Subcontract Report

NREL/SR-7A2-48814

September 2010

Compressed Natural Gas (CNG) 

Transit Bus Experience Survey

April 2009 — April 2010

R. Adams

Marathon Technical Services

Heidelberg, Ontario, Canada

D.B. Horne

Clean Vehicle Education Foundation

Acworth, Georgia
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-7A2-48814

September 2010

Compressed Natural Gas (CNG) 

Transit Bus Experience Survey

April 2009 — April 2010

R. Adams

Marathon Technical Services

Heidelberg, Ontario, Canada

D.B. Horne

Clean Vehicle Education Foundation

Acworth, Georgia

NREL Technical Monitor: M. Melendez

Prepared under Subcontract No. KLFH-9-88664-01
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

This publication received minimal editorial review at NREL

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
1

Executive Summary

The U.S. Department of Energy (DOE) and the National Renewable Energy Laboratory 

(NREL) commissioned this survey to collect and analyze experiential data from U.S.

transit agencies with varying degrees of compressed natural gas (CNG) bus and station 

experience. This information helps DOE and NREL determine areas of CNG transit bus 

success and priority areas for which further technical or other assistance might be 

required to enable success.

Clean Vehicle Education Foundation (CVEF) staff and subcontractors developed a 

battery of questions and identified 10 transit agencies to represent all U.S. transit 

agencies that use CNG buses, accounting for the diversity in characteristics such as fleet 

size, management system, station ownership and operation, and geographic location. The 

survey was conducted onsite.

The study found that the average fuel economy in CNG buses is approximately 20%

lower than in diesel buses, although there is reason to believe that this gap will be

narrower for new CNG and diesel engines. Based on a recent 12-month period—and after 

adjusting for energy content, bus fuel efficiency, station maintenance and power costs, 

and fuel-cost subsidies—the average CNG cost was $1.06 per diesel gallon equivalent.

The survey also captured qualitative information from the transit agencies in the 

following areas:

• CNG bus specification

• CNG bus operations

• CNG station specification/design

• CNG station operation

• CNG garage upgrades.

Transit agencies that are operating newer CNG buses reported adequate power and 

improving fuel economy. The following were reported as the main engine issues 

requiring additional development:

• Reliability of engine sensors

• Further fuel economy improvement

• Increasing the number of mainstream engine suppliers (there is now only one) 

and support for engines supplied by companies that have left the CNG engine 

market

• Addressing high engine temperatures (developments such as electric-drive fan 

systems have addressed some high-temperature issues).

Most agencies had limited concerns related to the design and reliability of their CNG 

stations, although some acknowledge that an inoperable station could ground their fleet

(whereas a bus problem would ground only a few buses). Infrastructure observations 

included the following:
2

• Agencies with gas-engine-driven CNG compressors generally indicated a

future preference for electric-drive compressors to eliminate the high 

maintenance and low reliability related to gas-engine drives.

• Several agencies continue to see oil carryover problems that must be 

addressed through research.

• Agencies stressed the need for redundancy in station equipment and the need 

to ensure that dispensing systems are designed to independently fill all buses 

simultaneously to avoid batching, which slows fueling.

• Agencies employed a variety of CNG station operation strategies—some 

providing maintenance in-house and some contracting out. Agencies tended to 

be very satisfied with whichever strategy they had selected, and there was no 

clear best approach.

Agency-identified needs for government assistance included the following:

• Agencies were universally pleased with the $0.50 per gasoline gallon 

equivalent fuel subsidy, praising the extent of the subsidy and the efficient 

means by which it is delivered. Some agencies wanted the subsidy to be made 

permanent because their bus purchases are 12-year commitments, and they 

must quantify operating costs.

• Agencies identified a need for government to assist engine manufacturers in

entering the CNG market and developing durable products.

• Agencies indicated a need for more durability and fuel economy testing.

• Many agencies expressed a need for help in training existing employees and 

increasing the pool of CNG-trained technicians.

This report is limited to CNG-specific issues and does not address other bus issues. It is a 

summary of information collected from the surveys and thus does not necessarily reflect

the opinion of CVEF or the author, and it is not necessarily exhaustive and complete in 

all areas discussed. Therefore, this report should not be considered a code or a 

specification, and its use should not preclude the use of other resources and professional 

advice.

Acknowledgements

Although the individuals and agencies surveyed and quoted herein are not named (to 

allow them to remain anonymous), the author thanks the many staff members of transit 

agencies who gave their time. The author also thanks Charles (Brooks) McAllister, who 

played a very significant role in the planning of this survey and in the design of survey 

questions.
3

Table of Contents

Executive Summary ............................................................................................................1

Acknowledgements.........................................................................................................2

Table of Contents................................................................................................................3

Survey Background and Purpose ........................................................................................4

Survey Approach and Methodology....................................................................................4

Practical Limitations to Data Collection and Reporting .....................................................5

Analysis and Findings—Quantitative Data.........................................................................6

Fuel-Efficiency Data .......................................................................................................6

Average Natural Gas Fuel Cost.......................................................................................7

Station Power Costs.........................................................................................................8

Station Maintenance Costs..............................................................................................9

Total Delivered CNG Costs ..........................................................................................10

Bus Reliability...............................................................................................................10

Best Practices—Qualitative Data ......................................................................................10

Best Practices Related to Bus Specifications................................................................11

Best Practices Related to Bus Operations.....................................................................13

Best Practices Related to CNG Station Design.............................................................15

Best Practices Related to CNG Station Operation ........................................................18

Best Practices Related to CNG Garage Upgrades.........................................................20

Best Practices Related to CNG Training.......................................................................22

Perception of CNG Buses and CNG Incidents..................................................................23

Public and Staff Perception ...........................................................................................23

Incidents—Bus..............................................................................................................24

Incidents—CNG Station and Bus Garage.....................................................................24

Recommendations.............................................................................................................24

Industry Needs and Concerns Not Directly Related to the Government ......................24

Industry Needs and Concerns for Government Policy and Assistance.........................26

Conclusions.......................................................................................................................26

Appendix A: CNG Transit Experience Survey Template .................................................28
4

Survey Background and Purpose

The U.S. Department of Energy (DOE) and the National Renewable Energy Laboratory 

(NREL) provide technical research and implementation support for a variety of 

alternative fuels, including compressed natural gas (CNG). The transit industry is a major 

segment of the CNG vehicle market, with CNG bus purchases often accounting for 20%–

25% of U.S. transit bus sales over the past 10–15 years. Based on recent NREL data, the 

use of CNG in transit fleets has resulted in a reduction in petroleum consumption of more 

than 200 million gal/yr.

To illustrate lessons learned by CNG transit agencies and to help focus future support of

the CNG market, DOE and NREL sponsored the survey-based collection of quantitative

and qualitative information from a representative sample of CNG transit fleets, including 

the following:

• Vehicle and engine information: bus manufacturer, date purchased, engine 

make and model, fuel storage system design (tank size, make location, etc.), 

total mileage, and drive cycle

• Operational information: total mileage, duty cycle, major maintenance 

issues for each bus, additional preventive maintenance specific to the natural 

gas fuel system and engine, information (causes and solutions) about natural 

gas system failures

• Vehicle and engine modifications to support the natural gas system:

including modifications to the engine required by the original equipment 

manufacturer (OEM) or converter

• Fueling information: fueling station type, capacity, performance history, 

maintenance schedule and history, ownership, fueling location (indoors or 

outdoors)

• Facility information: facility modifications necessary for 

fueling/maintenance of CNG buses

• Other issues: outstanding issues with natural gas vehicle (NGV) technology 

(engines, storage, fueling systems, performance) that require additional 

development. 

• Future work could examine other CNG markets such as garbage trucks, 

school buses, and light-duty vehicle fleets.

