to cut range-eating aerodynamic drag, resulting in a drag coefficient of 0.281, better than GM’s previous best in the Corvette (0.29). The aero effort also reduced wind noise, which is critical on a “quiet” electric powered vehicle, and improved engine bay cooling airflow.
Finally, the only criticism of the book I could find was a couple of references to sidebars that directed the reader to an incorrect page. Other than that, you can spend an informed and entertaining evening or two with it—and amaze your friends with your Volt knowledge, which many people, not just techies, are interested in these days.
Some Volt facts
- The Volt has five cooling systems (and five heat exchangers), including ones for the electrical generator, battery, and power electronics.
- The internal combustion engine runs on premium gas and in 2012 will be able to run on E85 fuel.
- To extend battery life, only 9.4 kW-h of a total capacity of 16 kW-h are used.
- The battery pack weighs 375 lb (170 kg) and is a structural member.
- The engine starts every 45 days and runs for 10 minutes to circulate fuel and oil, and run checks and maintenance routines.
- Volt interior designers had to wrap the cabin around the T-shaped battery pack that runs under the rear seat and along the centerline.
Chevrolet Volt, Development Story of the Pioneering Electrified Vehicle is available from SAE International at http://books.sae.org/book-pt-149 (although the price for non-SAE members seems steep at $119.95, it does contain 12 most recent papers).