Comparison of on-road emissions for hybrid and regular transit buses

Abstract

Hybrid technology offers an attractive option for transit buses, since it has the potential to significantly reduce operating costs for transit agencies. The main impetus behind use of hybrid transit vehicles is fuel savings and reduced emissions. Laboratory tests have indicated that hybrid transit buses can have significantly lower emissions compared with conventional transit buses. However, the number of studies is limited and laboratory tests may not represent actual driving conditions, since in-use vehicle operation differs from laboratory test cycles. This paper describes an on-road evaluation of in-use emission differences between hybrid-electric and conventional transit buses for the Ames, Iowa transit authority, CyRide. Emissions were collected on-road using a portable emissions monitoring system (PEMS) for three hybrid and two control buses. Emissions were collected for at least one operating bus day. Each bus was evaluated over the same route pattern, which utilizes the same driver. The number of passengers embarking or disembarking at each stop was collected by an on-board data collector so that passenger load could be included. Vehicle emissions are correlated to engine load demand, which is a function of factors such as vehicle load, speed, and acceleration. PEMS data are provided second by second and vehicle-specific power (VSP) was calculated for each row of data. Instantaneous data were stratified into the defined VSP bins and then average modal emission rates and standard errors were calculated for each bus for each pollutant. Pollutants were then compared by bus type. Carbon dioxide, carbon monoxide, and hydrocarbon emissions were higher for the regular buses across most VSP bins than for the hybrid buses. Nitrogen oxide emissions were unexpectedly higher for the hybrid buses than for the control buses.

Implications:

The main reason agencies consider hybrid transit vehicles is fuel savings and reduced emissions. Hybrid electric buses offer an attractive option and have the potential to significantly reduce operating costs and emissions for transit agencies. However, purchase of hybrid buses cost 50–70% more than regular buses. As a result, agencies require more quantitative information about the on-road costs and benefits of hybrid buses. This study assessed on-road emissions for in-use transit buses. The resulting information can be used by transit agencies in assessing the likely emission reduction benefits for hybrid buses.

Introduction

Background

Hybrid transit buses have the potential to significantly reduce operating costs for transit agencies. The main impetus behind use of hybrid transit vehicles is fuel savings and reduced emissions. Wayne et al. (2009) conducted an evaluation scenario comparing use of hybrid transit buses with regular transit buses. They estimated that use of diesel-electric hybrid buses in just 15% of the U.S. transit fleet could reduce annual emissions by 1800 tons of carbon monoxide (CO), 400 tons of hydrocarbons (HC), 4400 tons of nitrogen oxides (NO_x), 200 tons of particulate matter (PM), and 491,400 tons of carbon dioxide (CO₂).

Several laboratory studies have been conducted to compare emissions between hybrid and regular transit buses. Chandler et al. (2002) conducted chassis dynamometer tests for 10 low-floor hybrid buses and 14 conventional high-floor diesel transit buses run by New York City Transit (NYCT). The buses were tested over three driving cycles: the central business district (CBD), New York bus cycle (NYBC) and the Manhattan cycle. The operating costs, efficiency, emissions, and overall performance were also compared while both types of buses were operating on similar cycles. Data were collected from 1999 to 2001. Results indicated that for the CBD cycle, emissions for the hybrid transit buses were 97% lower for CO, 36% lower for NO_x, 43% lower for HC, 50% lower for PM, and 19% lower for CO₂. Results from the NYBC showed a decrease of 56% for CO, 44% for NO_x, 77% for PM, and 40% for CO₂. HC emissions, however, increased by 88% for the hybrid buses. With the Manhattan cycle, the researchers found a decrease for the hybrid buses of 98% for CO, 44% for NO_x, 28% for HC, 99% for PM, and 33% for CO₂.