Survey Approach and Methodology

Given the depth and breadth of information desired, the project team decided that onsite 

interviews were the most practical and efficient method for collecting data from a 

representative sample of U.S. CNG transit fleets. The team assembled a battery of 

approximately 300 questions and data requests, understanding that not all respondents 

would complete all requests owing to applicability and data availability issues. NREL

and DOE reviewed the survey questions. Appendix A shows the final survey form.
5

The team chose a sample of transit agencies to meet the following requirements:

• Geographic and climatic diversity

• A range of fleet sizes

• Different fleet-management approaches

• Different CNG station operational approaches

• CNG programs with several to many years of experience.

Although both liquefied natural gas (LNG) and LNG-to-CNG stations are used in the 

transit industry, this survey focused solely on CNG stations to maintain consistency in 

the data collected. Of the 11 potential agencies originally selected for the survey, three 

were unable to participate and were replaced with two other agencies, for a total of 10 

survey participants. The participants chosen were representative of the requirements

listed above.

Several fleets operated a combination of heavy transit buses (including 35-, 40-, and 45-ft 

buses—referred to in this report collectively as 40-ft buses), articulated buses (referred to 

in this report collectively as 60-ft buses), and small buses such as cutaway vans for 

paratransit. To keep the focus on the target 35–60-ft buses, paratransit bus data were

removed from the survey data. To the extent possible, the fuel economy data for 40- and 

60-ft buses were recorded separately, although fleet-average fuel data comingles 40- and 

60-ft buses.

Survey questions were emailed to each participant shortly before the onsite survey. The 

onsite survey was conducted in person, usually on the transit property site with transit 

agency (and/or bus operations contractor) staff who are involved with bus maintenance, 

station operation, and general operations management. After the survey interview, the 

interviewer transcribed the information collected into a spreadsheet. This spreadsheet was 

emailed to respondents so they could confirm the transcribed data and provide data 

unavailable at the time of the survey.

The transcribed files were compiled into one master file for analysis. This master file is 

not provided with this report to maintain the anonymity of participants.

This final report summarizes data collected, itemizes lessons learned and best practices, 

and provides operational statistical information where available. This information 

informs current or prospective CNG transit agencies of design and operational issues that 

might affect the successful implementation of CNG in their fleets. This report also 

identifies areas in which further development of technology or policy could assist in the 

implementation of CNG as a transit bus fuel.

Practical Limitations to Data Collection and Reporting

In designing and executing a targeted small sample survey such as this, the project team 

acknowledged the limitations of data collected, including the following:

• Although many questions were asked, respondents tended to focus on items 

that are current issues. Thus, this report tends to reflect current issues and 
6

experience rather than issues that are no longer considered relevant or 

pressing. For example, a fleet may be experiencing an issue with older 

technology, but, because they understand the issue and know it is related to 

obsolete technology, they might not comment on it.

• All agencies were not expected to respond to all questions. Some data do not 

apply to some agencies, and some data that are not tracked directly would 

require prohibitive amounts of time to assemble. Efforts were made to collect 

readily available data without imposing onerous requests on the participants.

The project team also moved toward an emphasis on qualitative or anecdotal 

information—these “war stories” or lessons learned often summarized the 

most critical issues faced by agencies.

• There was interest in benchmarking CNG against diesel in terms of operating 

costs, fuel economy, and reliability. However, this comparison was found to 

be impractical with the sample fleets because many have retired all or most of 

their diesel fleet and, in many cases, the diesel buses that remain are not 

comparable to the CNG buses.

• There were limitations in comparing CNG fleets with each other in some 

areas. For example, different fleets calculate the commonly used benchmark 

mean distance between failure (MDBF) using widely varying criteria; a range 

of MDBF values from 1,778–7,376 miles was observed, precluding direct 

comparison.

• Recommendations in this report are a summary of responses and may not 

provide a comprehensive, exhaustive source of all best practices or industry or 

code standards. Readers are encouraged to use this report in combination with

other sources and professional advice.

Analysis and Findings—Quantitative Data

This survey included 10 transit agencies with CNG fleet sizes of 15–2,509 CNG buses.

The survey covered a total of 24 garages and 4,071 CNG buses.

To facilitate easier comparison with diesel data, CNG consumption is converted to diesel 

gallon equivalents (DGE) using a conversion of 137 standard cubic feet (scf) or 1.37 

therms of CNG per DGE.

Fuel-Efficiency Data

Benchmark drive train/fuel efficiencies (i.e., fuel economies) were taken from data 

supplied by North American Bus Industries for a number of bus manufacturers tested 

over several years at the Altoona Bus Research and Testing Center. These data may not 

exactly reflect the duty cycle of the buses in a given fleet application; however, the 

various tests were performed under controlled conditions, so the relative fuel economy 

from the tests should be accurate. Altoona CNG fuel economy lagged diesel by 20%, 

although anecdotal industry experience has indicated that CNG is closing the gap with 
7

diesel in terms of fuel economy.

1

Historical-Test Fuel Economy Data

The historical Altoona figures are summarized in the 

table below:

Altoona Bus 

Testing 

(mpg/mpg DE)

Efficiency 

Compared 

to Diesel

40-ft bus

Diesel 4.33 100%

CNG 3.48 80%

60-ft bus

Diesel 3.16 100%

CNG 2.51 79%

Fuel economy data collected in the survey ranged from 2.5 mpg DE (miles per gallon 

diesel equivalent) to 3.9 mpg DE, with an unweighted2 average of 3.2 mpg DE3

The previously mentioned value is based on raw numbers for total miles driven and total 

fuel consumed within each fleet for which data were provided, so this is a bulk average 

using a recent 12 months of data. The survey also asked “What is your average fuel 

economy?” In all cases for which both raw numbers and calculated fuel economy data (as 

reported by the agency on the survey) were provided, the raw data indicated higher fuel 

economy than the calculated data—by as much as 1 mpg DE. Thus, it appears that the 

CNG fleets are delivering higher fuel economy than the operators believe. The drivecycle fuel economy testing recommended in this report would help to determine a 

realistic fuel economy for new CNG bus fleets. 

—

corresponding closely with Altoona testing. Although this average does not include lightduty vehicles such as cutaway vans, some 60-ft articulated buses are included. The

survey attempted to collect data on individual bus/engine combinations to determine the 

effect of newer technology; however, this information was, in most cases, unavailable 

without significant analysis on the part of the agency. Recent tests at Altoona are a source 

for this type of data based on controlled testing and well-characterized bus technology.

As previously indicated, there were very few diesel buses left in the fleets included in the 

survey. Those that remained were not typically operating side by side with CNG buses, 

so a comparison of fuel economy would be flawed. 

Average Natural Gas Fuel Cost

The average fuel cost was calculated over a recent 12-month period by dividing the total 

gas cost by total gas use for each agency. The range of costs was $0.71–$0.94/DGE, with 

 1 New CNG engines are achieving efficiency penalties of less than 10% compared with their diesel 

equivalents in full-load laboratory engine dynamometer testing (data from Cummins for CNG and diesel 

ISL engines); there is a need to use chassis/drive-cycle testing to compare the fuel economy of current 

CNG and diesel buses.

2 This average does not adjust for the number of buses in various fleets and thus it is “unweighted”—the 

per-bus fuel economy for a 50-bus fleet is given the same weight as the per-bus fuel economy of a 2,000-

bus fleet. This approach more accurately reflects drive-cycle and bus-equipment differences between fleets. 3 Caution must be observed in calculating fuel economy. If gas engines are used to power the CNG 

compressors, their fuel consumption must be either backed out of the total or separately metered. Not 

accounting for this could increase the apparent bus fuel consumption by approximately 5%–10%. This is 

addressed in the fuel economy numbers presented herein.
8

an unweighted average of $0.85/DGE, for commodity gas delivered to site.

4

These fleets purchased little or no diesel fuel, so no direct fleet comparison of diesel cost 

to CNG cost was attempted. The study did not research outside fleet diesel fuel costs for 

comparison, although this would be useful for future studies.

Several 

factors account for this range (outlined below); there is also additional cost for properties 

that are more remote from the source.

