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Study Finds Fuel Economy of Gasoline-Electric Hybrid Buses Lower Than That of Conventional Diesel

Nrellbt1
The gasoline hybrids exhibited an average 4.3% lower fuel economy over the 24-month evaluation period than the conventional diesel buses. Click to enlarge.

A two-year evaluation (July 2005 through June 2007) of gasoline-electric series hybrid buses in operation with Long Beach Transit (Long Beach, CA) found that on a volumetric basis, the average fuel economy for the gasoline hybrid buses is 3.35 mpg—4.3% lower than that of the conventionally-powered diesel buses also in service with Long Beach Transit.

The study by the Fleet Test and Evaluation Team at the National Renewable Energy Laboratory (NREL) concluded that the difference is likely due to the lower efficiency of a throttled, spark-ignited engine as well as the lower energy content of a gallon of gasoline versus a gallon of diesel. On an equivalent energy per volume basis, the hybrids had an 8.5% mpg increase.

During the evaluation period, gasoline at LBT cost an average of $2.49 per gallon and diesel cost an average of $2.29 per gallon. This lower fuel economy, combined with a higher fuel cost for gasoline, resulted in fuel costs per mile being $.74 per mile for the hybrids as compared to $.65 per mile for the diesels.

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Energy flow in the ThunderVolt hybrid drive. Click to enlarge.

The hybrids were New Flyer low floor buses equipped with ISE Corporation’s ThunderVolt Hybrid Drive propulsion system (earlier post), which combines a Ford ULEV 6.8-liter V-10 gasoline engine with a 650V, 110 kW (nominal), 145 kW (peak) Siemens ELFA generator and motor system. Dual drive motors each deliver 85 kW (nominal), 150 kW (peak) of power and nominal torque of 220 Nm, peak torque 530 Nm. The LBT hybrids each used two roof-mounted 360V Maxwell ultracap enclosures for energy storage.

ISE says that the typical fuel economy for the gasoline ThunderVolt system can range between 3.5-4.5 mpg US.

The ThunderVolt gasoline hybrid system was certified by the California Air Resources Board to 0.6g/bHp-hr NOx and 3.7g CO, making it a very low emission 40-foot bus drive system. PM is not measured on gasoline systems. By contrast, the diesels in the study are certified to 4.0 g/bHp-hr NOx, 0.05 g/bHp-hr PM and 0.5 g/bHp-hr CO.

As expected with hybrids in general, brake system maintenance costs were dramatically lower for the hybrid propulsion systems with regenerative braking. The hybrids had brake system maintenance costs that were about 90% less than that of the diesel buses with no relines to date on the hybrids. Miles between road call (MRBC) for the hybrids was lower than for the diesels, indicating a lower level or reliability.

Overall, said the report, LBT has been happy with the performance of the original forty-seven 40-ft hybrid gasoline-electric buses over the first two years of service. The transit authority has taken delivery of 15 more in the third quarter of 2007 and has requested an additional 25 for 2008.

The evaluation team selected 10 vehicles from the hybrid group of 47 vehicles and 10 vehicles from the diesel group (MY2002) of 138 vehicles for analysis. The number of vehicles in the study was sufficient to provide some degree of statistical significance to the results obtained, according to NREL.

LBT operates 228 buses out of two facilities and averages almost 40,000 miles per bus annually for both facilities. The average speed of the 40-ft bus fleet is 13.8 miles per hour (mph) with an average of 8 stops per mile.

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Comments

aym

I have no idea why it is designed to run on gasoline. Wouldn't it be better to have bio-diesel bus hybrids? Just so refueling is less and keeping particulate and other emissions low.

Anyway, the the title is a little misleading since it is comparing two totally different things. On a volume basis, the hybrid had lower mileage. On an energy basis, the hybrid had an increase of mileage of 8.5%. They should be using MPGGE to make it less ambiguous.

I would like to see better modeling of the routes to maximize the use of these buses. It wouldn't take alot to GPS the routes in actual traffic to map out the best routes for the hybrid characteristics. I'd like to think that they do this but somehow from what I've seen and read, they don't.

At least they're happy with the performance, which will only get better with feedback and tinkering.

Elliot

Is there a reason they couldn't have a hybrid diesel experiment? I mean is it really that hard to engineer? Also, although the mpg was lower in the gasoline hybrid one, would the high cost of diesel negate those gains?

