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Final NREL Report on GM Hybrid Buses in Seattle

The study found the hybrids delivered 27% better fuel economy than the conventional diesels. Click to enlarge.

The National Renewable Laboratory has recently published its final report on the GM-Allison hybrid electric buses in service at King County Metro (KCM) Transit in King County, WA. The report includes 12 months of performance data—April 2005 through March 2006—on ten 60-ft New Flyer buses using the two-mode hybrid powertrain. (Earlier post.)

The study, which focuses on a comparison of diesel buses operating from KCM’s Ryerson Base and hybrid buses operating from Atlantic Base because of their similar duty cycles, found that the hybrid buses from Atlantic Base had a 27% higher fuel economy on average compared with that of diesel buses at Ryerson Base.

Over the 12-month period, the diesel buses also carried a 4.5% higher maintenance cost than the hybrids: $0.46 per mile for the diesels, compared to $0.44 per mile for the hybrids.

Combining fuel costs with maintenance costs, an evaluation of total cost per mile gave the hybrids a distinct operational economic advantage: $1.06 per mile compared to the diesel $1.25 per mile.

Results of laboratory emissions testing using four different test cycles. Click to enlarge.

The NREL team performed emission testing on one hybrid bus (of the 235) and one conventional bus (of the 30) during May-June 2005 using four driving cycles: Central Business District (CBD), Orange County (OCTA), Manhattan, and a custom test cycle developed from in-use data on the King County Metro (KCM) fleet’s operation.

To evaluate the effects of additional engine and vehicle loading due to grade, select cycles were repeated with and without this added load.

Detailed results of this testing—which show a marked benefits to the hybrids except in one test combination (PM, KCM cycle)—are published in a separate report, King County Metro Transit: Allison Hybrid Electric Transit Bus Laboratory Testing.




Very good results. I would like to see a total accounting, it appears that GM has done more to reduce fuel consumption overall with the Hybrid bus than Toyota has done with the Prius, but Toyota gets the PR win as the public perception is just the opposite.

Fewer busses, running all the time, VS more cars but much less use.
Also, not sure what the majority of Prius owners replaced to purchase a Prius so fuel savings would be difficult to estimate.


Yup. This is very cool. Hybrid powertrains in larger vehicles like commuter buses, school buses, UPS and FedEx trucks, cargo hauling semis, etc, etc will have a MUCH bigger impact on saving fuel consumption (and in the end, cost to the companies using them) than consumer autos like Ford Escapes and Toyota Camrys.

This sort of thing on a large scale will result in a very big net decrease in the US's dependence on foreign oil. Of course it will take a long time for this to be large scale, as so many buses and shipping trucks are not going to be replaced until their lifecycles have come to an end. Eventually, shipping and trucking companies will be crazy to NOT by a hybrid system.

And hey, lower shipping costs means less inflation on consumer goods. Last I checked, that's a pretty damn good thing.


2 years ago in Seattle the results were just the opposite.


Gary, if memory serves, and as the article states if you read way down near the bottom, Seattle tried to use hybrid busses to do long haul routes to the Airport and out in the suburbs. Long haul express routes for a bus with regenerative braking... Oddly enough, the hybrids weren't at their best. Way at the bottom of the article the author admits that the new routes with stop and go instead of long haul might just be a better use of the hybrids. And he also admitted that the hybrids are cheaper to maintain, even if they are used for inappropriate routes. Hybrids aren't perfect for every application, but for the ones they fit, they fit very well indeed.


"...an evaluation of total cost per mile gave the hybrids a distinct operational economic advantage: $1.06 per mile compared to the diesel $1.25 per mile."

While this is pretty impressive, I think one big expense probably not captured in such a 12 month study is depreciation of the batteries in a hybrid. Not sure how long the batteries are supposed to last (6 years? 8 years?), but this is one major cost that the diesels won't ever have.


Of interest from the study -- "The new hybrid articulated buses cost $645,000 each. The diesel articulated buses, which were essentially identical to the hybrid buses without the electric propulsion system, cost $445,000 each."