Emission tests for one diesel hybrid-electric bus and two diesel buses (Orion V, with and without catalyzed diesel particulate filters [DPFs]) were evaluated using a chassis dynamometer in Ottawa, Canada (Battelle, 2002). The buses were tested on the CBD cycle using ultra-low-sulfur diesel (ULSD) no. 1 fuel. The researchers indicated that the hybrid bus had 94% lower emissions for CO, 49% lower emissions for NO_x, 120% higher emissions for HC, 93% lower emissions for PM, and 37% lower emissions for CO₂ than the diesel bus without a catalyzed DPF. Emissions for the hybrid bus compared with the diesel bus with catalyzed DPF installed were 38% lower for CO, 49% lower for NO_x, 450% higher for HC, 60% lower for PM, and 38% lower for CO₂. Tests were conducted in February 2000.

In another study, two buses (one from a conventional diesel fleet and another from a hybrid fleet) were tested using a chassis dynamometer at the National Renewable Energy Laboratory's (NREL's) ReFUEL facility in Golden, Colorado (Chandler and Walkowicz, 2006). The buses were tested over several drive cycles, including Manhattan, Orange County Transit A, CBD, and King County Metro (KCM). Results in percent difference in emissions are shown in Table 1 for each drive cycle. As indicated, emissions were lower in all cases for the hybrid bus, except for the cases where the differences were not statistically significant (indicated as NS in the table).

Table 1. Comparison of hybrid versus regular buses over various test cycles

	Test Cycle
Pollutant	CBD	NYBC	Manhattan	OCTA	KCM	UDDS
CO2	−19% ^a	−40% ^a	−33% ^a	−35% ^d	−24% ^d
	−37%^b		−44^d
	−38%^c
	+35%^d
CO	−97%^a	−56%^a	−98%^a	−32%^d	−60%^d	−47%^e
	−94%^b		NS^d	−82%^e
	−38%^c		−73%^e
	−48%^d
	−72%^e
NOx	−36%^a	−44%^a	−44%^a	−29%^d	−18%^d	−60%^e
	−49%^b		−39%^d	−42%^e
	−49%^c		−52%^e
	−27%^d
	−45%^e
HC	−43%^a	+88%^a	−28%^a	NS^d	−56%^d
	+120%^b		NS^d
	+450%^c
	−75%^d
PM	−50%^a	−77%^a	−99%^a	−51%^d	NS^d	approx. −90%^e
	−93%^b		−93^d	approx.
	−60%^c		approx. −90%^e	−90%^e
	−97%^d
	approx. −90%^e
Notes: NS = no statistically significant difference. ^a Chandler et al. (2002). ^bCompared with diesel bus without catalyzed DPF (Battelle, 2002). ^cCompared with diesel bus with catalyzed DPF (Battelle, 2002). ^dChandler and Walkowicz (2006). ^e Wayne et al. (2004).

Clark et al. (2006) 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 transportable heavy-duty emissions testing laboratory. Buses were tested over a driving cycle representative of Mexico City transit bus operation, which was developed using global positioning system (GPS) data from in-use transit buses. Depending on how emissions were compared, the hybrid bus and one of the CNG buses had the lowest NO_x emissions of the nine buses tested. Particulate emissions from the hybrid bus were less than 10% of the average PM emissions for the diesel-powered buses. The hybrid bus and one of the CNG buses had the lowest CO emissions, and the hybrid bus and buses equipped with continuously regenerated trap (CRT) exhaust aftertreatment had hydrocarbon emissions that were below the detectable limit of the instrument used.

Wayne et al. (2004) evaluated three transit vehicles, a series-drive hybrid-electric, a conventional drive diesel-powered, and a conventional-drive liquefied natural gas (LNG)-powered bus. They evaluated the buses on a chassis dynamometer over several cycles. In all cases, the conventional diesel-powered bus had the highest NO_x emissions. The hybrid had the lowest NO_x emissions, which were 52%, 60%, 42%, and 45% lower for the Manhattan, Urban Dynamometer Driving Schedule (UDDS), Orange County Bus Cycle (OCTA) drives, and CBD, respectively. PM emissions were around 90% lower for all cycles. The hybrid-electric bus had CO emissions that were 73%, 47%, 82%, and 72% lower for the Manhattan, UDDS, OCTA, and CBD drives, respectively. CO emissions were reported as 98% lower than the conventional diesel bus.

