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Chapter 13: Natural Gas Vehicles and Engines

13.1Detailed Life-Cycle Analysis

We conducted a review to assess the lifecycle impacts of natural gas used by the heavyduty truck sector. We also present the results of an analysis by the Energy Information 

Administration projecting the future use of natural gas by heavy-duty trucks. Finally, we list a 

number of potential technologies which could help to reduce the methane emissions from natural 

gas trucks.

13.1.1 Upstream Emissions

Upstream methane emissions, occurring in the natural gas production, natural gas 

processing, transmission, storage and distribution sectors, are estimated and summarized in an 

annual report “Inventory of U.S. Greenhouse Gas Emissions and Sinks” (GHG Inventory) for the 

United Nations Framework Convention on Climate Change (UNFCCC).1 As a basis for 

estimating the life-cycle impact of natural gas use by heavy-duty trucks, we used the year 2012 

methane emission estimates in the most recent GHG Inventory, published in 2014. The GHG 

Inventory also includes the quantity of carbon dioxide which is co-produced with methane 

throughout the natural gas system and emitted to the atmosphere through venting, flaring, and as 

fugitive emissions. 

The GHG Inventory is updated annually to account for new emission sources (e.g., new 

natural gas wells), updated data, emission factors and/or methodologies, and to account for 

changes in emissions due to changes in policy, regulations and industry practices. The GHG 

Inventory reflects emission reductions due to existing state regulations, National Emission 

Standards for Hazardous Air Pollutants (NESHAP) promulgated by EPA in 1999, the New 

Source Performance Standards (NSPS) promulgated by EPA in 2012, and Natural Gas Star (a 

flexible, voluntary partnership that encourages oil and natural gas companies to adopt proven, 

cost-effective technologies and practices that improve operational efficiency and reduce methane 

emissions)

Emission estimates in the GHG Inventory are generally bottom-up estimates which are 

per-unit (compressor, pneumatic valve, etc.) emission estimates based on measured or calculated 

emission rates from such emission sources. 

In addition to the national-level data available through the GHG Inventory, facility-level 

petroleum and natural gas systems data is also available through EPA’s Greenhouse Gas 

Reporting Program (GHGRP). These data represent a significant step forward in understanding 

GHG emissions from this sector and EPA expects that this data will be an important tool for the 

Agency and the public to analyze emissions, and understand emission trends. For some sources, 

EPA has already used GHGRP data to update emission estimates in the GHG inventory, and 

EPA plans to continue to leverage GHGRP data to update future GHG Inventories. 

The natural gas which comprises CNG is expected to be off-loaded from the natural gas 

system where the vehicles using CNG are refueled. This is because the natural gas used as CNG 
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is compressed at the retail stations and fleet facilities which fuel the CNG vehicles. To get the 

natural gas to the CNG retail facilities, the natural gas must be shipped through the distribution 

system downstream of the natural gas transmission system. When the natural gas is transmitted 

through the distribution system, the methane emissions are higher because the methane 

emissions from the distribution system are added to the rest of the upstream methane emissions. 

Because LNG plants are located separate from the retail facilities, they can be located to 

access the lowest cost feedstock. This means the natural gas for LNG can be sourced from the 

larger natural gas transmission pipelines which are upstream of the distribution pipelines. This 

provides two advantages for LNG: 1) by avoiding the natural gas distribution system, the natural 

gas is priced lower, and 2) avoiding the natural gas distribution system avoids the methane 

emissions which occur from the distribution system. Table 13-1 contains the 2012 methane 

emissions estimate for the UNFCCC document. 

Table 13-1 Methane Emissions from the Natural Gas System in 2012

EMISSION POINT FROM NG 

FACILITIES

METHANE EMISSIONS 

(GIGAGRAMS)

Field Production 1858

NG Processing 892

Transmission and Storage 2071

Subtotal without Distribution 4821

Distribution 1231

Total with Distribution 6052

The GHG Inventory also includes the quantity of carbon dioxide which is co-produced 

with methane throughout the natural gas system and emitted to the atmosphere through venting, 

flaring, and as fugitive emissions. This quantity is summarized in Table 13-2.

Table 13-2 Carbon Dioxide Emissions from the Natural Gas System in 2012

EMISSION POINT FROM NG 

FACILITIES

CARBON DIOXIDE EMISSIONS 

(GIGAGRAMS)

Production 13,659

NG Processing 21,469

Transportation and Storage 63

Distribution 37

Total 35,228

In the GHG Inventory, EPA assessed the amount of uncertainty with its emission 

estimates and provided a lower and upper bound estimate for its emission estimates. The lower 
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bound emission estimate is 19 percent lower than the best case estimate in Table 13-1 and the 

upper bound estimate is 30 percent higher than the best case estimate. 

In the Climate Action Plan, EPA projects that methane emissions will increase in the 

future due to increases in natural gas production.2 Table 13-3 summarizes the projected increase 

in US methane emissions from the Climate Action Plan and the projected increase in natural gas 

production referenced from the proposed power plant rulemaking,

3 which is likely the most 

consistent natural gas production estimate with the projections made in the Climate Action Plan.

Table 13-3 Projected Natural Gas Production Volume and Methane Emissions (g/million BTU)

YEAR 2012 2020 2025

Methane Emissions 

Teregram CO2eq. 

145 140 151

Natural Gas Production (dry)

trillion cubic ft

24.1 26.6 29.3

As Table 13-3 shows, methane emissions from natural gas facilities are expected to 

increase from 145 teregram CO2eq. in 2012 to 151 teregram CO2 eq. in 2025, about a 4 percent

increase. At the same time, natural gas production of dry natural gas is expected to increase 

from 24.1 trillion cubic feet in 2012 to 29.3 trillion cubic feet in 2025, about a 22 percent

increase. When estimating the methane emissions on the same natural gas production basis, the 

methane emissions are projected to be 14 percent lower in 2025 than 2012.A

In the GHG Inventory, emissions associated with powering the units or equipment (i.e., 

compressors, pumps) used in natural gas production, processing, transmission and distribution 

are aggregated with all the other fossil fuel combustion activities. Rather than attempt to 

disaggregate those specific GHG emissions from the rest of the process emissions in the GHG 

Inventory, we instead used the estimated emissions for these sources provided by GREET.4 

Table 13-4 summarizes the process energy consumed to produce and process natural gas.

 

A

 The 14% reduction figure is calculated by multiplying the methane emissions estimate in 2025 by the ratio of 

2012 natural gas production over the 2025 natural gas production (151x24.1/29.3) and the resulting value is 115, 

which is 85.5% of 145, or 14% less.
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Table 13-4 Process Energy Demand by the Natural Gas System (BTU/million BTU)

FUEL TYPE PRODUCTION NATURAL 

GAS 

PROCESSING

TRANSMISSION/ 

DISTRIBUTION

TOTAL

Conv 

Wells

Shale 

Wells

Weighted 

Avg

Natural Gas 22,016 20,955 21,307 26,123 0 47,430

Diesel 2816 2680 2725 272 0 2997

Electricity 256 244 248 816 0 1064

Gasoline 256 244 248 0 0 248

Resid 256 244 248 0 0 248

Totals 25,600 24,367 24,777 27,211 0 51,988

Table 13-5 contains the factors we used to convert the GREET process energy demands 

used to operate the equipment used to produce, process and distribute natural gas to carbon 

dioxide emissions for those process fuels.5

Table 13-5 Carbon Dioxide Emission Factors for Process Fuel Consumption 

PROCESS FUEL GCO2/BTU

Natural Gas 0.0398

Diesel 0.0555

Electricity 0.1549

Gasoline 0.0535

Resid 0.0563

Table 13-6 summarizes the total estimated methane and carbon dioxide emissions emitted 

by the upstream natural gas system. Two estimates are provided, one of which includes the 

emissions from the distribution system representing the upstream emissions for CNG. The 

second estimate summarizes the emissions excluding the emissions from the distribution system 

representing the upstream emissions for LNG, since it is expected to access the natural gas from 

the transmission portion of the natural gas system. 
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Table 13-6 Summary of Year 2025 Emissions from the Natural Gas System (grams/million BTU)

METHANE EMISSIONS CARBON DIOXIDE

CNG Analysis (includes CH4

emissions from the distribution 

system)

242 3881

LNG Analysis (does not include 

CH4 emissions from the 

distribution system)

192 3881

13.1.2 Downstream Emissions

The GHG Inventory does not estimate the methane emissions for natural gas once the 

natural gas is diverted for use by the transportation sector, thus, we obtained information from 

other sources. Natural gas can be used by vehicles either as a compressed gas (CNG) or as 

liquefied natural gas (LNG). We discuss the emissions of both. 

13.1.2.1 Compressed Natural Gas (CNG)

To make CNG available to trucks, the natural gas must be compressed from the pressure 

that it is available from the distribution pipelines to a pressure over 3600 psi to enable filling the 

truck CNG storage tanks. We used the compression energy available from GREET for this step 

which reflects national-average emissions for electricity generation.6 The emissions associated 

with natural gas compression for CNG are summarized in Table 13-7.

Table 13-7 Estimated Emissions for Electricity Generated to Power CNG Compressors (g/million BTU)

METHANE CARBON DIOXIDE NITROUS OXIDE

Emissions 6.9 3988 0.06

An important advantage that CNG has over LNG is that only a single facility, the retail 

outlet, is required for distributing CNG, while LNG requires both a liquefaction plant and a retail 

outlet. The simplified logistics of providing CNG also provides fewer opportunities for 

emissions and leakage to the environment.

We are aware of the following two types of emissions for CNG which are not estimated 

in the lifecycle analysis due to lack of quantifiable data. The first is CNG refueling emissions. 

CNG trucks are refueled at the retail stations providing CNG. When the refueling hose is 

disconnected from the connection fitting on the vehicle, a small amount of natural gas is released 

to the atmosphere. This CNG refueling vented gas has not been estimated and therefore not 

included in the lifecycle analysis. 

The second is fugitive emissions from small leaks in the CNG fuel storage system. While 

CNG has an advantage over LNG because it is contained in a sealed system, the very high 

pressure at which CNG is stored dramatically increases fugitive emissions if a fitting pipe were 

to develop a leak. The level of fugitive emissions for a certain sized hole is directly proportional 

to the pressure. We do not have any data on the fugitive emissions from CNG trucks, therefore, 

in our lifecycle analysis, we assume that CNG fugitive emissions are zero which likely 

underestimates the methane emissions from CNG trucks.
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13.1.2.2 Liquefied Natural Gas (LNG)

The first step in making LNG available to trucks is the liquefaction step. As discussed 

above, the liquefaction plant is likely to be constructed near natural gas transmission pipelines to 

access the natural gas at the lowest price point. The liquefaction step involves the removal of 

heat from the natural gas until it undergoes a phase change from a gas to a liquid at a low 

pressure. Once the natural gas is liquefied, it is stored in an insulated storage tank to keep the 

LNG liquefied. 