Also mention of a bus life span of 12 years. (Pg. 7)

"KC Metro wanted the hybrid buses to be able to operate in the [1.3 mile long] tunnel only on electric power, without any engine operation, to reduce tailpipe and noise emissions in the tunnel. The Allison engineers determined that the hybrid buses could operate in the tunnel on battery power only, but this would severely reduce battery life..."


3500 miles per month x 12 months x 12 years = 504,000 miles.
$200,000 extra cost / 504,000 miles = 39.7 cents per mile + ? for round of battery replacement.


DOE did a study on Prius NiMH battery life and quit after 160K mi when the batteries showed little-to-no loss in capacity. Deep cycling the current batteries would definitely be a bad thing. The prius battery stands up so well because it is babied. With future chemistries this will likely change. Battery tech is definitely a moving target... as is the price of diesel.

Harvey D.

What would happen to the total economic picture if avoided GHG (at about $85/tonne) + other air pollution (PM etc) were factored in?

One could always add associated health cost and other secondary pollution side effects.

The 40 cents/mile differential may be fully nullified when all factors are duly considered.

Bud Johns

Dave, it as apparent that you don't own a Prius. People that claim to get only 40 or so mpg in the car must be terrible drivers. I get a realistic 56 mpg around town where I live, and 51 on the highway is a given in normal conditions.


Frankly, I would think that would have been a good idea to continue collecting side-by-side data over the full 12 year lifespan of the buses. Conventional buses require midlife engine rebuilds, which cost a good deal. What do hybrid buses requre at midlife? Perhaps new batteries -- not sure. Engine rebuild? Maybe not, as the hybrid system might allow the diesel to operate more smoothly, extending its life. I will assume at this point that consumables such as oil and brake pads have been counted into the 5% operational savings. What about reliability? Are hybrids more reliable than conventional buses now? When both types are older? How much might that be worth? What about the value of the bus used (after 12 years) or as scrap? Are hybrids worth more or less because of their batteries?

The point is, we can speculate and project about the total life cycle costs of all these elements, but continued tracking of the Seattle buses would provide a much better idea of the reality behind these issues.


Bud, glad you able to achieve that mpg.
As you are probably aware there is great real world data on what other folks are getting with the Prius at http://greenhybrid.com/compare/mileage/toyota-priushsd.html
The mpg folks get looks close to being a normal distribution centered at 48 mpg. Some in the low 30’s some higher than 60. I would guess that this data is biased upward by the fact that the sample is of folks interested in achieving the best mpg they can and posting it. Taking the time to collect and post the data shows more concern about mpg than a random sample of the population…


Here is another NREL Transit bus evaluation report, for New York City, Hybrid vs. CNG --


Of note -- "Engine -- NYCT plans to replace the smaller than standard transit bus diesel engine used in the hybrid buses at approx. year 6 out of the 12 year life. A standard diesel or CNG bus would typically have one engine overhaul during the 12 year life without any planned engine replacements." (Pages x, xi)

"Traction Batteries -- The traction batteries for the NYCT hybrid buses are sealed gel lead-acid batteries... 46 battery modules total. These batteries (based on recommendations from the manufacturer), are expected to last at least 3 years, and then they will need to be replaced (or three sets of traction batteries during the 12 year life of the bus)." (Page xi)

"Brakes -- During this evaluation, the hybrid buses had brake repair costs that were 79% lower than for the CNG buses (CNG $0.18 per mile vs Hybrid $0.04 per mile)." (Page xi)



The New York figures are useful, but cannot be directly compared to the Seattle figures. The New York hybrids use a different drivetrain than the Seattle buses (BAE Hybridrive in the first, GM/Allison in the second). Also, the New York hybrids are being compared against CNG conventional buses, while the Seattle hybrids were compared against diesel conventionals. Finally, I think the New York test program used a different mix of route conditions than Seattle did.