Laboratory tests may not represent actual driving conditions, since in-use vehicle operation differs from laboratory test cycles. Several studies have used on-road measurements to compare emissions. Shorter et al. (2005) used a chase vehicle sampling strategy to measure NO_x from 170 in-use New York City transit buses. The authors sampled emissions from conventional diesel buses, diesel buses with continuously regenerating technology, diesel hybrid-electric buses, and CNG buses. The authors found that NO_x emissions from CNG buses and hybrid buses were comparable. NO_x emissions for the hybrid buses were approximately one-half of those for conventional transit buses.

In contrast, Jackson and Holmen (2009) collected second-by-second particle number (PN) emissions from four conventional and one hybrid transit bus in Connecticut over six predefined test routes that had multiple road types and ranges of driving conditions. For most of the routes, few differences were noted between the conventional and hybrid transit buses. However, the hybrid had higher emission rates on two routes with steep uphill grades, and PN emissions were 51% higher on one route and 24% higher on the other.

Choi and Frey (2010) evaluated emissions for a plug-in hybrid (PHSB) and conventional school bus (CDSB) using a portable emissions monitor. Results were presented by type of route. Depending on the route, they found that 1.4–8.7% lower CO₂ tailpipe emissions for the PHSB than for the CDSB; 3.3–8.5% lower NO_x tailpipe emissions; a 0.1–3.6% reduction in HC tailpipe emissions for all but one route, with the PHSB having 0.4% higher HC emissions than the PHSB for one route; a 0.6–5.4% reduction for CO tailpipe emissions; a 1.3–8.7% decrease in SO₂ tailpipe emissions; and a 5.1–17% decrease in PM tailpipe emissions.

Project objectives

A few laboratory tests have indicated that hybrid transit buses can have significantly lower emissions compared with conventional transit buses. However, the number of studies is limited and laboratory tests may not represent actual driving conditions, since in-use vehicle operation differs from laboratory test cycles.

The objective of the project described in this paper was to evaluate the in-use emission differences between hybrid-electric and conventional transit buses for the Ames, Iowa transit authority, CyRide. This report summarizes the results of a study that evaluated on-road emissions for three hybrid transit and two control buses, as described in the following sections.

Fuel economy was also calculated as part of the study but is not the subject of this paper (Hallmark et al., 2012). Average fuel economy in miles per gallon was calculated for each bus group overall and by season. Hybrid buses had a fuel economy that was 11.8% higher than control buses.

Study background

CyRide, the city bus system for Ames, Iowa, is operated through collaboration between the city and Iowa State University (ISU). CyRide reported 5,447,289 passengers for fiscal year 2011 and posted 1,185,089 revenue miles (CyRide, 2012).

CyRide operates around 79 buses and purchased 12 hybrid transit buses using a Transit Investments for Greenhouse Gas and Energy Reduction (TIGGER) grant. The grants were made available under the America Recovery and Reinvestment Act, which funded programs to decrease energy use and reduce greenhouse gas emissions.

In addition to the hybrid buses, regular diesel buses were selected from among the regular diesel buses in the CyRide fleet to serve as controls. These buses were selected, since they have very similar bus specifications in terms of manufacture, model year, and engine size as the hybrid buses, as shown in Table 2.

Table 2. Specifications of both hybrid diesel buses and conventional diesel buses

Specification	Hybrid Diesel Buses	Conventional Diesel Buses
Engine year	2010	2010
Capital cost	$521,970	$367,115
Manufacture	Gillis Hybrid	Gillig
Bus type	Low floor	Low floor
Engine	Cummins '10 ISL 280 HP,in line six cylinders	Cummins '10 ISL 280 HP,in line six cylinders
Transmission	Voith DIWA Parallel Hybrid	Voith D864.5 4-speed
Aftertreatment	Particulate filter (CPF)	Particulate filter (CPF)
Governed speed	65 mph	65 mph
Start date	June 28, 2010	June 28, 2010
Frontal area	113.5 × 102 ft	113.5 × 102 ft
Length × Height	40 ft × 138 in	40 ft × 138 in
Curb weight	29,500 lbs	25,000 lbs

Data Collection

Data were collected using a portable emissions monitoring system (PEMS), the Axion System from Clean Air Technologies (www.cleanairt.com). The PEMS is equipped with a computer and can be quickly installed on a variety of vehicles, without physical modification to the vehicle. The system is designed for a range of testing scenarios, from short tests in the laboratory to extended field testing. The system also has a global positioning system (GPS) to record the spatial position of the vehicle being tested that can be used to locate where the vehicle was on the roadway during testing.

