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Charging Forward OPEN ACCESS

As Automobiles become More and More Electric, Some of the Design Challenges are Taking Engineers by Surprise.

[+] Author Notes

Jack Thornton is a technology consultant in Santa Fe, N.M., and a frequent contributor to Mechanical Engineering.

Mechanical Engineering 134(08), 36-39 (Aug 01, 2012) (4 pages) doi:10.1115/1.2012-AUG-2

Abstract

This article outlines the challenges for automobile engineers in designing electric-drive vehicles. Understanding the way noise travels differently through an electric-drive vehicle is one of the main challenges for engineers as they design this new generation of vehicles. Moreover, bodies and chassis are evolving away from traditional sheet metal to more exotic materials, and consequently, the whole production process is being re-engineered. The power density of even the best battery is small when compared to the chemical energy in an identical volume of gasoline. Hence, electric vehicles (EVs) can at most eke out only around 100 miles per charge. Overcoming that challenge is the subject of decades-long research projects. The lithium-ion batteries found in most EVs generate so much heat in use that they require their own cooling systems. Temperatures of all cells within the battery pack also must be held within a few degrees of each other, lest internal current loops form that may slash battery life. Some other issues include cost, service life, and safety.

Article

Electrically powered cars have a reputation for being quiet, sometimes too quiet. But as electric-drive vehicles, in hybrid, plug-in hybrid, and pure electric form, become more mainstream, engineers are discovering that these cars are not as soundless as first thought. There may not always be an internal-combustion engine rumbling under the hood, but there are plenty of parts in motion, and this provides an opportunity for unwanted noise.

One ongoing challenge “is muting the high-pitched noise of the power-control electronics as the electric motor changes torque,” said Kevin Dietrich, hybrid control systems integration manager for the Chevrolet Volt, a plug-in hybrid car introduced by General Motors in 2010.

“Hybrids and EVs have all sorts of considerations in noise, vibration, acoustics that go all the way into the vehicle's structure, and these differ with each vehicle model,” Dietrich said.

Understanding the way noise travels differently though an electric-drive vehicle is just one of the challenges for engineers as they design a new generation of vehicles—a generation that represents the dawn of a new technological era in the transportation system. The upheaval goes beyond replacing the venerable ICE in some cases with all-electric powertrains. Bodies and chassis are evolving away from traditional sheet metal to more exotic materials, and consequently, the whole production process is being re-engineered. There are also new supply chains and carefully monitored demonstration programs.

The result is that the industry's longstanding paradigms are being overturned. Making a car will never be the same.

Some people have argued that cars have been at least partly hybrid-electric since Charles Kettering's electric starter motor, first installed on 1912 Cadillacs, became more or less standard. (Those first starter motors actually worked as generators, similar to the regenerative braking systems on modern hybrids.) But in the popular concept of the term, hybrid-electric vehicles, which combine a conventional internal combustion engine propulsion system with a battery-powered electric propulsion system, hit the market in 1997 with the Toyota Prius. Toyota has sold more than 3 million Priuses worldwide since then.

While today there are dozens of hybrids, car companies are making vehicles with a larger and larger share of the propulsion being performed by the electrical drive. The Chevrolet Volt is a plug-in hybrid, which enables motorists to top off their batteries with grid current in addition to being recharged by the gasoline motor. The Nissan Leaf and the Ford Focus EV are powered solely by batteries.

Compared to the Prius, however, these more electric-oriented cars are still niche products. GM sold 7,700 Volts in the U.S. in 2011, and that same year Nissan sold nearly 10,000 Leaf EVs.

The differences in design complexity between hybrid-electrics and full electric vehicles are stark. Hybrids, with their dual powertrains—both mechanical and electrical—have auto industry mechanical and electrical engineers working together as never before to integrate the two systems. The result is a powertrain that is more complicated than that of any type of conventional vehicle, gasoline-powered, diesel, even natural-gas-powered.

EVs are noteworthy for their relative simplicity. Chuck Gray, chief engineer for core electrification engineering at Ford Motor Co. and a veteran of its powertrain operations, points out that not only do EVs not have internal combustion engines, they don’t have the things that go with them. The list of redundant parts includes mechanical transmissions, starter motors, spark plugs and their wires, fuel injectors, fuel pumps, fuel filters, radiators, water pumps, hoses, and timing belts. Also, Gray said, EVs weigh less than ICE-powered cars and require far less maintenance. All that means EVs should be much simpler to engineer and assemble than ICE-powered vehicles.

