natural gas internal combustion engine hybrid passenger vehicle

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2008; 32:612–622Published 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 dependenceon imported crude oil, addressing the detrimental environmental impact of increasing petroleum usage worldwide, andsustaining the national economy. The question is not whether changes should be made, but instead centers onidentifying pathways that will lead to the greatest environmental and economic benefits. To avoid misuse of limitedinfrastructure investment, the objective of this research is to consider a broad range of relevant factors to determinedesirable power plant–fuel combinations for hybrid electric vehicles. In the long term, fuel cells may dominate thisapplication, but at least in the short term, proton exchange membrane fuel cells (PEMFCs) will not likely offerimmediate substantial benefit over internal combustion (IC) engines. Environmentally friendly operation of thePEMFC 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 fromhybrid electric vehicle technology and alternative fuels. Consequently, the fuel flexibility of IC engines provides anadvantage over the relatively fuel inflexible PEMFC. The methane/hythane IC engine hybrid option, as developed andpresented here, is a promising pathway that avoids the barriers encountered with conventional non-hybrid natural gasvehicles, namely range, power and fueling infrastructure difficulties. Dynamometer testing of the natural gas hybridprototype on the certification FTP-72 duty cycle revealed very low emissions and mileage greater than 33 miles pergallon gasoline equivalent. This hybrid option utilizes a domestic, cost-effective fuel with renewable sources. Withmulti-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

Energy supply and fuel production traditionallyplayed a crucial role in the sustainability of theU.S. and world economies. Recent emphasis on

our national dependence on imported crude oil hashighlighted the need to develop domestic fuels.The percentage of crude oil imported in the U.S. issteadily increasing and will soon reach 60%. Inaddition, the competition for world oil supplies is

*Correspondence to: S. Wright, Department of Mechanical Engineering, University of Colorado at Denver and Health SciencesCenter (UCDHSC), 1200 Larimer Street, Campus Box 112, Denver, CO 80217-3364, U.S.A.yE-mail: [email protected]

Received 8 January 2007Revised 27 August 2007

Accepted 28 August 2007Copyright # 2007 John Wiley & Sons, Ltd.

steadily intensifying, particularly due to theincreasing usage in developing countries. Forexample, crude oil imports in China increased by31% in 2003. As recoverable reserves decrease,competition for the remaining oil will continue toincrease, driving oil prices even higher and furtherjeopardizing America’s energy security. The totalsocietal cost of using petroleum includes manyindirect costs that are not accounted for in thepump price of these fuels [1, 2]. One of the mostcost-effective means of counteracting the growth inthe use of petroleum is implementation of hybridelectric vehicle (HEV) technology.

HEV technology provides both a means ofgreatly reducing fuel usage while also decreasingemissions per gram of fuel consumed. The hybriddesign combines the energy storage advantages ofan electric vehicle with the high capacity or rangeof a conventional vehicle that consumes fuelduring operation. In contrast to conventionalfriction breaking, regenerative breaking saves fuelby recovering a significant portion of the vehicle’skinetic energy. Fuel usage is also decreasedbecause the energy storage unit, usually advancedbatteries or ultra-capacitors (UCAPS), acts as abuffer between the wheels and the power plant.That is, the power plant is more efficient because itcan operate closer to design conditions rather thandirectly following the load at the wheels.

HEV technology also allows the engine to besized to accommodate average load rather thanpeak load, reducing the weight and cost of theengine. The additional cost of the hybrid system,primarily for the energy storage unit, is somewhatoffset by this decreased engine size in addition tofuel savings. Consequently, the benefits of hybridvehicle technology will likely result in wide scalemarket penetration. As a result, power plant–fueloptions discussed in this work will assume opera-tion in the unique operating environment of thehybrid vehicle design. A variety of power plantoptions can be utilized including spark ignition ICengines, compression ignition IC engines, gasturbines and proton exchange membrane fuel cells(PEMFCs).