HC, CO, CO₂, O₂, and NO_x concentrations are sampled using a dual-five-gas analyzer system with a tailpipe probe. The analyzers self-calibrate in the field using ambient air as a benchmark. Particulate matter concentration is quantified using a laser light scattering measurement subsystem. Speed, engine revolutions per minute (RPM), intake air pressure (manifold absolute pressure), and other engine operating parameters are collected to determine intake air mass flow. Using intake air mass flow, the known composition of intake air, measured composition of exhaust, and user-supplied composition of fuel, a second-by-second exhaust mass flow is calculated. The exhaust mass flow is multiplied by the concentrations of different pollutants to provide emissions in grams per second or milligrams per second (Clean Air 2007). The system synchronizes the different data streams (second-by-second engine data, emissions, and GPS).

Frey and Rouphail (2003) conducted a number of on-road emissions tests using a similar PEMS and indicated that the precision and accuracy of the equipment is comparable to that of laboratory instrumentation. They indicated that CO and CO₂ were accurate to within 10% when compared with the measurement of average emission rates for dynamometer tests. They also indicated that NO was measured using an electrochemical cell in the PEMS and reported that NO reported as equivalent NO₂ was accurate to ±10%. PM is measured using a light scattering method, which, according to Frey, is analogous to opacity and as such can be used to make relative comparisons of PM (Frey et al., 2008).

Data were collected for three hybrid and two control buses. Originally the intent was to test three control buses as well. However, the equipment malfunctioned during data collection for the third control bus and the PEMS had to be shipped back to the manufacturer. By the time the equipment could be fixed, it was well into summer and weather conditions were significantly different from when the other buses were collected (early spring). Each bus was instrumented with the PEMS and emissions were collected for at least one operating bus day. In some cases, issues arose with the equipment and data were collected for additional days.

CyRide operates with 12 fixed routes. The fixed routes operate every day of the year except Thanksgiving, Christmas, and New Year's Day. CyRide rotates buses into and off the system to meet peak travel demands. Buses are driven over several routes according to a prescribed schedule (route pattern), depending on when the bus comes into and leaves the system. In general, the buses are randomly cycled through the various route patterns.

Each bus tested was evaluated over the same route pattern. This route pattern utilizes the same driver unless that driver is sick or scheduled for vacation. Table 3 summarizes route characteristics. As noted, the total route is approximately 15.8 miles. Around 35.6% of the route is urban arterial with posted speed limits of 35 or 40 mph. A significant portion of the route is along a collector in a primarily residential area (around 40%) with posted speeds of 30 mph and approximately 11% is in a residential area with posted speeds of 25 mph. Finally, the bus traverses the ISU campus for around 13% of the route. This section of the route is characterized by a significant number of stops and typically lower than average speeds. Table 4 provides an estimate of the amount of time a bus spent in a given speed range for the route. As indicated, the bus was stopped or idling around 43% of the time and traveling >0 to <10 mph for 13% of the trip. The bus was traveling 10 to <20 mph for around 20% of the trip and between 20 and <30 mph for 19% of the trip. Finally, the bus traveled between 30 and 40 mph for about 4% of the trip.

Table 3. Route characteristics

Route	Miles	Fraction of Route
Urban arterial 35 mph	5.08	32.2%
Urban arterial 40 mph	0.54	3.4%
Residential 30 mph	6.22	39.5%
Residential 25 mph	1.79	11.4%
Campus 25 mph	2.13	13.5%
Total route	15.8

Table 4. Speed ranges for route

Speed Range	Average Percent of Trip
0	43%
>0 to <10	13%
10 to <20	20%
20 to <30	19%
30 to <40	4%

Grade could not feasibly be collected using traditional methods, such as surveying, given project resources and the distance of the route. Although the GPS does provide altitude, it is not of sufficient accuracy to calculate grade. As a result, grade was not incorporated into the model. However, no significant grade was present over any of the routes. The entire route pattern was characterized by fairly flat terrain.

