PowerSource

Coincidentally, no sooner had I learned about the popularity of hybrid technology in hybrid buses than I read in the newspaper that a local municipality was unveiling its brand new hybrid bus at a ribbon-cutting yesterday. So I ran over and took some pictures of the bus and its guts. I assumed that since Maxwell Technologies had only recently introduced its new HTM 390 ultracap aimed at the industrial/transportation market that this hybrid would be of the old-style lead acid battery type, but no, the hybrid electro-mechanical subsystem contractor, ISE, is in tight with Maxwell and used ultracaps on this bus. Look at the picture – the large wart on the bus’s roof is full of ultracaps – about 280 soda can-sized ones. I hopped in for the inaugural ride which wasn’t eerily quiet only because the air conditioning is so noisy. It has a fantastically smooth acceleration – you know how diesel buses shudder when they accelerate? This one was smooth as silk.

Two surprising points about hybrids and buses: The first is that better fuel efficiency is just a small part of the reason the government is subsidizing the move to hybrid buses. Average mileage for a diesel bus is about 3-5 mpg. This bus, which uses a Ford Triton V10 gas engine, gets about 5mpg. Nice, but not earth-shaking. It’s in the emissions category that hybrids blow conventional technology away: A gas hybrid eliminates the diesel particulates that conventional city buses produce while providing better mileage and hauling the same payload. Similarly, gas hybrids produce only about a fifth of the NOx emissions of a modern standard diesel.

Second point: City buses have accelerate/decelerate profiles that vary based on the terrain of their city, so it’s almost impossible to come up with a one-size-fits-all design for a city bus. For example, Grover City has a fairly flat terrain, with no major highways that require rapid acceleration up to a high speed. A bank of ultracaps that can provide 25 seconds of bus acceleration before switching over to the Triton suits this terrain just fine. Oakland, on the other hand, has some very hilly terrain, and 25 seconds of acceleration isn’t enough. In fact, the whole gas engine-ultracap mix isn’t a good mix; Oakland’s hybrids will be fuel cell-based. And just to make things more complicated, Palm Springs has a wind generator-based hydrogen fuel producing facility which makes hydrogen-fueled engines or even hydrogen fuel cells practical, so the city of Palm Springs has hydrogen-fueled buses.

The design requirement that the bus platform be so very flexible as to its power source led ISE to use a series rather than a parallel hybrid design. A parallel hybrid – like the familiar Toyota Prius -- has the engine/power plant as part of the drive train. In a series hybrid, the engine/power plant powers a generator which then powers the motors that in turn actually drive the wheels. Since the power plant isn’t part of the drive chain, it’s relatively easy for ISE engineers to swap out a gas engine for a hydrogen-fueled engine, or a hydrogen fuel cell, say.

Many thanks to ISE’s service manager, Harry Meyer, who gave me a great tour of a very cool technology.