INTERNATIONAL JOURNAL OF ENERGY RESEARCH

Int. J. Energy Res. 2008; 32:612–622

Published online 1 October 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/er.1369

Natural gas internal combustion engine hybrid passenger vehicle

S. Wright*,y and A. Pinkelman

Department of Mechanical Engineering, University of Colorado at Denver and Health Sciences Center (UCDHSC),
1200 Larimer Street, Campus Box 112, Denver, CO 80217-3364, U.S.A.

SUMMARY

The implementation of hybrid electric vehicles powered with alternative fuels is critical in reducing national dependence
on imported crude oil, addressing the detrimental environmental impact of increasing petroleum usage worldwide, and
sustaining the national economy. The question is not whether changes should be made, but instead centers on
identifying pathways that will lead to the greatest environmental and economic benefits. To avoid misuse of limited
infrastructure investment, the objective of this research is to consider a broad range of relevant factors to determine
desirable power plant–fuel combinations for hybrid electric vehicles. In the long term, fuel cells may dominate this
application, but at least in the short term, proton exchange membrane fuel cells (PEMFCs) will not likely offer
immediate substantial benefit over internal combustion (IC) engines. Environmentally friendly operation of the
PEMFC results partly due to low-temperature operation but primarily due to the requirement of a clean fuel, hydrogen.
In addition, the differential benefits from power plant choice can be overshadowed by the advantages obtained from
hybrid electric vehicle technology and alternative fuels. Consequently, the fuel flexibility of IC engines provides an
advantage over the relatively fuel inflexible PEMFC. The methane/hythane IC engine hybrid option, as developed and
presented here, is a promising pathway that avoids the barriers encountered with conventional non-hybrid natural gas
vehicles, namely range, power and fueling infrastructure difficulties. Dynamometer testing of the natural gas hybrid
prototype on the certification FTP-72 duty cycle revealed very low emissions and mileage greater than 33 miles per
gallon gasoline equivalent. This hybrid option utilizes a domestic, cost-effective fuel with renewable sources. With
multi-fuel capability (methane, hythane and gasoline) it is also designed for use within the emerging hydrogen market.
This hybrid option offers reliability and cost-effective technology with immediate wide spread market availability.
Copyright # 2007 John Wiley & Sons, Ltd.

KEY WORDS: hybrid vehicle; alternative fuels; natural gas; CNG; hydrogen; fuel cell; combustion

1. INTRODUCTION our national dependence on imported crude oil has

highlighted the need to develop domestic fuels.
Energy supply and fuel production traditionally The percentage of crude oil imported in the U.S. is
played a crucial role in the sustainability of the steadily increasing and will soon reach 60%. In
U.S. and world economies. Recent emphasis on addition, the competition for world oil supplies is

*Correspondence to: S. Wright, Department of Mechanical Engineering, University of Colorado at Denver and Health Sciences
Center (UCDHSC), 1200 Larimer Street, Campus Box 112, Denver, CO 80217-3364, U.S.A.
y
E-mail: Sean.Wright@cudenver.edu

Received 8 January 2007

Revised 27 August 2007
Copyright # 2007 John Wiley & Sons, Ltd. Accepted 28 August 2007
 NATURAL GAS INTERNAL COMBUSTION ENGINE 613