A technician was present with the equipment at all times. The technician monitored the equipment for problems and malfunctions and also recorded information such as time and the number of passengers who entered or exited the bus at each stop. Data were collected approximately from 8 a.m. to 4:30 p.m. each day. All emission data were collected in April 2012.

The PEMS was cleaned and calibrated according to manufacturer specifications, which was typically every 1–2 days. Data were downloaded at the end of each day of data collection.

Data reduction

The PEMS outputs emissions, GPS, and other data, such as manifold absolute pressure (MAP), on a second-by-second basis. A spreadsheet was created for each bus using output from the PEMS. Each observation output (row) represented 1 sec of data. Speed and acceleration were calculated by the system's GPS.

Ridership data were collected to the nearest second and were synchronized with the PEMS data sets using time stamp and bus stop. As a result, each row of data had a corresponding passenger load.

Each worksheet was reviewed to ensure data quality. Since there are a large number of errors that can occur with PEMS, each row of data in each sheet was manually checked. The data preparation and quality assurance methodologies are described in the following paragraphs. Data were corrected when possible and invalid data were removed when they could not be resolved.

Potential errors in the data sets have been discussed by Frey et al. (2001), Ensfield (2002), and Saeed et al. (2008). Potential errors were also discussed as they arose with the PEMS manufacturer during the course of data collection. Technicians were present with the data collection equipment and when problems were noted, adjustments or fixes to the equipment were made as needed. If data for a particular time period were suspect, they were discarded. A number of minor issues occurred and were discussed more fully in a final report (Hallmark et al., 2012). A discussion on how relevant errors is provided in the following paragraphs.

Gas analyzer errors result when zeroing occurs during a run and no engine or emission data are recorded during the zeroing event, which leads to data gaps. The researchers found a number of instances when the equipment reported “NA” instead of the corresponding emission value. When this occurred, the data were removed. Due to random measurement errors, concentrations (especially HC with diesel emissions) can have negative values or values that are not statistically different from zero. This occurs during zeroing when the reference air has significant amounts of a pollutant, resulting in negative emissions. When pollution concentrations were less than zero, those data cells were not used. If it appeared that equipment malfunctions were present for a portion of the route, all of the corresponding data were removed and the equipment malfunction addressed. This was based on use of PEMS in other studies. Ensfield (2002) evaluated on-road emissions for 15 gasoline-powered passenger cars and 15 heavy-duty diesel vehicles using an on-board emissions monitoring system. They identified instances where engine and emission parameters were well outside physical limits, including negative and improbably high values. Saeed et al. (2008) evaluated on-road emissions from excavators using the Montana Universal System, which is similar to the PEMS used in this study. During data reduction, observations with missing MAP values, negative pollutant concentrations, and other data errors were flagged as “invalid.” Frey et al. (2001) indicated that negative emission values are typically due to unacceptable instrument drift or improper zeroing of the instrument. When negative emission measurements were present, they either assumed them to be zero or were not included.

In several cases, emission values from one of the two sensors would spike to abnormally high values. For instance, HC values spiked to 100 times the normal values in several instances. The team could not determine the source of the error. If the abnormal concentrations persisted over some period of time and appeared to be equipment malfunction, they were removed. In all other cases, the high values were retained in the data set. Results with and without the high concentrations were compared and the high readings did not appear to affect the results in any meaningful way.

On-Road Emissions Analysis and Results

Vehicle emissions are correlated to instantaneous engine load demand, which is a function of factors such as speed, acceleration, road grade, and air conditioning use. Vehicle-specific power (VSP) has been used as a proxy variable for power demand or engine load (Frey et al., 2007; Zhai et al., 2008). VSP is the instantaneous power per unit mass of the vehicle. Huai et al. (2005) indicates that the advantages of using VSP as an independent variable for studying hot stabilized emissions are that specific power can be directly measured, it captures most of the dependence of emissions on engine operating parameters, and certification driving cycles can be specified in VSP.