LNG plants are configured depending on their ultimate capacity. World class LNG 

plants produce 5 million metric tons, or more, per year of LNG and the economy of scale of 

these large plants support the significant addition of capital to reduce their operating costs. An 

LNG plant solely producing LNG for truck fuel is expected to be significantly smaller than the 

world class LNG export plants and so the capital invested is expected to be much lower, thus, 

their operating costs would be expected to be much higher, and their energy efficiency much 

lower on a percentage basis. The California Air Resources Board estimates the liquefaction 

plants used for producing truck LNG fuel are 80 percent efficient, compared to 90 percent

efficient for world class LNG plants.7 In our lifecycle analysis of LNG as a truck fuel, we also 

assumed that LNG plants are 80 percent efficient. For our GHG analysis, we estimate the carbon 

dioxide emitted when 20 percent of the natural gas is combusted to provide the energy required 

to liquefy the natural gas to LNG. The upstream emissions associated with the natural gas used 

in the liquefaction process must be accounted for and added onto the LNG produced by the plant. 

These emissions are included as indirect emissions. Table 13-8 summarizes the GHG emissions 

attributed to the liquefaction plant. 

Table 13-8 LNG Liquefaction Plant Emissions (g/million BTU)

METHANE CARBON DIOXIDE

Direct Emissions 0 15,175

Indirect Emissions 48 970

Total Emissions 48 16,145

To transport the LNG to the retail station, the LNG is loaded into an insulated horizontal 

trailer designed specifically for transporting LNG. If the LNG in the trailer were to warm 

sufficiently to cause the LNG to reach the pressure relief valve venting pressure, there would be 

boil-off emissions from the trailer. However, since the LNG is super cooled, boil off events are 

likely to be rare. We used a CARB estimate of boil-off emissions for LNG transportation 

between the LNG plant and retail outlets.8 Table 13-9 contains the estimate of boil off emissions 

and the emissions from the vehicle transporting the LNG to retail. 
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Table 13-9 Boil-Off Emissions Estimate for LNG Transportation to Retail (g/million BTU)

METHANE CARBON DIOXIDE NITROUS OXIDE

Fuel Use (Diesel Fuel) 0.45 378 0.009

Methane Boil Off 

Emissions

0.43 0 0

Total 0.88 378 0.009

LNG is stored in the insulated storage tank at the retail facility. Heat gain in the storage 

tank could eventually lead to boil-off emissions. Service stations with little LNG demand are at 

a higher risk of boil-off emissions compared to service stations which have a significant 

throughput volume. LNG stations could be configured to avoid boil-off events to the 

atmosphere, such as venting to a co-located CNG facility, or venting to a nearby natural gas 

pipeline. We used a CARB emission estimate to provide an estimate of the boil-off emissions 

from LNG retail facilities.9 Table 13-10 summarizes the estimated boil off emissions for LNG 

retail facilities.

Table 13-10 Boil-Off Emissions Estimate for LNG Retail Facilities (g/million BTU)

METHANE 

EMISSIONS

LNG Retail Boil-Off 

Emissions

11.1

The total well to tank emissions for CNG and LNG are summarized in Table 13-12. 

These emissions represent the total of upstream and downstream emissions which includes 

delivering the fuel to the truck fuel storage tank. 

Table 13-11 Total Well to Tank Emissions Estimate for CNG and LNG (g/million BTU)

METHANE CARBON DIOXIDE NITROUS OXIDE

CNG 249 7869 0.07

LNG 251 20,405 0.009

13.1.3 Vehicle Emissions

13.1.3.1 Vehicle Configurations

There are several different ways that diesel heavy duty engines can be configured to use 

natural gas as a fuel. The first is a spark ignition natural gas (SING), Otto cycle SING heavy

duty engine burns the fuel stoichiometrically and uses a three-way catalyst and some also add an 

oxidation catalyst to provide the greatest emissions reduction. Stoichiometric combustion is 

used in most light-duty SING engines. Problems with thermal stress and low power density have 

favored the use of the lean-burn combustion system in heavy duty engines. The use of cooled 

EGR provides further potential to increase the engine output and, at the same time, decreases 

NOX emissions. In this case the engine compression ratio is reduced similar to that of a gasoline 
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engine, about 12 to 1 or more, and thus its thermal efficiency is lower than a diesel-like engine 

by about 10 - 15 percent, depending on the driver.

The second is a direct injection natural gas (DING), diesel cycle. The DING engine uses 

a small quantity of diesel fuel (pilot injection) or a glow plug as ignition sources. As the 

injection system for the diesel fuel does not have the capability of greater injection quantities, 

this option has no dual-fuel properties. On the other hand, an optimization of the pilot injection 

can be made to achieve lower emissions. An advanced high pressure direct injection (HPDI) fuel 

system combining the injection of both diesel fuel and natural gas can be used for lean burn 

combustion. This enables the engine to maintain the efficiency advantage of a compression ignition 

engine while running mainly CNG/LNG.

The third is a mixed-fuel natural gas (MFNG), diesel cycle. In a mixed-fuel engine, 

natural gas is mixed with intake air before induction to the cylinder and diesel fuel is used as 

ignition source. Mixed-fuel vehicle/engine means any vehicle/engine engineered and designed 

to be operated on the original fuel(s), or a mixture of two or more fuels that are combusted 

together. Mixed-fuel system means that a diesel engine works with two types of fuels together. 

In fact the engine is a diesel thermodynamic cycle and the energy is given by the diesel and the 

natural gas fuel. In mixed-fuel conversion the original engine is not modified in any way, a 

conversion system is installed in order to permit the engine to run on both fuels. The conversion 

of the engine is totally reversible, in fact it is possible to choose the mode how to run the engine 

(diesel / mixed-fuel). When the engine runs in diesel mode, the engine runs in the same way as 

per the original configuration. Engine results showed that the efficiency of the engine could 

decrease by about 2-5 percent in mixed-fuel mode compared to diesel mode and that the diesel

replacement was approximately 40-60 percent efficient.

Each of these natural gas engine types has its merits. The SING engine is less costly, but 

is less fuel efficient and because of the lower compression ratio it has less torque than the two 

diesel cycle engines. The DING engine is likely the most expensive because of the special 

natural gas/diesel fuel injection system and large required amount of natural gas (LNG or CNG) 

storage since the truck must run on natural gas. However, because the truck can run almost 

completely on natural gas, the DING engine has the potential to more quickly pay down the 

higher investment cost of the natural gas truck. The MFNG engine provides the truck owner the 

flexibility to operate on natural gas or diesel fuel, but at the expense of a slower natural gas 

investment pay down rate because it can operate at most 50 percent of the time on natural gas.

An important advantage of LNG is the increased energy density compared to CNG. At 

present, CNG stored at its maximum storage pressure is only 25 percent of the energy density of 

diesel fuel, while LNG contains about 60 percent of the energy density of diesel fuel. Because of 

its higher energy density, LNG is favored over CNG for long-haul trucking. An adsorbent for 

natural gas (ANG) material technologyBcalled metal organic framework (MOF) for storing CNG 

 

B Menon, V.C., Komarneni, S. 1998 “Porous Adsorbents for Vehicular Natural Gas Storage: A Review”, Journal of 

Porous Materials 5, 43-58 (1998); Burchell, T “Carbon Fiber Composite Adsorbent Media for Low Pressure Natural 

Gas Storage” Oak Ridge National Laboratory
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has been invented and is being tested for large scale use. The technology involves filling the 

CNG tank with a specially designed substance which looks similar to a pelletized catalyst. The 

substance establishes a matrix which causes the methane molecules in natural gas to become 

better organized and store the same quantity of natural gas in a smaller volume at the same 

pressure, or store the same density of natural gas at a lower pressure. This MOF could improve 

the energy density of CNG which would make it a better candidate for natural gas storage for 

long range combination trucks, while avoiding the boil-off events that are a risk with using LNG.

13.1.3.2 Tailpipe Emissions

When assessing the methane emissions from both CNG and LNG trucks, it is important 

to separate those trucks built or converted before 2014 to those built or converted in 2014 and 

later. The trucks built before 2014 are only required to meet a nonmethane hydrocarbon 

(NMHC) standard, which means that the methane emissions from these trucks are unregulated. 

Our certification data shows that the methane tailpipe emissions from these trucks/buses ranges 

from 2 – 5 g/bhp-hr for both spark ignition (gasoline type) and compression ignition (diesel type) 

engines. 

For 2014 and later OEM compression ignition natural gas trucks or natural gas 

conversions of 2014 and later diesel trucks, the trucks must meet a 0.1 g/bhp-hr methane 

emission standard in the case of a larger truck engine tested with an engine dynamometer, and a 

0.05 g/mile methane emission standard in the case of smaller trucks tested on a chassis 

dynamometer. For spark ignition (gasoline style) engines, the standards take effect in 2016.10 

The natural gas truck manufacturers are allowed to offset methane emissions over the standard 

by converting the methane emission exceedances into CO2 equivalent emissions and using CO2

credits. For the initial natural gas engine certifications that EPA received for 2014, the truck 

manufactures chose to continue to emit high levels of methane (around 2 g/bhp-hr) and use 

carbon dioxide credits to offset those emissions. We don’t know if this practice of using CO2

credits to offset high methane emissions will continue in the future, however, for evaluating the 

lifecycle impacts of natural gas heavy-duty vehicles, the 2014 and later natural gas heavy-duty 

trucks may in fact have an emissions profile more like the pre-2014 trucks and not like the 2014 

and later trucks as depicted below in the figures. Furthermore, our emissions analysis assumes 

that these trucks are emitting GHG emissions as designed. In cases when these trucks experience 

an increase in emissions due to deterioration or malfunction of the engines, fuel supplies or 

associated emission control devices on these trucks, the methane emissions could be higher than 

estimated. Table 13-12 summarizes the emission standards and the estimated methane emissions 

from heavy-duty trucks assumed in the analysis.
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Table 13-12 Methane Emission Standards and Estimated Emissions from Heavy-Duty Trucks

PRE-2014 2014 AND LATER

Methane Standard None g/bhp-hr

Estimated Emissions g/bhp-hr 2 – 5 0.05

g/million BTU 214 – 534 5.3

13.1.3.3 Boil-off, Venting and other Fugitive Emissions

Truck drivers requiring LNG fuel drive up to an LNG retail outlet or fleet refueling 

facility and fill up with LNG fuel. When the refueling nozzle is disconnected from the LNG tank 

nozzle, a small amount of methane is released to the environment. In addition, prior to refueling 

it may be necessary or advantageous, due to high pressure in the truck’s LNG tank, to reduce the 

pressure in the truck’s LNG tank to speed up the refueling process. In some cases the retail 

station is equipped with another hose and associated piping to vent the excess gas to the retail 

stations’ storage tank, or perhaps to a natural gas pipeline. However, for those retail outlets 

without such vent lines to the storage tank, the truck driver may simply vent the truck’s storage 

tank to atmosphere. As part of a sensitivity analysis for our lifecycle analysis, we estimate the 

emissions for venting an LNG tank prior to refueling.