A number of automotive manufacturers haveintroduced gasoline- or ethanol-powered sparkignition hybrids on the market. Research vehicles

have included hydrogen-powered IC engine hy-brids (such as the Ford Motor Company’shydrogen-powered IC engine Escort), a numberof hydrogen-powered PEMFC hybrids, dieselcompression ignition hybrids (such as GMC’sOpel Astra diesel hybrid or VW’s Golf dieselhybrid) as well as some ethanol-powered IC enginehybrids. The primary reason for lack of interest innatural gas was negative feedback from users ofnatural gas-powered conventional vehicles. Dri-vers often saw a 15–20% reduction in enginepower and reduced vehicle range. However, theseproblems can be effectively eliminated whennatural gas is used in a hybrid vehicle operatingenvironment.

2. ANALYSIS

2.1. Overview of optimal alternative fuel hybridvehicle configurations

The premise of this research is to avoid the misuseof limited infrastructure investment resources byconsidering a broad range of criteria to determineoptimum power plant–fuel combinations forHEVs. That is, identifying pathways that will leadto the greatest environmental and economicbenefits. The optimum choices for hybrid powerplant–fuel combinations depend on balancingmany technological factors including the following:

* Practical power plant performance and effi-ciency currently achieved.

* Performance potential (efficiency and power) inthe unique hybrid vehicle operating environment.

* Total environmental impact from fuel produc-tion to end use (cradle-to-grave).

* Unique equipment requirements and mainte-nance personnel training.

* Power plant reliability, longevity and cost-effectiveness.

* Consumer acceptance and convenience of fueland vehicle.

* Infrastructure requirements; fuel productionviability, distribution and cost.

* Enhanced domestic energy self-sufficiency.* Effect of fuel choice on air conditioning

parasitic load.

NATURAL GAS INTERNAL COMBUSTION ENGINE 613

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:612–622

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At present, PEMFCs are expected by many to bethe dominant power plant choice; as a result, thereis a nationwide focus on the development of fuelproduction and distribution infrastructure forfuels that are compatible with PEMFC, a rela-tively fuel inflexible choice. Considering the gen-erally limiting or prohibitive cost of infrastructuredevelopment, it is imperative that our choice ofoptimal power plant–fuel combinations be basedon the consideration of a broad range of criterialisted above. This is important, because on a localgovernment and fleet operation level, alternativefuel production and distribution efforts are gen-erally guided by little more than personal pre-ference.

2.2. PEMFC or IC engines?

PEMFC designs possess a number of positivecharacteristics for automotive applications. Theseinclude low emission capability, hybrid vehiclecompatibility (for systems with electric energystorage), modularity, desirable efficiency–powercharacteristics,z high efficiency, minimal movingparts and quiet operation. However, at least inthe short term, PEMFCs may not offer animmediate substantial benefit over spark andcompression ignition internal combustion (IC)engines for a variety of reasons in addition to theexpense of design and implementation of a newtechnology [3].

One common misconception is that fuel cellshave an inherently higher theoretical efficiencythan heat engines. For example, Larminie andDicks state that ‘It is quite well known that fuelcells are not subject to the Carnot efficiencylimitation’ [4]. However, contrary to this commonmisconception, fuel cells have the same theoreticalperformance potential as heat engines. The second

law governs the electrochemical conversion pro-cess in fuels cells to the same degree as the Carnotefficiency limitation for the combustion and heatengine process in IC engine [5]. In addition,practical efficiencies for IC engines can be compar-able to those achieved for PEMFCs. See, forexample, the experimental results of Blarigan andKeller [6] for a hydrogen-fueled diesel engine withlow emissions and an efficiency that is verycompetitive (approximately 40%) with the rangeof efficiencies being reported in the literature forPEMFCs.

