Ammonia as an internal combustion engine fuel.

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JUN 23 1965

4-/lRARIES

AMMONIA AS Aiv INTERNAL COMBUSTION ENGINE FUEL

H. FRANK SHAW

Submitted in Partial t-ulfillment

of the Requirements for the

Degree of Bachelor of Science at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

June, 1965

Signature

Certified

of Author...-N...- .

Uepartment of Mechanical Engineering

by... ... ...

h1jsisB upervisor

Accepted butt.ea-.', ...

ACMNOILEDGEMENTS

The author would like to extend his thanks to

Professor A. R. Rogowski for his help and overall guidance in this project, to Professor A. L. Hesselschwerdt for

his advice on the handling of ammonia, to the staff of

the Engineering Projects Laboratory _{and the Sloan Laboratory}

particularly Mr. J. A. Callogero, Roy Andrew, and Eddie

Clarke, who were most helpful throughout the experimental

P~

TABLE OF CONTENTS

Introduction 5

The uombustion of Ammonia 7

Experimental Apparatus 8

The Performance of Ammonia 10

General Observations 26

uonclusions 27

bibliography 28

-4- :

ABSTRACT

Studies were conducted to evaluate the perfomrmene

of ammonia as an engine fuel. The thermal efficiency, IMEP,

and ISFC were measured for variows operating conditions. mlthough satisfactory full throttle performance was achieved on a sligntly modified engine, there was not enough flexibility

-5-INTRODUCT ION

The evaluation of ammonia as an alternate fuel for

spark-ignited engines was prompted by the Energy Depot Concept proposed for the United States Army by the

Allison Corporation.

At present, the Army's mobile capability is severely limited by the necessity of maintaining fuel supply lines

to all its forward units. In the Energy Depot Concept,

a portable nuclear reactor located in the field is the

primary source of power; since the reactor concentrates

a large amount of energy into a reasonably mobile and

compact volume, the need for supply lines to the rear is eliminated. The energy of the reactor is used to manufacture a chemical fuel out of locally available materials. Of the possible chemical fuels, the

synthesis of NH3 from nitrogen in the air and hydrogen obtained from the electrolysis of water seems to be the most practical process, both from considering

the ease of manufacture and the specific energy content

bf the fuel produced.

The purpose of this thesis is two-fold. One objective is to evaluate the performance of a spark ignition engine

-6-possible to bring the performance of an engine while

burning ammonia to the same level as when burning gasoline

-7-THE COMBUSTION OF AMMONIA

The table below lists some of the properties of

ammonia and a typical gasoline which are related to the combustion process.

NH₃ Gasoline

Density, lb/gal _{5.1 6.1}

Boiling point, F - 28

Vapor pressure ( 70 F ), Btu/lb 128.8

Heat of combustion ( lower heat

value - gas ), Btu/lb 8000 18,900

Chemically correct fuel-air ratio .165 ..067

Uhemically correct heat release

Btu/ft 3 of mixture 77.3 96.5

The reaction for the combustion of ammonia is

21H3

₊

_{1.502 = 3H20 +} N2

The heating value of NH3 is about 8000 Btu/lb, 2.4 times less than gasoline. However, the stochiometric fuel-air

ratio for NH₃ is .165 compared to about .067 for gasoline.

Thus the heat released in the combustion of a stochiometric mixture of NH3 and air is 77.3 Btu/lb of mixture or

80 _{% of a gasoline-air mixture. Since the power output}

of a reciprocating engine is a function of its air

capacity, assuming constant efficiency, the power

obtained from burning NwH3 can only be 60 % of that

-8-obtained from an engine burning gasoline and running at the same speed.

LXPERIMENTAL APPARATUS

The engine used in these experiments was a single cylinder uooperative Iuels xesearch engine, having a

bore of 3.25", a stroke of 4.50", for a total displacement of 37.4 cubic inches. The compression ratio of the

engine is continuously variable between 4.0:1 and 10.0:1 The standard ignition system described in the appendix was used. The engine test stand was so designed that most operating parameters can be observed while running the engine. 6are was taken to keep all operating conditions such as water temperature, oil temperature, etc. constant during the runs.

The air and fuel flow rates were both measured with standard mSME orifices. The engine is connected to an electric generator which acts as a dynamometer, giving the brake horsepower developed.

Iuel system

Liquid ammonia was stored in 50 lb. cylinders. The vapor pressure of the ammonia in the cylinder was sufficient to force gaseous ammonia into the engine.

ihe cylinders were placed in a water bath kept at 70 t in order to provide enough heat to maintain the pressure

in the cylinders. m regulatorpwas used to drop the 1H3

-9-pressure to about 5 in. of ng before being injected

into the engine. !he giseous ammonia was injected into tne inlet air stream just before it entered the engine. cxcept for one series of runs, the inlet air was preheated to 170 F in a mixing chamber normally used to vaporize the gasoline. In being throttled down from cylinder pressure, the ammonia temperature decreased somewhat, but it absorbed sufficient heat tnrough the walis of the piping system

to bring it back to room temperature before entering

the engine.

serious difficulty was experienced in leakproofing the fuel system because of ammonia's tendency to leak through very small openings, and because it is very objectionable even when present in small quantities.