A major GHG issue for LNG trucks is boil-off emissions from the trucks themselves. 

When the liquefied natural gas is pumped into the truck LNG tanks, it is “supercooled,” meaning 

that the temperature of the LNG is well below the boil-off pressure and temperature. If the truck 

is driven extensively the drawdown of liquid level will cause some of the fuel to boil off and thus 

cool the rest of the liquid in the LNG storage tank. It is possible that the fuel would maintain its 

supercooled temperature or possibly even cool further below its supercooled temperature all the 

time until the LNG is completely consumed. 

If the truck is not driven or is driven very little, the very low temperature LNG warms 

through ambient temperature gradient through the tank wall causing the temperature and pressure 

of the LNG to rise. When the pressure reaches a maximum of 230 psi there is a safety release 

valve on the LNG storage tank which releases the methane gas directly to the atmosphere until 

the pressure drops to 170 psi, the pressure at which the safety release resets. There are two 

industry standards used to design tanks to reduce the temperature increase, one for a 3 day hold 

timeCand one for a 5 day hold time.D Hold time is the minimum time elapsed between when the 

truck’s LNG tank is refueled and when it begins to vent. 

If there is a boil-off event, a large amount of methane would be released. If aware of the 

impending boil-off such as when the truck is being maintained, the truck driver could hook up 

the LNG tank to a hose which would vent the natural gas emissions to a CNG system which 

 

C National Fire Protection Association 52, Compressed Natural Gas (CNG) Vehicular Fuel System Code, 2002 

Edition 

D

SAE International (2008) SAE J2343: Recommended Practice for LNG Medium and Heavy-Duty Powered 

Vehicles. Warrendale, Pennsylvania.
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would reuse the boil-off natural gas as CNG, or vent the natural gas emission to a natural gas 

pipeline. Otherwise the boil-off emission would simply vent to the atmosphere. 

When an LNG fuel tank venting (refueling venting or boil-off) incident occurs, there are 

two separate processes which occur that contribute to methane emissions during the venting. 

The most obvious process is the pressure drop, from 230 to 170 psi, in the gaseous space above 

the liquid. The volume of gas vented is proportional to the reduction in absolute pressure in the 

tank. Since the drop in absolute pressure is 244 to 184 psi (14.7 psi is added to the 230 and 170 

psi gauge pressure), about 25 percent of the gas in the tank is vented (184 psi is 25 percent of the 

way from 244 psi to zero pressure). The second process is the vaporization of liquid during the 

pressure reduction in the LNG tank. The boiling point of any liquid decreases as the pressure 

decreases. Thus, when the LNG undergoes the pressure reduction during a venting/boil-off, the 

boiling point of the methane decreases and to balance the system, some of the liquid methane 

must boil off to cause the cooling of the liquid. The quantity liquid methane which must boil off 

from the liquid is calculated from methane’s heat of vaporization over the boiling point 

temperature change, which drops from -178 F to -189 F as the pressure drops from 230 to 170 

psi. 

The amount of natural gas which boils off during a venting event varies based on the 

quantity of liquid in the LNG storage tank. The greatest amount of natural gas which is lost 

during a venting/boil off event occurs when the tank is closest to being full. For a 200 gallon 

tank system, each boil off event has the potential to release on the order of 3-9 gallons or 5,300 –

15,800 grams of CH4 which translates to 132 – 400K grams of CO2-equivalent emissions, 

assuming methane has global warming potential (GWP) of 25 over a 100 year lifetime. If the 

vehicle continues to sit after boil-off events begin to occur with boil-off events each day and up 

to several boil-offs per day, as much as million grams of CO2-equivalent emissions may be 

emitted over the twenty or so days at which point the vehicle LNG tank would be completely 

empty.

Table 13-13 summarizes the starting and ending conditions and the loss from the tank for 

venting incidents (200 gallon LNG tank decreases in pressure from 230 to 170 psi) when the 

LNG tank is 90 percent, 50 percent and 10 percent full. A refueling venting event is more likely 

to occur when the tank is mostly empty, so the 50 and 10 percent cases are the most likely cases 

to consider. 

Table 13-13 Estimated Quantity of Boil-Off from a 200 Gallon LNG Fuel Tank for a Single Boil-Off Event

PERCENT FULL 

(INITIAL) 

PERCENT FULL 

(FINAL)

LIQUID LOSS 

(GALS)

TOTAL MASS 

LOSS (LBS)

Boil-off Scenarios

90 83.2 13.6 38.7

50 46.2 7.6 24.8

10 9.3 1.5 11.0

Table 13-13 shows that if a truck had 200 gallon of LNG storage capacity, the estimated 

quantity of liquid boil-off volume would range from 2 to 14 liquid gallons of LNG depending on 

the fill level of the LNG tank. When the quantity of LNG gas loss is included, the total loss 

ranges from 11 to 39 lbs. 
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The quantity of LNG tank boil-off or venting per distance driven by the truck depends on 

the frequency of boil-off or venting incidents. As described above, a truck’s driving profile 

plays a key role in determining the boil-off risk from LNG trucks. Fleets which purchase LNG 

trucks do so with the intent of driving the LNG truck extensively to pay off the much higher 

purchase price of the LNG truck. For this reason, there is likely to be few boil-off incidents, 

except for cases when the truck is forced out of its routine. Examples of when the truck might be 

sidelined include times when the truck is being maintained, the immediate period after the truck 

is involved in an accident or perhaps when the owning company experiences a loss of workload 

or files for bankruptcy. We have no data which would allow us to estimate the frequency when 

these sorts of incidents would occur, and even if we did, we still could not estimate the frequency

of boil-offs that occur in these cases. 

As the truck ages, it likely would be sold by the company which originally purchased the 

truck to avoid having to deal with the increased maintenance that occurs with older trucks. 

Figure 13-1 shows the estimated vehicle miles traveled by class 8 trucks as they age (the data is 

from the MOVES Model. 

Figure 13-1 Vehicle Miles Traveled by Combination Trucks in 2014

Figure 13-1 shows that the mileage driven by combination trucks decreases as they age. 

By the time that a combination truck is about 17 years old, it is driven about half the number of 

miles per year as a new truck. It would seem that the risk of boil-off incidents increases with 

these older trucks.

Venting incidents during refueling can occur at any time, and there is an incentive to do 

so when it is time to refuel. The decision to vent an LNG tank in most cases is solely up to the 

truck driver who is often under pressure to complete his work in less time to maximize profits. 

There is a lot of uncertainty in estimating the quantity of boil-off and venting from an 

LNG truck. To reflect this uncertainty, we assume two different boil-off/venting emission 

estimates. The low estimate assumes that 35 grams per million BTU of fuel consumed is 

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

2015 2010 2005 2000 1995

Miles Driv

e

n

p

er Year

Truck Year
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13-13

emitted, which is from GREET. The high estimate assumes a boil-off event and a venting event 

each time the truck is refueled and before that tank full of LNG is used up, and this quantity is 

estimated to be 734 g per million BTU of fuel consumed. 

The crankcase of these engines receives leakage from across the piston rings, which can 

contain methane. The crankcase of the spark ignition engines is normally vented into the intake 

of the engines, thus, any methane emissions from the crankcase which is not combusted in the 

engine would be accounted for in both the engine-out and tailpipe emissions. For compression 

ignition engines, however, the crankcase emissions are typically vented into the exhaust pipe 

downstream of the aftertreatment devices although they are accounted for in addition to the 

engine-out emissions during certification. Engine-out emissions are subjected to deterioration 

factors based on well-established procedure, which may make them more robust than 

deterioration factors for vented crankcase emissions. Moreover, deterioration of crankcase 

emissions may be more variable. Thus, sealed crankcases would achieve more robust control of

methane emissions. 

Another potential source of methane emissions from CNG and LNG trucks is leaks from 

the fuel piping which to the engine which is another source of fugitive emissions. Thus, either 

while parked or operated, the vehicle fuel and engine systems could leak methane to the 

environment. We do not have nor did we attempt to estimate this type of methane fugitive 

emissions from CNG or LNG trucks.

Table 13-14 summarize the estimated tailpipe emissions for CNG trucks, and Table

13-15 summarizes the estimated tailpipe and boil-off and venting emissions for LNG trucks. 

Table 13-14 Estimated Tailpipe Emissions for CNG Trucks (g/mmbtu)

METHANE CARBON 

DIOXIDE

NITROUS 

OXIDE

2014 and Later Direct 5.3 60,702 2

Indirect 0.1 3 0

Total 5.4 60,705 2

Pre-2014 Direct 374 60,702 2

Indirect 6 189 0

Total 380 60,891 2

Table 13-15 Estimated Tailpipe and Boil-Off Emissions for LNG Trucks (g/mmbtu)

METHANE CARBON 

DIOXIDE

NITROUS 

OXIDE

2014 and Later

assuming low 

Venting and BoilOff Emissions

Direct 5.3 60,702 2

Indirect 0.07 5.5 0

Total 40.4 60.707 2

2014 and Later 

Assuming High 

Venting and 

Direct 5.3 60,702 2

Indirect 0.07 5.5 0

Total 739 60,707 2
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13-14

Boil-Off 

Emissions

Pre-2014 

Assuming Low 

Venting and 

Boil-Off 

Emissions

Direct 374 60,702 2

Indirect 4.8 386 0

Total 413 61,088 2

13.1.3.4 Thermal Efficiency

While not an emission source per se, the thermal efficiency of the natural gas engine also 

plays a role in the lifecycle emissions of the truck. Thermal efficiency is defined by the amount 

of energy that is obtained to propel the truck compared to the energy consumed by the engine. If 

a fuel-engine is less thermally efficient, then it consumes more fuel, or more BTUs, to travel the 

same distance, thus, emitting more carbon dioxide per distance traveled, or work performed. 

We estimate that SING engines can be as much as 15 percent less efficient than 

compressed ignition engines which operate on diesel fuel. Conversely, DING and MFNG 

engines which operate at a higher compression ratio, are estimated to be 5 percent less energy

efficient compared to a diesel engine. In our lifecycle analysis, we provide two different 

sensitivities for natural gas vehicles assuming that they may be 5 percent and 15 percent less 

efficient.

