,*
\ST. OF
E8
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,
Signature
of
1965
Author.............-N...-
.
Uepartment of Mechanical Engineering
Certified by...... ...........
.............
h1jsisB upervisor
Accepted butt.ea-.',
G~hairman, Uepar m
..................
tal Committee an Theses
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
work,
and to Miss Julie Cole for her encouragement.
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
Appendix
29
-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
in operating conditions to allow use in any practical application.
-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
while burning NH3 . The other is to determine if it is
-6-
possible to bring the performance of an engine while
burning ammonia to the same level as when burning gasoline
by making minor modifications to the engine.
MEN
-7THE 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
Density, lb/gal
5.1
Boiling point, F
- 28
Vapor pressure ( 70 F ), Btu/lb
128.8
Gasoline
6.1
Heat of combustion ( lower heat
value - gas ), Btu/lb
8000
Chemically correct fuel-air ratio
18,900
.165
..067
77.3
96.5
Uhemically correct heat release
Btu/ft 3 of mixture
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 NwH
3 can only be 60 % of that
memo
-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
1 H3
-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.
-12-
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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-15Performance 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 -,am
3 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
is almost the same as that of gasoline. mt lower compression
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-19ratios, 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 4H3
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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-21-
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 observations 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.
tT
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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
itr
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.
-27ODNCLUSIONS
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
3
as a fuel in any practical application without additional
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".
mcGruw-Hill,
Nvew
York, 1955.
-29-
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