-U ... - q

-10-HLRtORMANU OF AMMONIA

From the literature on the subject, it was thought

that ignition of ammonia could be achieved in a conventional engine designed for gasoline, uonsequently, it was first attempted to evaluate the oerformance of ammonia without

making any modifications to the engine. This was not

successful. ihe ignition system was then modified to

increase the spark energy by various means. This approach

was sucessful; ignition of ammonia was achieved, and the

performance of the engine was evaluated.

Initial Uperation on Ammonia

It was initially tried to burn wH3 with the engine

set up as it would be for operation on gasoline. The standard ignition system was used with a spark plug gap of .030". The compression ratio was 6.0:1.

The test procedure was to start and run the engine on gasoline and gradually reduce the amount of gasoline fed into the engine while simultaneously increasing the

amount _{of .H}

3'

When injected in moderate amounts, the ammonia lowered the output of the engine and did not seem to contribute to the combustion process. However, adding more than about 15% by weight ammonia would stall the engine. It was thought that this was due to two effects occuring simultaneously. First, reducing the amount of gasoline reduced the amount of energy introduced into the cylinder

-11-po:r stroke. iecond, the ammonia displaced some of the air in the charge, so that less air was available for the combustion of gasoline.

In order to promote-the combustion of ammonia, the

compression ratio was increased to 9.5:1. The same procedure

of leaning out the gasoline and increasing the flow of

ammonia was used. Ignition of the tH3 alone was not achieved; when the gasoline-air ratio was made too lean, the engine would stall even though the correct amount of iH3 was

present. However, the presence of ammonia did substantially contribute to combustion, as it was possible to run the

engine on a gasoline-air ratio as lean as .014, and still

produce about 2.5 IHP.

It was thought that the gasoline acted as an intermediary agent in the combustion of ammonia, providing the extra

energy needed to start the process. In other words, the spark alone does not have enough energy to set off the

am3 , but it does set off the gasoline; the gasoline in

turn has enough energy to set off the ammonia.

It may also be that the presence of gasoline helps promote the combustion of the ammonia by providing free

hydrogen to act as a catalyst in the reaction.

From these results, it was thought that NH3 alone

could ue burneo in the engine by providing additional energy in the spark.

Ignition bystem Modifications

In order to supply sufficient spark energy to ignite

the ammonia mixture, the following ignition system modifications were tried:

a) the spark plug gap was gradually increased to .050";

tne engine was run on gasoline to make sure that the plug was firing in spite of the increased gap; this produced no

improvement in performance wnen using ammonia.

b) a second spark plug was installed, also gapped

at .050"; this improved the performance to the extent that

the engine would occasionally fire a single time, but it could not be made to run regularly; the voltage from the

regular ignition system was not sufricient to make the

spark plug fire accross gaps greater than about .050".

c) a 220 volt ut; power supply was substituted for the

usual 110 volt ul; line used to drive the primary side of the ignition coil; this had the effect of approximately doubling the voltage seen by the spark plug.

d) the spark plug gap was further increased, using

the high voltage supply on a single plug; the engine started

developing useful power at a gap of .060" and ran best

witn a gap of around .085"; the ignition system was not capable of firing across gaps greater than .090". A plot

of the performance versus the spark plug gap is given in

lig. 1, and the effect of spark plug voltage on indicated power is given in tig. 2.

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-15-Performance of Mmmonia

With _{the ignition system modified as described previously,}

the engine ran regularly and smoothly enough so that its performance could be evaluated.

rullthrottle engine tests were made while burning

ammonia to determine the indicated horsepower (IHP), indicated mean efrective pressure (IMLP), and indicated

thermal erficiency of the engine as a function of speed and compression ratio. these results, together with typical performance data for the engine operating on gasoline are plotted in tigs. 3,4,and 5.

ihe ammonia and gasoline data are not strictly comparative because tne gasoline data is taken at a lower compression

ratio tnan any of the ammonia data. nowever, the knock limit

of #H3 is much higher than that of gasoline; therefore

it is possible to use much higner compression ratios with ammonia with no detrimental effect to the engine. Although

both -,am3 and gasoline could successfully be operated at

higher compression ratios, the relative values of the compression ratios used in these tests, namely 6.0:1 for gasoline and 9.5:1 for ammonia reflect a pradtical comparison of normal operating conditions.

ms can be seen from itne tlot of IHP versus rpm, at

a compression ratio of 9.5:1, the performance of ammonia

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-19-ratios, the performance of NH3 drops off rapidly; at the same compression ratio, the peak power output on ammonia

is only 60* of that possible while runninq on nasoline.

there is a rapid narrowing of the useful power range with lower compression ratio. this is probably due to the inability to combust ammonia at low compression ratios. bince these runs were made with the spark advance set at best power, the effect of burning time losses is minimized;

however, the output stillorops off markedky with engine speed. urom the plot of optimum spark advance versus rpm given intHig. 6, one can see that the flame speed of 4H 3 though somewhat slower than gasoline, is not extraordinarily

slow.

