Clean-Sheet Design for Proterra's All-Electric City Bus

October 16, 2015

Kami Buchholz

The Proterra Catalyst 40-ft (12-m) transit bus was driven more than 250 mi (400 km) between charges during evaluation runs at the Michelin test track in South Carolina. Said Proterra's Matt Horton, "We broke four performance records with this vehicle. And while this won't get pulses racing, the Proterra Catalyst is the fastest acceleration transit bus on the market." The Catalyst's 0 to 20 mph (32 km/h) time is 6.7 s.

The Proterra Catalyst all-electric transit bus nabs double-digit miles per gallon equivalent (MPGe), vastly outperforming its diesel, compressed natural gas, and diesel hybrid-electric rivals.

“At 22 MPGe, the Proterra Catalyst is a huge step-change for this industry. It’s kind of an amazing accomplishment because this is a 40-ft vehicle for 77 passengers and it’s about as fuel efficient as a small SUV,” Matt Horton, Senior Vice President of Sales and Marketing for Proterra, said in an interview with SAE Magazines.

Battery-electric and other electric drive configurations (plug-in hybrid, hybrid, and fuel cell) are poised for substantial growth, according to Genevieve Cullen, President of the Electric Drive Transportation Association.

“The combination of technology advances, policy drivers—including new federal standards for medium- and heavy-duty vehicles—and the increasing understanding of the duty-cycle benefits of electrification are contributing to the very positive market outlook,” Cullen pointed out.

A recent Navigant Research report forecasts that the global sales of electric drive and electric-assisted medium- and heavy-duty commercial vehicles will grow from less than 16,000 in 2014 to nearly 160,000 vehicles in 2023. The Asia-Pacific region likely will get the majority of the volume, but sales in Western Europe and North America are expected to be significant.

The bus market has dramatically shifted over the past five years with the entry of U.S.-based Proterra and Chinese manufacturer BYD both producing dedicated electric buses. There are also electric bus development projects at all three mainstream North American bus manufacturers (New Flyer, Gillig, and Nova Bus), according to Joshua Anderson, President of Desch Systems LLC, a Charlotte, NC vehicle projects company focused on EV and HEV technology integration and system development.

“From a manufacturing and supply chain perspective, it makes sense for these large-volume bus builders to develop EV variants of existing models, while Proterra has the ability to start with a ‘clean sheet’ approach,” noted Anderson.

Proterra’s first all-electric Catalyst bus, fitted with advanced lithium titanate oxide batteries and a fast-charge energy storage system, reached the U.S. market in 2010. In the first quarter of 2015, the company added an extended-range version fitted with lithium nickel manganese cobalt oxide (NMC) batteries.

According to Proterra’s chief engineer John Sleconich, the high-energy density NMC batteries were selected in large part because “the technology has been proven to be safe, durable, and cost-effective.” The extended-range bus battery packs can store between 53 and 321 kW·h on a single charge.

The standard configured Catalyst carries eight uniform battery packs under the bus body between the axles, lowering the center of gravity and separating the passengers from the batteries, according to Sleconich.

“Proterra’s proprietary battery packs were designed to incorporate automotive-grade [supplier provided] battery modules into our energy storage system with each battery pack holding 16 modules,” noted Sleconich.

The firm’s design philosophy centered on creating one vehicle platform that could use various types of batteries interchangeably. Location and design of the packs make them easily accessible for maintenance and upgrades as technology advances, noted Sleconich.

“By completely redesigning the interior configuration of the extended-range battery pack, we were able to get approximately 2.5 times as much energy storage into the same space when compared to our fast-charge batteries,” he said.

Roush Industries, AVL, and other companies have collaborated with Proterra engineers.

Said Horton, “Proterra is the lead designer and developer of all the vehicles that we build,” noting that the Proterra-designed bus bodies are being manufactured by specialized composite suppliers.

“With regard to our drivetrain, we focus our efforts on system controls to make our partner-supplied components function optimally as a heavy-duty EV product,” added Horton.

Utility companies, venture capitalists, and GM Ventures are among the Greenville, SC-headquartered Proterra’s investors.

“As an investor, General Motors has provided us with far more than capital to fuel our growth. They have been an invaluable resource in numerous areas, particularly in design and manufacturing advice and support,” said Horton.

The 27,500-lb (12,475-kg) curb weight Catalyst bus is a market leader on several fronts, including energy efficiency, acceleration, and hill climbing, according to Sleconich.

“For our next-generation vehicles, we are relentlessly working to improve the energy efficiency of our vehicle as well as reduce the cost and increase the energy density of our battery systems,” noted Sleconich. “Greater energy efficiency and higher energy density will enable our next generation of vehicles to tackle any bus route currently served by fossil fuels.”