steadily intensifying, particularly due to the have included hydrogen-powered IC engine hy-
increasing usage in developing countries. For brids (such as the Ford Motor Company’s
example, crude oil imports in China increased by hydrogen-powered IC engine Escort), a number
31% in 2003. As recoverable reserves decrease, of hydrogen-powered PEMFC hybrids, diesel
competition for the remaining oil will continue to compression ignition hybrids (such as GMC’s
increase, driving oil prices even higher and further Opel Astra diesel hybrid or VW’s Golf diesel
jeopardizing America’s energy security. The total hybrid) as well as some ethanol-powered IC engine
societal cost of using petroleum includes many hybrids. The primary reason for lack of interest in
indirect costs that are not accounted for in the natural gas was negative feedback from users of
pump price of these fuels [1, 2]. One of the most natural gas-powered conventional vehicles. Dri-
cost-effective means of counteracting the growth in vers often saw a 15–20% reduction in engine
the use of petroleum is implementation of hybrid power and reduced vehicle range. However, these
electric vehicle (HEV) technology. problems can be effectively eliminated when
HEV technology provides both a means of natural gas is used in a hybrid vehicle operating
greatly reducing fuel usage while also decreasing environment.
emissions per gram of fuel consumed. The hybrid
design combines the energy storage advantages of
an electric vehicle with the high capacity or range 2. ANALYSIS
of a conventional vehicle that consumes fuel
during operation. In contrast to conventional 2.1. Overview of optimal alternative fuel hybrid
friction breaking, regenerative breaking saves fuel vehicle configurations
by recovering a significant portion of the vehicle’s
The premise of this research is to avoid the misuse
kinetic energy. Fuel usage is also decreased
of limited infrastructure investment resources by
because the energy storage unit, usually advanced
considering a broad range of criteria to determine
batteries or ultra-capacitors (UCAPS), acts as a
optimum power plant–fuel combinations for
buffer between the wheels and the power plant.
HEVs. That is, identifying pathways that will lead
That is, the power plant is more efficient because it
to the greatest environmental and economic
can operate closer to design conditions rather than
benefits. The optimum choices for hybrid power
directly following the load at the wheels.
plant–fuel combinations depend on balancing
HEV technology also allows the engine to be
many technological factors including the following:
sized to accommodate average load rather than
peak load, reducing the weight and cost of the * Practical power plant performance and effi-
engine. The additional cost of the hybrid system, ciency currently achieved.
primarily for the energy storage unit, is somewhat * Performance potential (efficiency and power) in
offset by this decreased engine size in addition to the unique hybrid vehicle operating environment.
fuel savings. Consequently, the benefits of hybrid * Total environmental impact from fuel produc-
vehicle technology will likely result in wide scale tion to end use (cradle-to-grave).
market penetration. As a result, power plant–fuel * Unique equipment requirements and mainte-
options discussed in this work will assume opera- nance personnel training.
tion in the unique operating environment of the * Power plant reliability, longevity and cost-
hybrid vehicle design. A variety of power plant effectiveness.
options can be utilized including spark ignition IC * Consumer acceptance and convenience of fuel
engines, compression ignition IC engines, gas and vehicle.
turbines and proton exchange membrane fuel cells * Infrastructure requirements; fuel production
(PEMFCs). viability, distribution and cost.
A number of automotive manufacturers have * Enhanced domestic energy self-sufficiency.
introduced gasoline- or ethanol-powered spark * Effect of fuel choice on air conditioning
ignition hybrids on the market. Research vehicles parasitic load.

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
614 S. WRIGHT AND A. PINKELMAN