13.1.4 Results of Life Cycle Analysis

To estimate the lifecycle impact of natural gas used by heavy-duty trucks, we totaled the 

carbon dioxide, methane and the nitrous oxide emissions for the upstream and downstream 

portions of the natural gas system. The methane and nitrous oxide emissions are converted to 

carbon dioxide-equivalent emissions using global warming potentials ((GWPs); these are a 

measure of the relative contribution of global warming of emissions of a given gas in comparison 

to that of carbon dioxide over a given time period). The GWPs EPA is currently using is from 

the AR4 (2007) IPCC report for 100 year timeframe, which is 25 and 298 for methane and N2O, 

respectively. 

To establish the impacts of natural gas use in the heavy-duty fleet, it was necessary to 

compare the lifecycle impacts of natural gas against its replacement, which is a diesel fueled 

heavy-duty truck. The lifecycle impact diesel fuel was estimated by the National Energy

Technology Laboratory (NETL) for the production and use of diesel fuel in 2005. EPA used this 

lifecycle assessment for the Renewable Fuel Standard Rulemaking. We used this NETL diesel 

fuel lifecycle estimate for the baseline for comparison with the natural gas lifecycle assessment. 

The vehicle nitrous oxide and methane emissions are from the MOVES vehicle model developed 

by EPA. Table 13-16 summarizes the lifecycle emissions for diesel fuel estimated by NETL.

Table 13-16 Estimated Diesel Fuel Lifecycle Greenhouse Gas Emissions (g/million BTU)
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13-15

CARBON 

DIOXIDE

METHANE NITROUS 

OXIDE

TOTALS

CO2EQ*

Well to Tank 15,838 98 0.3 18,377

Tank to Wheels 78,308 1 2 78,929

Well to Wheels 94,146 99 2.3 97,306

Note:

*The totals are calculated using 25 and 298 for the GWPs for methane and nitrous oxide, respectively.

NETL is in the process of updating its lifecycle analysis from the 2005 analysis year to 

2009 as the analysis year. While the revised lifecycle analysis is not yet available, one of the 

authors of the analysis explained that the 2009 analysis appears to be quite similar to the 2005 

analysis.11 

To illustrate the relative full lifecycle impact of natural gas-fueled heavy-duty vehicles 

versus diesel fueled heavy-duty vehicles, we assessed a couple different scenarios. The first 

scenario is a conversion of a diesel engine to use CNG. Of the tens of thousands of heavy-duty

natural gas trucks currently in use, over 90 percent are of this type. These are conversions of 

older trucks so they are not regulated by the 2014 methane standard. For future year heavy-duty

trucks, we also estimated the lifecycle emissions if the trucks were meeting a 0.1 g/bhp-hr or a 

0.05 g/mile methane tailpipe standard. We provide two estimates for the lower thermal 

efficiencies of CNG trucks, one assumes that the truck is 5 percent less thermally efficient and 

the second assumes that the truck is 15 percent less thermally efficient (10 percent less efficient 

than the 5 percent less thermally efficient case). The estimated lifecycle emissions of CNG 

trucks, assuming projected upstream emissions in 2025, is summarized in Table 13-17.
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13-16

Table 13-17 Full Lifecycle Analysis of a CNG Truck (g/million BTU)

TRUC

K 

TYPE

EMISSION 

CATEGOR

Y

CARBO

N 

DIOXID

E

METHAN

E

NITROU

S OXIDE

TOTA

L 

CO2

EQ.*

THERMAL 

EFFICIENC

Y 5% AND 

15% 

CO2EQ.*

TOTALS 

INCLUDING 

THERMAL 

EFFICIENC

Y IMPACT 

CO2EQ. *

Pre2014 

CNG 

Truck

Well to 

Tank

7869 249 0.07 14,107 705

2116

14,812

16,223

Tank to 

Wheels

60,891 380 2 70,980 3548

10647

74,528

81,627

Well to 

Wheels

68,760 628 2 85087 4254

12,763

89,379

97,850

2014 or 

Later 

Truck

Well to 

Tank

7869 249 0.07 14,107 705

2116

14,812

16,223

Tank to 

Wheels

60,704 5 2 61,436 3072

9215

64,508

70,651

Well to 

Wheels

68574 254 2 75,544 3777

11,331

79,321

86,875

Note:

*The CO2eq. totals are calculated using 25 and 298 for the GWPs for methane and nitrous oxide, respectively.

The CNG lifecycle assessment relative to a diesel truck lifecycle analysis is shown in

Figure 13-2. 

Figure 13-2 Full Lifecycle Analysis of a CNG Truck 

(Projected Upstream Methane Emissions in 2025, Methane GWP of 25)

In the first two bars of Figure, it shows that based solely on tailpipe emissions (with and 

without thermal efficiency adjustments and assuming no increased methane emissions at the 

0

20,000

40,000

60,000

80,000

100,000

120,000

Diesel

Tailpipe

CNG

Tailpipe

Diesel CNG pre2014CNG 2014

and later

Emissions (gC

O2eq/m

m

Btu)

Thermal High

Thermal Low

CH4:&N2O

CO2
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13-17

truck), natural gas trucks are estimated to emit about 20 percent less GHG emissions than diesel 

engines. But this advantage decreases if the natural gas engine is less thermally efficient. The 

three full lifecycle analyses represented by the right three bars in the figure shows that pre-2014 

CNG trucks are estimated to emit about the same GHG emissions as diesel trucks, although if 

their thermal efficiency is much lower or if a higher GWP for methane were used, they would 

likely be somewhat higher emitting in GHG emissions. When such trucks are complying with 

the 2014 and later methane emission standards, their methane emissions are much lower and 

these trucks are expected to be lower emitting than diesels, even considering if they are less 

thermally efficient. 

The second scenario is a combination truck which is assumed to be in compliance with 

the 2014 methane standard. Because it is high mileage truck, the truck most realistically must 

use LNG as a fuel to provide the necessary range for the dedicated natural gas engine. We make 

two different assumptions with respect to refueling and boil off emissions. In the natural gas 

average case, we assume a modest quantity of refueling and boil-off methane emissions which is 

equal to the combined boil-off emissions from the liquefaction, transportation and retail station 

as estimated by GREET. The second boil-off emission estimate is based on venting the LNG 

storage tank to the atmosphere each time the driver refills his tank, and one LNG boil-off event 

between each time the driver must refuel his tank. As we discussed in the discussion about 

refueling and truck boil-off emissions, we don’t expect this to be as common practices for newer 

trucks that are operated regularly. However, as the use of these trucks decreases as they age and 

are sold into the secondary market, the risk for refueling and boil-off emission events increases –

this estimate provides a simple sensitivity emission estimate. The estimated lifecycle emissions 

of LNG trucks, assuming projected upstream emissions in 2025, is summarized in Table 13-18.

Table 13-18 Full Lifecycle Analysis of an LNG Truck (g/million BTU)

TRUCK 

TYPE

EMISSION 

CATEGOR

Y

CARBO

N 

DIOXID

E

METHAN

E

NITROU

S OXIDE

TOTALS

*

CO2EQ.

THERMAL 

EFFICIENC

Y 5% AND 

15% 

CO2EQ.*

TOTALS 

INCLUDIN

G 

THERMAL 

EFFICIENC

Y IMPACT 

CO2EQ.*

Pre-2014 

LNG 

Trucks 

assumin

g low 

Venting 

and BoilOff 

Emission

s

Well to 

Tank

20,405 251 0.01 26,693 1334

4004

28,027

30,697

Tank to 

Wheels

61,088 413 2 72,896 3645

10,934

76,542

83,831

Well to 

Wheels

81,494 665 2 98,732 4935

14,807

103,668

113,540

2014 and 

Later 

LNG 

Trucks 

assumin

Well to 

Tank

20,405 251 0.01 26,693 1334

4004

28,027

30,697

Tank to 

Wheels

60,707 40 2 62314 3116

9347

65,429

71,661

Well to 81,113 291 2 89,024 4450 93.475
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13-18

g low 

Venting 

and BoilOff 

Emission

s

Wheels 13,350 102,375

2014 and 

Later 

LNG 

Trucks 

with 

High 

Venting 

and BoilOff 

Emission

s 

Well to 

Tank

20,405 251 0.01 26,693 1334

4004

28,027

30,697

Tank to 

Wheels

60,707 990 2 86,037 3035

9106

89,072

95,143

Well to 

Wheels

81,113 990 2 112,720 4370

13,109

117,090

125,829

Note:

*The totals are calculated using 25 and 298 for the GWPs for methane and nitrous oxide, respectively.

The LNG truck lifecycle analysis relative to a diesel truck lifecycle analysis is shown in 

Figure 13-3.

Figure 13-3 Full Lifecycle Analysis of an LNG Truck 

(Projected Upstream Methane Emissions in 2025, Low and High Refueling and Boil-Off Emission, 

Methane GWP of 25)

Figure 13-3 shows that LNG trucks have about the same greenhouse gas footprint as 

diesel trucks when we assume a low quantity of refueling and boil-off emissions. In comparing 

CNG to LNG, the LNG trucks appear higher emitting than CNG trucks because of the low 

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

Emissions (gC

O2eq/m

m

Btu)

Thermal High

Thermal Low

CH4:&N2O

CO2
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-19

thermal efficiency of the small liquefaction facilities. If these LNG trucks emit high levels of 

methane when refueling and by experiencing boil-off events, their GHG emissions can 

potentially be much greater than that from diesel trucks.

It is important to point out the uncertainties associated with the lifecycle estimates 

provided Figures 13-2 and 13-3. As discussed above, there is uncertainty in both the upstream 

and downstream methane emission estimates for natural gas facilities and equipment, and the 

trucks that consume natural gas. In the GHG Inventory, EPA estimates a range of natural gas 

emissions from the upstream natural gas production sector. The range varies from -19 percent to 

+30 percent relative to the principal estimate. To illustrate the impact the range has on the 

relative life cycle impacts of natural gas versus diesel trucks, Figure 13-4 shows the impact on 

the relative life cycle emissions for CNG trucks when the low and high methane emissions are 

compared to the best estimate case we used in the above analyses for a CNG truck complying 

with the methane emissions standards. 

Figure 13-4 Full Lifecycle Analysis of a CNG Truck - Low, Avg and High Upstream Methane Emissions

(Projected Upstream Methane Emissions in 2025, Methane GWP of 25)

Figure 13-4 shows that higher and lower upstream emissions, based on the uncertainty 

factors provided in the GHG Inventory, does impact the relative GHG lifecycle impact of CNG 

trucks, but the effect is quite modest relative to the other emissions effects depicted in the figures 

presented earlier. 

