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The Technology of Diesel Hybrid-Electric
Buses 1Features at a glance:
5How They Work: NYC will use Orion (models VI and VII) buses that are equipped with BAE SYSTEMSí HybriDriveTM propulsion system. Model VII buses will be delivered between 2002-2004.
5Introduction and Goals of the Orion VI Hybrid Program: At the time that the 10 Orion VI hybrid
buses were delivered, this was the largest standard 40í hybrid
bus fleet in the U.S. Hybrid buses were not a standard configuration
for transit applications. BAE SYSTEMS, Orion, and NYCT set out
to design, build, and implement the use of hybrid buses into standard
transit service. All parties were aware that this was an investment
into unproven technology and the project was intended to develop
this technology. Goals:
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Reduce emissions,
specifically oxides of nitrogen (NOx <15 g/mi) and particulate
matter (PM <0.06 g/mi).
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Significantly
increase fuel economy.
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Show that the
hybrid buses can operate in regular revenue service with no route
or driver restrictions.
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Show that the
hybrid bus performance was equal to or better than that of conventional
diesel buses and that drivers can switch from one to the other
with no significant difference in operation.
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Significant
increase in brake life.
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Help put the
industry in a position to build and sell production hybrid buses. 2,3Specifications ñ Orion Model VI Hybrid Electric Bus: Vehicle Size
Hybrid Power Plant
4Hybrid-Electric Drive Heavy Duty Vehicle Testing Data ñ Emissions data from the Northeast Advanced Vehicle Consortium: Description of the fort-foot buses tested under the NAVC program:
A ñ The NovaBus was tested on DI, and MossGas diesel fuels. B ñ The Orion-LMCS bus was tested on D1, low sulfur D1, and MossGas diesel fuels. C ñ The Nova-Allison bus was tested on low sulfur D1 diesel fuel. CNG ñ compressed natural gas. 4Various drive cycles were used to simulate actual road conditions:
The Following charts represent actual test results obtained from the New York Bus Cycle by the NAVC. They represent various emission levels from different types of buses as well as the fuel economy for each. Due
to the voluminous amounts of data collected for each cycle and
the lack of time only the NY Bus cycle is represented in the charts.
This cycle was developed using real-world speed-time data from
heavy-duty vehicles, such as transit buses and trucks, in service
in New York City. For the full report visit the NAVC link at the
bottom. 4Explanation of Terms and Acronyms: Particulate matter (PM) from internal combustion engines is composed of a combination of carbon particles, on the surface of which, organic compounds are adsorbed. This organic fraction poses the largest toxic risk associated with the particulate. NOx are oxides of nitrogen and are regulated as precursors of ozone. These compounds are grouped together with VOCs (volatile organic compounds), which include many fluorinated hydrocarbons, methane, ethane, carbon monoxide, carbon dioxide, acetone, and several others. NMOCs are non-methane organic compounds CH4 ñ Methane CO ñ Carbon monoxide CO2 ñ Carbon dioxide mpg ñ miles per gallon ppm ñ Parts per million g/bhp-hr ñ Grams per brake horsepower hour HC - Hydrocarbons NAVC ñ Northeast Advanced Vehicle Consortium WVU ñ West Virginia University NYCT ñ New York City Transit MTA ñ Metropolitan Transit Authority SOC ñ State-of-charge 4Description of fuels: Diesel
fuel is a complex mixture of hydrocarbon molecules produced by
blending byproducts of crude oil refining. After crude oil is
distilled into different components, usually several refinery
streams are recombined along with additives to produce commercial
diesel fuel. The composition of diesel fuel has remained largely
unchanged with the only substantial difference being lower sulfur
levels. D1 ñ Transportation grade diesel fuel
(~300 ppm sulfur) MossGasÆ - A synthetic diesel fuel (10% aromatic
blend) manufactured using olefin distillate derived from natural
gas. Contains a very low sulfur content, near zero, and can be
used in unmodified diesel engines. CNG ñ Compressed natural gas. A simple
hydrocarbon fossil fuel composed primarily of methane. Contains
essentially no sulfur and is clean burning and relatively inexpensive
and abundant. Abbreviations for buses in charts: (A) Orion-LMCS VI Hybrid Diesel (B) Orion-LMCS VI Hybrid MossGasÆ (C) Nova-Allison RTS Hybrid LS Diesel (D) NovaBUS RTS Diesel Series 50 (E) NovaBUS RTS MossGasÆ Series 50 (F) Neoplan AN440T CNG L10 280G (G) New Flyer C40LF CNG Series 50G (H) Orion V CNG Series 50G Charts: 4Note: In an ideal world when the bus finishes
an emission test cycle, the batteries would have the same State-Of-Charge
(SOC) at the end of the test as at the beginning. This would allow
the data to be used uncorrected. When a hybrid-electric bus operates
in a charge-depleting mode, it effectively borrows energy from
the battery to complete the drive cycle test. This, therefore,
skews the results to seem better (higher fuel economy, lower emissions).
The results seem better than reality because less fuel is being
used to cover the same distance. Conversely, when the engine puts
more energy into the batteries than needed the results are skewed
to seem worse since more fuel was used to cover the cycle. To evaluate the performance of the
hybrid-electric buses relative to the conventional and alternatively
fueled buses, the fuel economy and emissions data for each hybrid-electric
bus were corrected to account for fluctuations in the battery
SOC.
