Comparison of In-Use Operating Costs of Hybrid-Electric and Conventional School Buses

doi:10.4236/jtts.2012.22017

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Journal of Transportation Technologies, 2012, 2, 158-164

http://dx.doi.org/10.4236/jtts.2012.22017 Published Online April 2012 (http://www.SciRP.org/journal/jtts)

Comparison of In-Use Operating Costs of Hybrid-Electric

and Conventional School Buses

Shauna Hallmark, Robert Sperry

Center for Transportation Research and Education, Iowa State University, Ames, USA

Email: [email protected]

Received November 15, 2011; revised January 10, 2012; accepted February 15, 2012

ABSTRACT

Hybrid-electric school buses became available in the US through a national consortium designed to bring hybrid-elec-

tric school buses to market by creating enough demand among school districts to encourage manufacturers to invest in

development of the technology. A number of school districts in the US joined the HESB project to purchase plug-in

hybrid-electric school buses. Sixteen hybrid-electric school buses were purchased and piloted in 11 states. Two of the

hybrid-electric school buses were purchased by the Nevada and Sigourney school districts in the state of Iowa, US.

In-use fuel economy and electricity operating costs were monitored for the two Iowa hybrid school buses and two con-

trol buses (one in each district). Fuel consumption and other operational metrics were calculated and compared for each

school district. The hybrid buses were deployed in January 2008 and data were recorded through May 2010. Valuation

of the data indicated that the Nevada HESB had 29.6% better fuel economy than the control bus and the Sigourney

HESB had 39.2% better fuel economy than the control bus. Electrical costs per mile were also calculated for the two

hybrid-electric school buses. Total operating costs per mile were calculated based on fuel use per mile for all buses and

electrical costs for the hybrid-electric school buses. The cost to operate the hybrid bus in Nevada was 37 cents/mile

while the control bus cost 42 cents/mile, making the hybrid bus 13% less expensive to operate. The hybrid bus in Si-

gourney was 27 cents/mile while the control bus was 34 cents/mile, making the hybrid bus 21% less expensive to oper-

ate. All values are in US dollars.

Keywords: Hybrid Vehicles; Fuel Consumption; School Bus; Energy

1. Introduction

Around 26 million school children are transported to

school annually in the United States on school buses re-

sulting in 5.8 billion miles of travel per year [1]. Al-

though transporting school children on school buses is

significantly more fuel efficient than transportation in

private vehicles, school buses still consume around 822

million gallons of diesel fuel annually. The cost of pro-

viding school transportation can be overwhelming for US

school districts as fuel costs rise and school budgets are

reduced due to eco nomic downturn. In response to rising

transportation costs, some school districts are cutting

services which increase the number of private vehicle

trips which can increase overall fuel use considerably.

The American School Bus Council [1] estimates that if

children who customarily ride the school bus took those

school trips in private vehicles, the school trips would

consume an additional 2.3 billion gallons of fuel. This

would result in even more consumption of non-renew-

able resources and continued reliance on foreign energy

sources. As a result, even though it may be challenging

for agencies, such as resource strapped school districts,

to invest energy and resources, it is important that agen-

cies take the lead in finding sustainable solutions. It is

also particularly important that school districts take the

lead in demonstrating sustainable practices to school

children.

One sustainable solution to school transportation prob-

lems is use of hybrid school buses, which have the po-

tential to reduce emissions and overall life-cycle costs

compared to conventional diesel school buses. Prior to

2006, the hybrid technology was only available in transit

buses and passenger vehicles with both having a track

record of fuel economy and emissions benefits. Hybrid

technology has recently become available in the school

bus market.

In order to create enough demand to bring hybrid

school buses to market, a national consortium was

formed in the US. The Hybrid-Electric School Bus

(HESB) Project was organized by Advanced Energy. The

project was designed to bring hybrid school buses to

market by creating enough demand among school dis-

tricts to encourage a manufacturer to invest in the devel-

S. HALLMARK ET AL. 159

opment of the technology [2]. The expected benefits of

the hybrid-electric school buses are fuel efficiency, re-

duced fuel costs, and decreased emissions.

A number of school districts joined the HESB project

and purchased plug-in hybrid-electric school buses. Six-

teen hybrid-electric school buses were purchased and

piloted in 11 states. Two of the hybrid-electric school

buses were purchased by Iowa school districts.

