Houston turning to diesel-electric hybrid buses after costly CNG/LNG fiasco

Diesel Fuel News, June 10, 2002 by Jack Peckham

Houston's Metropolitan Transit Authority just voted to convert four liquefied natural gas (LNG) buses to ultra-clean diesel-electric hybrid service, tapping $1.2 million in federal Congestion Mitigation Air Quality (CMAQ) funds as well as $600,000 of its own funds.

The move comes on the heels of MTA's decade of disappointing, costly experience with natural gas technology, both CNG and LNG. The latest diesel-electric hybrids, equipped with diesel particulate filters (DPFs) and ultra-low sulfur diesel (ULSD), are just as good or better than CNG/LNG not only on PM emissions, but also on nitrogen oxides (NOx).

Extra weight of CNG also cuts bus standing-load capacity, hence a clean-diesel bus can carry more passengers without exceeding weight limits, Houston MTA found. That means fewer emissions per passenger carried.

In the mid-1990s, the Texas legislature had pushed transits to adopt CNG or LNG technology due to a perceived excess gas capacity from shut-in state wells.

"We thought it was going to be cheap," MTA maintenance director Jim Patrick said, "but it turned out not to be so."

MTA also had hoped that gas engine makers would make big cost and quality breakthroughs on LNG technology, but "it didn't happen." As a result, MTA eventually scrapped and sold its high-cost, problematic LNG refueling infrastructure.

On the other hand, the alternative CNG technology requires "an ungodly amount of cost for [electric] compression" for the required fast-fill for transit bus service, Patrick said. The electric power emissions to compress CNG usually are ignored, too.

Ironically, politicians elsewhere -- like Washington (DC) Metro Area Transit Authority (WMATA) chairman Chris Zimmerman -- claim that CNG buses are lower in PM/NOx emissions than clean-diesel technology (see Diesel Fuel News 4/29/02, p7).

Zimmerman, neither an engineer nor an emissions technology expert, leads a push to force Metro to spend vast sums on CNG refueling garages (close to $50 million) and pay much higher costs for CNG buses compared to DPF-equipped clean diesels. This at a further cost to delayed maintenance on metro's parking garages serving passengers for its zero-emissions electric trains. What's more, Metro is facing huge costs for fixing leaking subway tunnels, with no apparent funds in sight.

Hundreds of new clean-diesel buses could be put on the streets with the taxpayer funds diverted to CNG infrastructure, while improving public transit service and slashing total emissions dramatically. But once certain "green" groups and elected politicians (like Zimmerman) latch-on to the CNG monopoly "gospel," facts don't get in the way of decisions, as New York City Transit expert Dana Lowell explains.

Besides converting its entire diesel fleet to ULSD and DPFs, NYCT also plans to buy 325 diesel-electric hybrids over the next few years, following pioneering real-world experience with a fleet of 10 diesel-hybrids in operation over the past three years.

More good news: New diesel buses with exhaust gas recirculation (EGR) could be combined with diesel-electric hybrids to slash NOx emissions even further, Lowell explains.

"The problem [with CNG] is the political equation wagging" some decision-makers, he said. "A lot of people fervently believe natural gas is the only way to go and they're not listening to anything else."

Aside from the CNG emissions "gospel," there's also the "energy security" gospel claiming gas is some sort of hedge against oil imports. This despite abundant press reports of new LNG export projects gearing-up for the U.S. market, from some of the same OPEC oil export countries.

Meantime, tests at New York City Transit show that diesel-electric hybrids are not only excellent at reducing fuel consumption (hence lowering possible OPEC exposure), but actually matching or beating CNG not only on PM but also NOx (see Diesel Fuel News 10/29/01, p9). Similar ultra-low emissions are expected in the new Houston hybrid tests, as MTA's Patrick explains.

GM-Allison is providing a parallel hybrid technology on the model year 1998, 40-feet New Flyer buses, with the installation being performed by Denver-based Stewart & Stevenson, a DDC engine/rehabilitation shop, Patrick said.

Nothing on the hybrid is really "space-age" technology, so Houston MTA figures that hybrids eventually will prove themselves on serviceability, maintenance, and efficiency, the latter in part due to regenerative braking. "We're interested to see how they'll perform in this heat and humidity" during the torrid, lengthy Houston summers, he said.

"Ours will probably be the first [hybrid] on a bus rehabilitation, and the first parallel [both engine and batteries provide power to wheels] hybrid bus," he said.

This scheme will employ torque-blending between the mechanical power of the engine and the rechargeable electric power from a nickel-hydride battery. It allows both engine and battery to operate in their most efficient power ranges, while slashing emissions compared to conventional straight-diesel, especially at start-off from idle.