JI

4 CLEARI NGU OUSE I_KI FOR PED~ ~ C NTIFIC AND

TECHNICAL 1NFORMATIONGARABEDIAN and JOiMSON Hrdeopy IMiorof iehel I

+ + "THE THEORY OF OPERATION OF AN AMMlONIABURNING INTERNAL COMBUSTION ENGINE"

CHRLES G. GARABEDIAN and JOHN H. JOHNSON

HQ* US ARMY TANK-AUTOMOTIVE CENTERWARREN, MICHIGANJ00 JUL 61 T

INTRODUCTION L L d--,- .

+-- Logistics studies of Army operations in World WaLE

II and Korea established that approximately 65% of thetotal tonnage required for support of combat operationsSconsisted of fuels and lubricants. To compound thisalready heavy logistical burden, future Army conceptsenvision increased mechanization and greater emphasis onmobility and dispersion. Faced with these problems, theArmy searchee for other materials and devices for vehiclepropulsion. Nuclear energy seemed the apparent answer.

Analysis proved that direct use of nuclear energypresented serious problems of application to ',ehicles.Attention was then turned to other potential applicationsof a nuclear energy power source. The Army in a coopera-tive research and development effort established thepracticality of mobile nuclear reactors as a source ofenergy in the field. These studies indicated threepossible approaches wherein nuclear energy could be usedwith direct or indirect energy conversion devices or as

- the power source could provide the energy to synthesizechemical fuels. Further studies indicated that to realizeearly payoff the latter approach held the most promise.it was decided that a nuclear power source could providethe energy to synthesize chemical fuels with air andwater as the on-site raw materials. This concept to pro-vide on-site manufacturing of fuels is referred to as theMobile Energy Depot (MED). Materials showing the greatest

. potential were hydrogen, ammonia, hydrazine and hydrogen

41I

GAP?BEDIAN and JOHNSON

peroxide. Factors concerned with physical and chemicalproperties, handling, storage and dispensing of the fourfuels led to the choice of ammonia as the fuel with thegreatest potential.

As part of the overall feasibility study of theMobile Energy Depot concept, the U.S. Army Tank-Automotive Center was assigned the task to investigate

* the use of anhydrous ammonia as an alternate fuel forreciprocating engines in military automotive equipment.

Two basic approaches are presented for achievingcombustion of ammonia in internal combustion engines.The first approach is one of integrating the ammoniacombustion capability into the research and developmentof engines for the 1975-80 time frame. This approachwould have the objective of building a multi-fuel enginewhich could use MED or commercially produced ammonia.line second approach is one of converting existing enginesby the uste of modification kits. This approach could beapplied to engines in the 1965-70 time frame, and wouldappear to require the use of ammonia produced commer-cially since the MED concept has application in the 1980time frame.

PAST WORK

The use of ammonia as a fuel for internal combus-tion engines has been around at least since the year 1935(1). A more extensive use of ammonia as a fuel was under-taken on vehicles in Belgium in 1942 (2). In this caseammonia vapor plus coal gas was burned in the engine.More recent data on the MED concept and single cylinderand multi-cylinder research work is presented inReference (3). In Reference (3) several practical methodsfor improving engine performance while burning ammoniaare described which include increased spark ener-y,increased compression ratio, engine supercharging, andhydrogen addition to the fuel. This study indicates thatsatisfactory engine performance can be obtained whileburning ammonia.

A more recent paper (4) considers both teory andapplication of ammonia for spark ignition engines. The

. results of this work showed that spark timing for maximiu

", i, " GARABEDIAN and JOHNSON

performance must be advanced slightly for ammonia butsensitivity to spark timing is slightly greater than with

- -- :hydrocarbons. The theory showed that maximum indicated

" output using ammonia vapor is about 77% of that with

.,_. hydrocarbons and specific fuel consumption increases two

times. In addition, the data showed a 2 2 time increase

in specific fuel consumption at maximum economy.

The U. S. Army Fuels and Lubricants Research

Laboratory has recently presented data (5) on the compat-

ibility of ammonia and its combustion products with enginematerials and lubricants. In addition, a further studyof compression ignition engine perf:-rmance was made toascertain the ability of ammonia to be pumped in existinginjection systems and various means of achieving ammoniacombustion were explored.

we Several of the general conclusions of this paperwere:

I. Compatibility of ammonia and its combustion productswith engineering materials and lubricants presents nosubstantial problem.

2. Ammonia-only combustion requires high compressionratios and temperatures (35:1 compression ratio, 300OFair and coolant).

