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NREL Publishes Final Version of Study Comparing Series-Hybrid and CNG Transit Buses

The fuel-economy of the hybrids surpassed that of the diesel and CNG buses. Click to enlarge.

The US Department of Energy’s National Renewable Energy Laboratory (NREL) has published the final version of a study comparing 10 new compressed natural gas (CNG) and next-generation diesel series-hybrid electric bus propulsion systems operated at New York City Transit (NYCT) against a baseline of conventional diesel buses.

The report is part of a series of evaluations from NREL that track and evaluate new propulsion systems in transit buses and trucks. DOE/NREL evaluated the original 10 prototype diesel-hybrid buses from Orion and BAE Systems (model Orion VI buses) operated at New York City Transit (NYCT) .

That evaluation was reported in July 2002 and provided results from the prototype buses from 1998 through 2001. This report focuses on 10 new compressed natural gas (CNG) buses and hybrid propulsion systems.

This report describes the evaluation results for new Orion VII low floor buses at NYCT with CNG propulsion (equipped with Detroit Diesel Corporation Series 50G CNG engine) and new hybrid propulsion (equipped with BAE Systems’ HybriDrive propulsion system). These final results represent a 12-month evaluation of these two groups of buses (October 2004 through September 2005).

The buses evaluated are the same age, the same bus platform, have been operated on similar duty-cycles, and experienced similar maintenance practices.

Among the study’s findings:

  • With a fuel economy of 1.70 miles per diesel-equivalent gallon, the CNG study group has a 25%-28% lower fuel economy than the diesel buses. The low average speed of the NYCT operation is the key to this significantly lower fuel economy—spark ignited (SI) natural gas engines typically have a lower thermal efficiency at low speed and load than compression ignition (CI) diesel engines. Consequently, lower natural gas fuel economy is expected in this type of operation.

  • The hybrid study fleet—with average fuel economy of 3.19 mpg) achieved 37% higher average fuel economy that the diesel buses, and exhibited 88% higher fuel economy than the CNG study fleet.

  • The hybrid bus average fuel economy had a much larger decrease/fluctuation in fuel economy in the summer months (June through September) than any of the other three study bus groups. According to BAE Systems, much of this decrease is due to an increase in energy consumption for air conditioning.




It sounds like you might want a series hybrid CNG diesel bus that loads the engine properly for maximum mileage and gets the economy of a hybrid. These are good studies that show the magnitudes of performance with various configurations.


Notice the effects of warm/hot weather on the Orion VI. The need for cooling takes a toll on fuel mileage as the diesel engine runs to keep the bus cool. Perhaps a electric compressor, running off of stored regen braking energy, could be used to run the AC during summertime. Larger, better battery packs would be needed during hot months.


With all the high grade waste heat, they could use abosorption cooling and use not extra power at all. It is a bus, with a lot more room than a truck or car for the cooling unit.


That may be feasible w/a modular pack that can be removed during cool/cold months. It might be a single unit, or 2 part, with one onboard and the other in on trailer.


wow, so the thing that i've been saying all along, this whole time, has finally been proven by months of testing – a CNG bus is simply not a "clean air" vehicle. i still cannot understand for the life of me why people want to insist on buying CNG buses instead of going for the diesel-hybrid route.


I see nothing in the article about clean air, just the mileage figures are shown. I would imagine CNG is much cleaner than diesels.
As far as absorption cooling from waste heat, a 10 ton cooling unit is smaller than the average refrigerator. I can think of several places under the floor of a bus for such a unit.


CNG is considered "clean air" because it releases far less PM and other local pollutants, relative to conventional diesel buses, even given its lower fuel economy. Cleaner diesel technology has narrowed the gap, but CNG still has the edge there.

Hybrid diesels that combine low emissions and high economy are nearly as clean as CNG in terms of local pollutants per mile driven, but cost a whole lot more to buy on the front end. They can make that up in fuel and repair cost savings (hybrid buses wear out their brake pads much more slowly than conventional buses), but only if they are substituted into appropriate routes -- stop and go city center routes, not high speed, hilly or more suburban ones.