At present, PEMFCs are expected by many to be law governs the electrochemical conversion pro-
the dominant power plant choice; as a result, there cess in fuels cells to the same degree as the Carnot
is a nationwide focus on the development of fuel efficiency limitation for the combustion and heat
production and distribution infrastructure for engine process in IC engine [5]. In addition,
fuels that are compatible with PEMFC, a rela- practical efficiencies for IC engines can be compar-
tively fuel inflexible choice. Considering the gen- able to those achieved for PEMFCs. See, for
erally limiting or prohibitive cost of infrastructure example, the experimental results of Blarigan and
development, it is imperative that our choice of Keller [6] for a hydrogen-fueled diesel engine with
optimal power plant–fuel combinations be based low emissions and an efficiency that is very
on the consideration of a broad range of criteria competitive (approximately 40%) with the range
listed above. This is important, because on a local of efficiencies being reported in the literature for
government and fleet operation level, alternative PEMFCs.
fuel production and distribution efforts are gen- The direct chemical to electrical conversion in a
erally guided by little more than personal pre- fuel cell requires an electrically powered air
ference. compressor with unique design requirements and
associated cost, as well as substantial parasitic
load during operation. Heat rejection is also a
2.2. PEMFC or IC engines?
difficulty for practical operation, caused by the low
PEMFC designs possess a number of positive operating temperature and poor heat transfer
characteristics for automotive applications. These characteristics. In practice this results in unaccep-
include low emission capability, hybrid vehicle tably high operating temperatures under certain
compatibility (for systems with electric energy driving conditions or the requirement of unusually
storage), modularity, desirable efficiency–power large heat exchangers. Also, although stack power
characteristics,z high efficiency, minimal moving densities are impressive, the power densities for the
parts and quiet operation. However, at least in complete fuel cell systems are not. Further,
the short term, PEMFCs may not offer an PEMFCs are easily poisoned or contaminated by
immediate substantial benefit over spark and impurities, including carbon monoxide, and can be
compression ignition internal combustion (IC) severely damaged by membrane dry out due to hot
engines for a variety of reasons in addition to the spots that tend to lack sufficient hydration at
expense of design and implementation of a new specific locations in individual cells. Issues such as
technology [3]. these raise questions of in service reliability and
One common misconception is that fuel cells service life.
have an inherently higher theoretical efficiency PEMFCs also currently have fairly high require-
than heat engines. For example, Larminie and ments for platinum group metal (PGM) catalysts.
Dicks state that ‘It is quite well known that fuel The world market of PGMs is volatile, indepen-
cells are not subject to the Carnot efficiency dent of the introduction of fuel cell technology.
limitation’ [4]. However, contrary to this common Supply of platinum and palladium is limited
misconception, fuel cells have the same theoretical primarily to South Africa and Russia, these two
performance potential as heat engines. The second sources account for 92% of worldwide production
of both these metals. A recent study by Tonn and
z
Note that the efficiency–power characteristics of fuel cells are Das [7] considered a number of factors contribut-
better suited for stop-and-go driving than combustion engines ing to the economic feasibility of adequate
in non-hybrid vehicles, that is, vehicles without regenerative platinum supply for large-scale introduction of
breaking and energy storage capability. However, fuel cells
offer no advantage, in this regard, in a series hybrid designs, light duty fuel cell vehicles. These factors include
and limited advantage in parallel hybrid configurations. Also, changes in other PGM demands worldwide,
fuel cells have limited performance (relatively low efficiency) at expected increase in the demand for light duty
high power demands. As a result, the power–efficiency
characteristics of fuel cell power plants are not as desirable vehicles (LDVs) in developing countries, different
under highway driving conditions. scenarios for market penetration of PEMFC

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
 NATURAL GAS INTERNAL COMBUSTION ENGINE 615

vehicles, and expected PGM loading in future from coal and steam methane reforming (SMR).
PEMFC designs. SMR is currently the most cost-effective method for
Currently, the typical catalytic converter in wide-scale hydrogen production but results in
today’s vehicles is loaded with approximately substantial environmental emissions [9]. For exam-
1.5 g of platinum or an equivalent 5 g of palladium. ple, a typical hydrogen-powered PEMFC prototype
However, regulatory requirements for low emis- vehicle using SMR hydrogen will produce higher
sion vehicles in developing countries are expected nitrogen oxide and carbon dioxide emissions than
to result in an increase of PGM loading in catalytic the gasoline powered Toyota Prius [3].
converters by a factor of three (or 4.5 g of Similar to SMR hydrogen for total emissions
platinum per converter). PEMFC designs cur- from fuel production to end use, ethanol-powered
rently have platinum loading of approximately vehicles also have high cradle-to-grave emissions
100 g for LDVs. The US DOE’s long-term PNGV and actually are likely to be higher than their
goal is to reduce platinum loading to 10 g per gasoline counterparts [10]. Total emissions include
vehicle [8]. In the Tonn and Das study [7], future emissions from sources, such as diesel farm
designs are expected to have between 5 g (best case tractors and from fertilizer production. The cost-
scenario) and 30 g of platinum loading. Even in the effectiveness of producing corn ethanol is also
best-case scenarios of all four factors listed above, questionable [10]. The production of ethanol is
they conclude that production capacities must most viable from by-products of the agricultural
increase much more than the historically manage- industry rather than cash crops, such as cellulosic
able few percent per year in the industry. They ethanol production processes. Research efforts are
question whether South Africa and Russia be focusing on improving the efficiency and cost-
willing to ramp up for a boom in PGM demand effectiveness of these processes.
and risk the consequences of a relative bust. They
conclude that PGM supplies will likely remain
2.3. Hydrogen fuel production
tight, prices will remain high and market penetra-
tion of new auto technologies based on PGMs will Hydrogen is a powerful, versatile, and clean fuel.
be difficult at very least. Hydrogen acts as an energy carrier and can be
PEMFCs require the use of pure hydrogen, and produced from a variety of energy sources [11].
the clean operation of the PEMFC can be mostly Unlike the current energy system, the use of
attributed to the requirement of a clean fuel.} hydrogen would allow any energy source to be
However, IC engines powered with hydrogen can used for transportation, such as solar and nuclear
also operate in an environmentally benign manner power. The U.S. DOE hydrogen energy directive
in a hybrid vehicle operating environment, reach- [12] primarily represents a focus on hydrogen
ing the requirements of the California Super Ultra- production from coal, nuclear energy and refor-
Low Emission Rating II (SULEV II) rating. IC mation of natural gas (methane). For the most
engines are fuel flexible and thus offer much part, the underlining tenure is that this directive
greater flexibility in choosing between cost-effec- seeks to reduce national dependence on imported
tive infrastructure options. IC engines can operate crude oil while neglecting or aggravating environ-
on liquid and gaseous fuels already in use with mental concerns and public health effects. The
existing infrastructure. It should be emphasized DOE directive for hydrogen production en-
that, even though a hydrogen PEMFC can operate courages pathways that overwhelmingly appear
with no emissions other than hydrogen and water, to be either not cost-effective or environmentally
environmental impact also depends on emissions less favorable than current vehicle-fuel life cycle
from fuel production, such as hydrogen production emissions.
The methods for production of hydrogen from
} coal and nuclear energy considered in the DOE
Specially designed PEMFCs can operate directly on methanol,
but reported efficiencies are low compared with hydrogen- directive require massive infrastructure invest-
powered PEMFCs. ments, particularly for distributing hydrogen to