The 12-month period used was closely overlapping but not identical for all agencies. This 

influences fuel costs because the cost of natural gas has been unusually volatile during

the past 2 years.

A number of gas commodity purchasing plans are used by the agencies surveyed, 

including the following:

• No contract—monthly purchases from the local utility

• Six- to 12-month contract for half or all of expected consumption, additional 

fuel purchased monthly at spot prices

• All commodity gas purchased through contract, usually with a state agency 

managing purchasing for a large group of state and municipal fleets.

There does not appear to be a singular correct approach to gas purchasing because each 

approach can be the low-cost method, depending on the timing and trajectory of gas 

commodity costs. In the time frame included in this survey, it appears that agencies that 

purchased gas monthly had lower gas costs than those that contracted. This is intuitively 

correct because the past 12–18 months have seen a rapid decline in gas commodity prices 

followed recently by a modest rebound. Conversely, contracts may have been signed at 

higher prices, and new contracts would price in this volatility by charging a premium to 

cover the risk of commodity price escalation.

Agencies that had part or all of their gas under contract enjoyed protection from spiking 

prices in 2008, and they reported an additional benefit of being able to accurately budget 

for fuel costs over the next 12 months.

Station Power Costs

The survey collected a small sample of data on electrical costs to operate a CNG station 

from agencies that meter this power separately from the rest of their facility. The costs 

include energy and demand charges, metering charges, and any penalties for peak-period 

operation. The costs were $0.13–$0.23/DGE, with an unweighted average of 

$0.18/DGE. Electrical costs are driven primarily by several factors:

• Certain areas, such as California and New York, tend to have very high 

electrical costs and very high peak-period charges. This factor is essentially 

out of the agency’s control.

 4 Caution must be exercised in calculating the gas cost because there are agencies that bundle this cost with 

maintenance and capital recovery for station construction. It is believed that the costs provided are net of 

these factors, which are addressed separately. The costs include any pipeline or utility transportation and 

metering charges.
9

• Station inlet gas pressure can have a marked effect not only on the power 

required but also on the capital cost of compression equipment. Using all 

available gas pressure at a site, or bringing in higher-pressure gas, can pay 

dividends initially, and every day the station is operated by reducing the 

compressor horsepower required to provide a specified flow. However, there 

may be an additional “pressure charge” from the gas utility, or a capital cost to 

pay for the extension of high-pressure lines to the site, for any increase in 

normal service pressure.

• Hours of station operation/type of operation affect the electrical cost.

Agencies may consider limiting the amount of station equipment permitted to 

operate during high or mid peak periods to control this aspect of the cost. This 

factor should be considered in the design of a new station.

Of the surveyed agencies, the fueling sites included 11 electric-drive compressor stations 

and 13 gas-engine-driven compressor stations. Although the survey team was unable to 

collect actual natural gas consumption and cost data for natural-gas-engine-powered 

compressors, previous calculations have indicated that the engines consume 5%–10% of 

station throughput. Based on the average commodity gas cost of $0.85/DGE, the fuel gas 

cost would be in the range of $0.043–$0.085/DGE. As was the case with electric motors,

this is influenced by local gas rates, inlet gas pressure, and the efficiency of the station 

design. Although the energy cost of using gas engines is attractive, there are significant 

other costs and complications, which are addressed later in this report.

Station Maintenance Costs

Care was taken in collecting maintenance costs, because there are many factors that could 

skew the costs from one agency to the next, such as the following:

• Agencies that perform most of their own maintenance did not always include 

this staff time in the maintenance total. This was added to these numbers to 

ensure uniformity.

• Some agencies that contract out maintenance on a throughput-charge basis 

have capital-recovery and/or power costs included in these numbers. Because

these individual cost components were not available, these numbers were 

removed from the average calculation and are addressed separately.

• Some agencies have an effective extended warranty included in their bundled 

maintenance costs. These total costs were used for analysis because agencies 

that performed their own maintenance included all subcontracting and parts 

even if those included major repairs in a given year. Costs were averaged over 

a 3-year period to smooth out unusual or infrequent expenditures.

Comprehensive station maintenance cost was calculated on a per-DGE basis for those 

agencies not currently contracting on this basis and was compared with agencies that did 

contract on a per-unit-of-throughput basis. The range of values was $0.06–$0.23/DGE. It 

is believed that the $0.06/DGE number may not be fully representative; removing this 

value from the range results in an unweighted average cost of $0.18/DGE. Some of the

maintenance data involve very-long-term station maintenance contracts, whose cost may 

not be reflective of the current market. Work done outside of this survey suggests that 
10

current maintenance contracts are generally in the $0.20–$0.30/DGE range. One agency 

reported much higher rates, but this included power and capital recovery.

Total Delivered CNG Costs

Based on the cost factors listed above, and assuming that a sum of average costs is a valid 

total cost, the average compressed cost of CNG at the nozzle is $1.30/DGE (using 

$0.27/DGE for maintenance) without accounting for capital cost recovery, which may be 

primarily paid through non-agency funds (e.g., Federal Transit Administration funding).

For accurate comparison to diesel, the difference in fuel economy must also be included.

Using a 20% efficiency penalty results in an energy- and efficiency-equivalent cost of 

$1.63/DGE for CNG. The current $0.50 per gasoline gallon equivalent (GGE) fuel tax 

credit would reduce this cost by $0.57/DGE to give a net energy—and efficiency—

equivalent cost of $1.06/DGE. It should be noted that in many cases the price for diesel 

includes only the commodity cost, applicable additives plus delivery, and any surcharges 

that might be levied in a given state. Ongoing maintenance is traditionally absorbed in a 

general shop maintenance budget and not calculated in the overall per-diesel-gallon cost. 

Bus Reliability

Most respondents stated that diesel buses would achieve higher MDBF than CNG buses.

In fleets where both diesel and CNG data were available, the CNG MDBF was as low as 

58% of the diesel value and as high as 80% of the diesel value. Respondents indicated 

they were not surprised by this statistic because the conventional (older-technology) 

diesel buses being compared to the CNG buses are simpler, with no ignition systems and 

very few engine sensors—the two areas that cause most CNG breakdowns.

MDBF addresses the entire bus and not just the CNG components, leaving the possibility 

that CNG buses have non-CNG components that might be reducing their MDBF. Several 

agencies estimated that about 30% of their breakdowns were related to CNG 

components—primarily sensors and ignition. Adjusting for this 30% would make CNG 

and diesel MDBF approximately equivalent. Two respondents expressed concern that 

new clean diesel buses using sophisticated exhaust aftertreatment systems are likely to be 

similar, or perhaps lower, in reliability than current CNG buses. This reinforces the 

importance of comparing equivalent-generation technologies to make robust comparisons

between diesel and CNG buses. With 2010 bringing a significant change in diesel 

technologies, previous data must be used carefully.

Best Practices—Qualitative Data

The qualitative survey data and some numerical data have been distilled into 

recommended best practices related to new equipment purchases. Many of the 

recommendations below have become common practice in the industry and are often 

included in the applicable specifications. The recommendations are a summary of advice 

and comments provided by survey respondents—they are not intended to be exhaustive 

specifications. Readers of this report are encouraged to consult other sources of 

information, including qualified professionals, when specifying or operating CNG 

equipment.
11

Best Practices Related to Bus Specifications

Roof-mounted cylinders

Roof-mounted cylinders are widely used to facilitate lower floors in modern transit 

buses. The placement of cylinders on the roof has also proven to be desirable from a 

cylinder-safety perspective because agencies feel that these cylinders are less 

vulnerable to impact from road debris or collision. Although less vulnerable to 

normal hazards, several agencies related stories of impacts when the vehicles were 

being towed or when they were redirected to unapproved routes—standard 

operating procedures should be developed and implemented to mitigate these 

impact risks. Although not specifically raised as a current problem, given the 

history of problems with old pressure relief device (PRD) designs and installations, 

bus purchasers should ensure that PRDs are installed and vented in a manner that 

ensures that the gas release blows clear of the bus and cannot accumulate moisture 

on the vent side of the PRD.

