Two-Mode Hybrids

If you ever drive up to Echo Summit in California’s majestic Sierra Nevada range, or up any extended steep grade, for that matter, sooner or later you are going to see one of those hybrids limping along in the slow lane. It is common knowledge that hybrids get better mileage in the city than on the freeway, and their poor performance on extended inclines is part of the same problem.

With a standard hybrid, if the power requirements of the vehicle exceed what the gasoline engine can offer, the electric motors provide assistance. When a standard hybrid goes up a hill, or into a headwind, or pulls a load, or drives at the speed limit, the electric motors are helping to turn the wheels, and the on-board batteries are draining. Eventually, the batteries run out of charge, and the vehicle limps along until the duty cycle changes.

This is why the “two-mode hybrid” is a significant development. While the two-mode design will not eliminate this inherent problem with standard hybrids, it minimizes it. The way this is done is through using slightly smaller electric motors, and a highly advanced mechanical transmission that is more efficient in transferring onboard power to traction on the road. For more on the two-mode technology, there is a pretty good write-up in Wikipedia. As they state: “This system amplifies the output of the electric motors similarly to the way in which a conventional transmission amplifies the torque of an internal combustion engine. It also transfers more of the engine’s torque to the wheels, making the transmission more efficient even without the electric motors in use.”

Today General Motors announced the largest hybrid bus order in history, receiving an order from King County, Washington for 500 busses. These busses will use the two-mode hybrid technology, meaning their 30% better mileage (compared to conventional busses) will apply on the freeway as well as in heavy stop-and-go traffic. Currently there are 700 hybrid busses operating in the United States, still a small percentage of the estimated 800,000 busses currently in operation in the U.S.

Since standard busses get about 3.8 MPG, and hybrid busses get about 5 MPG, and since the average commercial or municipal bus travels 250 miles per day, converting the entire bus fleet in the U.S. to hybrid drive would save roughly 320,000 barrels of oil per day. Since the U.S. imports about 12 million barrels of oil per day, this single step could reduce U.S. oil imports by nearly 3.0 percent. Not all that much, but imagine if the 250 million automobiles in the USA had their fuel efficiency improved by 30%. (ref. US Bureau of Transportation Statistics) And needless to say, these hybrid busses are also considerably cleaner than conventional busses.

The two-mode hybrid innovation, pioneered by General Motors in partnership with Daimler and Chrysler, is finding its way into General Motors lines of pickups and SUVs in 2008. What would you prefer to be driving, when you want to tow a boat up to Lake Tahoe (elevation 6,600 feet)? These vehicles are also planned to have a plug-in option, and that combined with the two-mode technology definitely means the next iteration of hybrids are here.

One Response to “Two-Mode Hybrids”
  1. Joe Goodwin says:

    The tow-mode hybrid drive-train is a real innovation that is imparting an advantage to cars that carry its design. A mechanical linkage transmits more energy to the wheels from the motor than continuously variable electronic transmissions, and a smaller pair of on-board electric engine/generators means less dead weight when the batteries are spent on a climb. A new iteration is a fair way to characterize two-mode hybrid designs. The “Volt,” by contrast, would be a new generation of car design.


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