As new methane emission information becomes available, we will update our methane 

emission estimates which would reduce the uncertainty. In addition to the new methane 

emissions information from the GHG Reporting Program, there will likely be other studies as 

well. For example, a number of studies are being conducted to quantify the methane emissions 

and life cycle impacts of natural gas by Environmental Defense Fund (EDF). The final reports 

for these studies have not yet been released but we will review them once they are available. 

The GWPs used to assess the relative climate impacts of methane and nitrous oxide can 

also effect the relative life cycle impacts natural gas trucks compared to diesel trucks. The 

0

20,000

40,000

60,000

80,000

100,000

120,000

Diesel

Tailpipe

Nat Gas

Tailpipe

Diesel Nat Gas

Low

Nat Gas

Avg

Nat Gas

High

Thermal High

Thermal Low

CH4:&N2O

CO2
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-20

GWPs of methane and nitrous oxide vary based on the timescale assumed. To illustrate this 

point, we added two more sets of figures as sensitivities for comparing the life cycle impacts of 

CNG and LNG natural gas trucks to diesel trucks if the greenhouse gas emissions are evaluated 

over a different lifetime. The GWPs that we use are the two alterative GWPs reported by IPCC 

in its 4

th Assessment Report evaluated at 20 year and 500 year GHG lifetimes. Table 13-19

summarizes the GWPs at the different lifetimes along with the GWPs used in the primary

analysis summarized above.

Table 13-19 Summary of GWPs

PRIMARY 

ANALYSIS

SENSITIVITY ANALYSES

100 Year 20 Year 500 Year

Methane (CH4) 25 72 7.6

Nitrous Oxide 

(N2O)

298 289 153

It is important to point out that while there are fairly significant differences in methane 

emissions between the various natural gas cases being studies and compared to diesel trucks, the 

nitrous oxide emissions vary very little across all the cases. Therefore, when comparing the 

relative lifecycle impacts using different GWPs, the impact on relative lifecycle emissions is 

almost exclusively due to changes in the methane GWP. Figures 13-5 through 13-8 show the 

relative lifecycle effects of natural gas trucks compared to diesel trucks when the GWPs used are 

based on 20 year and 500 year lifetimes. 

Figure 13-5 Full Lifecycle Analysis of a CNG Truck 

(Projected Upstream Methane Emissions in 2025, Methane GWP of 72)

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

Diesel

Tailpipe

CNG

Tailpipe

Diesel CNG pre2014CNG 2014

and later

Emissions (gC

O2eq/m

m

Btu)

Thermal High

Thermal Low

CH4:&N2O

CO2
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13-21

Figure 13-6 Full Lifecycle Analysis of an LNG truck

(Projected Upstream Methane Emissions in 2025,

Low and High Refueling and Boil-Off Emission, methane GWP of 72)

Figure 13-7 Full Lifecycle Analysis of a CNG Truck 

(Projected Upstream Methane Emissions in 2025, Methane GWP of 7.6)

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

Emissions (gC

O2eq/m

m

Btu)

Thermal High

Thermal Low

CH4:&N2O

CO2

0

20,000

40,000

60,000

80,000

100,000

120,000

Diesel

Tailpipe

CNG

Tailpipe

Diesel CNG pre2014CNG 2014

and later

Emissions (gC

O2eq/m

m

Btu)

Thermal High

Thermal Low

CH4:&N2O

CO2
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-22

Figure 13-8 Full Lifecycle Analysis of an LNG truck

(Projected Upstream Methane Emissions in 2025,

Low and High Refueling and Boil-Off Emission, Methane GWP of 7.6)

Figures 13-5 through 13-8 show that when evaluated over a shorter timescale, the higher 

GWP for methane increases the relative lifecycle impact of natural gas trucks compared to diesel 

trucks. Conversely, when evaluated over a longer timescale, the lower GWP for methane 

decreases the relative lifecycle impact of natural gas trucks compared to diesel trucks.

We compared our lifecycle emission estimates for natural gas, relative to diesel fuel, with 

the estimates provided by the California Air Resources Board (CARB) for its Low Carbon Fuel 

Standard (LCFS). For our emissions estimate used in the comparison we used the carbon 

dioxide-equivalent (CO2eq) emissions estimated for 2014 and later engines, which must comply

with a methane tailpipe emissions standard, and assumed that the engine was 5 percent less 

thermally efficient than a comparable diesel engine. Both analyses used GWPs based on 100 

year timescale (i.e., a GWP of 25 for methane and 298 for nitrous oxide). For the CARB 

emissions estimates, we used the estimates made for what it terms purposes” using the 2013 

version of the CARB GREET model as published in August, 2014.12 CARB estimates that CNG 

engines emit 76 percent of the CO2eq emissions as a diesel truck, while our analysis estimates 

that CNG engines emit 81 percent of the CO2eq emissions as a diesel truck. The most likely

explanation for CARB’s lower estimated CO2eq emissions for CNG engines is that a much larger 

portion of the electricity used to compress natural gas is renewable in California than the rest of 

the country. CARB estimates LNG engines emit 94.5 percent of the CO2eq emissions, as a 

diesel truck while our analysis estimates LNG trucks emit 96 percent of the CO2eq emissions as 

a diesel truck. CARB assumes no boil-off or venting emissions from LNG trucks and for this 

comparison, we used our more modest boil-off and venting assumption, as described above, 

0

20,000

40,000

60,000

80,000

100,000

120,000

Emissions (gC

O2eq/m

m

Btu)

Thermal High

Thermal Low

CH4:&N2O

CO2
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-23

which is close to CARB’s. Overall, our estimates are very similar to those estimated by CARB 

and when there are differences, the differences are as expected.E

A UC Davis report recently released estimated that CNG and LNG trucks using spark 

ignition engines (SING) emit about the same amount of CO2-equivalent emissions, and these 

emissions are slightly higher than that of diesel engines.13 The HPDI engines (DING) fueled by 

LNG are estimated to be the lowest emitting of the several scenarios analyzed by the study. 

Because the study did not discuss vehicle boil-off emissions, it is likely that the study either 

assumed that these emissions are zero or assumed the default vehicle boil-off emission estimates 

made by GREET. It is likely that the study assumed that the liquefaction plants are 90 percent 

efficient as this is the default assumption in GREET, which leads to lower GHG emissions by

LNG trucks.

13.2Projecting Natural Gas use in HD Trucks

EPA reviewed several information sources and projections to estimate how much natural 

gas is currently being used and is projected to be used by heavy-duty trucks. An obvious set of 

projections to review was the set of projections provided in the National Academy of Sciences 

(NAS) report.14 The NAS report attached a figure, sourced from Citi Research, which provided 

projections by ACT, PACCAR, Frost and Sullivan and the National Petroleum Council.15 This 

figure is reproduced below as Figure 13-9. 

 

E

Per Anthy Alexiades of CARB: CARB is planning to propose a new draft lifecycle analysis for CNG and LNG 

trucks at an April 2015 public meeting. While the CNG lifecycle is expected to be about the same, the LNG 

lifecycle analysis is expected to have lower emissions based on using a 90% efficiency for liquefaction plants 

instead of the 80% efficiency it was using previously. Lifecycle emissions for both CNG and LNG trucks will be 

adjusted to be 10% higher if using a spark ignition engine to account for their lower thermal efficiency. These 

estimates are solely for hypothetical analyses. LCFS credits are awarded based on GHG emissions for each specific 

application. 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-24

Figure 13-9 Near-Term Class 8 Natural Gas Penetration Forecasts

The first observation we can make about all these reports is that they start out assuming 

that natural gas use is 2 percent of the Class 8 heavy duty truck fleet in 2012. However, that 

level of natural gas vehicle penetration of the heavy-duty fleet is not supported by other data 

sources. In the Energy Information Administration’s Annual Energy Outlook 2014, EIA shows 

natural gas use comprising about only 0.2 percent of total heavy duty fuel consumption in 2012, 

and natural gas use by Class 8 trucks is under 0.1 percent.

16

 In 2014, AEO 2014 shows natural 

gas comprising about 0.4 percent of total heavy-duty fuel demand and about the same for Class 8 

heavy-duty truck demand. 

One estimate of the number of natural gas trucks supports this level of fuel demand made 

by EIA. In a meeting with the Natural Gas Vehicle for America (NGVA), NGVA presented 

their estimate that 62,000 heavy-duty trucks are fueled by natural gas. The MOVES data base 

estimates that there are 12.4 million heavy-duty trucks in 2014. Combined, the NGVA and 

MOVES numbers estimate that natural gas heavy-duty trucks comprise 0.5 percent of the heavyduty truck population. 

We also evaluated the growth rates for natural gas trucks, including reviewing two of the 

studies referenced in the NAS report. The ACT Research study shows the most aggressive 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-25

growth rate for natural gas heavy-duty trucks. The ACT Research projection did not seem to 

consider the economics of natural gas versus diesel fuel. Instead, the ACT projection seemed to 

be based on a consumer acceptance profile of a new technology, presumably assuming that the 

technology is already economically competitive. In a recent ACT press release for a more recent 

report, it was acknowledged that the growth rates ACT projected earlier were too aggressive and 

a more modest growth rate is more likely.

17

 The NPC projection shows a similar growth rate as 

that estimated by ACT Research, but NPC’s projection for increased uptake of the natural gas 

technology begins in 2015 instead of 2012. In its study, NPC assumed that the increased capital 

cost for a natural gas truck compared to a diesel truck study decreases from $60,000 to $20,000 

by 2040.18 This cost decrease seems excessive, and it is likely an important reason why the NPC 

study shows such a large increase in natural gas use by heavy-duty trucks. We did not have 

access to core assumptions used in the PACCAR and Frost and Sullivan projections to assess 

their viability. 

We searched for the Citigroup report on the Web and in addition to the figure provided in 

the NAS report, we found Citi Group’s projection shown in the context of the other projections 

referenced by NAS from the Citi bank report in Figure 13-10.

19

 Citi Group’s projection is less 

optimistic than the ACT projection, but is more optimistic than the NPC reference case 

projection.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-26

Figure 13-10 Long-Term Class 8 Natural Gas Penetration Forecasts

In its Annual Energy Outlook, EIA projects the use of different fuels by the 

transportation sector. 20 This projection was not referenced in the NAS report, but our review

found it to be especially credible.

First, as described above, EIA estimates that natural gas fueled 0.4 percent of the energy

use of heavy-duty trucks in 2014 and this estimate is consistent with the fraction of heavy-duty 

fleet which are fueled by natural gas. 

Second, the EIA projection is based on an economic analysis which considers the 

increased cost of manufacturing a natural gas truck over a diesel truck, the fuel savings for using 

natural gas instead of diesel fuel, and whether the payback time of the fuel savings against the 

increased truck cost would trigger purchases of natural gas trucks. As part of this analysis, EIA 

assumes that lighter heavy-duty trucks would use CNG which is a lower cost technology suited 

for the shorter driving distances for these trucks. The long haul trucks, however, require larger 

stores of fuel to extend the driving range which is satisfied by storing the natural gas as a liquid. 