1.1. Performance of Hybrid School Buses

Although hybrid technology was relatively new in school

buses, a few studies are available which have assessed

the fuel economy and emissions impact of hybrid school

buses. It should be noted that most of the studies cited in

the following sections were assessments of buses that

participated in the HESB project.

Enova tested a plug-in hybrid school bus using a chas-

sis dynamometer to evaluate fuel economy under con-

trolled test conditions. The company used the West Vir-

ginia University Suburban Cycle (WVUSC) and Tor-

rance California Test Cycle (TCTC) [3]. With the

WVUSC, the research ers found that the hybrid bus had a

fuel economy of 12.70 mpg, while a conventional bus

used for comparison had a fuel economy of 7.10 mph; a

79% improvement. For the TCTC, the researchers found

a 57% improvement in fuel economy for the hybrid bus,

with 12.80 mpg for the hybrid and 8.10 mpg for the con-

ventional bus.

Hybrid-electric school buses in Vermont were tested

using on-road emissions equipment [4]. It was estimated

that the hybrid-electric school bus consumed 28.7% less

fuel than a standard diesel school bus. Additionally, the

researcher found that NOx emissions were 49% lower for

the hybrid-electric school bus than for the conventional

school bus, CO emissions were 72% lower, and HC

emissions were 49% lower.

Data were collected on a conventional charge-sus-

taining (hybrid) bus, a standard diesel control bus, and a

charge-depleting (plug-in hybrid) bus for the New York

Power Authority (NYPA) [3]. Fuel consumption for

three test routes was measured (urban, suburban, and

rural) and they found that the plug-in hybrid school bus

had a fuel economy 51% to 131% higher than the other

buses, depending on the test route. However, this was

only observed while the HESB operated in charge-de-

pleting mode. Once it began operating in charge-sus-

taining mode, it performed similar to the conventional

hybrid and diesel control buses.

In another study, a plug- in hybrid schoo l bus w as co m-

pared to a control bus by The University of Texas. The

study authors found that fuel economy for the conven-

tional bus was 6.9 mpg, while fuel economy was 9.0 mpg

for the HESB (an increase of 30%). The researchers also

reported that the control bus emitted approximatel y 6 3 kg

of CO2 per day while the HESB emitted 51 kg. When the

researchers factored in pollution for electricity regenera-

tion to recharge batteries, this represents a reduction of

23.5% [3].

Advanced Energy [3] has also been collecting in-use

fuel economy data from each school district involved in

the HSEB project across the US. Advanced Energy’s

researchers evaluated the fuel economy comparing hy-

brid-electric school buses and regular school buses.

Overall, the hybrid buses have done much better than the

control buses. Three school districts reported decreases

in fuel economy, 5%, 6%, and 12% respectively. The

remaining school districts have reported improved fuel

economy for the hybrid-electric school buses over con-

trol buses. Two districts reported improvements between

6% and 8%, three reported improvements from 11% to

14%, one had a 23% savings, one had a 47% improve-

ment, and one school district saw a fuel economy im-

provem ent of 57%.

Choi and Frey et al. [5] conducted a study which com-

pared energy use for a plug-in parallel-hybrid diesel-

electric school bus (PHSB) to a conventional school bus

using five real-world driving cycles. They found that the

direct diesel fuel economy was 3.2% to 9.5% better for

the PHSB compared to the conventional bus.

The National Renewable Energy Laboratory [6] ana-

lyzed real-world school bus drive cycle data for a

first-generation PHEV, a more recent PHEV, and a con-

ventional school bus. Depending on the drive cycle,

PHEV fuel savings ranged from 30% to 50% while the

buses were in charge depleting mode. While in charge

sustaining mode the PHEV showed only small fuel sav-

ings compared to the conventional bus.

1.2. Hybrid Performance in the Transit Market

Although only a few studies have been conducted to as-

sess the fuel economy or emissions for hybrid school

buses, transit buses have used hybrid technology for

some time and have overall shown significant fuel

economy and emissions improvements over conventional

buses.

Chassis dynamometer tests were conducted for 10

low-floor hybrid buses and 14 conventional high-floor

diesel transit buses run by New York City Transit [7].

Buses were evaluated over three driving cycles including

the Central Business District (CBD), New York bus cy-

cle, and the Manhattan cycle. The operating costs, effi-

ciency, emissions, and overall performance were also

compared while both types of buses were operating on

similar routes. They found that fuel economy was 48%

higher for the hybrid buses.