The direct chemical to electrical conversion in afuel cell requires an electrically powered aircompressor with unique design requirements andassociated cost, as well as substantial parasiticload during operation. Heat rejection is also adifficulty for practical operation, caused by the lowoperating temperature and poor heat transfercharacteristics. In practice this results in unaccep-tably high operating temperatures under certaindriving conditions or the requirement of unusuallylarge heat exchangers. Also, although stack powerdensities are impressive, the power densities for thecomplete fuel cell systems are not. Further,PEMFCs are easily poisoned or contaminated byimpurities, including carbon monoxide, and can beseverely damaged by membrane dry out due to hotspots that tend to lack sufficient hydration atspecific locations in individual cells. Issues such asthese raise questions of in service reliability andservice life.

PEMFCs also currently have fairly high require-ments for platinum group metal (PGM) catalysts.The world market of PGMs is volatile, indepen-dent of the introduction of fuel cell technology.Supply of platinum and palladium is limitedprimarily to South Africa and Russia, these twosources account for 92% of worldwide productionof both these metals. A recent study by Tonn andDas [7] considered a number of factors contribut-ing to the economic feasibility of adequateplatinum supply for large-scale introduction oflight duty fuel cell vehicles. These factors includechanges in other PGM demands worldwide,expected increase in the demand for light dutyvehicles (LDVs) in developing countries, differentscenarios for market penetration of PEMFC

zNote that the efficiency–power characteristics of fuel cells arebetter suited for stop-and-go driving than combustion enginesin non-hybrid vehicles, that is, vehicles without regenerativebreaking and energy storage capability. However, fuel cellsoffer no advantage, in this regard, in a series hybrid designs,and limited advantage in parallel hybrid configurations. Also,fuel cells have limited performance (relatively low efficiency) athigh power demands. As a result, the power–efficiencycharacteristics of fuel cell power plants are not as desirableunder highway driving conditions.

S. WRIGHT AND A. PINKELMAN614


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vehicles, and expected PGM loading in futurePEMFC designs.

Currently, the typical catalytic converter intoday’s vehicles is loaded with approximately1.5 g of platinum or an equivalent 5 g of palladium.However, regulatory requirements for low emis-sion vehicles in developing countries are expectedto result in an increase of PGM loading in catalyticconverters by a factor of three (or 4.5 g ofplatinum per converter). PEMFC designs cur-rently have platinum loading of approximately100 g for LDVs. The US DOE’s long-term PNGVgoal is to reduce platinum loading to 10 g pervehicle [8]. In the Tonn and Das study [7], futuredesigns are expected to have between 5 g (best casescenario) and 30 g of platinum loading. Even in thebest-case scenarios of all four factors listed above,they conclude that production capacities mustincrease much more than the historically manage-able few percent per year in the industry. Theyquestion whether South Africa and Russia bewilling to ramp up for a boom in PGM demandand risk the consequences of a relative bust. Theyconclude that PGM supplies will likely remaintight, prices will remain high and market penetra-tion of new auto technologies based on PGMs willbe difficult at very least.

PEMFCs require the use of pure hydrogen, andthe clean operation of the PEMFC can be mostlyattributed to the requirement of a clean fuel.}

However, IC engines powered with hydrogen canalso operate in an environmentally benign mannerin a hybrid vehicle operating environment, reach-ing the requirements of the California Super Ultra-Low Emission Rating II (SULEV II) rating. ICengines are fuel flexible and thus offer muchgreater flexibility in choosing between cost-effec-tive infrastructure options. IC engines can operateon liquid and gaseous fuels already in use withexisting infrastructure. It should be emphasizedthat, even though a hydrogen PEMFC can operatewith no emissions other than hydrogen and water,environmental impact also depends on emissionsfrom fuel production, such as hydrogen production

from coal and steam methane reforming (SMR).SMR is currently the most cost-effective method forwide-scale hydrogen production but results insubstantial environmental emissions [9]. For exam-ple, a typical hydrogen-powered PEMFC prototypevehicle using SMR hydrogen will produce highernitrogen oxide and carbon dioxide emissions thanthe gasoline powered Toyota Prius [3].