LNG has about 60 percent of the energy density of diesel fuel, compared to CNG which has only 

25 percent of the energy density of diesel fuel. To satisfy the long driving range of the long haul 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-27

trucks, EIA assumed that they would use LNG as a fuel. The assumptions used by EIA for 

conducting its economic analysis all seem reasonable. 

Third, EIA is one of the several well-respected organizations in the world for collecting 

and analyzing today’s fuel prices and projecting future fuel prices. According to the Alternative 

Fuels Data Center, one of the most important assumptions in projecting the future use of natural 

gas in the transportation sector is the relative price of natural gas to the price of diesel fuel. 

Figure 13-11 summarizes the total retail prices and the cost components that make up the final 

average year 2014 retail prices of diesel fuel, CNG and LNG, whose prices are expressed on a 

diesel gallon-equivalent basis.

Figure 13-11 Relative Retail Cost of CNG and LNG to Diesel Fuel ($/gal dge)

In 2014, the natural gas price purchased by industrial users was about $6 per million 

BTU, which corresponds to $0.60 per diesel gallon equivalent. The price of crude oil has been 

volatile during 2014 as the Brent crude oil price started at about $110 per barrel ($2.38/gallon), 

but decreased to under $50 per barrel towards the end of 2014. From EIA’s website, the average 

retail diesel fuel price in the first part of 2014 was about $3.80 cents per gallon. When 

comparing the natural gas spot market price on a diesel equivalent basis to the diesel fuel price, it 

appears that natural gas is priced about one quarter of the diesel fuel price. However, if used as 

compressed natural gas, the natural gas must be distributed through smaller distribution pipeline 

system that exists in cities, which dramatically increases the price of the natural gas to $1.15 per 

DGE. Then the natural gas must be compressed and stored at a retail outlet which adds another 

$1.30 per DGE. The estimated retail price of CNG is $2.35 on a diesel gallon equivalent basis 

(DGE), or about $1.45 DGE less than diesel fuel. 

Similarly, if natural gas is converted to LNG, the resulting retail LNG price is much 

higher than the raw natural gas price. LNG liquefaction plants are assumed to be located close to 

large transmission pipelines away from cities, thus, they would likely pay the same low price as 

industrial users. However, for producing LNG, the natural gas must be liquefied which adds 

about $0.75 DGE. When the LNG is transported to retail outlets and marked up, the LNG is 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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priced $60 per DGE higher. The tax applied to LNG is on a per gallon basis, thus, is much more 

than on a DGE basis because of LNG’s lower energy density. All these steps add substantially to 

the price of the LNG and the estimated retail price of LNG is $2.65 DGE, or $1.15 DGE less 

than diesel fuel.

In its projections, EIA estimates that crude oil prices in the upcoming years will decline 

modestly until after 2020 when they start increasing until they reach $140/bbl in 2040. Natural 

gas prices are expected to only slightly increase over this period. 

The fifth reason why the EIA projections seem reasonable is because the payback hurdle 

assumptions assumed for truck fleet owners seem reasonable. EIA projects that natural gas 

trucks begin to be purchased when the payback times are 4 years or less based on a survey

conducted by the American Trucking Associations. The ATA survey found that 24 percent of 

respondents would choose natural gas trucks over diesel trucks if the payoff is 4 years, another 

57 percent would choose natural gas if the payoff is 3 years, the next 15 percent would choose 

natural gas if the payoff is 2 years and the last 5 percent would choose natural gas if the payoff is 

1 year or less.22 This is consistent with some conversations we have had with some fleet owners.

The NAS cites the pay back for the extra cost of natural gas trucks as 2 years, but other sources 

report a longer return closer to 4 years.F 

The results of EIA’s economic analysis and projected natural gas use in heavy duty trucks 

presented in the 2014 Annual Energy Outlook is presented in Figure 13-12.

23

Figure 13-12 EIA Projection of Fuel Prices and Natural Gas Use by HD Engines

Figure 13-12 shows, as we discussed above, that natural gas currently supplies only about 

0.5 percent of total heavy-duty truck fuel demand and it expected to continue to do so until about 

 

F Early LNG Adopters Experience Mixed Results; Truck News, October 1, 2013

0

1

2

3

4

5

6

7

8

9

0.00%

1.00%

2.00%

3.00%

4.00%

5.00%

6.00%

7.00%

8.00%

9.00%

2010 2015 2020 2025 2030 2035 2040

C

ost ($

/

gal)

Natural Gas P

erce

nt of total H

D

D

e

man

d

Year

NG percent of

HD Demand

NG Price DGE

Diesel Price
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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2023. Starting in 2023, EIA estimates that the rising price of diesel fuel relative to that of natural 

gas, which begins to change about 2019, creates the economic incentive to purchase natural gas 

trucks. As expected, the EIA projection that the price differential between natural gas and diesel 

fuel continues to increase results in the effect that the uptake of natural gas use in the heavy-duty 

truck fleet accelerates as the price differential increases.

A very interesting conclusion of the EIA projection is the natural gas penetration 

differences between the different heavy-duty truck classes. Figure 13-13 summarizes the 

projected use of natural gas by the AEO for different truck classes.

Figure 13-13 EIA Projection of NG use by Truck Weight Class

Figure 13-13 shows that the only heavy-duty sector which is projected to see a large 

penetration of natural gas is the heavy, heavy-duty sector which increases to 10 percent by 2040. 

The light and medium classes of the heavy-duty truck fleet only show modest increases in 

natural gas use. The likely reason EIA’s analysis shows little CNG or LNG use by light and 

medium heavy-duty trucks is because they are driven far less and their use does not justify the 

higher purchase price. According to the Vehicle Inventory and Use Survey, light and medium 

heavy duty trucks average less than 1/3rd the annual mileage of the heaviest trucks.24 EIA is 

using a distribution of VMT for new class 7 and 8 trucks as shown in Figure 13-14.

0.00%

2.00%

4.00%

6.00%

8.00%

10.00%

12.00%

2005 2010 2015 2020 2025 2030 2035 2040 2045

Percentage

Year

Heavy HD

Medium HD

Light HD
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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Figure 13-14 Percent of Class 7 and 8 Truck Fleet by Annual Miles Traveled

Figure 13-14 shows that although about half of class 7 and 8 trucks are driven less than 

60 thousand miles per year, the other half is driven from 60 thousand to over 200 thousand miles 

per year. It is these high mileage long haul trucks which are prime candidates for using LNG 

because of their ability to pay down the high marginal natural gas truck cost. 

Since EIA does not report the payback times as an output of its projections, we conducted 

our own analysis of sample payback times solely for illustrative purposes. We assessed the time 

required for the lower fuel cost of LNG to payback the incremental truck cost of using LNG

assuming that a truck averages 120,000 miles per year. There were several important aspects of 

the payoff analysis that we conducted. First, based on the EIA analysis which found that the 

heavy, heavy-duty trucks sector is the only one which will see natural gas use increase 

dramatically, we solely studied the payback of natural gas in this truck sector. Second, as 

concluded by EIA, we also assume that the higher energy density of LNG will make it the most 

likely natural gas fuel type used by the heavy, heavy-duty trucks. Third, the higher natural gas 

truck cost was approximated from the analysis EIA conducted for its Annual Energy Outlook. 

Fourth, the analysis presents a simple payback as well as a discounted payback using a 7 percent

discount factor. Fifth, we evaluated the payback in 2014, and we also assessed what the payback

might be in 2020 and 2030 and assume some changes in the future years as discussed in some 

example evaluation cases below. 

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

0 50 100 150 200

P

erce

ntag

e

of Truck Fle

et

Annual Miles Traveled (thosand miles)
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Table 13-20 Payback Analysis

CASE 1 

2014 DUEL 

FUELED

CASE 2 

2014 HPDI

CASE 3

2020 HPDI

CASE 4

2030 HPDI

Miles per Year 120,000 120,000 120,000 120,000

Miles per Gallon 6.7 6.7 7.1 7.2

Incremental NG Truck Cost 55,000 70,000 65,000 60,000

Incremental NG Maintenance 

Cost per year

970 1613 1935 1935

Diesel Fuel Price ($/gal) 3.80 3.80 4.16 5.65

Natural Gas Price ($/gal DGE) 2.64 2.64 2.66 2.83

Diesel Fuel Cost per Year 68,263 56,886 70,608 94,167

Natural Gas Fuel Cost Per Year 60,062 43,978 50,614 53,810

Lower NG Efficiency (%) 5% 5% 5% 5%

Vehicle NG Use (%) 50% 95% 95% 95%

Simple Payback (years) 6.7 4.5 3.3 1.5

Discounted Payback (years) 9.1 5.5 3.7 1.6

We evaluated two different cases for 2014. Case 1 assumes an LNG fueled, heavy-duty 

truck which exceeds 26,000 gross vehicle weight rating and averages 120,000 miles per year. It 

is a mixed-fuel (MFNG) natural gas truck and is assumed to operate 50 percent on natural gas 

and 50 percent on diesel fuel. But because this truck can operate on diesel only, the truck can 

manage with a more modest storage quantity of LNG, thus reducing the cost of LNG storage. 

When this truck is fueled by LNG, it is estimated to be 5 percent less thermally efficient. The 

fuel costs are consistent with the prices during the first part of 2014. Accounting for the fuel cost 

savings based on the average fuel economy and also accounting for the increased maintenance 

cost for operating on LNG,Gthis truck only achieves a discounted payback time of 9.1 years for 

paying down the $55,000 increased cost for this truck. 

The second case we evaluated for 2014 is a direct injection natural gas (DING) truck. 

Because this truck must fuel on LNG or be parked (the diesel fuel is simply used to enhance the 

combustion process), there must be more LNG storage capacity and the truck purchase price is 

estimated to be $70,000 more than a diesel truck. This case also assumes 120,000 miles 

accumulated per year. The discounted payback time is 5.5 years, which is less than the first case 

because the truck runs more of the time on natural gas.

If we used the lower diesel fuel prices that we experienced later on in 2014 and early 

2015 ($2.90/gallon), the payback time would be much longer. Even on a simple payback basis, 

the payback time is over 20 years for both Case 1 and Case 2. 

For Case 3, we assessed a 2020 case using EIA fuel price projections. Like the second 

case, the truck is a DING truck, but because it is six years later, we assumed a modest cost 

 

G

FPInnovations estimates that natural gas truck maintenance costs are $0.01/per kilometer more than diesel trucks. 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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reduction due to a learning curve. Due to the large price spread between diesel and natural gas, 

this truck’s discounted payback time is 3.7 years. 