A study by Battelle [8] tested emissions using a dy-

S. HALLMARK ET AL.

160

namometer for one diesel hybrid-electric bus and two

regular diesel buses (with and without catalyzed diesel

particulate filters [DPF]). The researchers reported that

fuel economy for the hybrid bus was 54% higher than the

two regular diesel buses. In another study, two buses

were tested using a dynamometer at the National Re-

newable Energy Laboratory’s (NREL’s) Refuel facility

in Golden, Colorado [9]. One bus was a conventional

diesel and the other was a hybrid bus and both were

tested over several drive cycles including Manhattan,

Orange County Transit A, CBD, and King County Metro.

Results are shown in Table 1 for each drive cycle. The

table shows percent difference in fuel economy among

the buses (fuel economy was reported as miles per gal-

lon). Fuel economy was 30.3% to 74.6% higher for the

hybrid bus compared to the conv entional buses.

In another study, Clark et al. [10] evaluated six transit

buses with traditional diesel engines, two powered by

spark-ignited compressed natural gas (CNG), and one

hybrid transit bus in Mexico City using a mobile heavy-

duty emissions testing lab. Buses were tested over a drive

cycle representative of Mexico City transit bus operation,

which was developed using GPS data from in-use transit

buses. Depending on how fuel economy was evaluated,

the hybrid bus ranked 4th and 1st in fuel economy.

1.3. Objective

Two school districts in Iowa, (US) participated in the

Hybrid Electric School Bus Project (Nevada and Sigour-

ney) and each purchased one hybrid-electric school bus.

The Center for Transportation Research and Education

(CTRE) at Iowa State University (ISU) monitored and

evaluated bus performance for the two Iowa HESBs from

2008 to 2010. The project was funded by the Iowa En-

ergy Center (IEC). The team used information provided

by the school district to calculate fuel economy and elec-

tricity use for the two hybrid-electric school buses and

two traditional diesel school buses which were used as

controls.

The objectives of the research were to monitor fuel

and electricity use and assess the on-road operating costs

of the hybrid buses compared to conventional school

buses.

Tab le 1. Percentage chan ge comp ared to conventional buses .

Cycle Fuel economy

Manhattan +74.6%

OCTA +50.6%

CBD +48.3%

KCM +30.3%

1.4. Iowa Buses

Each of the two school districts purchased one hy-

brid-electric school bus with funding assistance from the

Iowa Energy Center (IEC). Each school district was also

asked to select a control bus which had similar charac-

teristics to the hybrid-electric bus (size, year manufac-

tured, etc.) and operated on similar routes. The hy-

brid-electric school buses were manufactured by IC Bus.

The school bus body is the same as that of a standard

school bus and bus specifications include the following

[2]:

 6.4 L 210 hp diesel

 Hybrid system with plug-in capability

 Post-transmission parallel drive

 80 kW electric motor

 35 kWh Li-ion phosphate battery pack

Both of the Iowa HESB buses are 2009 International

65-passenger buses with an International MaxForce (V8)

6.4 L engine with an Allison automatic 2000 transmis-

sion. The hybrid function can be turned off so that the

bus runs on the internal combustion engine (ICE) only.

The Nevada control school bus is a 2009 Bluebird

65-passenger bus with a Cummins ISB07 engine (220

HP, Allison automatic GEN4, 2500 PTS, TC 221). The

Sigourney control bus is a 2005 Bluebird, 65-passenger

with a Caterpillar C-7 (V6) engine (210 HP) with an Al-

lison automatic 2000 transmission.

The Nevada School District is located in Nevada, Iowa,

about 10 miles west of Ames, Iowa. The district has

about 13 operational buses and transports around 828

school children per day. The district uses a 20% biodiesel

blend. The blend amount varies, but the same fuel was

used in both the h ybrid and contro l buses. The Sigourney

School District is located about 90 miles southeast o f Des

Moines, Iowa. The district has about 11 buses and trans-

ports around 320 school children per day. Both locations

are considered to be rural co mmunities.

2. Methodology to Evaluate Operating Costs

Both school districts began recording information for the

hybrid buses in February of 2008. The Nevada school

district also began reporting for the control bus in Febru-

ary 2008 while the Sigourney school district did not se-

lect a control bus until October 2008. Data were recorded

for all buses through May 2010. Each time the buses

were fueled, school districts reported the date, odometer,

gallons of fuel added, fuel cost, and any maintenance or

other issues.