After passenger data were entered and data quality assurance was completed, VSP was calculated for each row of data using eq 1 (U.S. Environmental Protection Agency [EPA], 2010).

1

where

VSP = vehicle-specific power in kilowatt/metric ton

v = velocity in meters/second

M = mass in metric tons

g = acceleration due to gravity (9.8 m/sec²)

a = acceleration in meters/second²

sin θ = fractional road grade

A = road load coefficients for rolling resistance (kilowatt second/meter)

B = road load coefficients for rotating resistance (kilowatt second²/meter²)

C = road load coefficients for drag resistance (kilowatt second³/meter³)

Road load coefficients were obtained from the MOVES user guide for transit buses (EPA, 2010): A = 1.0944 kW-sec/m, B = 0 kW-sec²/m², and C = 0.003587 kW-sec³/m³. The bus mass was provided by the manufacturer, as listed in Table 3. Each passenger was assumed to weigh 150 pounds (0.068 metric tons) as used by O'Keefe et al. (2002) and others. Road grade was not collected and could not be included in the calculations, as discussed in the data collection system.

Emission rate, speed, and acceleration are reported by the PEMS in 1-sec interval. The number of passengers between stops was recorded so the passenger load by time was also known. VSP was calculated for each 1-sec inteval using eq 1.

VSP bins were developed by taking into account that bins should have a statistically significantly different average emission rate from each other and no single mode should dominate the estimate of total emissions, as suggested by Zhai et al. (2008). Based upon these considerations, VSP bins were defined using a preliminary analysis of the data for all pollutants considered. The same bin definitions were used for all the pollutants and buses to facilitate comparison. The final bins are shown in Table 5.

Table 5. Bins used to evaluate bus emissions by VSP range

Bin	VSP Range (kW/metric ton)
lower than −10	VSP < −10.5
−10 to 0	−10.5 ≤ VSP < 0
0	0 ≤ VSP < 0.5
1–2	0.5 ≤ VSP < 2.5
3–4	2.5 ≤ VSP < 4.5
5–6	4.5 ≤ VSP < 6.5
7–8	6.5 ≤ VSP < 8.5
9–10	8.5 ≤ VSP < 10.5
11+	11.5 ≤ VSP

Data were averaged for each VSP bin for each bus and standard error was calculated. Results are presented by pollutant in the following sections.

Carbon dioxide

Results for carbon dioxide (CO₂) are shown in Figure 1. At the lower VSP categories (lower than −10; −10 to < 0; 0), CO₂emissions were higher for control buses but are within 0.2 g/sec for the two bus types. Emissions were 0.36 g/sec lower for VSP bin 1–2 and 0.32 lower for bin 3–4 (21% and 11%, respectively) for control buses than for hybrid buses. Emissions were within 1% for the hybrid buses and control buses for VSP bin 5–6. In the higher VSP ranges (7–11+), emissions were 0.67–2.64 g/sec lower for hybrid buses than control buses (13–46%).

Figure 1. CO₂ emissions by VSP bin and bus type.

CO₂ emissions are lowest in the lower VSP bins for both bus types. However, whereas CO₂ emission are highest in the mid-VSP ranges for hybrid buses (bins 3–8), they are highest for control buses in the higher VSP bins (bins 8–11+).

Carbon monoxide

Carbon monoxide (CO) emissions in milligrams per second (mg/sec) are shown by VSP bin in Figure 2. CO emissions were lower for the hybrid buses than the control buses for all scenarios except for VSP bin 0. It is not know why this is the case. It is possible that this is due to computer software tweaks by the manufacturer to optimize fuel economy, which resulted in inefficiencies elsewhere. Emissions were 0.36 mg/sec lower for hybrid buses compared with control buses for VSP bins lower than −10 and 0.74 mg/sec lower for VSP bins −10 to <0 (37% and 62%, respectively). For VSP bin 0, emissions were 1.31 mg/sec lower for the control buses than for hybrid buses (65%). Emissions were lower for hybrid buses than for control buses for all other VSP bins. Emissions were 1.24 and 0.66 mg/sec lower for VSP bins 1–2 and 3–4 (66% and 44%). Emissions were 1.88 mg/sec lower for VSP bin 5–6 (65%). For the remaining bins 7–8, 9–10, and 11+, CO emissions were 3.38, 3.26, and 2.87 mg/sec lower (74–77%).