It may be that the burning process is such that the

reaction goes only a small portion of the way towards

completion during the passage of the flame front. the

speed of the remainder of the combustion process, and therefore the location of the pressure peak would not be so much

a function of the spark advance as of the initial spark energy. 1his agrees with the previous observations showing the marked effect of spark intensity 'n power.

The best power fuel-air ratio as a function of rpm is also plotted in 1ig. 6. Judging from the way the optimum fuel-air ratio decreases with speed, it may be that some of the inefficiency at high speed is due to poor mixing of the ammonia and air. However, this behavior could also

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-substantiahe the previously mentioned possibility that the combustion reaction does not go to completion rapidly enough. If the reaction is not fast enough, additional fuel would depress the rate of reaction even further, so in ordet tol rbihiimize burning time losses and keep best power, the mixture should be made progressively leaner at higher speeds.

tuel-air Ratio and :park Advance

the fuel-air ratio and spark advance requiremsmts of the engine while burning ammonia are given in Figs. 7

and 8. The runs were made keeping the engine speed constant at 1000 rpm with all the other parameters besides the one being varied adjusted to give maximum power. It can be seen that the point of max. IMEP occurs at progressively leaner fuel-air ratios. this agrees with the hypothesis that the rate of reaction decreases with lower compression ratios and richer fuel-air ratios.

cffect of Inlet Temperature

It was attempted to run the engine without preheating the inlet air to the normal operating temperature of 170 F. The engine would not start to fire until the inlet air

temperature reached 120 F and ran very rough between 120 F and 150 t-; between 150 F and 170 F, the performance gradually reached the normal operating conditions.

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-24-Part Throttle Performance

The part throttle IMEP and indicated specific fuel

consumption of the engine running on ammonia are given in Fig. 9. Although the engine ran at part throttle and

obser-vations wereh made, this should not be ccnstrued to mean

that satisfactory performance was achievea. ihe engine ran jerkily and misfired frequently; the values given represent the time average conditions observed rather than a steady output.

Indicatar Observations

In order to gain some insight into the deterioration

of the combustion process when the operating parameters

were not adjusted for optimum performance, pressure versus crank angle observations were made with the M.I.T. Point

by Point Indicator. The wide scatter of data points in

the region where combustion occurs indicates that only

a fraction of the charge is burned on most power strokes.

This irregularity from cycle to cycle increased as the operating conditions were made progressively worse.

Copies of the indicator cards taken for various conditions are given in the appendix.

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-26-UEIERAL OBSERVATIONS

Ammonia Lontamination

At Lhe conciusiun of the tests, the engine was

examined for any deterioration or corrosion due to the ammonia. wo such evidence was found either in the cylinder or in the exhaust system. The ammonia had no noticeable effect on the engine oil.

nnock Limit of Ammonia

The General Motors Laboratory report un ammonia

as a fuel indicates that the knock limit of evH , although

not precisely known, is much higher than that of gasoline.

1* knock was observed during any of these tests. However,

when the engine was run at off-peak conditions, a dull

heavy pounding noise was occasionally heard. This is probably explained by the great variatiuns in peak pressure from

cycle to cycle.

uual Ignition Operation

It was tried to operate the engine with two spark plugs gapped at .070" located on opposite sides of the

cylinder. sov improvement in performance was noticed;

however, no conclusion should be drawn from this fact because there was no way of making sure that uoth plugs were firing properly.

-27-ODNCLUSIONS

Although the full throttle performance of ammonia approached that of gasoline, its performance at off-peak conditions was not adequate enough to permit the use of

ivH as a fuel in any practical application without additional

3

improvemebtents in the engine.

The inability to obtain satisfactory performance was thought to be due to the lack of suffictent external energy at the start of combustion to initiate the reaction at

a high enough rate. There are several ways in which this

additional energy might be provided.

uecause or tne hign knock limit of ammonia, -he engine

could have a higher compression ratio. zupercharging could also be a likely way to improve the performance. One could take advantage of the fact that the hH' is usually stored

3

under pressure to

supercharge

without drawing power from

the engine by means of an ejector.

Additional ignition system modifications, especially dual ignition could be investigated.

the combustion chamber could be redesigned to provide greater turbulence.

Though the author doubts that ammonia will find

many applications as an engine fuel because of the handling difficulties associated with it, with certain modifications to the engines, ammonia could become practical for some specialized uses.

-28-bIBLIOGRAPHY

1. Walter Uornelius, L. William Huellmantel, and Harry R.

Mitchell, "Ammonia as an tngine tuel", bAE Meeting,

uetroit, 19b5

2. K. 0. Lindell, "Introduction to niuclear Powered Energy Depot Concept", SAE Meeting, uetroit, 1965

3. m. R. Rogowski,"clements of internal U'ombustion tngines".

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