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UConn Study Finds No PM Emissions Reduction from GM Parallel Hybrid Buses

PM filter mass results from study. The mass from hybrids was greater than or equal to that from conventional diesel. Click to enlarge.

Newly completed research conducted by the University of Connecticut finds that particle emissions from two GM parallel hybrid diesel-electric buses purchased by CTTransit and emissions from two comparable CTTransit diesel buses were statistically identical.

The research also found that fuel economy for the hybrids was only some 10%–15% better, compared to the claims of up to 50% improvement from the manufacturer.

Other cities—notably Seattle—have found similar, or even worse, performance on fuel consumption with the GM-Allison hybrids, but the findings on PM emissions is a surprise.

The ultra fine particle data were surprising because we expected significant reductions for the hybrid technology. That proved not to be the case.

—Britt Holmén, UConn professor of civil and environmental engineering and lead researcher

In June 2003, CTTransit, which oversees bus service in Connecticut and is operated by the state Department of Transportation (DOT), purchased two of the GM-Allison parallel hybrids. The Connecticut Department of Transportation and the Joint Highway Research Advisory Council retained UConn’s School of Engineering to test the particle emissions of the hybrids as well as two conventional diesel buses manufactured about the same time that have the same emissions standards and, to compare the results.

The conventional diesel buses, manufactured in 2002, used a standard 280-hp Detroit Diesel Series 40 engine. The GM hybrids, manufactured in 2003, used the 280-hp Cummins ISL engine combined with the Allison Ep 40 electric drive, NIMH battery pack and dual electric traction motors.

The researchers used three fuel and aftertreatment configurations: No. 1 Diesel; ULSD; and ULSD plus a DPF (diesel particulate filter). UConn researchers tested all four buses between January and November 2004 on actual roadways, as opposed to using a dynamometer.

To my knowledge, this is the first study to evaluate hybrid bus particulate emissions while the buses were operating on actual bus routes. We found that the only testing variable that reduced particulate emissions was use of a diesel particulate filter, and these reductions of more than 90 percent were similar for both the hybrid and conventional diesel bus types. Without the particulate filter, the particulate emissions from the hybrid buses were not statistically different from the particulate emissions of the diesel buses, regardless of fuel sulfur content.

The good news is that our measurements show that diesel particulate filters can remove even toxic nanoparticles—those with a diameter of less than 50 nanometers—from engine exhaust on both the diesels and the hybrids. My hope is that the hybrid parallel-design technology can be improved to give better fuel economy while maintaining the low-noise and smooth ride performance of these buses.

I suspect the hybrid manufacturer had these parallel-drive hybrid buses programmed for optimal performance rather than optimal emissions benefits because they were capable of freeway commuter-express route speeds of 65 mph and the hybrid bus speeds were not affected by travel over Avon Mountain. These parallel design hybrids are unlike the series-drive hybrids that have been extensively studied previously and are in-service in cities like New York. We hope to test a series-drive hybrid bus in the future using the same experimental methods to test this hypothesis.

—Prof. Holmén

The parallel hybrid drive in the transit bus is the basis for development of the two-mode hybrid drive being developed by GM, DaimlerChrysler and now BMW for applications in light duty vehicles.




Why would someone think this GM hybrid design was going to be that good for MPG? The engines on Both the standard bus and the hybrid had the same HP rating. I thought the theory, like the other newer, non GM design uses a smaller engine but runs it at a more effienct/pollution free speed to charge batteries?


can you say ENOVA!!!!!!!!

Harvey D

Mild parallel hybrid with the same or similar high power ICE may not be the best technology for fuel economy and pollution reduction but is may be much cheaper to implement......
Has anybody heard of adding direct hydrogen injection (from an on-board hydrogen generator) to increase efficiency and reduce pollution of diesel/gasoline engines?

Ron Fischer

This does seem to call into question GM's so-called 'dual mode' hybrid design. Wasn't this debuted on a light truck concept, calling it the ParadiGM system? It was slated for Saturn Vue, but that was nixed when Bush Jr. started first term and California ZEV mandate was squashed. ...which makes me wonder where the problem lies: is the basic concept flawed? Possibly mild-parallel hybrids just don't really work for buses? Or is the engineering of this iteration half-baked? GM may have underfunded development because they never believed in hybrids and are now trying to make up for that through the multi-corporate 'dual mode' alliance.


GM??? Oh! You mean the junk bond company that will soon go bankrupt and leave Toyoto the world's largest auto company.

Wonder what they did different and if it's too late to do something realistic now?

Mikhail Capone

If we learn from what Honda has been doing with their mild hybrids, it seems that a big part of the gains come from things that don't have to do with the extra-propulsion; idle-stop, downsizing the engine, cylinder-deactivation, reduced vehicle weight, low resistance tires, CVT transmission, etc.

If GM hasn't tried to gain efficiency in these areas, it's no surprise that their mild hybrid shows its limitations.


GM for decades has relied on their marketing muscle/dominance to makeup for their lack of
true desire to risk profits for design leadership.
Everytime GM had to play "catchup" it the marketplace,
they had to rush out a product to save marketshare, and to GENERATE PRESS RELEASES to keep the stock price pumped up.
In rushing out the product, it had not been the best.
Rushed out the covair in early '60s to beat VW bug.
Rushed out converted gas block engines to make diesel engines after the first gas crisis in the 70's.
Plainly, GM does not use their R&D research to drive actual production innovation. Honda and Toyota have with their hybrids.
Lucas, may be correct. GM could be headed for bankrupcy.
Patent and Produce, or Perish!


I'm very interested to see how the series hybrid model compares with the GM hybrid, so far in terms of fuel efficiency they look good.


I have always doubted the claims of the GM Allison hybrid buses. Everything is the same with a regular disiel engine except for the extra electric motor. Like mikhail said, Honda's IMA gained from many other sources which GM did not consider. Its just a pity to the companies that fell for GM's marketing to purchase the expensive buses. The much cheaper Orion Series Hybrid buses used in New York and Vancouver are much better. they've shown a steady 28% improvement in economy and are even quieter and more reliable since the gearbox is taken out of the equation.

GM and its partner's "Two Mode" alliance would also suffer a similar faith. They all seem to be pusuing performance and not economy. The recent hybrids from Frankfurt have engine improvements like Direct injection for petrol engines to boost the economy.

I guess GM would never learn.


In the past GM treated its engineering innovations as a hobby. Consider:
They sold the first air suspensions - which leaked.
They made the first engine from a high-carbon Reynolds aluminum alloy needing no steel liners. Too bad it was the Vega, and faulty. Later this technology turned up in the Porsche 928.
They introduced a clever front engine rear drive transaxle in the Pontiac Tempest. Whoops, that ended up in the 928 too.
They invented the quiet belt-driven overhead camshaft in the '60s. Yankees preferred big pushrod V-8s, and the Japanese took over the technology big-time.
And the first turbocharger in a car - the Corvair.
I guess engineering and marketing were not talking to each other, and marketing was definitely in (mind) control. Does GM still have enough engineers to invent stuff this advanced? Does it matter when GM's middle-class constituency thinks that the solution to every problem is a 5000 pound tank?


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