For Case 4, we assessed a 2030 case using EIA fuel price projections. Like the previous 

two cases, the truck is a DING truck, and we assumed a further modest cost reduction due to a 

learning curve. This truck is also assumed to accumulate 120,000 annual miles fueled on LNG. 

Due to the even larger price spread between diesel and natural gas, this truck’s discounted 

payback time is 1.6 years. 

The payback time for both 2014 high mileage heavy-duty truck cases we evaluated are 

over 4 years. Since fleets become interested in purchasing natural gas trucks purchased when the 

payback times are 4 years or less, it explains the low penetration of natural gas in the heavy-duty 

sector. 

Given the apparently poor payback times for natural gas vehicles in 2014, it suggests that 

existing subsidies for natural gas likely play an important role in encouraging its use. According 

to EIA, half the natural gas consumption by cars and trucks is in California and may be partially

due to subsidies and other incentives California offers. California subsidies the purchase price of 

natural gas vehicles, and also offsets the cost of natural gas dispensing stations. The Low 

Carbon Fuel Standard (LCFS) in place in California also incentivizes natural gas use because 

natural gas is considered to cause less of an impact on the climate than petroleum-based gasoline 

and diesel fuel. The majority of the other half of the NG fleet is also in states which subsidize 

the natural gas truck or service station costs. 

Based on the EIA projections for crude oil and natural gas prices, the payback time of 

LNG trucks is expected remain long (more than 4 years) until sometime around 2020 when crude 

oil prices are projected to begin increasing. Thus, natural gas use by heavy-duty trucks is not 

projected to increase above 1 percent of the heavy-duty fuel demand until after 2025. Even if the 

economics improve for using CNG and LNG in the heavy-duty fleet, another hurdle is fuel 

availability since these fuels are not already widely available. Figure 13-15 shows the number of 

CNG and LNG public and private service stations relative to the number of gasoline and diesel 

fuel service stations and truck stops, respectively.
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13-33

Figure 13-15 CNG and LNG Availability at Service Stations

As Figure 13-15 shows, CNG and LNG fuel availability at service stations is 1 percent or 

less of the availability of gasoline and diesel fuel. Even if a business owner finds the purchase of 

one or more new natural gas trucks an attractive investment, if the fuel is not available in the 

area, the business owner may have to forgo purchasing the natural gas trucks. A fleet owner 

might be in the position to also install a natural gas service station or establish a contract with a 

third party fuel provider to provide the fuel, but that may require making a large purchase of 

trucks to justify the installation of the service station or the establishment of the contract. If the

fleet owner would need to build a CNG or LNG refueling station to enable purchasing the natural 

gas trucks, then the combined cost of the service station installation and the natural gas trucks 

purchase could make the prospect uneconomic even if the natural gas truck purchase by itself is 

justified. LNG availability is particularly challenging because in addition to an LNG service 

station, a LNG liquefaction plant would be needed as well. If the economics turn favorable for 

using natural gas in the truck fleet, the conversion to natural gas is likely to be slow due to the 

need to build out the fuel availability.

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Number of

Service Stations

Selling CNG

Total Number of

Service Stations

Number of

Trucks Stops

Selling LNG

Total Number of

Truck Stops

N

u

m

b

er o

f

S

ervic

e

S

t

a

tio

n

s

a

n

d

Tru

c

k

S

t

o

p

s

Planned

About 1%

of

Service Stations

~150,000 total 

Service Stations

less than 1%

of

Truck Stops
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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13.2.1 Dimethyl Ether

Although NAS focused its recommendations on natural gas, it also discussed dimethyl 

ether (DME), which is a potential heavy-duty truck fuel sourced from natural gas. Dimethyl 

ether has a high cetane number (more than 55), although its energy density is about 60 percent of 

that of diesel fuel. Dimethyl ether is a volatile fuel, like liquid petroleum gas, that can be stored 

as a liquid at normal ambient temperatures under moderate pressure. Typical DME fuel tanks 

would be designed to prevent any significant evaporative emissions.

A DME fueled truck is only modestly more expensive than a diesel fuel truck. The fuel 

tank is more expensive than a diesel fuel tank, but much less expensive than an LNG tank since 

it does not need to be heavily insulated. The engine modifications to enable using DME are also 

modest. Because DME does not have carbon-carbon bonds that form particulate particles during 

combustion, the particulate filter, which is normally installed on recent year diesel trucks, can be 

eliminated. This offsets some of the increased DME engine and fuel tank costs.

Although DME is sourced from cheap natural gas, the conversion of natural gas to DME 

and moving the fuel to retail outlets greatly increases the cost of the fuel. As Figure 13-16

shows, DME is more expensive than LNG, but still lower in cost than diesel fuel. Similar to 

Figure 13-11, the diesel fuel price used in Figure 13-16 is based on crude oil prices in early 2014.

Figure 13-16 Relative Retail Cost of DME to CNG, LNG and Diesel Fuel ($/gal dge)

DME is estimated to cost $3.50/ DGE, or $0.30 DGE less than diesel fuel. 

Because there is very little DME use in the US (there is only a very small fleet of DME 

trucks being contemplated in California), we did not conduct a lifecycle assessment of DME. 

We will, however, discuss a few aspects of a lifecycle analysis for DME. First, since DME is 

sourced from natural gas, the upstream methane emissions from the natural gas industry would 

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

CNG LNG Diesel DME

Cost ($/gal)

Raw Fuel Cost/Crude Oil Liquifaction/Refining Distribution and Marketing Taxes
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-35

still be allocated to DME. Second, there are not venting issues associated with DME as with 

LNG or CNG refueling. Third, but very significantly, DME’s global warming potential is 

estimated to be 0.3 when assessed over a 100 year lifetime, which is about 1 percent of 

methane’s GWP.

25

13.3 Natural Gas Emission Control Measures

13.3.1 Proposed Control Measures

EPA is proposing some control measures to reduce potential methane emissions from 

natural gas vehicles. The cost discussion for each is below.

13.3.1.1 Crankcase Emissions

The proposal would require that all natural gas engines have closed crankcases, rather 

than continuing the provision that allows compression-ignition engines to separately measure 

and account for crankcase emissions that are vented to the atmosphere. This allowance has 

historically been in place to account for the technical limitations related to recirculating 

crankcase gases with high PM emissions back into the engine’s air intake. Natural gas engines 

have inherently low PM emissions, so there is no technological limitation that would prevent 

manufacturers from closing the crankcase and recirculating all crankcase gases into the engine’s 

air intake. The methane standard that was introduced in Phase 1 of this rule accounts for 

crankcase emissions by requiring methane in crankcase emissions be measured and included. 

However, there can be significant deterioration with respect to volatile crankcase emission such 

as methane, and it is difficult to ensure that all deterioration is fully reflected in the 

manufacturer’s deterioration factor. When the system is sealed and emissions are routed to the 

engine intake, crankcase emissions are zero and deterioration ceases to be a concern. See the 

Preamble Section II. D. for a description of the proposed requirement. 

Most (if not all) NG engines are derived from either a gasoline engine or a diesel engine.

Since it is already required for gasoline engines to seal the crankcase, it is not necessary to 

propose any crankcase changes for gasoline-derived engines. Diesel engines are not required to 

seal the crankcase, but are required to include the crankcase emissions in the emissions test. 

Many OEMs already close the crankcase for their engines, and EPA projects the average costs to 

comply with this proposed requirement would be negligible. 

13.3.1.2 Require 5 Day Hold Time

Boil-off emissions from LNG vehicles were not addressed nor accounted for in Phase 1 

of this rule. As more testing has been done in this area since that time for this emerging issue, a

minimal requirement EPA is proposing as described in the Preamble Section XII is to require 

manufacturers to follow current industry recommended practice, SAE J2343 for five day hold 

time to limit boil-off emissions from LNG vehicles. The ANSI standard for NG vehicles was 

adopted in 1994 when we required evaporative standards for NG vehicles (59 FR 48472 

September 21, 1994). This was updated to the more recent ANSI NGV1-2006 in the Tier 3 Rule 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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(79 FR 23414, April 28, 2014) to the proposal would adopt the analogous requirements for LNG, 

found in SAE J2343 in Section 4.2.

The specifications of this safety related standard will only affect new LNG vehicles to 

prevent boil-off initially and does not address aging vehicles as their insulating properties 

diminish such as loosing vacuum over time and may eventually result in much shorter hold 

times. The SAE J2343 test is done at 72oF, therefore it is reasonable to assume that hold times 

will be shorter for very hot summer days and high solar loading.

Since the majority or all of the NG vehicles are already compliant with the NFPA 52 and 

SAE J2343 recommended practices for 3 and 5 day hold times, there would appear to be zero to 

minimal costs for the requirement for a 5 day hold time.

13.3.2 Potential Controls

EPA is also investigating additional controls to further reduce potential methane 

emissions from natural gas vehicles. 

There are not well defined cost estimates for many of these options since they are ideas 

that will require research and development. 

13.3.2.1 Update to CO2 Credits Program

The Phase 1 Heavy-duty vehicle rulemaking establishing greenhouse gas emission 

standards included a compliance alternative allowing heavy-duty manufacturers and conversion 

companies to comply with the respective methane or nitrous oxide standards by means of overcomplying with CO2 standards (40 CFR 85.525 (ii)). The heavy-duty rules allow averaging 

only between vehicles or engines of the same designated type (referred to as an “averaging set” 

in the rules). Specifically, the Phase 1 Heavy-duty rulemaking added a CO2 credits program 

which allowed heavy-duty manufacturers to average and bank pollutant emissions to comply

with the methane and nitrous oxide requirements after adjusting the CO2 emission credits

(generated from the same averaging set) based on the relative GHG equivalents. To establish 

the GHG equivalents used by the CO2 credits program, the Phase 1 Heavy-duty vehicle 

rulemaking incorporated the IPCC Fourth Assessment Report GWP values of 25 for CH4 and 

298 for N2O, which are assessed over a 100 year lifetime.

Since the Phase 1 rule was finalized, a new IPCC report has been released (the Fifth 

Assessment Report), with new GWP estimates. This is prompting us to look again at the relative 

CO2 equivalency of methane and to seek comment on whether the methane GWP used to 

establish the GHG equivalency value for the CO2 Credit program should be updated to those 

established by IPCC in its Fifth Assessment Report. The Fifth Assessment Report provides four 

100 year GWPs for methane ranging from 28 to 36. Therefore, we not only request comment on 

whether to update the GWP for methane to that of the Fifth Assessment Report, but also on 

which value to use from this report. 