Similar to SMR hydrogen for total emissionsfrom fuel production to end use, ethanol-poweredvehicles also have high cradle-to-grave emissionsand actually are likely to be higher than theirgasoline counterparts [10]. Total emissions includeemissions from sources, such as diesel farmtractors and from fertilizer production. The cost-effectiveness of producing corn ethanol is alsoquestionable [10]. The production of ethanol ismost viable from by-products of the agriculturalindustry rather than cash crops, such as cellulosicethanol production processes. Research efforts arefocusing on improving the efficiency and cost-effectiveness of these processes.

2.3. Hydrogen fuel production

Hydrogen is a powerful, versatile, and clean fuel.Hydrogen acts as an energy carrier and can beproduced from a variety of energy sources [11].Unlike the current energy system, the use ofhydrogen would allow any energy source to beused for transportation, such as solar and nuclearpower. The U.S. DOE hydrogen energy directive[12] primarily represents a focus on hydrogenproduction from coal, nuclear energy and refor-mation of natural gas (methane). For the mostpart, the underlining tenure is that this directiveseeks to reduce national dependence on importedcrude oil while neglecting or aggravating environ-mental concerns and public health effects. TheDOE directive for hydrogen production en-courages pathways that overwhelmingly appearto be either not cost-effective or environmentallyless favorable than current vehicle-fuel life cycleemissions.

The methods for production of hydrogen fromcoal and nuclear energy considered in the DOEdirective require massive infrastructure invest-ments, particularly for distributing hydrogen to

}Specially designed PEMFCs can operate directly on methanol,but reported efficiencies are low compared with hydrogen-powered PEMFCs.



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fueling stations and certainly do not appear torepresent a viable short-term solution. Productionof hydrogen from coal either results in the releaseof extreme levels of carbon dioxide (approximately3.6 tons CO2 per ton of coal) or high costassociated with carbon dioxide sequestration.Use of coal-produced hydrogen results in therelease of an assortment of other environmentalcontaminants, such as heavy metals. At least in theshort term, hydrogen production from nuclearpower is also questionable. Nuclear power hasmany positive attributes, but various disadvan-tages appear to make the process presently non-viable, such as massive capital investment, publicresistance, extensive safety precautions and asso-ciated costs, creation of attractive terrorist targets,weapons grade nuclear fuel production, andnuclear waste disposal costs and concerns [3].

Renewables such as off peak hydroelectric, windpower and solar energy provide clean hydrogenproduction, but current cost-effectiveness com-pared with other options is the main obstacle. Inaddition, hydrogen production from sources suchas hydroelectric and biomass, even in the longterm, can only provide a small fraction ofhydrogen production required for the automotivesector. An obvious advantage of solar energy overbiomass-derived fuel production is more efficientuse of land area and the acceptability of wastelands rather than productive farm land. With allfactors considered, hydrogen derived from solarenergy through thermal processes is likely to bea winning hydrogen production process thatbalances cost-effectiveness and minimization ofenvironmental intrusion.

Thermal processes are by nature the most cost-effective and efficient solar conversion processesfor large-scale plants. The cost-effectiveness ofsolar parabolic trough systems, for example, isnearly competitive with conventional power pro-duction plants and are expected to steadilyincrease ($0.060 per kWh possible by 2015), asadvances in the technology over the last twodecades see application in new plants [13]. Otherprojections for wide scale implementation ofparabolic trough technologies estimate electricitycosts as low as $0.035 per kWh by 2020 withoutnew research breakthroughs [14]. The DOE report

for the Western Governors Association in 2005provided an assessment of the potential impact ofconcentrating solar power (CSP) systems. ThisDOE report estimated the potential power pro-duction, using only available and most suitableland with the most intense sunshine, as over6800GW of electricity generation in the Southwest[15]. In comparison, the current total U.S.electricity generating capacity is approximately1000GW. However, CSP technologies requirepolicy incentives for initial deployment. As aresult, the U.S. Federal government has createdan investment tax credit that encourages thedeployment of CSP. This has resulted in thedevelopment of new CSP projects in the states ofCalifornia, Arizona, and Nevada, totaling2000MW, and scheduled for completion by 2010.