Each of these differences cuts in different directions. For instance, the GM/Allison drivetrain, used in Seattle, has NiMH batteries, which may cost somewhat more to install but may last longer. The BAE hybrid uses a form of lead battery which has a shorter useful life but might be cheap to replace. Also, the diesel engines in each might be different, leading to different overhaul schedules, but I haven't had the chance to compare those closely. Notice finally that there seem to be much higher savings on the brakes in New York than in Seattle. I don't know why the numbers work that way. I'll stop discussing the possible impacts of each additional difference, and just leave it at the idea that you have to dig beneath the surface to draw the right lessons from the data.

This is exactly why I would have liked to see real-world full life data from each hybrid test program.


Always fan to see how brain-dead idiots discuss the economy merits of new emerging technology.


I'm with you on that one , its not about economy , its about wether or not we will be able to breathe in 20 years time . Sure the Prius is the first step , but its not the holy grail, 37% tank to wheel ratio, I am afraid I am not impressed!


I was surprised to learn that these buses get only 2.5 MPG. That would mean that a car getting 25 MPG and only one occupant is just as efficient as a bus carrying 10 people! A solo occupant hybrid at 50 MPG would be just as efficient as a fully loaded bus with 20 people!
(of course this is only looking at the fuel and not the emissions)



Remember, those fuel economy figures are for articulated buses which are used on high ridership routes and can easily fit 58 people in the seats (transit.metrokc.gov). I have ridden many of the Seattle transit routes around downtown Seattle and into the University district and at times the buses have so many people you have to stand (I'd estimate around 75 people on board). The times I have ridden the articulated buses (as early as 6:45am up to 7:00pm) I have never seen fewer than atleast 15 people on board and typically see most of the seats occupied.

tom deplume

While 12 years is the 'official' lifetime of a new bus I drove buses over 30 years old on a regular basis for over 5 years. Transit buses are used for over 25 years by being transfered to smaller transit systems. I'm certain that penny pinching politicians will squeeze as many years as possible out of those hybrids. A decade from now ultracaps may be a very cost effective retrofit for these hybrids.


It is easy for me to say that I am not impressed. It tries to establish me as someone of superiority, with higher standards. I am just happy that someone is trying to do something good with public money.



The economic merits of emerging technologies should not be discussed? By anyone? By people on this site? If not here, then where? If there is no point to discussion here, what are you doing on this site?


NYC is fairly flat and level while Seattle is a city built on hills (some New Yorkers would likely call them mountains) as are the surrounding areas. So it makes sense that NYC buses would stress their brake systems less etc.

I`m sure Metro will continue to track the data over the buses lifetime.

"As long as people believe in absurdities they will continue to commit atrocities." - Voltaire


Sorry for overemotional comment.


Do you really think that bus costs as much as 30 Corollas (with combined passenger capacity of 150 riders)? Or hybrid bus real price premium is 200 000 – 20 times more than hybrid price premium of three Priuses with comparable combined power output? I doubt that coming this year GM full size pick-ups with two-mode hybrid transmission, which is slightly scaled-down Allison unit used in hybrid buses, will carry price premium of anything like two or one hundred grand.

US/Canada taxpayers are routinely milked by hugely overpriced public transportation spending, with price of buses, trains, etc. being 3-5 times higher than buses, trains, etc. built with same engines (under licensing from western companies) in India or China.

Also note, that big companies are doing their R&D on money earned from past sales of their vehicles. They do capitalize on price premium of new superior technology (like Toyota does), but generally their R&D spending are already paid for, and there is no need to overprice their products to return R&D capital. The case with hybrid buses prices is clear continuation of overpricing practice carried out by government officials responsible for public transport.

It makes all back-envelope calculations of hybrid or CNG bus economy just silly.

John Schreiber

I am pretty sure that the Eaton hydraulic hybrid system would have better efficiency (both in terms of acquisition cost and operating cost) than either of these systems.
The stated cost of the hybrid part of the vehicle must have amortized most of the engineering cost over a very small number of vehicles.

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