The Ford Focus EV (top) and the Chevy Spark (left) have replaced internal combustion engines with batteries and electric motors. Plug-in hybrids such as the Chevy Volt (below) need both systems to operate.

Grahic Jump LocationThe Ford Focus EV (top) and the Chevy Spark (left) have replaced internal combustion engines with batteries and electric motors. Plug-in hybrids such as the Chevy Volt (below) need both systems to operate.

But electrics and plug-in hybrids (which can conceivably go days without running their engines) have their own challenges. The biggest is batteries. The power density of even the best battery is small when compared to the chemical energy in an identical volume of gasoline. That's why EVs can at most eke out only around 100 miles per charge. Overcoming that challenge is the subject of decades-long research projects.

The Volt's Burning Issues

Lost amid the reporting of the Chevy Volt fires Last year was the fact that after putting over 6 million miles on hybrids and EVs, not one has caught fire outside of a Lab. In contrast, the National Fire Protection Association reports there were roughly 184,000 highway vehicle fires in 2010, nearly all of them in gasoline-powered vehicles; 285 deaths resulted.

Almost as overlooked was the fact that widely reported Volt fire at a National Highway Transportation Safety Administration testing facility in Wisconsin occurred weeks after the Lithium ion battery was compromised. On May 12, 2011, a side-impact crash test poked a hole in the battery pack, breaking open some of the battery cells and puncturing coolant lines. That caused the explosion and fire—but not until June 4. The Volt was in the Lab's parking Lot; the fire destroyed that car and four nearby vehicles.

It took a crash and weeks of neglect before this Volt caught fire.

Grahic Jump LocationIt took a crash and weeks of neglect before this Volt caught fire.

The NHTSA cleared GM and the Volt by stating that it did not identify a safety defect and concluding that the car does not pose any unusual risk of fire. The agency also said it “remains unaware of any real-world crashes that have resulted in a battery-related fire involving the Chevy Volt or any other electric vehicle.” Further tests by GM and the NHTSA failed to reproduce the explosion and fire.

In any event, GM is retrofitting the roughly 8,000 Volts it has already sold and modified the battery packs in new vehicles coming off the assembly Line. The new structural reinforcement better protects the battery pack from punctures or a coolant Leak in a severe side-impact crash.

That's not the end of GM's battery woes. At the end of 2011, GM switched the Volt's battery supplier from LG Chem Ltd. of South Korea to A123 Systems in Waltham, Mass. On April 11, 2012, gases from a prototype A123 battery pack exploded and burned briefly in an enclosed test cell at GM's Alternative Energy Center in Warren, Mich.

Power density, however, is not the only challenge for automakers. The lithium ion batteries found in most EVs generate so much heat in use that they require their own cooling systems. Temperatures of all cells within the battery pack also must be held within a few degrees of each other, lest internal current loops form that may slash battery life.

Engineers at GM have found that the operating range of batteries varies so much due to temperature that they are cooling the packs with liquid. The extra cooling and thermal diffusion mean big engineering challenges in heat exchangers, and extra valving and plumbing, plus lots of thermodynamics and analysis with computational fluid dynamics.

Other issues include cost, service life, and safety. Industry engineers note that the best of today's lithium-ion batteries account for $10,000 to $15,000 of an EV's sticker price, which is three to five times the cost of an ICE and its mechanical transmission. As a result, EVs and hybrids cost roughly twice as much as comparable conventional vehicles. Owners looking to replace the EV battery packs after only three or four years—a consequence of the batteries’ short service life—face costs exceeding the expected Kelley Blue Book value of the vehicles. And if the lithium-ion batteries are punctured in an accident, dangerous materials may leak into the passenger compartment.

Batteries may have gotten the most attention in the press, but automakers are taking other factors just as seriously. “In hybrids you are adding an engine to the mix of all the variabilities of an EV,” said GM's Kevin Dietrich. “From an engineering standpoint, it is a big balancing game—motors, controls, engine, power electronics, and battery—across a wide spectrum of services, driver demands, and outside temperatures.”

Power electronics carry power back and forth between battery packs and motor/generators and control the power transfers, constantly adapting the powertrain to driving conditions. This requires precise control of voltage and current in all parts of the system. Hybrid and EV applications for powercontrol electronics “are huge and varied,” Dietrich said, “and an evolution is under way. What we think of as the second generation is in production now and work is well under way on the third.”

Optimizing these systems is a constant challenge. “There are many combinations of energy paths—electrical, mechanical, and thermal—to account for and analyze as part of maintaining drive quality,” Dietrich said.