The costs for changing the GWPs used for the CO2 Credits Program can be estimated by

the cost to the manufacturer for reducing CO2 emissions. If for example the GWP of methane is 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-37

increased from 25 to 34, then the number of CO2 credits that will be necessary to offset methane 

emissions above the standard can be calculated by this change. The cost involved would be the 

cost per ton of CO2 for the difference in CO2 credits. See chapter 7 for a discussion of our 

estimated cost per ton of CO2 credit.

13.3.2.2 Deterioration Factors for NG Tailpipe Emissions

The current deterioration factors are based on diesel technology. 

13.3.2.3 Vehicle Boil-Off Warning

A simple means to help limit boil-off emissions would be to require that natural gas 

truck drivers be alerted to expected near-future boil-off events. Such an alert could be in the 

form of a warning light and associated alarm that would indicate that the LNG storage tank is 

approaching a pressure which would require that the tank vent. Knowing this, the truck driver 

could take evasive action to prevent such a release. 

A second alarm could be required when the LNG tank is venting. This would alert the 

truck driver to take action to avoid the potential for an explosive environment forming from the 

vented natural gas emissions. 

To alert for a boil-off event a pressure sensor integrated with the vehicle’s horn would 

cost on the order of $5.

13.3.2.4 Extend 5 Day Hold Time

The specifications of the proposed 5 Day Hold Time SAE 2343 safety related standard 

will only affect new LNG vehicles to prevent boil-off initially and does not address aging 

vehicles as their insulating properties diminish such as loosing vacuum over time and may

eventually result in much shorter hold times. LNG tank manufacturers are further developing 

their technologies for improvement of hold times and reducing boil-off from LNG storage tanks 

on trucks. These improvements can be incorporated by requiring longer hold times. It may be 

possible using these improvements in technology to extend the hold times to ten days or longer.

One example of an existing technology to address boil-off emissions has shown a 10 day

or more hold time depending on ambient temperatures and solar loading experienced by the 

vehicle. Westport Innovations, Inc. Ice Pack technologyHis an integrated vehicle design which 

requires low pressure, lower temperature fuel, referred to as “blue” fuel, for the longer hold 

times. The system does accept the typical LNG fuel which is higher pressure and temperature, 

referred to as “green” fuel, at the expense of shorter hold times. Fleet owners of these innovative 

vehicles have installed refueling stations which service their own fleets. The Ice Pack 

technology used by Westport has been on the market since 2007 and has gone through several 

 

H Reiskin, Jonathon S. “Expensive Fuel Tanks, Systems Drive High Price of Nat-Gas Trucks”, Transport Topics 

Special Report “Alternative Fuels” December 2013
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13-38

revisions to work out issues. The technology involves a pump at the bottom of the tank which 

pumps the liquid fuel to the engine. Through a series of pressure changes and pumps, vapor is 

fed to the engine. At the same time a purge system takes the high pressure warming vapor in the 

fuel tank through a series of valves and feed it to the engine using the pressure. This purge 

system reduces the pressure of the fuel tank which further extends the hold time.

The range of enhancements necessary to achieve extended hold time of at least 10 days 

will range in cost from $5000 to $20,000 per vehicle.

I

 Ice Pack technology used by Westport is 

on the higher end of that range due to the additional vehicle pump and complicated engineering 

system which depressurizes the LNG for use in the engine while purging the gas during engine 

operation and keeping the LNG in the tank cold. Other approaches used by OEMs include 

increasing the vacuum size between the interior and exterior tank walls to maximize the 

insulation, keeping heat transfer to a minimum. The colder the LNG stays, the longer the hold 

time.

13.3.2.5 Capturing and/or Converting Methane Refueling or Boil-Off Emissions

A methane canister using adsorbents such as ANG (adsorbed natural gas) could be added 

to capture the methane which otherwise would be released to the environment during a refueling 

or boil-off event. Once captured, steps could be taken to route the methane to the engine intake 

once the vehicle is operating again, or to take steps to converting the methane to less GHGpotent CO2. ANG has been a patented technology since the 1950’s.

J

The Department of Energy

(DOE) Advanced Research Projects Agency (ARPA-E) has awarded over $10 million in 2012 to 

four different projects to develop new sorbent materials for on-board natural gas storage. Gas 

Technology Institute (GTI), a research and development organization which serves energy and 

environmental markets has been utilizing the grant to develop lightweight, affordable, natural gas 

tanks for natural gas vehicles. Methane will adsorb more efficiently with high pressure such as a 

boil-off event onto this material. 

As shown in Figure 13-17, for a 115 gallon LNG tank which would boil off completely in 

approximately 35 days

K

the boil-off methane emissions would be on the order of 18 lbs over two 

days, and close to 50 lbs in 5 days. This would require a canister of approximately 15-50 gallons 

of the adsorbent material for a 2-5 day hold time,

L

as show in Figure 13-18.

A methane canister using adsorbents such as ANG will need to be on the order of 15-50 

gallons for 2-5 day hold time. It is still early to say what the costs for this application will be 

because there is so much engineering development and testing yet to do, but it is estimated that 

the casing and adsorbents for this size of canister will range from $1,500-$8,000.

 

I As per confidential discussions with OEM’s, June 27, 2014, and November 20, 2014.

J Menon, V.C., Komarneni, S. “Porous Adsorbents from Vehicular natural Gas Storage: A Review”, Journal of 

Porous Materials 5, 43-58 (1998)

K

Powars, Charles A. “Best Practices to Avoid LNG Fueling Station Venting Losses”, St. Croix Research for 

Brookhaven National Laboratory, 2010

L LNG-ANG Venting Calculations Spreadsheet, “LNG-ANG Venting Calcs.xlsx”
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13-39

Figure 13-17 Boil Off Rate for a 90% Filled 115 gallon LNG Tank with 18 Watts of Heat Input

Figure 13-18 ANG Control of 115 gallon LNG Tank with 18 watts of Heat Input

If being discharged to the environment, the methane potentially could be burned to CO2

using a burner. Another potential option would be to convert the methane captured in a canister 

to CO2 over a catalyst. If a catalyst were to be developed, it would have to be designed to 

transform any escaping methane into carbon dioxide to reduce the global warming potential of 

methane down to that of carbon dioxide. 

A methane catalyst for a boil-off event on a vehicle has not yet been designed. We 

acknowledge that this would be a challenge and expect a good deal of engineering will be 

required for this application which will include an external heat source to be triggered upon a 

boil-off event initiation, possibly a pressure sensor which notices when the pressure builds to 

close to 230 psi when boil-off occurs.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

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13.3.2.6 Reducing Refueling Emissions

In addition to the boil-off issue is the recurrence of manual venting at refueling by LNG 

truck operators. Under high pressure circumstances, such as when the vehicle has been sitting for 

some time period in warmer temperatures, it is necessary to decrease the pressure in the fuel tank 

before new fuel can enter the tank. The recommended practice is to transfer the extra vaporized 

fuel to the gas station or natural gas pipeline, but this can take extra time. In some areas it has 

turned into common practice to just vent to the atmosphere to keep the down time at the 

refueling station to a minimum. In other areas there is an incentive to reroute the gas into the 

station storage tank or natural gas pipeline with credit towards the fuel purchase. One option 

would be to require a system for rerouting back to fuel storage tank, whether it is to a CNG tank, 

a CNG pipeline or re-liquefying system for an LNG storage tank. There are opinions within the 

industry that the technology at the refueling station and the vehicles are advancing so that the 

newer pumps and vehicles will automatically vent any pressurized gas back through the refueling 

nozzle which will quickly cool with LNG in the fuel line and go back into the vehicle as a liquid 

which ultimately lowers the pressure with the drop in temperature. Most LNG refueling stations 

are already equipped with a vent line to take excess vapors back to the station storage tank before 

refueling with the cold liquid fuel. For the older LNG stations which are not equipped with a 

vent line, there would be an overhead cost of approximately $10,000-$15,000 to install a vent 

line system at each pump. Advancing this feature to tie into a NG pipeline for monetary credit 

would add another overhead of similar magnitude. The technology exists to install a very small 

methane liquefaction facility at a retail station which would allow the retail outlet to re-liquefy

the vented gas and put it back into their storage tank.

Another option would be to control refueling emissions for LNG trucks with an on-board

vapor recovery refueling system similar to light duty gasoline vehicles. This would likely

involve a canister with carbon designed to adsorb methane specifically. See discussion in 

13.3.2.5.4.

Onboard refueling will require research and development for an onboard canister with 

methane adsorbents with a canister sized to take the vapors from a refueling event. The resulting 

canister and therefore costs will likely be similar to the discussion in Section B4.2.4. 

The recently promulgated Tier 3 rule requires use of the ANSI-NGV1-206 standard 

practice to meet the evaporative emissions refueling requirement. Small puffs of up to 200 cc/hr 

(which equates to 72 grams of methane per hour) of leakage are allowed with these tests. For 

CNG the current recommended practice for refueling involves a vent line taking a small amount 

of NG away from the operator for safety reasons, but this small amount is generally released to 

the atmosphere. Multiplied out for all of the refueling events of the CNG fleet this small amount 

can add up quickly especially with projected fleet expansion. When it is again multiplied by the 

methane GWP it would be worth investigating options for a system that would recompress the 

gas and reroute back into the CNG storage tank or pipeline.

Rerouting the vent line from a CNG refueling event will be a slightly more complex 

endeavor due to the mixing of air involved. In order to reroute the vent line to a NG pipeline for 

credit or to compress for usage will both require some engineering and development. This will 

be an overhead cost for the station. 
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-41

13.3.2.7 OBD Requirements for Fuel System Methane Leak Detection 

Onboard diagnostics (OBD) are required to detect and provide a warning for when 

methane leaks occur due to wear of connections and components of the CNG or LNG fuel 

system. Methane leaks occur due to wear of connections and components of the fuel system just 

as in gasoline or diesel systems. This can result in cracks, holes or structural breaks in the 

components. The HD OBD Rule requires comprehensive component monitoring with rationality

and functionality checks for all HD vehicles including NG, by 2013. By 2019 all HD vehicles,

including NG, must verify that their emissions control systems are functioning (74 FR 8310 

February 24, 2009. The implementation schedule is found in 86.010-18(o)). 

The requirements are already in place for methane leak detection. Therefore there would 

be minimal additional costs to include a pressure sensor and warning light for impending boiloff. The OBD code should already be programmed to record all associated methane leak events 

and keep a running record. Therefore adding accounting for boil-off venting should be minimal. 

There would be additional hardware and programming involved to detect whether the operator is 

venting high pressure gas to the refueling storage tank or to the atmosphere. Depending on the 

amount of engineering development required for a system of pressure and temperature sensors 

this approach could be relatively cost effective. The incremental costs for a vehicle with an 

OBD system already in place would be minimal, on the order of $5-10.
*** E.O. 12866 Review – REVISED – Do Not Cite, Quote, or Release During Review ***

13-42

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