Still, considering previous studies which examined the fuel economy performance of hybrid buses, this study is encouraging. Previous studies would often put hybrids into service on routes which did not take full advantage of their capabilities; this study seems to openly acknowledge the importance of route selection, and confirms the hybrid advantage on well chosen routes.


right, i didn't phrase that quite properly. NBK-Boston said my thoughts exactly, thanks man.

Rafael Seidl


an absorption chiller would be a great way to deliver airco in city buses. Since diesels are turbocharged, it could also be used to drive a chiller between the intercooler and the intake manifold to further cut the temperature of the fresh charge. With an air/water intercooler, the waste heat from the first stage can actually help drive the second. Charge chilling should boost rated power and/or reduce engine-out NOx levels.

The fly in the ointment is that traditional absorption chiller designs (other than small hotel/RV refrigerators) rely on the gravity-induced flow of a finely dispersed mist to achieve adequate re-absorption by the weak ammonia solution. Experiments on ships have shown that the chiller power of such designs is highly susceptible to vibration, making them unsuitable for mobile applications. The porous membrane absorber stacks under development in Germany are much more compact and impervious to vibrations.


Keep in mind, though, that absorption chillers are quite expensive. Once mechanically robust solutions are available, they should initially be considered for buses and refrigeration trucks with life expectancies of 1-1.5 million kilometers to permit amortization of the up-front premium.


Yazaki makes an absorption unit that works with lithium bromide and not ammonia. It runs on hot water temperatures down to 200F and if you have higher temperatures you get higher efficiencies. Yazaki makes components for the auto industry as well, so perhaps they could design a unit for busses.



Actually, numerous studies have shown that CNG is typically much worse in NMHC emissions, including the associated air toxics. Even when equipped with catalysts, CNG NMHC emissions are somewhat higher than clean diesel NMHC emissions (which are usually near or below detection limits) according to a study by CARB.

NMHC is the worse possible pollutant for urban air quality.


Here is a link to BAE Hybridrive


The Orion VII hybrid is a low floor bus. It may fit on the roof, but that could change the center of gravity. One may be able to fit a unit on the rear of the vehicle, or mounteed on a trailer fluid&electric w/couplings. The Orion Bus site has the Thermo King AC at 100,000+ BTU unit.
___The batteries could use updating. I went over the Resources link and the PDF states that they are lead acid units. Li-ion could be make large improvments to fuel mileage in stop and go driving. It would also:
a) drop the mass of electrical energy storage.
b) increase energy storage capacity.
c) lend to plug-in bus experimentation and implementation.
On the flipside, Li-ion units are expensive vs lead acid.
That is why they are found mostly on buildings. Many office towers/large buildings away from the waterfront in Manhattan use absorption chillers (+ heating during winter) powered via ~350 F steam from co-generation electric+steam plants.


Oh, I don't care where the heck they put it, just put it!


NMHC is the worst possible pollutant for urban air quality
What's NMHC?


The internet is the data keeper to most of your questions.


NMHC refers to "non-methane hydrocarbons," and Carl's phrasing should therefore read "NHMCs *are* the worst possible..."

But as to the substance of that statement, NMHCs are a range of compounds, and the statement that they are "the worst possible" form of local air pollution is a vague one at best. By what standard? Which NHMCs are particularly nasty, and under what circumstances? I am better acquainted with the dangers associated with PM and NOx emissions, so I would appreciate a link to the CARB study mentioned earlier in order to get more background information on these questions.


There are couple of defined emissions like HC (hydrocarbons), VOC (volatile organic carbons) and NMHC (non-methane hydro-carbons). Methane emissions, quite common to NG engines and somehow to propane engines are not really troublesome, because methane is not smog-forming emission, and naturally occurring methane emissions are way higher then produced from IC engines.

That’s why emission legislation usually targets VOC or NMHC. In case of gasoline or diesel engines it practically means all HC emissions.


