The Technograph

When you returned to campus this fall, you may have seen—but not heard—something new on the streets of Champaign-Urbana. The C-U Mass Transit District unveiled five hybrid buses in July and four more this past fall. The advantages that hybrid buses have over diesel buses are numerous: less noise pollution (hybrids are three decibels quieter); lower maintenance costs; fewer daily fill-ups; and an emissions reduction of about 90%. Hybrid buses even last for fifteen years, the same amount of time as diesel buses.

“Hybrid” is a buzzword that has been thrown around a lot in regards to today’s energy efficiency efforts. We all know that it refers to a vehicle with great gas mileage, but how does it really work? To be classified as a hybrid, a vehicle must have the three following capabilities: to shut off while idling; to convert kinetic energy from braking into electricity that can be used to drive the vehicle (this is referred to as “regenerative braking” or simply “regen”;) and to be powered by an electric motor (even if it is in assistance to a conventional motor like most hybrids are).

There are two types of drivetrains that hybrids can have: series drivetrains or parallel drivetrains. In a series hybrid, an electric motor provides the power that gets the vehicle’s wheels turning. To maximize efficiency, the battery is recharged by both the engine’s generator and regenerative braking. Incredibly, a series hybrid is at its most efficient in stop-and-go traffic because the gasoline engine is not coupled to the wheels, so the engine can operate in a narrow power range at near-optimum efficiency. It is easy to imagine why most buses are series hybrids.

Unlike series hybrids, both the engine and the electric motor generate the power that drives the wheels in a parallel drivetrain. A more advanced computer and a transmission are what makes these components work together. Parallel hybrids are much more highway efficient than series hybrids because they don’t convert mechanical power to electricity and back since the engine is connected directly to the wheels. The engine operates inefficiently in stop-and-go driving, however, because it is forced to meet the widely-varying power demands. The Insight, Civic, and Accord hybrids from Honda are parallel hybrids.

What about the other energy-efficient options available? Cynthia Hoyle, Transportation Planning Consultant for the C-U Mass Transit District, pointed out that “several options were considered, including compressed natural gas systems.” Compressed Natural Gas (CNG) vehicle systems are a viable option because natural gas is less expensive than gasoline and it has the highest energy to carbon ratio of any fossil fuel, meaning it produces less carbon dioxide per unit of energy. Several public transit agencies throughout the U.S. have introduced CNG buses, but CNG taxis and vehicles are most common in South America and Asia.

It may seem like there is a small number of hybrid buses compared to the total fleet of more than 80 buses, but in time, the entire fleet will consist only of hybrids: “New hybrid buses will be purchased each year replacing older buses,” says Hoyle. The C-U Mass Transit District’s initiative is an excellent investment in the future, and one that should make you feel good about not wanting to walk to class in the bitter, freezing winter.

Tagged with: bus, MTD, hybrid, green