But GM sees the quality of their power electronics as a competitive advantage. Controls, traction motors and batteries, and their integration “are very core technologies,” he said, “and GM is keeping these developments in-house.”

The charging system for electric-drive cars is also in need of optimization. The electric motors that propel the car do double duty as generators which recharge the car with energy harvested from regenerative breaking. Adding complication, plug-in vehicles need to accept grid electricity as well as that from their own generators.

Re-engineering the Engineers

The emergence of electric-drive vehicles is creating conflicting design and engineering requirements and the way forward will be bumpy and occasionally unclear. That lack of clarity extends to the impact on the number—and type—of engineering jobs in the automotive industry.

Opportunities in the auto industry have several large unknowns, from the impact of outsourcing work abroad to the number of newly minted engineers entering the workforce. But one thing seems clear: The role of computer-aided engineering will only get larger.

Al Houtman, vehicle performance manager for the Chevy Volt, explained that “balance” in the vehicle [or any complex product] requires extensive analysis to get the systems engineering and the systems integration right. “We have so much to balance on hybrids and EVs that the modeling and simulation never end. Some of the analyses have hundreds of variables to quantify and weight against each other.”

The more analysis GM does, Houtman added, “the more we are able to reduce the number of potential solutions and, beyond that, reduce the possible variations in the vehicle itself.”

This electric generator was designed in CAD.

Grahic Jump LocationThis electric generator was designed in CAD.

Kevin Dietrich, a hybrid control systems integration manager for the Chevy Volt, said, “Any new engineer here needs CAE skills in modeling and simulation. Most of the new engineers have Matlab and Simulink on their laptops and those laptops go every-where they go. Today's young engineers do far more analyses than we ever did when I was new on the job, back in the 1980s.”

In EVs, modeling and simulation “begin with picking the right basic components and matching them to get the most efficient mix,” Dietrich added. “I would advise MEs not to shy away from the electrical side of the profession. For MEs, avoiding electronics completely is a risky career choice.”

Engineers at Chrysler are working on a new system for recharging the batteries of its demonstration fleet of 165 plug-in hybrid-electric pickup trucks and minivans. Unlike the Pri-uses and Volts, the Ram truck and Town and Country minivan are plug-in workhorses. A Ram pickup can carry up to half a ton in the back, tow a 6,000-pound trailer, and handle 30 percent grades. One Ram 1500 PHEV was driven to the top of Pike's Peak in Colorado.

The centerpiece of the PHEV system is an on-board 6.6-kilo-watt ac battery charger that can switch between 240- and 110- volt power. The system can reverse itself so that, powered by the 5.7-liter gasoline engine, the Ram 1500 pickup can act as a generator and supply electricity to the power grid. “With a full tank of gas,” said Abdullah Bazzi, senior manager of Chrysler Group's advanced hybrid vehicle project, “a Ram 1500 PHEV can generate 6.6 kW continuous power for five days.”

That ability may seem a curiosity at best, but it has gotten the attention of the U.S. military. The Army is testing the system at Fort Carson in Colorado to see whether several Ram pickups could constitute a micro-power grid for battlefield medical installations. “The gasoline engine powers its own micro-grid,” Bazzi said, noting that several electric utilities are testing a related system.

While Chrysler's Town and Country PHEV minivans have essentially the same mechanical and electrical integration as the Ram 1500, “The anticipated uses of the minivans and their component packaging are quite different,” Bazzi said. The minivans use a smaller, 3.6-liter gasoline engine and won’t have the grid connectivity or the reverse power flow capability of the pickups.

Ford is also developing plug-in chargers for its EVs that communicate with the electric power grid. “It turns out that there are many new things involved for our engineers writing the software for the grid interface to validate,” Ford's Chuck Gray said. “Among them was the charger's response to fluctuating line voltages.”

But Ford is also working on another variable element: the driver. The company is offering instrument panel enhancements designed to coach motorists into improving their driving. New gauges and readouts keep the driver aware of the state of the vehicle's electrical systems, the condition of its battery, the electric motor, and the ICE.

One innovation is the Brake Coach: It helps drivers overcome any urge to delay braking so they won’t have they have to “stab the brakes” to slow or stop. “By starting to brake sooner and braking more slowly, the regeneration systems extract more kinetic energy for recharging,” Gray said.

Until all these engineering issues are resolved, U.S.-built EVs and plug-in hybrids will appear only in a few dealer showrooms in a handful of markets. We may love the idea of clean and quiet electric vehicles, but when it comes to the cars we drive, we want something we can rely on to take us as far as we want to go.

Copyright © 2012 by ASME
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