Initially, neither Iowa school district had a separate

electric meter for the hybrid bus. When a dedicated meter

became available, electricity use was recorded. However,

less electricity use information was available than for

fuel use. Additionally, although the districts did a good

S. HALLMARK ET AL. 161

job keeping track of major problems with the buses, they

did not regularly report minor maintenance, such as add-

ing oil. As a result, it was not possible to evaluate total

maintenance per mile.

The data collected for each period were compared

against school district notes to determine whether there

were any problems with the data that needed to be ad-

dressed. In several cases, the battery charging system on

the hybrid buses did not hold a charge. This occurred for

nearly six months for Nevada and seven months for Si-

gourney. In other cases, the schools indicated that data

were uncertain. (For instance, one bus was in the shop

for several days, and the school district did not know

whether additional fuel had been added). Data for which

the hybrid bus was not functioning properly or errors

were noted were removed from the data set. All costs

reported in the following sections are in US dollars.

2.1. Fuel Cost

Because temperature and weather conditions impact bus

fuel economy, data were first disaggregated and initially

evaluated by season. However since there was not suffi-

cient data to determine whether fuel economy between

the hybrid and controls buses was statistically different

by season, data were combined and annual average fuel

economy calculated.

Fuel economy for each time period was calculated us-

ing the following Equation:

 (1)

where:

m = fuel economy for period t (mpg)

t = miles driven in period t based on odometer read-

ing

= gallons of fuel used in peri od t

Average fuel economy was calculated using Equation

(2):

avg t

EFE



(2)

where:

avg

E = average fuel economy

In Nevada, the conventional bus had a fuel economy

of 6.35 mpg while the hybrid bus had an overall fuel

economy of 8.23 mpg for an increase of 29.6%. In Si-

gourney, the control bus’ fuel economy was 6.42 mpg

while the hybrid bus had an overall fuel economy of 8.94

mpg which is 39.2% higher. Results of a Wilcoxon

signed rank test indicated the differences were statisti-

cally significant at the 5% level of confidence for both

school districts. A more in-depth discussion of how fuel

economy was calculated and evaluated is summarized in

a correspondin g paper [11].

Fuel costs were averaged over the analysis period to

simplify estimation of costs. The average cost for a gal-

lon of diesel for Nevada over the analysis period was

$2.71, and the cost for Sigourney was $2.17. The average

fuel cost per mile was calculated using the following

equation:

fuel

mile avg

 (3)

where:

mile

C = fuel cost per mile

fuel

The cost for Nevada was $0.33/mile for the HESB and

$0.43/mile for the control bus. In Sigourn ey, the co st was

$0.24/mile for the HESB and $0.34/mile for the control

bus.

= average cost of fuel per gallon

2.2. Electrical Cost

Average electricity use for the hybrid-electric buses was

reported for times when the hybrid was functioning and

when electricity readings were available. As indicated,

the hybrid buses did not have a dedicated electricity me-

ter available when the buses were initially placed in op-

eration. As a result, data were summarized for periods

when the meters were functioning and it was assumed

that electricity use was constant across the study period.

Electric use per mile was calculated using Equation

(4):

KWH m

 (4)

where:

KWHm = kilowatt hours p er mile

KWHt = kilowatt use for period t

mt = miles driven in period t based on odometer read-

ing

Cost per mile for electricity use was determined using

the following equ ation:

ECKWH Ckwh



 (5)

where:

ECm = average cost of electricity per mile

Ckwh = average cost per kilowa t t hour

MidAmerican Energy [12], Iowa’s largest utility, re-

ported that the average cost per kilowatt-hour was $0.06

for retail organizations. This was the value used to cal-

culate electricity cost per mile since the school bus ga-

rages could not obtain th e actual cost paid by the districts

for electricity. Electrical cost was 5.0 cents/mile for the

Nevada bus and 4.0 cents/mile for the Sigourney bus.

Operating cost per mile for the control buses was simply

the fuel cost per mile. Operating cost per mile for the

hybrid bus was calculated by adding fuel cost per mile

S. HALLMARK ET AL.

162

and electricity cost per mile.