20-year-life cylinders

Given that transit agencies are experiencing budget compression, a number of 

agencies are contemplating extending their bus life beyond the originally planned 

12–15 years. If buses are kept beyond 15 years, the 15-year life on the cylinders 

will expire, and there is no mechanism available to extend the life of these 

cylinders. Agencies are recommending the purchase of 20-year-life cylinders on

new buses to guard against this potential future cost.

Maximized gas storage

A number of agencies promoted equipping buses with the maximum number of gasstorage cylinders possible (typically eight cylinders per bus with approximately 

3,000 scf each, a total of 24,000 scf on a 40-ft bus). This additional capacity ensures 

that all buses can operate on any route. The additional cylinders also allow some of 

these agencies the possibility of skipping a night’s fueling in the event of a hard 

station outage.

Cylinder types

Transit agencies often had a mix of buses, some with Type 3 gas storage cylinders

(lightweight aluminum liner with fiber overwrap) and some with Type 4 cylinders 

(polyethylene liner with fiber overwrap). None of the survey respondents indicated 

any concern or strong preference for one technology over the other.

External solenoid valves on each cylinder

Most CNG buses are now equipped with solenoid valves on each cylinder, and they

should be incorporated into the specification. These solenoid valves greatly reduce 

the maximum potential release or leak if a bus vents gas. This is particularly 

important for buses entering buildings.

Addressing higher operating temperatures

To address the higher temperatures in CNG engines and exhaust systems (the 

energy that is not converted to power—the efficiency penalty—is converted to 

heat):
12

• Some agencies have had to retrofit older buses to reduce heat trapped in 

compartments housing exhaust system components.

• On new designs, bus manufacturers should be required to validate the 

installation of natural-gas-powered engines into buses to ensure that a 

buildup of heat does not occur.

Hydraulic line placement

Specifications should require the placement/routing of wiring and hydraulic lines, 

particularly in the engine compartment, in a manner that minimizes the possibility 

that the lines or wires would be compromised by the high heat in that compartment.

Damaged hydraulic lines have been the cause of many of the fires that have 

occurred in CNG and diesel buses.

Fire-extinguishing system

Most agencies surveyed include a fire-extinguishing system on their CNG buses—a 

practice shared with most diesel buses. CNG systems often include gas (leak) 

detection sensors in the cylinder compartment and engine compartment with an 

alarm at the driver console. Dry-chemical extinguishing nozzles are generally 

provided in the engine and exhaust system compartments. Respondents advised that 

the location of the nozzles be reviewed because some of their buses had nozzle 

positions that did not address the highest-risk areas—the turbo and the exhaust 

manifold. Most of these systems include a manual release, often placed above the 

front bumper. Some early CNG buses with fire-extinguishing systems included 

nozzles in the cylinder compartment; this practice generally has been abandoned 

because there is little risk of a fire starting in this compartment.

Survey respondents indicated that bus fires are unusual on both diesel and CNG 

buses, and most respondents did not consider CNG buses to be any more likely to 

have a fire than a diesel bus; most did not consider their own CNG bus fires to have 

been CNG related. Fires on diesel and CNG buses are typically attributable to the 

following general categories:

• Exhaust system heat that builds up and causes the ignition of adjacent 

materials

• Hydraulic system leaks on hot components such as exhaust manifolds and 

turbochargers

• Electrical-system short circuits

• Operator error (brake or tire fire) or an accident.

Electric accessory systems

Four of the 10 agencies surveyed are testing electric accessory systems retrofitted to 

buses. The systems are supplied by Engineered Machined Products, Inc. and 

include a larger alternator, larger battery pack, and electric-drive radiator fan bank.

All the agencies surveyed were positive to enthusiastic in their assessment of such a 

system, which is reported to improve fuel economy in excess of 5%, improve

battery life (by keeping batteries charged more fully), and significantly reduce the 

possibility of an engine-compartment fire by eliminating the hydraulic hoses to the 
13

fan. All the agencies that are testing these systems indicated that they will specify 

this system on future bus orders. Two agencies indicated interest in expanding the 

system to include other parasitic loads (air conditioning compressor, air 

compressor, and power steering pump).

Fueling location

The fueling location on buses is typically curbside rear. Some early designs had the 

fill point on the rear of the bus and too high for easy access—most are now 

standardized to the side, in the area of the engine compartment. Most agencies 

require the fuel compartment door on the bus to be equipped with a proximity 

switch to prevent the bus from starting if it is shut down or to activate an alarm at 

the driver’s console and shut the bus down after 15 minutes if the bus is running.

Immediate shutdown was not recommended because agencies have experienced 

opening of this compartment by the public at bus stops.

Fueling connections

Most agencies now purchase buses with both the OPW 5,000 (high flow) and the 

NGV1 (light-duty vehicle) connectors. The light-duty vehicle fueling receptacle 

allows for off-site fueling, and, if the transit fueling station is equipped with a 

defueling system, the bus can be partially or fully defueled through the NGV1 

receptacle using an OPW defueling nozzle. The NGV1 receptacle should be the 

type with an internal check valve (rather than an in-line external check valve) for 

this defueling nozzle to function correctly.

Gas flow tubing

Gas flow tubing from the fueling connection to the rooftop cylinders is 

recommended to be a minimum of 3/4-in outside diameter to minimize pressure 

drop and fill time. Connections to individual cylinders may be 3/8-in or 1/2-in

outside diameter tubing, depending on the bus manufacturer. Valves and other 

components in the gas fill path should be selected to optimize fill speed.

Coalescing filters

Buses are now typically equipped with high-pressure and low-pressure coalescing 

filters on the lines feeding the engine. These filters are designed to operate at engine 

fuel gas flow rates and are intended as a last line of defense to remove CNG-station 

compressor oil from the gas stored on the bus. Several agencies have reported 

compressor-oil-related problems on engines—primarily with sensors and injectors.

Specifying oversized filters (preferably by requiring the filters to be rated for at 

least twice the maximum flow rate at the minimum expected gas pressure) on the 

bus, and that each filter is equipped with a manual drain valve that is easily 

accessible and robust enough to facilitate frequent drainage, will ensure that the gas 

stream is as oil free as possible.

Best Practices Related to Bus Operations

The overriding comment from many agencies was that, unlike conventional diesel 

technology, CNG engines are less forgiving of extended service intervals. This is 

particularly true of ignition-system maintenance (plugs, wires, and boots) because a 

failure of these components can result in a melted piston or other expensive problem. Bus 

operators are therefore strongly encouraged to follow manufacturer-recommended 

minimum service intervals and be vigilant in identifying buses that require unscheduled 
14

repairs—power loss is often the first sign of an impending problem. This section outlines

respondents’ typical approaches to several common maintenance issues, but agencies are 

encouraged to closely follow manufacturer recommendations if they are more stringent 

than common industry practice.

Fuel filter maintenance

Agencies reported a wide variety of bus gas filter-draining and media-replacement 

intervals, ranging from daily while fueling (which is reportedly advocated by one or 

more bus manufacturers), to monthly, to approximately each 6,000 miles 

coordinated with engine oil changes. The need for filter drainage is very dependent 

on the type of compressor oil used and the sophistication of the filtration system at 

the CNG station. It is not practical to recommend one interval as the best practice;

rather, based on suggestions by respondents, it appears that daily or weekly draining 

may be required initially (particularly because several agencies reported high 

quantities of oil from the trip from the bus manufacturer to the transit agency), then,

as experience dictates, this interval could be extended to monthly or to coordinate 

with oil changes or other scheduled maintenance. The main determinant in 

extending the inspection interval is the quantity of oil collected; only very small 

quantities should be present at the time of draining. Using the manufacturer’s 

recommendation as a guide, the experience of the fleet personnel should be the 

dominant factor in determining the selection of the maintenance frequency. This 

area may benefit from additional independent research. Additional comments are 

provided below in the station design and operations section.