SMR is currently the most cost-effective methodfor wide-scale hydrogen production. Reformation ofnatural gas by SMR to produce hydrogen at fuelingstations utilizing the existing natural gas distributionsystem is more viable than centralized reformationand distribution of hydrogen. However, direct useof natural gas as the vehicle fuel is more advanta-geous in that no reformation equipment is requiredon the vehicle thereby reducing complexity and cost.

Natural gas has renewable sources and there arevast quantities of fossil natural gas reservesworldwide. Proven worldwide natural gas reservesare presently over 6183 tcf (trillion cubic feet) [16].Proven sources are defined as reserves that can berecovered under current technology and currentprices. In comparison, world consumption was106 tcf in 2006. U.S. natural gas reserves, as statedby the U.S. Energy Information Administration(EIA), are 1191 tcf. The National PetroleumCouncil puts these reserves at over 1800 tcf. Incomparison, U.S. natural gas consumption hasbeen nearly constant for over a decade, at between21.8 and 23.0 tcf (21.8 tcf in 2006). At present theU.S. consumption rates, independent of renewablebiomass sources, proven the U.S. fossil reserveswould last between 55 and 83 years, assuming nonew proven reserves were identified. However,note that proven natural gas reserves have doubledover the last 25 years and are expected to continueto rise. International sources may also be utilizedas international transport of liquefied natural gas



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is steadily increasing (in 2006 14% of the U.S.natural gas imports where liquefied natural gasimported from overseas). In comparison, estimatesof worldwide peak oil range from the present to2037 at the latest.

3. RESULTS AND DISCUSSION

The lack of interest in natural gas as a hybridvehicle fuel is primarily due to negative feedbackfrom its use in conventional vehicles includingreliability and other performance issues (primarilydue to poor fuel control systems), lack of vehiclepower compared with gasoline, and limited range.However, these problems can be effectively elimi-nated when natural gas is used properly in ahybrid vehicle operating environment.

The most cost-effective means of producing anatural gas IC hybrid was to convert a newlyreleased gasoline hybrid to operate on the gaseousfuel. The conversion of a 2005 Ford Escapegasoline IC engine Hybrid was successfully carriedout in January 2005, in collaboration with ECOFuel Systems Inc. and the University College ofthe Fraser Valley (UCFV). This involved installa-tion of a composite fiber compressed gaseous fueltank, a sub-controller used to control the gaseousfuel system, gaseous fuel injectors, gas pressureregulator, gages, heaters and other equipment. Thegaseous fuel system was installed in completeredundancy with the gasoline fuel system so thatthe vehicle could operate on either fuel at the pushof a button installed in the driver area. The systemwas thus designed to operate on three fuels;gasoline, methane (or natural gas) as well ashythane (mixtures of hydrogen and methane). Thisis not only important for testing purposes but alsorepresents an important design feature for con-sumers; flexibility in fueling alleviates consumers’concerns regarding re-fueling availability, a pro-blem in the past for alternative fuels.

The vehicle range problem encountered withconventional CNG vehicles was overcome sincethe fuel mileage of the hybrid vehicle is muchhigher than conventional vehicles. As a result,there is no difficulty in obtaining an acceptablevehicle range with natural gas operation. The

hybrid prototype tested in this work had a rangegreater than 300 miles on CNG (with a 9 GGECNG tank) and an additional 450 miles ongasoline, based on the FTP72 duty cycle. Greaterrange can be obtained if larger or additional tanksare installed, or if higher pressures are used for fuelstorage. This vehicle range is well within theacceptable range expected by consumers. Inaddition, the dual fuel capability, and the possibi-lity of home fueling with natural gas, alleviates thedifficulty encountered with limited fueling infra-structure encountered in the past.

The CNG hybrid prototype had no noticeablelack of power when operated on natural gascompared with gasoline. This is due to the paralleloperation of the electric drive and the IC powerplant. The main implication of slightly reducedpower from the IC engine is that the IC engineturns off less frequently than when operating ongasoline. This can be eliminated, if desired, byforced air intake, increased compression ratio orby slightly increasing engine size. It is important tonote that the IC engine size (2.3 L) in the hybrid issubstantially less than in the conventional versionof the Escape, leaving ample space for a largerengine in the engine compartment.

