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NYC Hybrid Buses Improve Fuel Economy 45% Over Diesel, 100% over CNG

27 February 2006

The series-hybrid buses offer up to 45% better fuel economy than diesel, and 100% better than CNG.

Orion VII series-hybrid buses operated by New York City Transit (NYCT) on the city’s most severe duty cycles achieved up to 45% better fuel economy than diesel buses and 100% improvement compared to comparable natural gas buses on an energy-equivalent basis, according to the results of a study released by the National Renewable Energy Laboratory (NREL).

The evaluation is part of a series of evaluations of new propulsion systems in transit technologies performed by the lab. from the U.S. Department of Energy (DOE). NREL recently concluded an evaluation of the GM-Allison parallel hybrid buses in use in Seattle. (Earlier post.)

The Orion VII series-hybrid buses with the BAE HybriDrive combine a 5.9-liter, 260 hp (194 kW) Cummins ULSD (Ultra Low-Sulfur Diesel) engine with a 120 kW traction generator. The electric traction motor delivers 250 hp (186 kW) and 2,700 lb-ft (3,657 Nm) of low-end torque.

The hybrid fleet proved the most reliable in the study, with 7,000 miles between road calls, compared to 5,000 miles for natural gas and 4,000 miles for diesel. The hybrid propulsion system also performed better than the other propulsion systems, with 10,000 miles between calls, compared to 8,000 miles for CNG and 5,000 miles for diesel.

The evaluation compared Orion VII low floor buses at NYCT with CNG propulsion (Detroit Diesel Corporation Series 50G CNG) and hybrid propulsion (BAE Systems HybriDrive propulsion system) against conventional diesel buses.

The CNG buses’ average fuel economy was 25% lower than the diesel baseline buses—a typical difference in fuel economy for low-average-speed operation for the spark-ignited natural gas engines.

The hybrid buses’ average fuel economy was 45% higher than the diesel baseline buses (ranging from 32% to 52% better than the diesel baseline during the evaluation period). The diesel baseline buses for the hybrid bus evaluation have diesel engines without exhaust gas circulation (EGR). The addition of EGR for emissions control would tend to lower the diesel baseline fuel economy.

The reported results represent eight out of a planned 12-month evaluation of these two groups of buses. An additional evaluation of NYCT’s order of 200 Orion and BAE Systems hybrid buses will be reported separately.

The eight-month evaluation period does not include summer months, which could have reduced the hybrid bus fuel economy advantage from air conditioning loading and the ability to collect regenerative braking energy into the batteries. The summer-month fuel economy information will be provided in the final results report on this evaluation. The hybrid buses had an average fuel economy 100% higher than the CNG buses.

In October 2005, New York City transport services ordered 500 more Orion VII series-hybrid-electric buses from DaimlerChrysler Commercial Buses North America. New York City Transit ordered 216 units, and Metropolitan Transportation Authority (MTA Bus) 284. (Earlier post.)

The exploration of alternative fuel technologies for urban transit has been driven, up to now, by imperatives for emissions reductions.

EPA Emissions Requirements for Transit Buses
Model YearsCO
1990 15.5 1.3 6.0 0.60
1992–1992 15.5 1.3 5.0 0.25
1993 15.5 1.3 5.0 0.10
1994–1995 15.5 1.3 6.0 0.07
1996–1997 15.5 1.3 6.0 0.05
1998–2003 15.5 1.3 4.0 0.05
2004–2006 15.5 2.4 combined or 2.5 with a limit of 0.5 for NMHC 0.05
2007–2010 15.5 0.14 (NMHC) 0.2 0.01

Diesel hybrid bus propulsion systems offer improved fuel economy during a time of fuel economy penalties for emissions control.

An issue that requires resolution, however, is the EPA’s current lack of recognition of the emissions reduction from a hybrid bus. Under current regulations, the emissions profile of the bus—or other heavy-duty vehicle—is determined by evaluating the diesel as a stand-alone engine. In other words, from an EPA point of view, the emissions profile of a hybrid bus is the same as the emissions profile of a non-hybrid bus using the same engine.

The California Air Resources Board (CARB) has recognized the emissions savings offered by hybrids, and as granted hybrid bus propulsion systems a 25% blanket reduction in emissions that can be used in the state implementation plan for emissions reductions. Currently, EPA does not recognize this benefit.


February 27, 2006 in Diesel, Fleets, Hybrids | Permalink | Comments (16) | TrackBack (1)


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This is pretty significant. The new hybrid propulsion proves more reliable than either CPG (25% better) or diesel (100% better)! How much more reliable will it be 10 - 15 years from now?

What are the names of companies regardong hybrid vehicles I can invest in? Please specify.



IIRC, this system is fully series and should be quieter and less jerky than the mechanical buses.

engineer, you switchted your numbers. it's better by 100% than CNG, 45% than disel.
this only further supports my belief that CNG is not all the great hype it's made out to be. let's get huge numbers of diesel hybrid buses out there and see some real changes.