Spark plug replacement

Spark plug replacement intervals ranged from 10,000–36,000 miles, with 18,000 

miles being the most common. Intervals depended on engine type, and several 

agencies have adopted the use of iridium plugs to extend plug life. One agency 

indicated that, when it first used iridium plugs, it experienced very low life until a 

“clean” procedure was implemented—ensuring that the plug area was vacuumed 

and wiped, the boot was clean, and the new plug was not soiled prior to installation. 

This procedure resulted in a marked improvement in plug life to 30,000 miles.

Many agencies reported that, although they try to keep plug change-out intervals at 

18,000 miles or higher, they are often forced to change plugs at 10,000 miles. Other 

tune-up work, including valve lash adjustment, is generally scheduled to coincide 

with spark plug replacement at 18,000 or 36,000 miles.

Turbo chargers

There has been a significant history of failure of turbo chargers on Detroit Diesel 

Corporation (DDC) Series 50 CNG engines. These failures have escalated to bus 

fires on several occasions. Several agencies have attempted to implement proactive 

scheduled replacement campaigns to replace turbo chargers before failure. This was 

reported to be unsuccessful because the turbo life proved difficult to predict, and 

many failed before replacement. These agencies have reverted to a run-to-failure 

approach to this problem.

Hydraulic hoses

The hoses and wiring inside the engine compartment should be inspected at regular 

maintenance intervals and replaced if there is indication of wear or breakdown. One 
15

of the agencies indicated that it proactively changed hydraulic hoses within the 

engine compartment to reduce the risk of fire from hoses that have degraded.

Cylinder inspection

Most agencies have their own staff (or the staff of their bus operations contractor) 

trained to perform the required in-house inspection of CNG cylinders. The training 

program is provided by several entities and covers inspection techniques, various 

levels of damage, and safe cylinder disposal if a cylinder damage level is 

determined to require removal from service. Whether an agency uses its own staff

or outside contractors, it is very important that:

• All cylinders are inspected at 36-month or 36,000-mile intervals as 

required by the governing code (NGV2).

• Records for each cylinder inspection are kept on file until the bus is retired 

from service or longer if there is concern the cylinders may be repurposed.

Best Practices Related to CNG Station Design

A major concern that causes agency operation personnel to “lose sleep” is the operational 

reliability of the CNG station. Several agencies noted that a bus problem may ground one 

or more buses on a given day, but a serious station problem may prevent fleet rollout.

This concern has driven the need for redundancy of certain components, as noted below.

Expansion capability

In addition to providing some level of redundancy (as outlined below), agencies 

reported that they would advise new station owners to design for expansion 

capability. This might be done in several ways:

• Over sizing the station initially—this must be done carefully because

equipment that is too large will tend to cycle more, causing reliability 

problems.

• Sizing the dryer, piping, and electrical systems so that the redundant 

compressor(s) can be operated simultaneously with all other compressors.

• “Roughing in” an additional pad and services and sizing the dryer, piping,

and electrical systems for a future additional compressor(s).

• Providing space for additional dispensers and compressors to be installed 

at some future date.

Inlet dryer

An inlet dryer with a manual or automatic regeneration package is generally 

required to meet code-mandated gas quality and to prevent condensation problems

that would impact fueling and bus performance. Although both types of dryer will 

prevent condensation problems, agencies should give consideration to using a 

single-tower or twin-tower manually regenerated dryer to reduce initial cost and to

reduce the complexity of the dryer—leading to higher reliability. Manually 

regenerated dryers may be the best option in small- to medium-sized fleets and 

where gas water content is low.
16

Compressor size

Compressors are generally sized to meet the average fuel load per bus, multiplied 

by the number of buses fueled per night, divided by the productive time during a 

fueling shift. It is recommended that stations be equipped with enough compressors 

that the flow can be “scaled” to the fueling demand. For example, using one large 

compressor may require the compressor to cycle (load and unload) frequently when 

buses are fueling at less than design conditions, whereas using two half-size 

compressors allows the station to operate with one compressor only under partial 

load. Reduced cycling increases compressor reliability, and this arrangement may 

reduce electrical demand charges and increase the redundancy of the station.

Redundant compressors

It is preferable to provide redundancy by adding at least one more redundant 

compressor. For example, if the required flow to fill a fleet in the allotted time is 

2,000 scf per minute (scfm), then a preferred design would be to supply three 

1,000- scfm compressors—two to meet the fueling need plus one for redundancy.

This arrangement typically will be more reliable and potentially lower cost 

providing two 2,000 scfm compressors—one for flow and one for redundancy.

Filtration system

Gas-filtration-system design is often overlooked but can pay large dividends. Most 

natural gas compressors inject oil into the gas during compression to lubricate 

compressor parts. Most agencies reported evidence of oil downstream of the 

compressors. In some cases, the quantity of oil was extreme, and the presence of oil 

is commonly cited by engine manufacturers in refusing warranty claims. With 

regard to the filters required to remove oil from the gas stream (note that other 

filters on the dryer and compressor inlet are also required), the best practice noted 

by agencies in this survey and best practice in the industry is to:

• Install interstage separators on compressors that are automatically drained 

to a collection tank

• Specify a pre-coalescing filter and a coalescing filter in series on each 

compressor discharge—automatically drained to a collection tank

• Design and orient storage to allow drainage of any condensate in each 

vessel

• Include with each dispenser one or two coalescing filters in series that can 

be manually drained

• Oversize filters by a factor of two or more (on a flow basis) to increase 

efficiency and increase media life

• Specify filtration systems to protect against damage to the media from 

rapid depressurization.

This level of filtration is conservative but will provide the best chance to minimize 

oil carryover levels to the buses.
17

One other issue related to filtration is the use of non-lubricated compressors. At 

least two brands of non-lubricated compressors are still available, although less 

widely used, in the transit CNG station market. Non-lubricated compressors can 

still experience oil carryover because oil migrates from the crankcase into the 

compressor cylinders; thus, although the volume of oil should be significantly 

lower, non-lubricated compressors still require oil filtration in the gas stream.

Station owners have reported mixed reliability with non-lubricated compressors;

thus, this report does not make a purchasing recommendation for or against nonlubricated compressors.

Electric compressors

Seven of the 10 respondents either currently or previously owned CNG stations 

with natural-gas-engine-driven compressors. Two of these agencies have already 

converted to electric-drive compressors, and two more indicated that they would go 

electric if they could do it over. Electric-drive compressors have several advantages 

over engine-driven packages: they are smaller, quieter, more reliable, more durable,

have lower capital and maintenance costs, and require no special environmental 

permit. Engine-driven packages have the advantage of lower energy cost, no 

demand cost, and no need for a large power upgrade onsite during installation.

Overall, the current industry trend is toward electric-drive packages. One of the 

agencies that converted to electric from gas implemented several energy-costsaving technologies in its design to shift load away from peak periods.

System design

Two agencies expressed frustration with the design of the dispensing system. The 

system in question was a simple manifold that connected all of the compressors to 

the buffer storage and to all of the dispensers. When the system operates, it fills all 

of the connected buses at the same time, which results in a batching of the buses, in 

turn resulting in inefficient use of hostler labor. Several alternative dispensingcontrol systems are available—specifications should clearly require a system that 

ensures independent and simultaneous fueling of each bus and avoids batching.

Transit fleet personnel specifying the installation of the dispensing system should 

carefully review the needs of their fleet to assure a proper match because this 

decision has significant functional and economic implications.

Standby generator

Given the current trend toward electric-drive compressors, there is also a trend 

toward the inclusion of a diesel or natural gas powered standby generator, either 

permanently installed or trailered to the site to address power outages that would 

prevent fueling.

Enhanced infrastructure design

Although respondents that contracted out the maintenance of their stations had 

lower expectations for the level of CNG station control and instrumentation 

sophistication required, several of those that maintained their own equipment 

recommended “all of the bells and whistles” to allow enhanced remote support and 

troubleshooting. Current industry best practice that provides these “bells and 

whistles” is to include analog instrumentation of critical pressures and temperatures 
18

in the dryer, compressors, storage, and dispensers with a controller that can be 

remotely accessed for monitoring and control over the Internet.