Design for operation on hythane allows for thepotential utilization of any energy source, includingnuclear, hydroelectric and other renewable energysources such as solar and wind power. As a resultthe prototype has immediate market potential aswell as long-term viability as hydrogen productiondevelops. Operation on CNG provides immediatemarket viability as these vehicles can be conveni-ently fueled at home, for those residences withnatural gas supply, and additional local fuelingstations can be easily added to those alreadyavailable as underground natural gas infrastructureexists in most urban areas. Natural gas can also beproduced in agricultural regions or shipped bytanker trucks to any areas that require that mode offuel distribution. An overview of the advantages ofthis prototype is given in Section 3.4.

3.1. Dynamometer testing and results

Performance and emissions testing of this proto-type provide unique data for consideration of



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power plant–fuel options. Standardized dynam-ometer duty cycle testing of this vehicle on CNGand gasoline was carried out with the Environ-mental Protection Agency (EPA) certification dutycycle consisting of two back-to-back FTP-72 testswith a 10min delay between tests. The FTP-72 orLA4 cycle consists of the first 1372 s of the FTP-75standard test cycle. Road load coefficients wereobtained from Ford Motor Company and used tosimulate actual road conditions on the dynam-ometer.

The vehicle was tested on three sets of FTP-72tests at the State of Colorado Emissions Center(CFR 40 Parts 86 and 100 certification analysis)using a Horiba 48 in. electric 2WD dynamometerto simulate EPA hybrid certification. Electronicoverride of the PCM was required to allow twowheel drive testing without having onboarddiagnostic trouble codes (DTCs) and vehiclemalfunction. The exhaust sampler used was aBeckman constant flow venturi with a Horiba non-dispersive infrared analyzers for CO and CO2

(�1 ppm accuracy), Horiba flame ionization ana-lyzer for total hydrocarbons (THCs) (�1 ppmaccuracy), Horiba chemiluminescent analyzer forNO=NOx (�1 ppm accuracy) and a Horibamethane cutter for determination of non-methanehydrocarbons (NMHCs).

Figure 1 shows a plot of the vehicle speed tracefor the FTP-72 duty cycle that is being used forcertification testing of hybrid vehicles.

The FTP-72 duty cycle represents typical in citydriving. Figure 2 depicts a portion of the FTP-72

vehicle speed trace and a sample dynamometer testresult for the RPM of the IC engine. Figure 2shows regions where the IC engine is off eitherbecause power supplied by the electric drive motoris sufficient to power the vehicle, the vehicle is atrest and no power is needed, or when the vehicle isin a state of deceleration and regenerative breakingis charging the hybrid battery pack.

3.2. Emissions

Dynamometer testing was carried out with gaso-line as a baseline for comparison with CNG testresults. Carbon monoxide (CO), carbon dioxide(CO2), nitrogen oxides ðNOxÞ; NMHCs and THCswere measured for three sets of back-to-back FTP-72 tests. Figure 3 depicts the emissions for theHEV during these tests as well as California andFederal limits. Series 1 is the average emissionsfrom the three back-to-back FTP-72 tests, series 2is the California SULEV II emission limits forLDVs, and series 3 is for Federal bin 2 emissionlimits for LDVs. Note that the CO limits for bin 2and SULEV II are much higher than actuallydepicted.

As can be seen in Figure 3 the average COemissions were well within both the Federal bin 2and the SULEV II limits. However, the averageNOx and NMHC emissions were slightly higherthan the maximum limits for Federal bin 2 andSULEV II. It should be noted that these tests werecarried out in Denver, Colorado, with an altitudeof approximately 1 mile. The reduced ambient air

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Figure 1. FTP-72 duty cycle or vehicle speed trace.

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Figure 2. Sample IC engine RPM data for a portion ofthe FTP-72 test cycle.