3. None of the fuel additives investigated significantlylowered the energy level requirement for ammonia ignition.

4. Gases introduced into the intake manifold resulted in-. - ammonia combustion although amounts required were high

(10% hydrogen, 15-20% acetylene).

PROPERTIES AND. COMBUSTION CMARACTERISTICS OF AMMONIA

There are many problems in burning ammonia in

Spark-Ignition (SI) and Compression-Ignition. (CI) engines.Most of these problems carn be related to the propertiesand combustion characteristics of ammonia. The combustioncharacteristics that present the largest problems withammonia are the following:

(1). Very high autoignition temperature.

13

GARABEDIA} and Johnson

(2) Low flame speed.

(3) Narrow flammability limits.

(4) High heat of vaporization.

These properties are better illustrated in Table 1 wherea direct comparison is made of the properties of hydrogen,diesel, gasoline and ammonia fuels. Fuels ith a highautoignition temperature and long delay are good SI enginefuels while those of low autoignition temperatures andshort delays are good CI fuels. From Table 1 the highautoignition temperature and the narrow flammabilitylimits point out clearly why ammonia present5 manyproblems in the CI engine. Ammonia further t:omplicatesmatters in the CI engine with its high latent heat ofvaporization which causes very large gas temreraturedrops at the time of injection. The characteristics oflow flame velocity and narrow flammability linits presentthe main combustion problems in an SI engine althoughthey can be overcome with the proper design.

S// , Ii / /

• .. -

[£.. --

?I

4-

m m\: - - - -- - f- - - - -

-4mmc m F)> cc CM () > r m-r -o 09 0

z o z 0 0r) Z) Z 0- Z0 -(-4) -n C0-- ;;< < (A~

Omv rri 0i m-u3CA m nA ?; m nm -0 o (A Mz mL

-C >1 A 1 47 Z0 500~C > M- z. ;a 0 -In I- > 0g > ? 0 o

o 0 0>0 0 -

"n -n Z

00CA z

013

M I -< -> -

- 0 -40 0 0 - CA) C)ch '0 ;o ,

m m

0 0~z z"a

-4 m

*1 - > -n(A)

ODF CI N. w

10 0l I

1* 0 aD 40 CA) *

GARABEDIAN and JOHNSON

THE THEORY OF OPERATION OF AN AMMONIA BURNING SI ENGINE

When burning a stoichiometric mixture of ammoniaand air, the following chemical equation holds:

2 NH3 + 1.5 02 + 5.65 N2 -*3 H20 + 6.64 N2

Due to slow flame speed and narrow flammability limits,the complete combustion of ammonia in an ST engine is hardto achieve. The data in Reference (3) strongly suggestthat the approach of disasso ating the ammonia vapor toachieve approximately 2-3% hydrogen by weight would be apractical way to operate an ammonia burning SI engine.The use of hydrogen to promote the combustion of ammoniais a logical choice because theoretically it could beproduced on the vehicle itself by disassociating some ofthe ammonia fuel in a catalytic disassociator. As shownin Table 1, hydrogen has wide flammability limits, isignited easily and has a high flame speed. The dis-association of ammonia to nitrogen and hydrogen iscontrolled by the following equation:

2 NH3--oN 2 & 3 H2

The above equation shows that for each volume of ammoniadecomposed, one-half volume will be nitrogen and one andone-half will be hydrogen. In addition, for each per centby weight of hydrogen, 5.67% by weight -f ammonia willbave to be d3composed which will result in 4.67% ofnitrogen.

The approach to ammonia combustion in an SI enginein this paper is based on the use of hydrogen enrichment.The hydrogen is obtained by passing ammonia through adisassociator. Basically, the disassociator is a shelland tube type heat exchanger which uses a catalyst andengine exhaust energy to partially disassociate ammoniainto nitrogen and hydrogen. In order to achieve fullpower output from a conventional SI engine, a superchargerdriven off the engine is required. A schematic of thefuel tank-engine system is shown in Figure 1.

EXPERIMENT.AL11 DATA 01M T • A , T

For initial experimental ammonia combustion studiesa 4-cylinder, spark-ignition L-141t 65 HP gasoline mili-tary engine was used. Although the data is of a prelimi-n nary nature, several of the results are significant.

I

r -T

GARABEDIAN and JOHNSON

The L-141 engine has been set up and performancecurves determined for both combat gasoline and ammonia.The system for which the engine is designed was describedin thL- previous paragraph and the schematic of Figure 1.The data presented in Figure 2 is without the disassocia-tor system axld is for simulate. hydrogen enrichment, andshows various maximum power .urves for the L-141 engine.Initially, the engine was an on combat gasoline in theas received" condition. These data are shown by thesolid base-line curve in Figure 2. The L-141 engine wasfirst converted to ammonia operation by changing over tothe LPG carburetor and holding all other components fixed.This resulted in the engine being limited to a maximum of10 horsepower and speeds less than 2,000 rpm.