Figure 2. CO emissions by VSP bin and bus type.

CO emissions varied only moderately over most VSP bins for the hybrid buses except for VSP bin 0. CO emissions were highest for VSP bin 0 and were nearly twice that of any other bin for hybrid buses. In contrast, CO emissions for control buses are lower in the smaller VSP bins (0.71–.21 mg/sec) and then steadily increase in the higher VSP bins (up to 4.60 mg/sec for VSP bin 7–8).

Hydrocarbons

Average hydrocarbon emissions in milligrams per second by VSP bin are shown in Figure 3. In all cases except for VSP bin 0, control buses had higher HC emissions than hybrid buses. As for CO, it is not known why this occurs. As noted in the CO section, this may have been due to software changes made to hybrid buses to improve fuel economy. Differences are less marked in the lower VSP ranges. Hybrid buses had HC emissions that were 0.22 and 0.78 mg/sec lower for VSP bins lower than −10 and −10 to < 0 (17% and 48%, respectively) for control buses compared with hybrid buses. HC emissions are 0.72 mg/sec (41%) lower for control buses than for hybrid buses for VSP bin 0. Control buses have HC emissions that are 1.93–2.98 mg/sec lower for hybrid buses than control buses (72–78%) for VSP bins 1–2, 3–4, and 5–6. Emissions are 4.57–4.99 mg/sec lower for hybrid buses than control buses for VSP bins 7–8, 9–10, and 11+ (around 80% for each bin).

Figure 3. HC emissions by VSP bin and bus type.

Additionally emissions are highest for hybrid buses for the lower VSP bins, with highest emissions at VSP bin 0. Emissions were lower for the positive VSP bins for hybrid buses than for the lower VSP bins. In contrast, HC emissions were lowest for the negative VSP bins and are highest in VSP bins 7–8, 9–10, and 11+, with emissions peaking in VSP bin 7–8 for the control buses.

Nitrogen oxide

Average nitrogen oxide emissions in milligram per second are shown in Figure 4 by VSP bin. As noted, in the lower VSP bins (lower than −10, −10 to < 0, and 0), NO_x emissions are moderately higher for the hybrid than for the control buses (0.52–1.4 mg/sec). Differences were much greater and increasingly larger for the higher VSP bins. Emissions were 2.01, 2.95, and 4.82 mg/sec lower for control buses than for hybrid buses (80%, 78%, and 78%, respectively) for VSP bins 1–2, 3–4, and 5–6. Emissions for control buses were 6.89 and 6.95 mg/sec lower than for hybrid buses (81% and 79%, respectively) for bins 7–8 and 9–10. Emissions were 4.89 mg/sec lower (68%) for control buses compared with hybrid buses in VSP bin 11+.

Figure 4. NO_x emissions by VSP bin and bus type.

NO_x emissions are lowest in the lower VSP bins for both bus types. Emission peak in VSP bin 9–10 for hybrid buses and decline slightly in VSP bins 11+. NO_x emissions for control buses increase steadily from VSP bin 5 to bin 11+.

Summary and Discussion

Hybrid transit buses require a significant investment for transit agencies, with purchase price currently being approximately 50–70% higher than a conventional diesel bus. Reduced fuel consumption and emissions are the two main benefits attributed to hybrid buses that are used to justify the investment. Laboratory tests have indicated that hybrid transit buses can have significantly lower emissions compared with conventional transit buses. However, the number of studies is limited and laboratory tests may not represent actual driving conditions, since in-use vehicle operation differs from laboratory test cycles.