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
616 S. WRIGHT AND A. PINKELMAN

fueling stations and certainly do not appear to for the Western Governors Association in 2005
represent a viable short-term solution. Production provided an assessment of the potential impact of
of hydrogen from coal either results in the release concentrating solar power (CSP) systems. This
of extreme levels of carbon dioxide (approximately DOE report estimated the potential power pro-
3.6 tons CO2 per ton of coal) or high cost duction, using only available and most suitable
associated with carbon dioxide sequestration. land with the most intense sunshine, as over
Use of coal-produced hydrogen results in the 6800 GW of electricity generation in the Southwest
release of an assortment of other environmental [15]. In comparison, the current total U.S.
contaminants, such as heavy metals. At least in the electricity generating capacity is approximately
short term, hydrogen production from nuclear 1000 GW. However, CSP technologies require
power is also questionable. Nuclear power has policy incentives for initial deployment. As a
many positive attributes, but various disadvan- result, the U.S. Federal government has created
tages appear to make the process presently non- an investment tax credit that encourages the
viable, such as massive capital investment, public deployment of CSP. This has resulted in the
resistance, extensive safety precautions and asso- development of new CSP projects in the states of
ciated costs, creation of attractive terrorist targets, California, Arizona, and Nevada, totaling
weapons grade nuclear fuel production, and 2000 MW, and scheduled for completion by 2010.
nuclear waste disposal costs and concerns [3]. SMR is currently the most cost-effective method
Renewables such as off peak hydroelectric, wind for wide-scale hydrogen production. Reformation of
power and solar energy provide clean hydrogen natural gas by SMR to produce hydrogen at fueling
production, but current cost-effectiveness com- stations utilizing the existing natural gas distribution
pared with other options is the main obstacle. In system is more viable than centralized reformation
addition, hydrogen production from sources such and distribution of hydrogen. However, direct use
as hydroelectric and biomass, even in the long of natural gas as the vehicle fuel is more advanta-
term, can only provide a small fraction of geous in that no reformation equipment is required
hydrogen production required for the automotive on the vehicle thereby reducing complexity and cost.
sector. An obvious advantage of solar energy over Natural gas has renewable sources and there are
biomass-derived fuel production is more efficient vast quantities of fossil natural gas reserves
use of land area and the acceptability of waste worldwide. Proven worldwide natural gas reserves
lands rather than productive farm land. With all are presently over 6183 tcf (trillion cubic feet) [16].
factors considered, hydrogen derived from solar Proven sources are defined as reserves that can be
energy through thermal processes is likely to be recovered under current technology and current
a winning hydrogen production process that prices. In comparison, world consumption was
balances cost-effectiveness and minimization of 106 tcf in 2006. U.S. natural gas reserves, as stated
environmental intrusion. by the U.S. Energy Information Administration
Thermal processes are by nature the most cost- (EIA), are 1191 tcf. The National Petroleum
effective and efficient solar conversion processes Council puts these reserves at over 1800 tcf. In
for large-scale plants. The cost-effectiveness of comparison, U.S. natural gas consumption has
solar parabolic trough systems, for example, is been nearly constant for over a decade, at between
nearly competitive with conventional power pro- 21.8 and 23.0 tcf (21.8 tcf in 2006). At present the
duction plants and are expected to steadily U.S. consumption rates, independent of renewable
increase ($0.060 per kWh possible by 2015), as biomass sources, proven the U.S. fossil reserves
advances in the technology over the last two would last between 55 and 83 years, assuming no
decades see application in new plants [13]. Other new proven reserves were identified. However,
projections for wide scale implementation of note that proven natural gas reserves have doubled
parabolic trough technologies estimate electricity over the last 25 years and are expected to continue
costs as low as $0.035 per kWh by 2020 without to rise. International sources may also be utilized
new research breakthroughs [14]. The DOE report as international transport of liquefied natural gas