Next, the disassociator was simulated and theengine was given the optimum amount of hydrogen and besttorque spark advance. This resulted in the engine beinglimited to a maximum of 27.5 horsepower (at 2,000 rpm)and speeds less than 3,200 rpm as shown in Figure 2. Thehead on the standard L-141 engine was then changed to onethat resulted in a 12.6:1 compression ratio (CR). Whenoperating on ammonia vapor alone and a 12.6:1 CR, theengine was limited to 17 horsepower (at 1,850 rpm) andspeeds less than 2,300 rpm. Operating with the optimum

33

SPARK-INGNITION ENGINE FUEL SYSTEM SCHEMATICFUEL PUMP

F ~tMMONIADISSOCIATION

TANKPRO DUCTSCOOLER

PRESS. REG. '_____ S...I

MUFFLER

PREHEATER BPS

DISSOCIATOR

BYPASS ARI

EXHAUST GAS

Figure 1.

COMPARISON OF PERFORMANCEd* GASOLINE VS. AMMONIA, FUEL FOR L-141 ENGINE

0'- 07.5:1 C.R.l~---~ 12.6:1 C.R. 5.0

SESTIMATED ZT U.'

-70- 400

OPTIMUM H2% C

z60- NH-3 + OPTIMUM H2 20 y

U.'

-- 1.0

50- NjIN 3 + 4. 0% H2

00

N3 +. 0%H2

x NH3 + PTIMU H2U U

101 + OPTIMLB/HR.

lo--NH2 LB/HR.NOTE:H2

NH-3 ONLY

1200 2000 2600 3200 4000

ENGINE RPM

Figure 2.

GARABEDIAN and JOHNSON

hydrogen concentration at the 12.6:1 CR resulted in engine

performance up to 4,000 rpm. In addition, power outputwas above that which is predicted theoretically

(Reference 4). Although the power output on ammonia wasacceptable, engine operation was still very sensitive tothe hydrogen concentration. This can be seen in Figure2

where 2, 3 and 4% of hydrogen made great differences inoutput power.

The data on the L-141 engine is very encouraging

but will require much more work. Two pr-blP_,n areas thatrequire further investigation are the z.--alyst used inthe disassociator and the problem of ha-,ing a responsiveengine. Preliminary running with a di'sas.iociator hasshown that the catalyst used, triplo-promoted iron, isunacceptable for automotive engine application since itpowders from the engine vibrations. The problem of engineresponse is related to the disassociator system and thehigh heat of vaporization of ammonia. When a sudden de-mand for fuel (ammonia) by the engine is made, theincreased flow of ammonia apparently cools the systemexcessively, resulting in reduced hydrogen output. Thisin turn reduces exhaust temperatures which the disassocia-tor system depends on for the energy to disassociateammonia. This process is cumulative, resulting in markedreduction in the efficiency of the disassociator system.Work is now in progress to correct these two problemareas.

VEHICLE MODIFICATIONS FOR AMMONIA OPERATION

The components required to convert a conventionalSI engine to ammonia operation have been briefi y discussedin a previous paragraph. In addition to these components.new cylinder head (s), intake manifold (s), exhaust mani-fold(s), starting aids, cooling system modifications, andminor material changes would be required to convert agasoline burning SI engine to ammonia capability.Reference (6) presents data for the estimated costs ofconverting present engines from gasoline burning to

" ammonia burning. These cost figures showed that the con-versions would cost from 50 to 200% of the original costsof the standard engine.

In addition to the engine conversion, a completelynew fuel tank is required for ammonia. To determine thesize of ammonia fuel tanks for a particular vehicle, thebaseline used was to provide for the volume of ammoniafuel equal in heating value to that of the hydrocarbonfuel carried on the standard vehicle. Using the fuel tankcapacity of. the standard vehicle, and increasing it by

GARABEDIA'N and JOHNSON

2.8 times, a basic volume required for ammonia tanks wasdetermined. This 2.8 volume factor for going from hydro-carbon fuels to ammonia is one of the biggest disadvan-tages of ammonia as a military fuel since engine fuel

tank space is such a premium.