3. Other Costs

Several long term costs which are important factors in

assessing whether investment in hybrid buses makes

sense for a school district could not be included in the

analysis. Initial feasibility studies and marketing of hy-

brid buses indicate that the hybrid technology would re-

sult in less engine wear and regenerative braking is esti-

mated to result in less brake wear [13-16]. However, the

battery pack s ad d ap prox imately 2000 po und s [1 3] wh ich

may increase wear on some parts such as shock absorbers

and tires. Replacement cost of the batteries will also add

to the lifecycle cost of the hybrid buses. IC Bus [13] in-

dicates that the lithium-ion batteries have a life of 5 - 7

years. The replacement costs, however, are to some ex-

tent unknown at this point. Battery technology is rapidly

evolving so the cost of b atteries in the future will depend

on the latest technology and costs when it is time to re-

place the batteries.

One of the original project goals was to track mainte-

nance costs for the hybrid and control buses. However,

the buses were only followed for 2 years which was not

sufficient time to identify differences in normal wear.

During the study, a number of maintenance issues spe-

cific to the hybrid buses occurred. The initial charging

connections and power cords furnished with the buses

proved to be inadequate for the function for which they

were needed. After both districts’ chargers burned (or

shorted) out, all were replaced with heavier duty cords

and more positive (locking) connections. In addition, the

batteries required about eight hours to achieve a full

charge, so the intermediate charge between the typical

morning and af ternoon routes was insufficient to provide

full electrical power in the afternoon. During periods of

maximum electrical use (and minimal engine use), the

idling engine did not circulate and heat enough water in

its cooling system to pro vide adequate h eat to the interio r

of the bus. This was unacceptable to the driver and the

riders, so the attempted so lution was an auxiliary electric

heater. However, during the study, any maintenance

costs specific to the hybrid system was covered under the

vehicle warranty so the schools did not incur any actual

costs due to the maintenance issues. Additionally, it was

felt that the initial maintenance problems with the h ybrid

buses were generational and will be overcome in future

models so these types of costs are not expected to occur

with more recent hybrid school buses.

Reduced emissions are another cost that could be in-

cluded in a lifecycle analysis. Studies have shown sig-

nificant reduction in criteria pollutants for hybrid school

buses compared to regular school buses. Students on hy-

brid school buses would have lower exposure to pollut-

ants, particularly particulate matter, resulting in a posi-

tive health benefit. However, quantifying costs associ-

ated with a reduction in pollutants was beyond the scope

of this study. Additionally the benefits of these reduced

costs are not accrued to the school distri ct s them sel ves.

4. Results

Operating costs by category are provided in Table 2. As

shown, the cost to operate the hybrid bus in Nevada was

38.0 cents/mile while the con trol bus cost 43.0 cents/mile,

making the hybrid bus 13% less expensive to operate.

The hybrid bus in Sigourney was 28.0 cents/mile while

the control bus was 34.0 cents/mile), making the hybrid

bus 21% less expensive to operate. As noted in Section 4

none of the maintenance costs directly related to the hy-

brid buses were paid by the school, so these costs were

not factored in. This would be typical of any new vehicle

while it was still under warranty. Additionally, as indi-

cated, differences in regular maintenance were not noted.

Regular maintenance includes items such as an oil

change, changing an air filter, etc.

The Nevada HESB was driven an average 987 miles

per month while the control bus was driven an average of

705 miles. The average mileage for the Nevada buses

was 846 miles per month. Assuming a school year of 9

months, the average cost for operating a HESB over an

average route would be 9 × 1362 × $0.236 = $2893 an-

nually while the cost to operate a traditional bus on a

similar route is 9 × 1362 × 0.267 = $3273. Total savings

are $380 annually.

The Sigourney HESB was driven an average of 1339

Table 2. Operational metrics for Iowa buses (costs are in USD).

Avg fuel

economy in

km/l (mpg)

Avg fuel

cost in

$/liter ($/gal)

Avg fuel

cost per

km (cost/mile)

Avg kwh

per km

(per mile)

Avg cost

per kwh

Avg kwh

cost/km

(per mile)

Total avg

cost/km

(per mile)

Nevada HESB 3.50 (8.23) $0.72 ($2.71) $0.21 ($0.33) 0.53 (0.85)$0.06 $0.032 ($0.051) $0.24 ($0.38)

Nevada Control 2.70 (6.35) $0.72 ($2.71) $0.27 ($0.43) NA NA NA $0.27 ($0.43)

Sigourney HESB 3.80 (8.94) $0.72 ($2.71) $0.15 ($0.24) 0.37 (0.60)$0.06 $0.022 ($0.036) $0.17 ($0.28)

Sigourney Control 2.73 (6.42) $0.72 ($2.71) $0.21 ($0.34) NA NA NA $0.21 ($0 .34)

S. HALLMARK ET AL. 163

miles per month and the control bus was driven an aver-

age of 1010 miles per month for an average of 1175

miles per month. Based on this, the average operating

cost for a HESB would be 9 × 1625 × 0.174 = $2545

annually. The average operating cost for the control bus

on a similar route would be 9 × 1625 × $0.340 = $4972.