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
 NATURAL GAS INTERNAL COMBUSTION ENGINE 617

is steadily increasing (in 2006 14% of the U.S. hybrid prototype tested in this work had a range
natural gas imports where liquefied natural gas greater than 300 miles on CNG (with a 9 GGE
imported from overseas). In comparison, estimates CNG tank) and an additional 450 miles on
of worldwide peak oil range from the present to gasoline, based on the FTP72 duty cycle. Greater
2037 at the latest. range can be obtained if larger or additional tanks
are installed, or if higher pressures are used for fuel
storage. This vehicle range is well within the
3. RESULTS AND DISCUSSION acceptable range expected by consumers. In
addition, the dual fuel capability, and the possibi-
The lack of interest in natural gas as a hybrid lity of home fueling with natural gas, alleviates the
vehicle fuel is primarily due to negative feedback difficulty encountered with limited fueling infra-
from its use in conventional vehicles including structure encountered in the past.
reliability and other performance issues (primarily The CNG hybrid prototype had no noticeable
due to poor fuel control systems), lack of vehicle lack of power when operated on natural gas
power compared with gasoline, and limited range. compared with gasoline. This is due to the parallel
However, these problems can be effectively elimi- operation of the electric drive and the IC power
nated when natural gas is used properly in a plant. The main implication of slightly reduced
hybrid vehicle operating environment. power from the IC engine is that the IC engine
The most cost-effective means of producing a turns off less frequently than when operating on
natural gas IC hybrid was to convert a newly gasoline. This can be eliminated, if desired, by
released gasoline hybrid to operate on the gaseous forced air intake, increased compression ratio or
fuel. The conversion of a 2005 Ford Escape by slightly increasing engine size. It is important to
gasoline IC engine Hybrid was successfully carried note that the IC engine size (2.3 L) in the hybrid is
out in January 2005, in collaboration with ECO substantially less than in the conventional version
Fuel Systems Inc. and the University College of of the Escape, leaving ample space for a larger
the Fraser Valley (UCFV). This involved installa- engine in the engine compartment.
tion of a composite fiber compressed gaseous fuel Design for operation on hythane allows for the
tank, a sub-controller used to control the gaseous potential utilization of any energy source, including
fuel system, gaseous fuel injectors, gas pressure nuclear, hydroelectric and other renewable energy
regulator, gages, heaters and other equipment. The sources such as solar and wind power. As a result
gaseous fuel system was installed in complete the prototype has immediate market potential as
redundancy with the gasoline fuel system so that well as long-term viability as hydrogen production
the vehicle could operate on either fuel at the push develops. Operation on CNG provides immediate
of a button installed in the driver area. The system market viability as these vehicles can be conveni-
was thus designed to operate on three fuels; ently fueled at home, for those residences with
gasoline, methane (or natural gas) as well as natural gas supply, and additional local fueling
hythane (mixtures of hydrogen and methane). This stations can be easily added to those already
is not only important for testing purposes but also available as underground natural gas infrastructure
represents an important design feature for con- exists in most urban areas. Natural gas can also be
sumers; flexibility in fueling alleviates consumers’ produced in agricultural regions or shipped by
concerns regarding re-fueling availability, a pro- tanker trucks to any areas that require that mode of
blem in the past for alternative fuels. fuel distribution. An overview of the advantages of
The vehicle range problem encountered with this prototype is given in Section 3.4.
conventional CNG vehicles was overcome since
the fuel mileage of the hybrid vehicle is much
3.1. Dynamometer testing and results
higher than conventional vehicles. As a result,
there is no difficulty in obtaining an acceptable Performance and emissions testing of this proto-
vehicle range with natural gas operation. The type provide unique data for consideration of