Best Practices Related to CNG Station Operation

Respondents generally indicated satisfaction with the reliability of their CNG stations.

Agencies that manage their own maintenance were more aware of the various 

breakdowns and were more involved in rectifying station-down situations. These 

agencies generally indicated that the major station equipment operated with reasonable 

reliability.

Instrumentation

The area mentioned most frequently as a cause for station faults was 

instrumentation. It is recommended that agencies specify high-quality components 

that have a long history of successful use in the CNG industry. It is also 

recommended that analog components (such as thermocouples and pressure 

transducers) be used to replace discrete mechanical switches. The analog devices

have no moving parts, which enhances reliability and durability and enables remote 

monitoring—allowing agencies to have a remote contractor diagnose and correct 

some faults.

Inventory of key components

The agencies performing their own maintenance advised the purchase of an 

inventory of commonly used components including transducers, fuses, filter 

cartridges, control valves and actuators, relief valves, compressor valves, rings,

packing, and other parts that the manufacturer might recommend.

Inlet gas drying/filtering

Several agencies reported the presence of a water/oil mixture that degrades the 

performance of engine sensors. This situation requires vigilance to ensure that the 

station inlet gas dryer is fully functional and that the coalescing filtration system is 

adequately designed and maintained to ensure that oil is being effectively removed 

from the gas stream. Many stations with older compressors and dispensers may lack 

an adequately designed filtration system and may require an upgrade.

Although not specifically mentioned by any of the survey respondents, it is highly 

desirable to track the oil added to the compressors and similarly to track oil drained 

from the various condensate receivers, filters, and storage. It is recommended that 

this tracking be done on an ongoing basis and based on the weight of oil added and 

drained—due to difficulties in accurately measuring the volume of oil. Over time,

these records (kept in a spreadsheet) can be used to determine the efficiency of the 

filtration system.

Two respondents indicated that their dryers had been bypassed for extended periods 

due to unresolved repairs. Unlike the compressor, full dryer operation is not 

required to fuel buses—thus there is a temptation to de-prioritize dryer problems.

This is potentially dangerous because there have been dryer fires and other 

incidents linked to inadequate service. Also, there can be nuisance problems in the 

station and on the bus related to moisture in the gas, and there is potential for 

catastrophic failure of some components if moisture is present. The desiccant 
19

degrades over time and with each regeneration. Most dryer manufacturers claim a 

5-year lifespan on desiccant, at which point it will need to be safely disposed of (as 

a hazardous waste) and replaced with new desiccant, which should be immediately 

regenerated. Proper dryer service and repair is essential to meeting fuel 

specification codes—including SAE J1616, CARB, and NFPA 52—and to ensure 

that agencies do not experience collection of an oil-water emulsion that may plug 

bus fuel filters.

Compressor oil

CNG stations have long been known to contribute oil carryover, causing vehicle 

fuel-system problems. Previous research has indicated that compressor lubricator 

oil type would be a strong contributing factor in the degree of oil carryover.

Conventional mineral-based oils were known to vaporize under the pressure and 

heat present in a compressor; this vaporized oil cannot be filtered with coalescing 

filters, which will only remove fine mist or aerosol oil. Synthetic oils were less 

volatile and therefore less likely to cause oil carryover because the oil remains in 

liquid form and can be filtered, assuming the filtration system is adequately and 

conservatively designed. Based on research at the Gas Technology Institute in the 

1990s, a number of CNG station manufacturers and station operation contractors 

have adopted the use of polyalkylene glycol (PAG) based synthetic oils. PAG oils 

are very resistant to vaporizing in CNG applications, which allows them to be 

filtered very efficiently; however, they have shelf-life issues, and they are 

incompatible with some paints (used in crankcases), some seal materials, and 

mineral-based oils. 

Most respondents used synthetic oils. About half used PAG oils, which can be a 

viable solution for a new bus fleet with no history of mineral-based oils. The other 

half used Diester-based synthetic oils, which are more volatile than PAG oils but 

are also more forgiving because they are compatible with both PAG and mineralbased oils and do not have the solvent effect that PAG oils have on some paint and 

seals. Some industry maintenance personnel suggest the use of Diester oils if an 

agency is looking to transition to PAG in the future.

Any decision to change oil types should be approved with the oil manufacturer, the 

compressor and CNG station manufacturers, and the bus and cylinder 

manufacturers.

Station maintenance

Many agencies contract some or all of their CNG station maintenance work to third 

parties. This approach is favored by some agencies because there is a high technical 

skill requirement that is outside of the agency’s core competency and mission—to 

operate reliable bus service. Respondents generally adopted one of the following 

approaches:

• Provide all or most of the CNG station maintenance using in-house 

personnel

• Provide light maintenance (walk-around inspections and possibly oil 

changes) using in-house personnel and contract specialized or more 

invasive maintenance and repair tasks on a fee-for-service basis
20

• Contract out most or all maintenance based on a fee-for-service or annual 

contract, which may include what amounts to an extended warranty on 

equipment.

• Contract out all maintenance based on a fuel-throughput charge, with 

minimum throughputs guaranteed by the agency; this contract typically 

includes what amounts to an extended warranty on equipment.

Agencies surveyed had 5–20 years of CNG station ownership experience. Thus, all 

agencies had time to develop and stabilize their maintenance approach, and most 

agencies were satisfied with their current maintenance approach.

Of the surveyed agencies, three used approach “a” above—providing most of their 

own maintenance. A fourth agency had used this approach on one of its two 

stations, but the retirement of a key employee caused the agency to switch to 

approach “d.” Agencies using the “a” approach were among the longer-term owners 

of CNG stations and tended to be small- to medium-sized agencies. This group 

liked the control that they experienced using their own staff, and their maintenance 

costs tended to be the lowest in absolute dollars but higher on a per-unit-of-fuel 

basis (because these are smaller stations). Two agencies indicated that they would 

like to be able give this responsibility to an outside contractor, but they were 

concerned that an outside contractor may not feel the urgency to make a down 

station operational, and they were concerned about the extra cost.

One respondent was using approach “b”; several other agencies had started with 

this approach and migrated toward the “a” approach. The agency using approach 

“b” was a smaller agency, and it appears that its maintenance costs were among the 

lowest in the survey.

Three small- to medium-sized agencies used approach “c,” contracting out all 

maintenance. These contracts took different forms, from fixed price to time and 

materials. The agencies using the time-and-materials basis had the lowest absolute 

and per-unit maintenance costs of the survey.

Those using approach “d” tended to be the largest transit agencies. This throughputbased maintenance program is most viable where large throughputs can spread the 

costs. Agencies using this approach were generally satisfied and felt that this 

method allowed them to budget more predictably and avoid unexpected, expensive 

equipment failures. These contracts generally include some type of damages clause 

($X00.00 per bus that is not fueled by the end of the fill shift). There was no 

indication that any of the agencies had ever needed to claim these damages. Two 

large agencies that currently or previously used both approach “a” and “d” reported 

that they preferred approach “d” because it allowed them to focus on their core 

activities.

In summary, all agencies appeared to be satisfied with their current maintenance 

approach. Small- to medium-sized agencies tended to take on more of the 

maintenance responsibly in-house to reduce costs, whereas larger agencies tended 

toward contracting maintenance on a per-unit-of-fuel-throughput basis.

Best Practices Related to CNG Garage Upgrades

Data provided for this section of the survey tended to be of a general nature. Respondents 

were typically bus or station personnel with less involvement in the design, construction,
21

and operation of garage facilities. The surveyor was unable to get building-upgrade costs 

from any agencies because this work would have been administered by others and may 

not have been exclusively to address CNG upgrades.

All of the agencies surveyed performed some level of building upgrade or, in the case of 

new facilities, some customization for CNG. This section summarizes

features/approaches that were common to most facilities.