If I'm interpreting the milage chart correctly then the hybrid is using over 22 gal/100 miles less than the old diesels. A Prius only saves 2 gal/100 miles compared to my Kia. Considering that transit buses put on 3 times the mile/yr that the average car this can be a real financial benefit for the taxpayers.

"engineer, you switchted your numbers. it's better by 100% than CNG, 45% than disel (sic)."
No Robert, I did not. I was talking about reliability, not fuel economy.

"engineer, you switchted your numbers. it's better by 100% than CNG, 45% than disel (sic)."
No Robert, I did not. I was talking about reliability, not fuel economy.

First learn how to read the comments, Robert didn't say that lensovet did. Second I don't believe he meant it to be insulting just a friendly correction. I'm curious where you got your reliability info I really wanna read that I've wondered about how reliable hybrid electric systems would be.

OK, I got the author of the comments wrong. Apologies to Robert.

As for the reliability stats, it is right there in front of you: "The hybrid propulsion system also performed better than the other propulsion systems, with 10,000 miles between calls, compared to 8,000 miles for CNG and 5,000 miles for diesel."

Improvement of hybrid over CNG = (10,000 - 8,000)/8,000 = 25%
Improvement of hybrid over diesel = (10,000 - 5,000)/5,000 = 100%

Anything else you'd like to know?

engineer please chill, my bad.
my point about hybrids kicking ass still stands however.

Looking at these numbers one must conclude: A series hybrid CNG is the best of all!? But since the study did not include this type of vehicle(does it exist?), one is left with the wish that it did.

Yes, I was surprised to see how well hybrids did, and that for a relatively new technology. I think plug-in hybrids offer you the best of two worlds: use cheap electricity when convenient, or use gasoline/diesel when you need to go long distance. I suspect hybrids are going to be around for a while.

I think recent swings in natural gas prices shows the limitations of CNG. A liquid fuel has many advantages over a gaseous one.

They seem to ignore the fact that hybrid buses cost $500,000. Articulated hybrid buses are running $750,000. So we increase the fuel economy 45% and triple the cost. These buses do not pan out economically yet. And the Detroit Diesel is an old CNG engine I believe, Detroit Diesel does not even make CNG engines any longer. I believe its on only John Deere and Cummins Westport.

Let's take a closer look at the operating economics of a hybrid bus. The numbers out there are a little spotty, but here is the best data I could find.

1. Capital Cost: A conventional diesel bus costs around $300,000, while a hybrid is $500,000. That's a premium of $200,000 (or 40%), not 3x. See References #1 and #2.

2. Fuel Costs: Transit buses typically put on 30,000 miles per year. See Reference #3. At the fuel economy figures noted above, a conventional diesel bus would burn 14,285 gallons per year, while a hybrid would burn 8571 gallons per year, a savings of 5715 gallons per year. At current prices ($2.60 per gallon for diesel, guesstimated by checking "gas buddy" websites for a half-dozen major cities), that's a savings of $14,858 per year. Over a useful life of 15 years (see Reference #4), that is $222,873 in fuel savings.

3. Maintenance Costs: Overall per-mile maintenance costs for conventional diesel buses have been estimated at $0.24 per mile, while for hybrids it is pegged at $0.16 per mile. This is due to reduced wear and tear on the brakes, reduced lubricating oil consumption, reduced road-cals, and the like. See Reference #5, pg. 38. At 30,000 miles per year, that is a cost of $7200 for the conventional bus and $4800 for the hybrid, a savings of $2400. Over 15 years that equals $36,000.

4. The total savings of operating a hybrid bus for 15 years in place of a conventional bus is about $260,000. The total increased capital outlay is $200,000. However, you must account for the fact that in a hybrid the increased outlay is up front, while the savings comes slowly over the life of the bus. If you ammortize the increased capital outlay at 5% interest, you pay a total of around $280,000 in principle plus interest by the end of the 15 year lifespan.

5. On these numbers, hybrid buses nearly break even. For the reduction in air pollution they promise, for the increased rider comfort they provide (see Reference #5), and for the prospect of lowering costs as production techniques improve, I conlcude that it would be wise to encourage transit operators to order as many hybrids as possible, starting now, when making plans to renew their fleets. The numbers make sense.


NBK is right especially about point #5. This is how we can really save money over the long haul. Cut down on pollution and reduce long-term, the incidents of many diseases. Especially, asthma which in many inner cities is the no 1 reason for emergency room visits amongst children.

One thing transit operators must remember when purchasing and assigning these hybrid-buses to the fleet, is that the huge gains in efficiency happen on a local bus route, where the bus stops every few blocks. The longer distance express buses should be the last to be converted or replaced.

Also, does anyone know whether or not a hybrid engine can be retrofitted into a car or bus already on the road?

our company is specializing in manufacturing composite CNG cylinders in china ,welcome to contact us!
we are permitted to produce seamless steel cylinders, wrapped cylinders and cylinders for NGV. At present the company mainly produces steel lined carbon fiber hoop-wrapped composite cylinders for NGV .

Great info

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