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
618 S. WRIGHT AND A. PINKELMAN

power plant–fuel options. Standardized dynam- vehicle speed trace and a sample dynamometer test
ometer duty cycle testing of this vehicle on CNG result for the RPM of the IC engine. Figure 2
and gasoline was carried out with the Environ- shows regions where the IC engine is off either
mental Protection Agency (EPA) certification duty because power supplied by the electric drive motor
cycle consisting of two back-to-back FTP-72 tests is sufficient to power the vehicle, the vehicle is at
with a 10 min delay between tests. The FTP-72 or rest and no power is needed, or when the vehicle is
LA4 cycle consists of the first 1372 s of the FTP-75 in a state of deceleration and regenerative breaking
standard test cycle. Road load coefficients were is charging the hybrid battery pack.
obtained from Ford Motor Company and used to
simulate actual road conditions on the dynam-
3.2. Emissions
ometer.
The vehicle was tested on three sets of FTP-72 Dynamometer testing was carried out with gaso-
tests at the State of Colorado Emissions Center line as a baseline for comparison with CNG test
(CFR 40 Parts 86 and 100 certification analysis) results. Carbon monoxide (CO), carbon dioxide
using a Horiba 48 in. electric 2WD dynamometer (CO2 ), nitrogen oxides ðNOx Þ; NMHCs and THCs
to simulate EPA hybrid certification. Electronic were measured for three sets of back-to-back FTP-
override of the PCM was required to allow two 72 tests. Figure 3 depicts the emissions for the
wheel drive testing without having onboard HEV during these tests as well as California and
diagnostic trouble codes (DTCs) and vehicle Federal limits. Series 1 is the average emissions
malfunction. The exhaust sampler used was a from the three back-to-back FTP-72 tests, series 2
Beckman constant flow venturi with a Horiba non- is the California SULEV II emission limits for
dispersive infrared analyzers for CO and CO2 LDVs, and series 3 is for Federal bin 2 emission
(
1 ppm accuracy), Horiba flame ionization ana- limits for LDVs. Note that the CO limits for bin 2
lyzer for total hydrocarbons (THCs) (
1 ppm and SULEV II are much higher than actually
accuracy), Horiba chemiluminescent analyzer for depicted.
NO=NOx (
1 ppm accuracy) and a Horiba As can be seen in Figure 3 the average CO
methane cutter for determination of non-methane emissions were well within both the Federal bin 2
hydrocarbons (NMHCs). and the SULEV II limits. However, the average
Figure 1 shows a plot of the vehicle speed trace NOx and NMHC emissions were slightly higher
for the FTP-72 duty cycle that is being used for than the maximum limits for Federal bin 2 and
certification testing of hybrid vehicles. SULEV II. It should be noted that these tests were
The FTP-72 duty cycle represents typical in city carried out in Denver, Colorado, with an altitude
driving. Figure 2 depicts a portion of the FTP-72 of approximately 1 mile. The reduced ambient air

6
Speed*10-1 (mph) and RPM*10-3

60 Vehicle Speed *(10-1)

5
50
4
Speed (mph)

40 RPM *(10-3)
3
30
2
20
1
10
0
0 0 100 200 300 400 500
0 500 1000 1500 Time (seconds)
Time (seconds)
Figure 2. Sample IC engine RPM data for a portion of
Figure 1. FTP-72 duty cycle or vehicle speed trace. the FTP-72 test cycle.