The high vapor pressure values of ammonianecessitated pressurized vessel design. Reference (6)goes into greater detail for the design and installationof fuel tanks for eight typical military vehicles. Thetank design must consider both engine heat rejection loadsand solar radiation loads. A cylindrical tank design wasused on all wheeled vehicles studied in Reference (6), inorder to take advantage of high strength to weight ratios.In the tracked vehicles (6), because of space limitations,the hydrocarbon fuel tanks were replaced with ammonia fueltanks of similar configuration and were constructed withflat plate alloy steel. A wall thickness of 0.75 inch wasrequired in the flat plate design for structural integrityto withstand 310 psia pressure for ammonia at the 1250 Ftemperature encountered within the engine compartment.One inch vinyl insulation was used for all tracked-vehiclefuel tanks in Reference (6) to assure that ammonia fueltemperatures did not exceed 125 0F. The ammonia Euel tankweights for the wheeled vehicles varied from 2.2 - 2.8lbm/gallon of capacity and the tracked vehicles variedfrom 12 - 15 lbr1.'gallon of capacity (6). Figures 3 and 4show the ammonia fuel tank installation for the 1/4 tontruck, M151 and the 3/4 ton truck M37B1. These fuel tankinstallations removed approximately 15% of the available

* cargo space of these vehicles.

COMPRESS ION-IGNITION DATA

Due to the large number of problems of burningammonia in a compression-ignition (CI) engine and the low

*-probability.of success in the 1965-70 time frame, it wouldbe best to convert CI engines to operate as spark-ignitionengines for this period. Recently(7) some success hasbeen obtained on a single cylinder CFR engine using earlyliquid ammonia 4%uel injection and spark-ignition with aconventional diese. engine. Under these conditions ofspark operation, very early fuel injection is required tooperate the diesel engine on liquid ammonia since it ishypothesized the fuel charge must have sufticient timebefore ignition for evaporation and disassociation totake place (7). Further work will be required to provethe feasibility of this approach.

Another interesting approach to ammonia combustionin a CI engine is to introduce ammonia vapor at engineinlet terperature into the intake manifold of the engine

qo_

AMMONIA FUEL TANK INSTALLATION ON 1/4 TON TRUCK, M151

AMMONIA FUEL TANK CAPACITY 50 GAL.

AMMONIA .UEL TANK WEIGHT 140 LBS. (2 TANKS)

AMMONIA FUEL WEIGHT 253 LBS. TOTAL

TOTAL WEIGHT FUEL TANKS PLUS FUEL 393 LBS.

- 62-1/4

Figure 3.

AMMONIA FUEL TANK INSTALLATION ON 3/4 TON TRUCK, M37BI

AMMONIA t;" IEL TANK CAPACITY 67 GAL.

AMMONIA FUEL TANK WEIGHT 160 LBS. (2 TANKS)

AMMONIA FUEL WEIGHT 343 LBS.

TOTAL WEIGHT FUEL TANKS PLUS FUEL 503 LBS.52.4

16 DIA

Figure 4.

. •

>3

4 GARABEDIAN and JOHNSON

followed by a small amount of diesel fuel injected withthe conventional fuel injection system as the pilotcharge. The above experiment was carried out on the VeeTwin 1790 engine. The amount of diesel fuel was lessthan 4% of the ammonia vapor by weight. The maximumhorsepower curves for operation of the Vee Twin 1790 ondiesel fuel only and ammonia vapor plus diesel fuel areshown in Figure 5. It should be rioted that the operationon ammonia plus diesel fuel resulted in greater power out-put than with diesel fuel only. This is caused by betterair utilization under these conditions. Part-load datahave been taken using this approach and, base; on thenarrow flammability limits of ammonia, throttling will berequired.

This approach l.'s very good for the CI engine ifthe use of hydrocarbon fuels is allowed. Convertingpresent engines to this approach would cost nearly thesame as for converting them to SI engines. This CI engineconcept looks particularly good for future multi-fuelengines.

THE CONCEPT OF A HYDROCARBON-AMMONIA BURNING ENGINE

The concept of a reciprocating engine that couldburn both ammonia and hydrocarbon fuel may at first seeminsurmountable. The use of the Variable CompressionRatio (VCR) piston (8) would lend itself to the design ofa multi-fuel engine. The VCR piston i -in automatichydraulically actuated piston that prov.Jes a practicalmethod of obtaining a variable compression ratio engine.