Total savings are $2427 annually.

As indicated, the annual savings are modest. At the

time of the study, a hybrid electric school bus was ap-

proximately $80,000 more than a conventional school

bus. Consequently, it would take some time for the cost

to be realized. However as discussed in Section 4, other

costs, such as the impact of exposure to pollutants, are

not included.

5. Summary

This paper summarizes the evaluation results of an in-use

fuel economy and operating cost evaluation for two

plug-in hybrid school buses deployed in two different

school districts in Iowa. Each school district selected a

control bus that ran a route similar to that of the hybrid

bus. Odometer readings, and fuel consumption were re-

corded for each bus. The buses were deployed in 2008

and data were collected through May 2010.

The buses were part of the Hybrid-Electric School Bus

Project organized by Advanced Energy which was de-

signed to bring hybrid-electric school buses to market by

creating enough demand among school districts to en-

courage a manufacturer to invest in the development of

the technology. A number of school districts in the US

joined the HESB project to purchase plug-in hybrid-elec-

tric school buses. Sixteen hybrid-electric school buses

were purchased and were piloted in 11 states. Tw o of the

hybrid-electric school buses were purchased by Iowa

school districts, Nevada and Sigourney.

In-use fuel economy and electricity operating costs

were monitored for the two Iowa hybrid school buses and

two control buses (one in each district). Fuel consump-

tion and other operational metrics were calculated and

compared for each school district.

Evaluation of the data indicated that the Nevada HESB

had 29.6% better fuel economy than the control bus and

the Sigourney HESB had 39.2% better fuel economy

than the control bus. Electrical costs per mile were also

calculated for the two hybrid-electric school buses. Total

operating costs per mile were calculated based on fuel

use per mile for all buses and electrical costs for the hy-

brid-electric school buses. The cost to operate the hybrid

bus in Nevada was 38.0 cents/mile while the control bus

cost 43.0 cents/mile, making the hybrid bus 13% less

expensive to operate. The hybrid bus in Sigourney was

28.0 cents/mile while the control bus was 34.0 cents/mile,

making the hybrid bus 21% less expensive to operate.

Both hybrid buses had fuel economy and operating

costs that were well below those for the control buses.

However, fuel economy was not as high as had been ex-

pected by the school districts based on in itial estimates of

12 mpg. Fuel economy may have been impacted in part

by maintenance problems experienced by both school

districts. The charging system for buses required several

fixes and resulted in both buses running only on the ICE

for several months during the analysis period. This may

have impacted fuel consumption results if the bus opera-

tor did not notice and report the problem as soon as it

occurred. Additionally problems with the charging sys-

tem resulted in the buses not being able to fully recharge

for the afternoo n run. As a result, fu el economy would be

lower than if the buses had been fully charged. Conse-

quently, the HESB may have actually had better fuel

economy than was reported.

Although the HESB experienced an unusual number of

maintenance problems, which were frustrating for the

school districts, the team believes that this problem is

generational and can be overcome in future models.

Transit buses have utilized both plug-in and conv entional

hybrid techno logies fo r some ti me, and it does not appear

that they have experienced the same challenges. As a

result, it is believed that the potential exists for manufac-

turers to overcome the initial problems.

The operatin g costs for the HESB were lo wer for both

school districts than for the conventional buses used as

controls. However, even with the savings that could be

achieved it will still be difficult for schoo l districts in the

US to recuperate the additional purchase price of the hy-

brid school buses. As a result, the difference in purchas-

ing prices for hybrid school buses may need to be subsi-

dized in order for the technology to be cost effective for

school districts.

6. Acknowledgements

The research team would like to thank the Iowa Energy

Center for funding this project. They would also like

thank the Nevada and Sigourney bus garages, drivers,

and supervisors for their assistance in collecting data. In

particular, the team would like to thank Mr. Dennis

Kroeger, who conducted much of the work and provided

much of the vision to bring hybrid school buses to Iowa.

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S. HALLMARK ET AL.

164

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