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
 NATURAL GAS INTERNAL COMBUSTION ENGINE 619

density may cause the IC engine to run at higher automatic adjustments by the on-board computer,
rpm. However, automatic adjustments of the air- the test engine tended to run rich, particularly
fuel ratio, the amount of exhaust gas re-circulation during unsteady conditions. This resulted in no
(EGR), as well as other factors, will compensate noticeable increase in PM, NMHC or NOx
for the lower air density and mass air flow, such emissions. However, a slight increase in carbon
that the effects on fuel mileage and emissions are monoxide (CO) was observed compared with
minimal. Grabowski et al. [17] tested a natural gas operation at an altitude of 500 ft [17]. It should
closed-loop, spark ignition engine and an altitude be emphasized that the purpose of the present
of 1 mile (5280 feet). They found that despite dynamometer testing is not for certification
purposes or performance evaluation in an absolute
sense, but strictly for comparison of emissions and
performance with different fuel choices.
0.0500 Figure 4 depicts the CO emissions for the HEV
0.0450 2.1 1.0
grams/ gram/
during CNG testing. Series 1–6 are the CO
0.0400 emissions for each individual FTP-72 test, series
mile mile
0.0350
Emission
Type
7 is the average for all tests, series 8 is the
0.0300
California SULEV II emission limits for LDVs,
grams
mile

0.0250
and series 9 is for Federal bin 2 emission limits for
0.0200
LDVs. The net CO emissions are extremely low,
0.0150
0.0100
and in some cases the net emissions for a single test
0.0050
CO were negative (as low as 0:006 g=mile1 ), indicat-
0.0000
NOX ing that the exhaust flow had less CO than the
1 NMHC ambient air.
2
#1= Average of Six LA4 Tests 3 The average CO emission was 0:050 g mile1 ;
#2 = Bin 2 (Federal LDV) compared with 0:040 g mile1 for gasoline,
#3 = SULEV II (California LDV)
2:1 g mile1 for Federal bin 2, and 1:0 g mile1
Figure 3. Average emissions with gasoline. for SULEV II rating. Figure 5 depicts the

2.1

1.6

1.1
grams
smile

0.6

0.1

-0.4
1 2 3 4
5
CO

6
7
8
Test Number or 9
#7= Average of Six LA4 Tests
#8 = Bin 2 (Federal LDV)
#9 = SULE VII (California LDV)

Figure 4. Carbon monoxide emissions with CNG.

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
620 S. WRIGHT AND A. PINKELMAN

hydrocarbon emissions for the HEV during CNG line, and 0:010 g mile1 for SULEV II and Federal
testing. Methane and THC emissions are included bin 2. Figure 6 depicts the nitrogen oxide ðNOx Þ
in the figure, although they are not restricted by emissions for the HEV during CNG testing. The
the SULEV II or bin 2 ratings. average NOx emission was 0:079 g mile1 ; com-
The average NMHC emission was 0:0133 pared with 0:020 g mile1 for gasoline, and 0:020
g mile1 ; compared with 0:0123 g mile1 for gaso- g mile1 for SULEV II and Federal bin 2.

0.1

0.09

0.08

0.07

0.06
grams
mile

0.05

0.04
0.03
0.02
0.01
0
THC
1 2 CH4

Ty sion
3 4 5 NMHC

pe
6

is
7

Em
8 9
Test Number or
#7 = Average of Six LA4 Tests
#8 = Bin 2 (Federal LDV)
#9 = SULEV II (California LDV)
Note: no THC or CH4 limits on #8,#9

Figure 5. Hydrocarbon emissions with CNG.

0.14

0.12

0.1
grams
mile

0.08

0.06

0.04

0.02

0
1 2 3 4
X

5 6
NO

7
8
Test Number or 9
#7 = Average of Six LA4 Tests
#8 = Bin 2 (Federal LDV)
#9 = SULEV II (California LDV)

Figure 6. Nitrogen oxides ðNOx Þ emissions with CNG.