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density may cause the IC engine to run at higherrpm. However, automatic adjustments of the air-fuel ratio, the amount of exhaust gas re-circulation(EGR), as well as other factors, will compensatefor the lower air density and mass air flow, suchthat the effects on fuel mileage and emissions areminimal. Grabowski et al. [17] tested a natural gasclosed-loop, spark ignition engine and an altitudeof 1 mile (5280 feet). They found that despite

automatic adjustments by the on-board computer,the test engine tended to run rich, particularlyduring unsteady conditions. This resulted in nonoticeable increase in PM, NMHC or NOx

emissions. However, a slight increase in carbonmonoxide (CO) was observed compared withoperation at an altitude of 500 ft [17]. It shouldbe emphasized that the purpose of the presentdynamometer testing is not for certificationpurposes or performance evaluation in an absolutesense, but strictly for comparison of emissions andperformance with different fuel choices.

Figure 4 depicts the CO emissions for the HEVduring CNG testing. Series 1–6 are the COemissions for each individual FTP-72 test, series7 is the average for all tests, series 8 is theCalifornia SULEV II emission limits for LDVs,and series 9 is for Federal bin 2 emission limits forLDVs. The net CO emissions are extremely low,and in some cases the net emissions for a single testwere negative (as low as �0:006 g=mile�1), indicat-ing that the exhaust flow had less CO than theambient air.

The average CO emission was 0:050 g mile�1;compared with 0:040 g mile�1 for gasoline,2:1 g mile�1 for Federal bin 2, and 1:0 g mile�1

for SULEV II rating. Figure 5 depicts the

1 2 3 4 5 6 7 8 9

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Figure 4. Carbon monoxide emissions with CNG.

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Figure 3. Average emissions with gasoline.



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hydrocarbon emissions for the HEV during CNGtesting. Methane and THC emissions are includedin the figure, although they are not restricted bythe SULEV II or bin 2 ratings.

The average NMHC emission was 0:0133g mile�1; compared with 0:0123 g mile�1 for gaso-

line, and 0:010 g mile�1 for SULEV II and Federalbin 2. Figure 6 depicts the nitrogen oxide ðNOxÞ

emissions for the HEV during CNG testing. Theaverage NOx emission was 0:079 g mile�1; com-pared with 0:020 g mile�1 for gasoline, and 0:020g mile�1 for SULEV II and Federal bin 2.

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Figure 6. Nitrogen oxides ðNOxÞ emissions with CNG.

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Figure 5. Hydrocarbon emissions with CNG.



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Dynamometer testing revealed that the ICengine was running too lean at high rpm, resultingin lean misfire under certain conditions. Re-calibration of the CNG fuel controller for CNGfuel trim and adjustments to EGR are expected toreduce the levels of NOx below the 0:020 g mile�1

limit. The lean misfire also caused the emissions ofNMHC to be higher than expected and slightlyabove SULEV II and Federal bin 2 ratings.

3.3. Fuel mileage and economic perspective

The composition of the natural gas used during thetest was 93.36% methane, 3.52% ethane, 0.69%propane, 0.07% iso-butane, 0.11% n-butane,0.05% higher order alkanes, and 2.20% inerts(accuracy of concentrations is �0:01%). The lowerheating value of the fuel was 778 Btu mol�1; or 49:8MJ kg�1: Fuel mileage is determined based onfuel consumption as well as the net change in thestate-of-charge (SOC) of the hybrid battery.However, monitoring of the electric current to orfrom the hybrid battery revealed no measurablenet change in SOC during the 1372 s FTP-72 test,or during the 2744 s back-to-back double FTP-72test cycle. The average mileage for the FTP-72testing was 33.8MPGGE (miles per gallon gasequivalent), compared with 34.3mpg average forthe gasoline tests.

