Two modes are better than one for heavy-duty hybrids

After proving two-mode full hybrid technology in transit buses, GM has put a scaled– down version in its Chevy Tahoe and GMC Yukon SUVs. The Chevy Tahoe and GM Yukon will drive a bit differently beginning with the 2008 model year. That’s when GM debuts hybrid versions of its big SUVs. A GM-BMW-DaimlerChrysler collaboration designed the hybrid system, which GM has packaged in a standard-sized automatic transmission for the SUVs. The approach simplifies integration with an existing platform and is key to the hybrid’s ability to tow loads on par with conventional SUVs. “Mode,” in this context, describes the configuration of transmission gearing for optimal efficiency at varying speeds and power levels. Unlike conventional continuously variable transmissions, the 2-Mode full hybrid’s electrically controlled system uses no mechanical belts or bands. Shifts between modes are synchronous — no engine speed changes are necessary, so acceleration is extremely smooth. Here’s how it works: In mode 1, at low speed and light loads, the truck operates in one of three ways: electric power only, engine power only, or a combination of