HC

Comparing fuel consumption of two different fuels on volumetric basis is absolutely not appropriate. The report also compares orange to apple, as there is huge difference in NOx emissions, 0.6 vs 4.0 g/hp-h. I expect a national laboratory would have done a better job in comparison.

Bike Commuter Dude

Apples to Oranges? Perhaps. However, the important number to look at from the perspective of global climate change, is the CO2 emissions, as well as the NOx emissions.
Oxides of Nitrogen are a more ideal greenhouse gas than is CO2. They last longer in the atmosphere, and they are eight times more effective at holding heat.
If the CO2 emissions are increased by 4.3% from the gasoline engine(by the consumption of 4.3% more fuel than the diesel to perform the same work), that is more than offset by the 85% decrease in NOx.
The NOx emissions are also what concern the Californian government, as smog is a reality there, as well as chronic lung disease caused by particulate matter and NOx, as well as ground level O3 (ozone).
Other things to consider:
a)California's diesel buses are probably the cleanest available in the nation, so the benefits would be even more pronounced in other regions with older/dirtier diesels
b)They DO make diesel hybrids --such as the one from Gillig -- but CARB is more concerned with the other emissions.

Joseph


Yeah, CARB decided that it was better for you to buy a 25mpg gas Jetta than a 45mpg TDI Jetta or you could buy a 14mpg F250 but not the 19mpg F250D

Brilliant!!!

Lulu

The tag line should read "Study Finds Fuel Economy of Gasoline-Electric Hybrid Buses 8.5% Higher Than That of Conventional Diesel".

They were probably trying to lower smog by going with a gasoline engine.

Gerald Shields, Seattle, WA

Okay, I can go along with that. However, what of Diesel-Electric Hybrids v.s. Conventional Diesel? New Flyer has Diesel Buses equipped with GM's two mode Hybrid system. in Seattle. Those hybrid buses demonstrated significantly better fuel economy — on average 27 percent higher — with differences ranging from 24 to 30 percent during a evaluation period.

shane

I believe LBT was trying to lower criteria emmisions (NOx, SOx, PM, HC, CO, etc). For that, a gasoline engine is better than most diesels (at least the ones that were available when this effort was funded, probably 5+ years ago)

For CO2 gains, the diesel-hybrid would be better. In any case, it is all very doable if you have the checkbook - and southern CA does.

But - someone needs to decide if they want to minimize CO2 (diesel-hybrid) or criteria pollutants (gasoline-hybrid). Unless you really want to spend money and go to the fuel cell system. ($1-2M per bus I was told)

Brian P

I'm thinking that the way to go TODAY, is to use a modern diesel with particulate trap and de-NOx catalyst to simultaneously achieve the fuel consumption benefit and lower regulated emissions.

Hybrid or not, a gasoline engine makes no sense in that application.

JC7

Bike commuter Dude,
NOx (ie NO and NO2) is not considered a greenhouse gas, it is N2O that is the GHG that is 310X the global warming potential of CO2. N2O emissions are probably lower as a result of this project, but typically N2O emissions from combustion are not that significant compared to the CO2 emissions.

mki

I agree that with most of the people that diesel hybrid will be better solution then spark engine.

If they like to reduce emission and get high efficiency drive system (sound like contradiction) they have to look on external combustion engine (steam engine).
There couple of development that bit most spark engine hand down in efficiency and emission, and they have comparable or better efficiency then diesel engine.
One example is Cyclone Power Technology steam engine.
With efficiency between 36-46% and emission almost 10 time better then spark engine you can not discard such approach.
It look specially attractive for the buses, because of no idle cycle at the stops. I believe that it could be as much as 3 time more efficient in bus city drive cycle as conventional bus.
I also believe that most of the cities have to forget buses as a mean of public transportation.

The problem with buses is rolling resistance due to tires. 10 tone bus at 25 mph require around 20kW of power to move, where 17KW is the roiling resistance.

The solution if light railway or trams. You can see then in many European countries.
The roiling resistance could be up to 100 time smaller in trams.
The cities have to rebuild the tram system (that was destroy by GM ,Ford & Chrysler) that exist before WWII in US. That with new technologies will make very significant difference.