The single CNG tuffshell tank installed on thevehicle has a 9 GGE capacity. This results in avehicle range, based on the FTP-72 cycle, of morethan 300 miles on CNG and 450 miles on gasoline.The overall cost of using a fuel depends on mileageas well as fuel cost per unit energy released. Thealternative fuel data center (AFDC) averagenational fuel for prices, per gallon of gasequivalent (GGE), for June 2006, was $2.84 forgasoline, $3.43 for E85, $2.88 for propane, $2.67for B20 Biodiesel, $3.71 for B100 biodiesel, and$1.90 for CNG. It should be noted that the price ofCNG was $0.77 per GGE lower in price than thesecond cheapest fuel (B20 biodiesel), and $0.94 perGGE cheaper than gasoline. With these numbersfor CNG and gasoline and the average mileage foreach fuel during dynamometer testing of thehybrid, the cost of using CNG is 68% of the costof using gasoline.

3.4. Overall benefits of the CNG/hythane hybrid

The overall benefits of the hybrid prototype can besummarized as follows:

* Combines alternative fuel benefits and hybridvehicle technology.

* Personal home fueling of CNG possible, andlimited infrastructure required for local fuelingstations due to existing non-transportationinfrastructure.

* Domestic fuels (CNG and hythane) with renew-able sources.

* Low cost fuel; currently CNG is less than 68%the cost of gasoline.}

* Designed for use with emerging hydrogenmarket with its multi-fuel capability.

* No loss of power on CNG due to unique hybridoperating environment.

* Reliable, cost-effective with immediate marketavailability.

* Prototype has greater than 750 mile range withCNG and gasoline tanks full, more than 300miles on CNG alone.

* Optional fuel pressures and undercarriage tanklocations possible.

* Superlow emissions, SULEV II and Federal bin2 emission levels expected.

4. CONCLUSIONS

The overview of the results presented here demon-strates the importance of considering a variety ofrenewable fuels and power plant options to max-imize overall environmental and economic benefits.Efforts should not focus strictly on PEMFCdevelopment and wide scale production and dis-tribution of infrastructure to satisfy the strictrequirement of pure hydrogen. Also, the differentialbenefits from power plant choice can be over-shadowed by the advantages obtained from HEVtechnology and alternative fuels. Consequently, ICengines are advantageous because they are fuelflexible, while the PEMFC is relatively fuel inflexible.

}Cost comparisons based on latest AFDC pricing statistics,June 2006.



DOI: 10.1002/er

The methane/hythane IC engine hybrid is identi-fied as a promising pathway that avoids the barriersencountered with conventional non-hybrid naturalgas vehicles, namely range, power and fuelinginfrastructure difficulties. Initial dynamometer test-ing, on the certification duty cycle for hybrid vehicles,revealed emissions well below SULEV II for carbonmonoxide, 0:059 g mile�1 above for nitrogen oxides,0:003 g mile�1 above for NMHCs and a33.8MPGGE mileage rating (compared with34.3mpg on gasoline). With calibration and EGRadjustments it is expected that the SULEV IIemissions levels can be achieved. This hybrid optionutilizes a domestic, cost-effective fuel with renewablesources. With its multi-fuel capability (methane,hythane, and gasoline) it is also designed for usewith emerging hydrogen market. This is a reliable,cost-effective technology with immediate and widespread market availability.

NOMENCLATURE

Acronyms

AFDC ¼ Alternative Fuel Data CenterCNG ¼ compressed natural gasDOE ¼ Department of EnergyDTC ¼ diagnostic trouble codesEGR ¼ exhaust gas re-circulationEPA ¼ Environmental Protection

AgencyGGE ¼ gallons of gasoline equivalentHEV ¼ hybrid electric vehicleIC ¼ internal combustionLDV ¼ light duty vehicleMPGGE ¼ miles per gallon gasoline

equivalentNMHC ¼ non-methane hydrocarbonsPCM ¼ power control modulePEMFC ¼ proton exchange membrane

fuel cellRPM ¼ revolutions per minuteSMR ¼ steam methane reformingSULEV II ¼ California Super Ultra-Low

Emissions Rating IISOC ¼ hybrid battery state of charge

THC ¼ total hydrocarbonsUCAPS ¼ ultra-capacitors

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DOI: 10.1002/er

natural gas internal combustion engine hybrid passenger vehicle

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