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
 NATURAL GAS INTERNAL COMBUSTION ENGINE 621

Dynamometer testing revealed that the IC 3.4. Overall benefits of the CNG/hythane hybrid
engine was running too lean at high rpm, resulting
The overall benefits of the hybrid prototype can be
in lean misfire under certain conditions. Re-
summarized as follows:
calibration of the CNG fuel controller for CNG
fuel trim and adjustments to EGR are expected to * Combines alternative fuel benefits and hybrid
reduce the levels of NOx below the 0:020 g mile1 vehicle technology.
limit. The lean misfire also caused the emissions of * Personal home fueling of CNG possible, and
NMHC to be higher than expected and slightly limited infrastructure required for local fueling
above SULEV II and Federal bin 2 ratings. stations due to existing non-transportation
infrastructure.
3.3. Fuel mileage and economic perspective
* Domestic fuels (CNG and hythane) with renew-
able sources.
The composition of the natural gas used during the * Low cost fuel; currently CNG is less than 68%
test was 93.36% methane, 3.52% ethane, 0.69% the cost of gasoline.}
propane, 0.07% iso-butane, 0.11% n-butane, * Designed for use with emerging hydrogen
0.05% higher order alkanes, and 2.20% inerts market with its multi-fuel capability.
(accuracy of concentrations is
0:01%). The lower * No loss of power on CNG due to unique hybrid
heating value of the fuel was 778 Btu mol1 ; or 4 operating environment.
9:8 MJ kg1 : Fuel mileage is determined based on * Reliable, cost-effective with immediate market
fuel consumption as well as the net change in the availability.
state-of-charge (SOC) of the hybrid battery. * Prototype has greater than 750 mile range with
However, monitoring of the electric current to or CNG and gasoline tanks full, more than 300
from the hybrid battery revealed no measurable miles on CNG alone.
net change in SOC during the 1372 s FTP-72 test, * Optional fuel pressures and undercarriage tank
or during the 2744 s back-to-back double FTP-72 locations possible.
test cycle. The average mileage for the FTP-72 * Superlow emissions, SULEV II and Federal bin
testing was 33.8 MPGGE (miles per gallon gas 2 emission levels expected.
equivalent), compared with 34.3 mpg average for
the gasoline tests.
The single CNG tuffshell tank installed on the 4. CONCLUSIONS
vehicle has a 9 GGE capacity. This results in a
vehicle range, based on the FTP-72 cycle, of more The overview of the results presented here demon-
than 300 miles on CNG and 450 miles on gasoline. strates the importance of considering a variety of
The overall cost of using a fuel depends on mileage renewable fuels and power plant options to max-
as well as fuel cost per unit energy released. The imize overall environmental and economic benefits.
alternative fuel data center (AFDC) average Efforts should not focus strictly on PEMFC
national fuel for prices, per gallon of gas development and wide scale production and dis-
equivalent (GGE), for June 2006, was $2.84 for tribution of infrastructure to satisfy the strict
gasoline, $3.43 for E85, $2.88 for propane, $2.67 requirement of pure hydrogen. Also, the differential
for B20 Biodiesel, $3.71 for B100 biodiesel, and benefits from power plant choice can be over-
$1.90 for CNG. It should be noted that the price of shadowed by the advantages obtained from HEV
CNG was $0.77 per GGE lower in price than the technology and alternative fuels. Consequently, IC
second cheapest fuel (B20 biodiesel), and $0.94 per engines are advantageous because they are fuel
GGE cheaper than gasoline. With these numbers flexible, while the PEMFC is relatively fuel inflexible.
for CNG and gasoline and the average mileage for
each fuel during dynamometer testing of the
hybrid, the cost of using CNG is 68% of the cost }
Cost comparisons based on latest AFDC pricing statistics,
of using gasoline. June 2006.

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er
622 S. WRIGHT AND A. PINKELMAN

The methane/hythane IC engine hybrid is identi- THC ¼ total hydrocarbons

fied as a promising pathway that avoids the barriers UCAPS ¼ ultra-capacitors
encountered with conventional non-hybrid natural
gas vehicles, namely range, power and fueling
infrastructure difficulties. Initial dynamometer test-
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Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622
DOI: 10.1002/er