The objective of the project described in this paper was to evaluate the in-use emission differences between hybrid-electric and conventional transit buses for the Ames, Iowa transit authority, CyRide, which purchased 12 hybrid transit buses. Fuel economy was addressed in another paper (Hallmark et al., 2012b), which showed that hybrid buses had an on-road fuel economy that was 11.8% higher than control buses.

Three hybrid and two control buses were instrumented with a portable emissions monitor and each bus was evaluated in-use over a set route pattern in April 2012. Bus ridership was also collected during testing and correlated to emission data. Vehicle-specific power was calculated and emissions were compared by bus type and VSP bin by pollutant.

Results indicate that average CO₂ emissions were lowest for the hybrid buses for all VSP bins except for VSP bin 5/6 where hybrid and control buses had similar emissions. In the lower VSP bins, CO₂ emissions were within 0.2 g/sec. Emission differences were the most pronounced for VSP bins 7–8, 9–10, and 11+, with differences up to 46%. CO₂ emissions were in line with other studies, which have shown 24–40% lower emissions for hybrid buses depending on test type and drive cycle.

Similarly, average CO emissions were higher in all cases for control buses than for hybrid buses except for VSP bin 0 where emissions were 65% lower for control buses. CO emissions were within 37% and 62% lower for hybrid buses than control buses for the lower VSP bins. Emissions were around 65% lower for VSP bins 1–2 and 5–6. Emissions were 44% lower for VSP bin 3–4 for hybrids than for control buses. CO emissions were around 75% lower for hybrid than control buses for VSP bins 7–8, 9–10, and 11+. Other studies have reported 47–98% decreases for hybrid buses compared with control buses for CO. As a result, results are similar to those reported by other studies.

Average hydrocarbon emissions were also higher for control buses than for hybrid buses in all cases except for VSP bin 0 where emissions were 41% lower for control buses. At the negative VSP bins, emissions were 17% and 48% lower for hybrid buses than control buses. Control buses have HC emissions that are 72% and 78% lower for hybrid buses than control buses for VSP bins 1–2, 3–4, and 5–6. Emissions were around 80% lower for hybrid buses than control buses for VSP bins 7–8, 9–10, and 11+. HC emissions have also generally reported as lower for hybrid buses with decreases ranging from 28% to 56%. A few studies showed increases in HC emissions for hybrid buses compared with control buses.

Finally, average NO_x emissions were higher for all VSP bins for the hybrid buses than for the control buses. These results were somewhat unexpected. In the lower VSP bins, NO_x emissions were moderately higher for the hybrids than for the control buses. Differences were around 90% lower for control buses compared with hybrid buses for VSP bins lower than −10 and 0 and were around 80% lower for VSP bins −10 to < 0, 1–2, 3–4, 5–6, 7–8, and 9–10. Emissions were 68% lower for VSP bins 11+ for control buses than for hybrid buses. These results were somewhat unexpected, since other studies have shown decreases of 18–60% depending on the drive cycle and test type.

Study Limitations

The main study limitation is sample size. Ideally 10 or more buses of each bus would have been tested. However, as is the case with many research projects, collection of more data was not feasible with existing project resources.

NO_x emissions were higher than expected for the hybrid buses. In all other studies reviewed (as shown in Table 1), NOx emission were 18–60% lower for hybrids than for control buses. Unexpected results may have been due to unusual operating conditions during testing days (although this is not reflected in the other pollutants) or could be due to unusual factors associated with the individual buses tested that may have been minimized with a larger sample size. The hybrid buses had several adjustments made, since CyRide was not finding the expected fuel economy. The manufacturer adjusted electronic brake pedals, installed new programs, and made transmission adjustments. Since the programming adjustments were proprietary, the team was not able to determine the impact. However, the adjustments may have maximized fuel economy at the expense of NO_x.

Acknowledgment

The team would like to thank the Iowa Energy Center for funding this project. They would also like to thank CyRide for all of their assistance in giving feedback, providing data, setting up emission and GPS data collection, and allowing us to conduct the study. In particular, they would like to thank Sheri Kyras, James Rendall, Rich Leners, and all of the technicians who provided data for their assistance.