Methane is a GHG. If we have fuel cells with reformers to turn SNG into H2, we would have no tailpipe emissions other than CO2 (neutral) and H2O.


Interesting comments, as usual. A few additional points:

BAE has chosen to stick with lead acids based upon a cost model factoring in replacement frequency and cost. They model the costs to the customer as less, with the current battery technology landscape, than if NiMH were used. That said, they are testing NiMH, and will make the transition eventually.

The decision to go CNG or diesel hybrid is really specific to each fleet. There are infrastructure and facility conversion costs associated with CNG, as well as price volatility of fuel. Hybrids of course cost a bit more per unit right now. It is expensive to install fueling and convert old buildings in places like NYC, which is why they are moving more in the direction of hybrids. Other fleets may have a different situation, and CNG can make more sense.

It is important to note that the comparison of CNG to diesel hybrid to diesel is not a perfect one. NYCT has not purchased diesel buses in some time, so diesel baseline is comprised of older buses, with consequently higher maintenance costs. Diesels are not factored into the maintenance cost comparison. The CNG and diesel hybrids are of similar vintage, thus the maintenance cost comparison is made in addition to fuel economy. As noted above, the NYC duty cycle is severe, and not the best showcase for SI engine technology. Also, the CNG buses are powered by the DDC S50G, which is no longer produced, and was generally considered an inferior powerplant.

We take pains to match duty cycles when making comparisons. The question of series versus parallel, and what control strategy is employed, are very specific to to duty cycle and application. One size does not fit all.

BAE/Orion have a refined version of the hybrids we tested, which we are in the process of evaluating without a baseline. We also hope to emissions test one on our chassis dyno. Stay tuned.

Harvey D.

It seems to be an ideal vehicle for 140+ KWh Altairnano new quick charge-discharge batteries.

In city traffic situation, these batteries would allow much more breaking energy recuperation to operate the cabin heating/cooling system.

Some 40+ square meters of high efficiency (25% to 30% = about 1.1 x .25 x 40 x 6 = 66 KWh/day) solar panels mounted on the large bus roof would also help to keep the batteries charged when most needed = during the 6 to 8 hrs/day of bright sunshine.

An up-to-date very light weight, compact, very high efficiency 23-SEER, 8 to 10 Ton heat pump, (preferably 2 units x 4 to 5 Ton) equipped with variable ECM motors would to the HVAC job year round in most cities. Why re-event the wheel when it is there already.

This type of city bus could be built now and would need a much smaller diesel genset.
Dozens of USA/Canadian firms could do it with adequate R & D support.



To his credit, Andrey noted that methane is "not smog-forming," and therefore not a problem in the air quality arena. Its status as a greenhouse gas is well known.


I was not responding to anyone, just pointing out that methane is a GHG. Keep methane out of the air, simple as that.


NBK-Boston - you're correct, NMHCs are a class of compounds (hydrocarbons) so it should have been plural. Should QC my posts more closely.

Here's the link to the CARB study to which I referred:


See also:


My response was more to convey that CNG engines are not necessarily uniformly "cleaner" than low-emitting diesel engines based on most studies. My comment about NMHCs being the most problematic of the regulated emissions was in regard to urban ozone (smog) production.



The two presentations you posted are certainly eye-opening when it comes to comparing CNG to clean-diesel in the 2002 time frame. I wonder if newer CNG buses have oxydation catalysts (these presentations seem to imply that the older ones did not), and if so, whether that puts them decisively over 2002-era clean diesels as far as local air quality issues go. All the same, the newest diesel technologies can make the real target a moving one.

The cost issues identified in the second presentation in particular confirmed what I had long suspected, which is that a conversion to a CNG fleet is quite expensive. Taking into account infrastructure costs, my gut always leaned towards the diesel-hybrid approach, as against a conventional CNG drivetrain. Though a good series-hybrid layout might be able to take maximum advantage of CNG, and be worth the cost.

Thanks for posting a response.

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