Sloped roofs

Agencies building new CNG garages tended to use sloped roofs with relatively 

open structural systems to prevent pocketing. Agencies modifying garages tried to 

use garages with relatively open structural systems such as open-web steel joists. In 

garages with precast concrete roof structures, the pockets that were created were 

equipped with ducted exhaust systems pulling from each pocket. Some agencies 

included fire doors to seal the bus garage from the non-bus areas of the building.

Ventilation systems

Ventilation systems varied depending on the climate. Although current industry 

practice is to provide continuous ventilation with exhaust points at the ceiling and 

makeup air units at the floor (there were several excellent garages designed around 

this system), some properties operating in warm climates did not use baseline 

ventilation, but they did operate with the doors open nearly year round.

Heating systems

All of the agencies surveyed had addressed the heating system to eliminate the risk 

of the heating system providing a source of ignition for a leak. Agencies used 

indirect-fired rooftop warm-air systems, low-density infrared tubes fully vented to 

the outside, hazardous-locations-rated gas-fired infrared-plate heaters, and hotwater heating systems. In one warm-climate garage, no heating system was used.

Gas detection

All the agencies used combustible-gas-detection systems. These detectors used 

either infrared or catalytic technology. Agencies expressed a preference for both 

technologies, so there is no clear recommendation; catalytic sensors are lower cost 

initially but require quarterly calibration and sensor replacement every 2–3 years.

Infrared detectors do not require periodic replacement and require much less 

frequent calibration but are significantly more expensive initially.

The sensors are tied to a panel that typically sounds an alarm at 20% of the lower 

flammability limit (LFL; in this case, 20% LFL is 1% gas in air). In addition to 

alarming, the panel may open several overhead doors and start supplemental 

emergency exhaust systems. Some properties shut down power to certain building 

areas or circuits using breakers equipped with “shunt trips” that are activated by the 

gas-detection system at 40% LFL.

Electrical upgrades

The approach to electrical upgrades varied. Some agencies simply lowered lighting 

to below 18 in. from the ceiling, while others upgraded the lighting to sealed (not 

explosion-proof) fixtures. Other electrical equipment in the ceiling area was 
22

typically relocated or upgraded. No garage visited had upgraded or built the 

electrical system in the ceiling to be fully Class 1, Division 2 compliant—nor is this 

required by code if certain other precautions are taken.

Standby generators

A standby generator was recommended by several agencies to maintain ventilation

and gas-detection system operation, including power to doors, fans, and alarms.

Fall-arrest system

A small number of garages included one to two bays with a fall-arrest system to 

enhance the safety of workers on the roof of the bus during cylinder inspections and 

other rooftop work. This, or a work platform, is believed to be required by the U.S. 

Occupational Safety and Health Administration.

Best Practices Related to CNG Training

As with any new technology, it is critical to the success and safety of a CNG program 

that personnel and other stakeholders receive adequate training. Agencies noted that 

technicians that had more current computer/electronics training adapted better to CNG 

engine work than others that lacked this training. Most agencies stressed the need for 

initial and ongoing technician training to ensure that technicians are adequately prepared 

when new CNG buses arrive.

Training must include some level of practice/drill to provide the desired level of 

knowledge. Most agencies used equipment vendors to provide initial training and then 

provided annual/periodic refreshers and new employee training using in-house trainers.

The following is a summary of training best practices of the surveyed agencies.

Bus training

Bus training (usually provided initially by bus manufacturer) includes the following

considerations:

• 160–200 hours with each new bus order, of which approximately 40–60 is 

engine related

• Cylinder inspection training (typically 2 days) for two or more technicians 

that will be inspecting cylinders; it is useful to rotate new staff through 

this training annually to increase the number of qualified staff and to 

increase the number of informed eyes looking at cylinders between 

inspection intervals

• Driver training related to the unique aspects of operating a CNG bus, 

including emergency response.

Station training

Station training (usually provided initially by station manufacturer) includes the 

following considerations:

• Amount of training closely linked to the degree of agency involvement in 

the maintenance of the facility

• Basic training in the safe fueling of CNG buses for all affected staff
23

• Training in how to deal with emergency situations—emergency shutdown 

procedures, fire response, gas leak response, etc.—for all staff.

Emergency responder training

Emergency responder training (usually provided by the transit agency) includes the 

following considerations:

• Orientation shortly after commissioning of the CNG station for local fire 

departments, including location of shutoffs, hydrants, etc. (there will be

multiple shifts per department)

• Possible additional orientation for fire departments for each new series of 

bus, including general bus training to caution fire fighters against the use 

of an axe near cylinders or high-pressure lines and the proper function of 

the PRD

• One agency with a large bus population produced a binder with laminated, 

picture-rich, emergency-response information for each series of bus they 

operate—showing the location of isolation valves, fire-system controls, 

contact numbers, and other information of interest to a first responder.

These binders are available to the fire and police departments in their area.

This concept has been adopted by other agencies and has been appreciated 

by first responders

• Police should be contacted and trained initially and periodically as 

required.

Perception of CNG Buses and CNG Incidents

Public and Staff Perception

The survey looked at the general public as a distinct group from the ridership; however, 

agencies essentially found the two groups to be the same. In both cases, the agencies 

tended to enjoy a “halo” effect when they first introduced CNG buses into service. There 

was a positive reaction because the buses were noticeably cleaner than the old diesels that 

they replaced. As time has progressed, the public and ridership have come to accept the 

CNG buses as the norm and are indifferent—they no longer notice the benefit, and they 

do not appear to have any safety or other concerns regarding riding CNG buses.

Driver acceptance of CNG buses is very similar to the ridership acceptance. Most drivers 

now consider CNG to be business as usual. Driver preference is mostly driven by the 

newness of a particular bus and the available power. Early CNG buses were regarded by 

many drivers as being underpowered, but more recent buses are considered to be 

adequately powered. Underpowered complaints are now usually related to buses that

need repair.

Most agencies reported that technicians had openly accepted CNG technology early on 

and that many technicians find the learning curve to be interesting and fulfilling.
24

Incidents—Bus

There have been a number of bus fires in the agencies surveyed. Most of the agencies 

have considered this to be a normal occurrence because their experience indicated a 

similar percentage of fires in diesel buses.

Fires on CNG buses have been caused by (in approximately descending order of 

frequency):

• Hydraulic hose failure spraying oil on hot engine components

• Turbocharger failure resulting in oil leakage initiating a fire

• Heat from the exhaust system causing adjacent materials to ignite

• Impact of the cylinders on an overpass on an unapproved route

• Brake fire that engulfed the bus.

In each of these fires, the PRDs on the cylinders performed as designed and prevented a 

cylinder rupture under pressure. The bus incidents reported in this survey did not result in 

a serious injury or loss of life; however, one agency noted the inherent risk of having to 

quickly evacuate a bus on a highway or busy street.

The most significant non-fire bus incident among respondents involved the rupture of 

two Type 4 cylinders that were belly mounted, below the bus with virtually no protection 

from road debris. The cylinders failed during fueling presumably due to damage from 

road debris. No personnel were injured. The fleet inspected all similar buses and installed 

shields under the cylinders—these shields were inspected frequently with mirrors.

Personnel were not allowed inside this series of bus during fueling. Subsequent bus 

designs with roof-mounted cylinders are considered to have addressed this hazard.

These incidents underscore a need for personnel and emergency training.

Incidents—CNG Station and Bus Garage

Although there have been several incidents at other transit CNG stations and garages, the 

surveyed agencies had not experienced reportable incidents.

Recommendations

The survey included a block of questions for respondents to identify issues that need 

further technical research and development and areas where policy or financial support 

are required. The responses from these questions were combined with other issues 

identified in the technical portions of the survey and organized into the following 

recommendations. These recommendations have been broken down into areas where the 

government can directly provide assistance and areas where industry players will need to 

take a leading role and government can provide a supportive role.