The data in Figure 2 show that the performance ofthe SI ammonia engine is substantially better at the12.6:1 compression ratio (CR). This CR would be highlyobjectionable for operation on gasoline. Peak pressuresfor the 12.6:1 CR with ammonia are of the same order ofmagnitude as the 7.5:1 CR with gasoline. Hence, a multi-fuel SI engiie would be possible using the VCR pistons.In addition to the VCR pistons, means to advance the sparkfor operating on amonia would be required. Also both anLPG and .a conventional carburetor would. be required formetering the two fuels. This approach of building amulti-fuel engine would only be practical if this capa-bility was designed into the engine during the R&D phase.Admittedly, this multi-fuel SI engine .ould be anexpensive engine but it would still be cheaper than whatit would cost to convert engines in the present militaryfleet to ammonia only operation.

For the concept of a CI multi-fuel engine there are.several possibilities. If the use of small quantities of

MAXIMUM POWER DATA COM PARISON, AMMONIA VAPORAND DIESEL PILOT INJECTION VS. DIESEL FUEL ONLY

FOR V-TWIN 1790 (18.6.1 C. R.)

160

0 150-

0 140-J

130 - co

10-

-j-

-JJ'U

00zQ00

0

1.0

<**.C AMMONIAIELFULIOT9

00

LfFUEL UEL -JIL4LY V

6~~~~ 0'. DISLFULOL12001400160 180 200 220 200 -

3NIN SPE- r

B.S.F.Fiur - M O I LSDESLFE IO 9

GARABEDIAN and JOHNSON

diesel fuel is permitted, a system such as used for thedata of Figure 5 would be a possibility . This approachwould require an LPG carburetor to meter the fuel and a

LAU- preheater to vaporize the liquid ammonia and heat it toinlet air conditions. The preheater would use the exhaustenergy for heating the ammonia vapor. This approach woulduse a fuel injection system for metering the hydrocarbonfuel which would be as a pilot charge during ammonia oper-ation. This approach would be fairly straightforward andnot overly costly if designed into the basic engine.

Another approach to a CI multi-fuel engine would bein havina a fuel injection system that would handle eitherhydrocarbon or ammonia fuel. This looks like a possibili-ty in future fuel injection concepts. This with theaddition of a suitable ignition source and a variable fuelinjection timing mechanism would appear to give an enginewith hydrocarbon-ammonia multi-fuel capability. Tl,- pre-liminary data of Reference (7) would support thi._ approach.These various approaches to multi-fuel engines are onlyideas at this time and will require extensive exploratorydevelopment to prove their practicality.

CONCLUS IONS1. In order to convert existing engines to amonia

operation only, extensive modifications are necessary forspark-ignition and compression-ignition engines. A com-pression-ignitionengine must first be converted to aspark-ignition engine in order to operate with ammonia asan automotive fuel.

2. The conversion of the present military fleet toammonia operation is extremely costly and impractical.

3. The increased fuel requirements for ammoniaoperation with current vehicles would require carryingfuel tanks which would compromise present militarycharacteristics by incre;.sing size and waight of vehicleswith attendant reduction in vehicle peiformance.

4. If the requirement for ammonia operation wereto be integrated into the original design of futureengines and vehicles, considerable savings in weight,volume, cost and complexity could be achieved.

5. By the 1975-80 period, projected advancesin techniology ..... in -e Amfrzng.... hydocabo an mmoni afuel combustion research, will permit practical engineswith ammonia-hydrocarbon multi-fuel capability.

GARABEDIAN and JOHNSON

REFERENCES

1. Patents of Ammonia Casale (French), 799,610 and

802,905.

. Koch, E., "Axmonia - A Fuel for Motor Buses,'Journal of the Institute of Petroleum, Vol. 31,pages 213-223, 1945.

3. "Energy Depot Concept," SAE SP-263,November 1964.

4. Starkman, E. S., Newhall, H. K., Sutton, R.,Maguire, T. and Farbar, L., "Ammonia as a Spark IgnitionEngine Fuel: Theory and Application," Presented at SAEAutomotive Engineering Congress, Paper No. 660155,Detroit, Michigan, January 1966.

5. Gray, J. T., Dimitroff, ', Meckel, N. T. andQuillian, R. D., "Ammonia Fuel - Enqine Compatibility andCombustion," Presented at SAE Automotive EngineeringCongress, Paper No 660156, Detroit, Mich., January 1966.

6. Garabedian, C. G., and Johnson, J. H., "Reporton the Feasibility ot 'sing Anhydrous 7 mmonia as Fuel forReciprocating Engines," Technical Report No. 9049, U. S.ATAC, Components Research and Development Laboratories,November 1965.

7. Ammonia Fuel Monthly Progress Report No. 16,Contract No. DA-04-200-AMC-791(X), 7 January 1966.

8. Wallace, W. A. and Lux, F. B., "A VariableCompression Ratio Engine Development", SAE Paper 762A,1963.