The Seattle Post Intelligencer reports today in a lengthy piece that the diesel hybrid buses purchased by  King County Metro with the expectation of using up to 40% less fuel than existing buses have fallen far short of that. At times, the New Flyer buses, using the GM Allison hybrid system (earlier post), have gotten worse mileage than the buses they replaced.

On the positive side of the ledger, however, maintenance costs are down (although not nearly enough to offset the higher price of the hybrids) and the buses have exceed expectations for reducing emissions.

King County Metro was the first agency in the country to buy a 60-foot articulated diesel hybrid bus. It ordered 235 of them and now now has the largest fleet of hybrid buses in the world.

But the expected fuel efficiency has not been there. One apparent culprit is stricter federal emissions standards. Another could be that the hybrids are used on routes—suburban express routes with more highway mileage—where their advantages don't shine.

...mileage performance varies from bus to bus, from route to route, and season to season...

In January 2003, Todd Gibbs, manager of the hybrids project, said on a posting on Metro’s Web site that the hybrid bus was achieving 40 percent better fuel economy than the Breda, even though it was overloaded with the water barrels. “We expect the numbers to go even higher,” he said.

As the tests continued, Metro staff members called the results “impressive” and “remarkable.”

But in July 2003, almost at the end of its testing period for the hybrid buses, Metro suddenly announced that it needed to switch engines.

The federal government had imposed stricter exhaust emissions standards, and the Cummins engine was not federally certified. Metro sent the bus to the Winnipeg, Manitoba, manufacturer to have a certified Caterpillar engine installed in its place.

The fuel economy results were never the same after the switch to the Caterpillar engine. Boon said it wasn’t just a switch in the engine but also a switch in the emissions control system.

The entire article is worth reading—it highlights the variability of fuel economy based on different types of duty and driving cycles, the effect that emissions control has on fuel economy, and, above all, that there still is a great deal to learn about the real world application of these technologies.

The timing on this story could have been a bit better from GM’s point of view, as the day it broke was the day GM announced its partnership with DaimlerChrysler on a full hybrid system based on the concept and architecture used in these transit buses.