Clee

Why does the fuel efficiency of the gas/electric hybrid gradually increase over the two years until it matches that of the diesel? If the study is continued for another two years, will the hybrid fuel efficiency continue to rise above that of the diesel?

marc

ouch 3.35 mpg..? there have been times i've ridden a bus and been near the only person on it. better to just drive the truck at that point..

Joseph


LuLu, your tagline is incorrect. The electric hybrid bus would be 8.5% higher IF gasoline was equivalent to diesel, but it's not. Ya know, if frogs had wings...

Diesel Electric Hybrid with particulate filter and NOx trap running on reclaimed vegetable oil would be a winner.

sulleny

Which is why too much "behavioral engineering" is unwarranted.

Bernard

I think that they a favoring NOx emissions simply because they have to look at the entire city as an ecosystem.
How many additional hospital visits (which are strongly correlated to NOx emissions) does it take to offset a $0.10 higher fuel cost per mile?

If fuel cost is the only factor, hybrid diesel might come-out ahead. If you factor-in childhood asthma, smog and lower life expectancy, then an extra dime per mile looks like a bargain.

arnold

Aren't the nox aftertreatment devices and emission legislations including biofuels and ethanol additives bringing the NOx emissions down to 'Acceptable' limits ?
Then.. we would be comparing apples to oranges.

Brian P

The 3.35 mpg bit is why one shouldn't jump to an automatic conclusion that public transport (of any sort) is automatically "better" than individual transport (of any sort). Buses inherently aren't full of people all the time and, if you look at the individual trips that each passenger is making, aren't necessarily taking the optimum path for each one.

My Honda CBR125 motorcycle is capable of doing better than a mostly-full bus on a passenger-mile-per-gallon basis ... and my Jetta TDI with me only does better passenger-mile-per-gallon than that bus with 13 people on it. If I could coax people into car pooling (which does not work for the type of work that I do and neither does public transit but I digress) then the Jetta with 4 people does better than a *full* bus, and the CBR125 with a passenger would be about the same ...

J T

Ok kids ... what is it with the misleading headline?

This is black journalism. An oughtright fib of misrepresentation that only gets corrected in the last line of the article.

Shame on the editors at GCC for copying it uncorrected.

Andy

"During the evaluation period, gasoline at LBT cost an average of $2.49 per gallon and diesel cost an average of $2.29 per gallon."

Either they have a heck of a fuel contract or the evaluation period is pretty unrepresentative of what they are experiencing today.

Tom

4 mpg just plain out sucks. sounds like a bad solution. fix the bus motor or do something else. buy 10 prisus, and run those instead of the bus - just kidding. would be nice to scale up the prius while keeping some semblence of its mpgs

George

Three and a half miles per gallon. But do you guys have any idea how many people can ride on a 40 foot bus? I know it's not full all the time, but it only needs 4 passengers to beat a lot of SUVs.

George

Joseph writes: Yeah, CARB decided that it was better for you to buy a 25mpg gas Jetta than a 45mpg TDI Jetta or you could buy a 14mpg F250 but not the 19mpg F250D

Brilliant!!!

Joseph, do you think it's possible that CARB cares more about criteria pollutants, which injure and kill people when concentrated by LA's peculiar geography, than they do about CO2? Ya never know...

Anne

@Bike Commuter Dude:

Did you take the effort to lookup the amount of CO2 emitted by gasoline vs diesel? I bet not. Gasoline emits around 10% less CO2 per litre than diesel. So from a GHG point of view the hybrid gasoline is better.

GCC has a miseleading header. GCC, if you are reading this, please change it.

If you look at diesel/gasoline as an energy carrier, then one litre of diesel contains more energy than one litre of gasoline. So indeed, bike commuter dude, it is comparing apples to organges.

clett

Mean gasoline engine efficiency: 25%

Mean diesel engine efficiency: 40%

Given the low likely thermal efficiency that is inherent to the drive cycle of a bus (ie mostly idling or low load) it's frankly remarkable that the hybrid system managed to get the mpgs of the gasoline version close to that of the diesel!

Also, @Anne, it takes a lot more energy and fossil fuel to refine one litre of gasoline than it does one litre of diesel. On a well to wheel basis, the CO2 outputs of the fuels are similar assuming complete combustion per litre.

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