Industry Needs and Concerns Not Directly Related to the Government

A major concern for most agencies is that there is only one domestic engine supplier at 

this time. When John Deere and DDC exited the market, they left legacy issues that 

agencies continue to struggle with. Agencies feel very vulnerable with only one supplier 

because there is reduced incentive for Cummins to improve their product. The entry of a 
25

new engine supplier is welcomed, and the early reports from respondent agencies are 

very promising with regard to Doosan engine power and fuel economy. However, several 

agencies are concerned that Doosan will also split the already very small market with

Cummins, and this could reduce Cummins’ interest in the market.

Most agencies noted engine reliability is lacking. Some agencies are seeing the 

newest engines as less reliable than slightly older engines (the last Cummins 8.3 

engines being held as an example of a reliable engine). Engine reliability 

complaints centered around:

• Sensors that fail frequently

• Short turbo charger life in some cases

• Failure of parts that should be utterly reliable such as exhaust manifolds 

(DDC—followed by a non-availability of replacement part manifolds) and 

gas high-pressure regulators

• Engines that must be more tolerant of compressor oil

• Improved life of frequent replacement parts such as spark plugs.

Several agencies noted that CNG buses are more expensive to operate in large part 

because of the premium price on engine parts. Agencies complained that they are 

not allowed to rebuild certain parts—they must buy a new assembly, or they must 

buy OEM parts such as spark plugs and turbo chargers where non-OEM plugs are 

much cheaper (and likely comparable), and diesel turbo chargers are much lower 

cost.

Agencies would like to see engines and buses designed to manage heat more 

effectively to reduce fire risk and perhaps to reclaim heat lost in exhaust.

Although most agencies noted that significant improvement in power and fuel 

economy has already taken place, they would still like to see improvement in 

performance—particularly in fuel efficiency. The CVEF believes that the industry 

would benefit from testing equal CNG and diesel engines in chassis under 

controlled drive cycles to determine the real fuel economy difference.

In CNG stations, several agencies still view oil carryover as a problem that must be 

addressed. Previous work has been done on oil vaporization and dew-point 

requirements; however, it is apparent that the industry would benefit from a more 

comprehensive best practices document addressing dryer selection and 

maintenance, station filtration system design and operation, and best oil type for 

specific applications. This report could be used by agencies when specifying new 

station equipment or when contemplating an upgrade to station equipment to 

address an oil carryover issue.

All agencies surveyed used OPW Series 5000 nozzles for their transit bus fueling.

This nozzle is essentially the industry standard and has evolved to a design with 

acceptable durability. Agencies indicated that they would like to see a lighterweight high-flow nozzle, perhaps with a swivel to make operation more 

ergonomically friendly. A new nozzle manufactured by Weh of Germany claims to 

include these attributes, but the author has no direct knowledge or experience with 

its use, safety, or durability.
26

Industry Needs and Concerns for Government Policy and Assistance

There was unanimous approval of the federal $0.50/GGE fuel subsidy. This 

program was a pleasant surprise to many of the agencies that were already involved 

in CNG, and it has made the business case to remain with and expand CNG use in 

their fleet much more compelling. The only concern was that the agencies wanted 

to see this program locked in over the long term because the decision to continue 

with CNG is a 12–15 year commitment.

A number of agencies called for permanent funding assistance in capital areas,

including infrastructure cost, incremental bus cost compared with diesel (this may 

be addressed if currently proposed federal legislation is passed), and, in California,

the very high impending cost as agencies attempt to comply with the 15% zeroemission-vehicle requirement coming in 2012.

A number of California agencies were very concerned about environmental 

regulations at the state level that require engine manufacturers to continue to tweak 

and redesign engines. This has prevented the engine design stability necessary to 

reach designs that are reliable and durable.

The point above has also contributed to the exit of two of the three domestic natural 

gas engine suppliers. Several agencies suggested that the government continue to

assist, or expand their assistance, for engine suppliers to adapt engines for CNG or 

to improve the reliability of existing designs.

Agencies supported continued investments in technician training so technicians can 

become more than “parts changers.” This would apply to transit agency and engine 

supplier staff.

More rigorous testing of the reliability and durability of engine components was 

desired—perhaps by funding additional testing at Altoona. Again it was noted that 

the transit market is very small, and there are limited interest and resources with 

engine manufacturers to develop CNG bus engines. One respondent suggested that 

greater assistance to the trash industry would spur that industry to use CNG, and 

this would improve demand for heavy-duty CNG engines. 

Developing bus designs that address the additional heat generated by natural gas 

engines—and possibly capturing and using the additional heat—could merit a 

focused research project. Areas such as under-hood temperatures, airflow patterns, 

and under-hood material specifications could be studied. Note that the areas of bus 

incidents were dominated by non-natural gas related causes.

Agencies that currently have buses with 15-year gas-storage cylinders would like to 

find a way to extend this life to 20 years without cylinder replacement. At this time, 

no such mechanism exists. One suggestion was made to address timed-out cylinders 

with a disposal credit from the government to subsidize the cost of new cylinders—

similar to the recent “Cash for Clunkers” automobile trade-in program.

Conclusions

This survey was intended to focus on the problems, mistakes, and lessons learned the 

hard way. By focusing on and reporting the negative experiences, it is hoped that others 

can avoid these pitfalls and learn from the experience of the pioneers in the industry.
27

For this reason, the report concludes with several positive comments. The survey asked 

what respondents felt were the unexpected benefits of CNG. The following is a sample of 

the responses:

• Fuel costs were sometimes much lower than diesel and then further subsidized by 

the federal rebate of $0.50/GGE.

• The cost of CNG has been much more stable than the cost of diesel, and it appears 

that it will remain at relatively low levels for some time to come. Agencies have 

enjoyed the ability to budget short and medium term with stable or locked-in fuel 

prices.

• Some respondents indicated a preference for CNG as a domestically sourced fuel 

that displaces foreign fuel.

• Operators of certain engines (particularly the Cummins 8.3) felt that the durability 

of the engine was very high. One agency indicated that it had avoided mid-life 

overhauls that would have been required for diesel, while another indicated that it 

was seeing engine life comparable to or better than conventional diesel. Still 

another agency with some clean diesel engines indicated that CNG may prove 

more reliable than new diesel technology.

• Agencies liked the fact that there were no issues of diesel spills—agencies had 

experienced the cost and inconvenience of tank replacement in the 1990s.

• Respondents noted much improved indoor air quality and cleanliness in their 

garages.

• Agencies feel that CNG has an environmental benefit and that their use of CNG 

demonstrates environmental stewardship that gives their transit agency brand a 

positive environmental image.
28

Appendix A: CNG Transit Experience Survey Template
29
30
31
32
33
34
F1146-E(10/2008)

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

September 2010

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Compressed Natural Gas (CNG) Transit Bus Experience Survey: 

April 2009--April 2010

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

R. Adams (Marathon Technical Services)

D.B. Horne (Clean Vehicle Education Foundation)

5d. PROJECT NUMBER

NREL/SR-7A2-48814

5e. TASK NUMBER

FC08.0063

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Marathon Technical Services

Box 318 Six Venus Crescent

Heidelberg, Ontario Canada

8. PERFORMING ORGANIZATION

REPORT NUMBER

KLFH-9-88664-01

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NREL/SR-7A2-48814

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: M. Melendez

14. ABSTRACT (Maximum 200 Words)

This survey was commissioned by the U.S. Department of Energy (DOE) and the National Renewable Energy 

Laboratory (NREL) to collect and analyze experiential data and information from a cross-section of U.S. transit 

agencies with varying degrees of compressed natural gas (CNG) bus and station experience. This information will be 

used to assist DOE and NREL in determining areas of success and areas where further technical or other assistance 

might be required, and to assist them in focusing on areas judged by the CNG transit community as priority items.

15. SUBJECT TERMS

National Renewable Energy Laboratory; NREL; Clean Cities; Compressed Natural Gas; CNG; Transit Bus; 

Experience; Survey

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL

18. NUMBER 

OF PAGES

19a. NAME OF RESPONSIBLE PERSON

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified 19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18