4,5Results and Conclusion of Testing: Positive:
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NYCT, BAE SYSTEMS,
and Orion are committed to operating the Orion VI diesel hybrid-electric
buses in service, as well as the new Orion VII, which is currently
on order. The Orion VI hybrid bus has met all basic performance
expectations and requirements such as gradability, acceleration,
low noise and emissions.
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Facility conversion
for accommodating hybrid buses was minor compared to preparing
for compressed natural gas (CNG) vehicles.
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The hybrid buses
had a fuel cost per mile 9% lower than the NovaBUS RTS diesel
buses.
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The hybrid buses
had a rate of miles between road-calls that was 54% lower than
the NovaBUS RTS diesel buses for all road-calls and 80% lower
for engine and fuel related road-calls.
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PM emissions
from the hybrid vehicles were generally 50%-70% lower than a conventional
diesel. In several cases the equipment was not sensitive enough
to detect the mass emission from the hybrids. The ability to utilize regenerative braking, less transient
engine management and the regenerative particulate trap control
are responsible for the reductions.
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NOx emissions
from the Orion-LMCS hybrid buses were 30%-40% lower than a conventional
diesel vehicle. This result is interesting given these hybrid
buses utilized diesel engines certified to the same NOx standard
of 4.0 g/bhp-hr. Only about a third of this benefit is attributable
to regenerative braking. As these vehicles were not equipped with
after treatment for the control of NOx, the remaining NOx benefit
must be attributed to differences in engine operation.
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The emission
testing performed on the Orion VII diesel hybrid buses had 94%
lower CO, 49% lower NOx, 120% higher HC, 93% lower PM, and 37%
lower CO2 than the Orion V diesel without the catalyzed diesel
particulate filter (DPF). The model VII had a 38% lower CO, 49%
lower NOx, 450% higher HC, 60% lower PM, and 38% lower CO2 than
the Orion V diesel with the catalyzed DPF. Negative:
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The hybrid buses
had lower mileage (miles driven) on a monthly basis compared to
the NovaBUS RTS diesel buses at the same NYCT depot. The reduction
in usage of the hybrid buses was a direct result of the need to
service the buses and the extra time required to coordinate with
the manufacturers to troubleshoot and fix those problems. The
lower mileage for the pre-production hybrid buses was expected
by NYCT and the manufacturers.
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Maintenance
costs for the hybrid buses were 76%-150% higher than those of
the NovaBUS diesel buses, in a way negating the savings in fuel
costs. Maintenance costs were much higher for all bus subsystems,
even those that have nothing to do with the hybrid propulsion
system. These costs are expected to decrease with the full-scale
deployment of the model VII.
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The hybrid buses
had operating costs 46%-92% higher than the NovaBUS RTS diesel
buses. 4Each of the Vehicles tested under this program was equipped with an oxidation catalyst for the control of CO, HC, and PM. In the cases of the hybrid electric buses a particular trap integrated with oxidation catalyst material was used. While the HC emissions from a diesel engine are already low, including the catalyst helps the particulate trap regenerate by converting NO in the engine exhaust to NO2. The NO2 then helps oxidize carbon particles caught in the trap. The combined values of NOx and NMOC emissions are shown in the following chart from the NAVC.
Fuel Economy
4The fuel economy benefits of hybrids can be seen in the above graph. There were consistent fuel economy improvements of nearly one mile per gallon for the Orion-LMCS Hybrid over conventional buses on the NY Bus, CBD and Manhattan cycles. While these may seem like small numbers keep in mind that on the NY Bus cycle the best performing diesel bus only achieved 1.4 mpg fuel economy vs. 2.3 mpg for the Orion-LMCS hybrid-electric bus. This equates to about a 65% fuel economy improvement on the NY Bus cycle over a conventional diesel. It is also important to understand that the savings in fuel are offset by the higher price tag of these hybrid buses so fuel economy, while still a significant factor, is not as important in the cityís decision to convert to hybrids as is a major decrease in emissions that hybrid buses can achieve. 4The hybrid-electric vehicles tested under the NAVC project are essentially conventional buses with hybrid-electric drive systems. As a result the hybrid-electric buses weigh more than conventional diesel buses. Much of the additional energy used for accelerating this weight can be recovered via regenerative braking in the hybrid-electric vehicles. The manufacturers are currently working to reduce the overall vehicle weight to improve performance and efficiency. Even so, the Orion-LMCS VI Hybrid-Electric bus outperformed the other buses in nearly every test. This technology shows great promise for New York City in the near future and the tests performed by the NAVC reaffirm this with scientific evidence showing a significant reduction in air pollution from hybrid-electric buses as well as decreased fuel consumption. The expense of these buses and whether the city government is willing to spend hundreds of millions of dollars converting their giant fleet in an uncertain economy is the major issue facing this technology and is currently being debated. Research conducted by Alexander Kaysin. Research
Sources:
2. http://www.evaa.org/evaa/pages/ele_product_bus.htm 3. http://www.ngv.org/ngv/ngvorg01.nsf/bytitle/OrionBusOrionVIILowFloor.htm 4. http://www.navc.org/Navc9837.pdf The custom charts on this page were constructed using data from the above web site from the NAVC. It is a 74-page report on the emission levels and performance of five heavy-duty diesel, CNG, and diesel-electric powered buses. Other sources:
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