Discussion of papers 650050 (p. 274), 650051 (p. 281). and 650052 (p. 300),
American Oil Co.
THIS AUTHOR GENERALLY concurs with the remarks within the scope of the presentation. Several comments should be made.
The first relates to the choice between converted-internal-combustion and fuel-cell power units for vehicles. This choice might be influenced by the attractive possibility of a simple conversion kit allowing use of standard hydrocarbon-fueled engines. However, this convertability may not be unique to IC engines in view of other work directed toward fuel cell vehicles powered by steam reformed petroleum fuels. Such vehicles would be readily converted by changing to ammonia cracking.
The second relates to scope. While the application detailed in the paper, vehicles, is one of the most difficult, an army requires energy for various other purposes, ranging from cooking and lighting to communications power. Safe efficient burners for depot fuels will be needed.. A variety of electric power supplies of capacity or type not possible to be provided by batteries in the isolated arena will also be needed. In many of these cases, the projected fuel cell, with suitable power processing auxiliaries, will probably be more efficient than corresponding converted engine generator sets.
The third relates to the direct ammonia fuel cell. In the paper, this approach was discarded for consideration at this time because of lagging technical advance. Yet this system holds a potential advantage which warrants at least continued attention to its possible development. This advantage is high efficiency on idle. The rate of the ammonia cracker cannot be changed rapidly, whereas the fuel consumption of the direct cell readily drops to a low level on idle. Important fuel savings can result. In addition, an ambient-temperature, direct ammonia cell would not require 25% of the ammonia's hydrogen to heat the ammonia cracker.
The fourth relates to the projection of the fuel cell performances . The projections presented do not seem too optimistic. But, for the benefit of those more casually acquainted with fuel cells, it should be. noted that the comparison with present state of art should be made for power densities at the required operating efficiency, not at the maximum efficiency. Fig, A summarizes the power den -sity data of Fig. 9 and 11 of the paper and shows the operating points selected in the design study. The voltage efficiency lines which have been superimposed on the curves reveal that the design points are all in the 60-70% range. The corresponding point on the 1963 state-of-art curve given is well below 100 wft.2
Finally, in a truly isolated arena, some unexpected materials can become "fuels" in the sense that a supply of them can be used up. For example in the processes detailed, air purification and ion exchange water purification are required. Taking the KOH requirements for air. purification given in the paper, some 30 tons/year of KOH would be re-quired for the utilization in fuel cells of the yearly reactor output. Chemical needs for regeneration of the ion exchange bed would depend on available water and on undisclosed process details, but could be substantial and would apply whether engines or fuel.cells were used. Use of thermally regenerable treatment chemicals would help alleviate a, problem in this area.
THIS PAPER IS an important contribution to the literature on combustion in reciprocating engines as,well as a significant collection of data on the possible utility of ammonia -in the energy depot concept.
The authors have clearly pointed out many of the important factors that must be considered if ammonia is to be viewed as à serious contender for use as a military fuel. From their data, it is obvious that, in contrast with gasoline, ammonia suffers from low heat of combustion, difficult ignitability and low flame speed. However, these problems can be overcome to some extent by use of supercharging, high compression ratios, ignition system modifications, and partial dissociation of the fuel before induction. As the authors' data show, ammonia-air mixtures are hard to ignite. Although minimum ignition energies are difficult to reproduce and to interpret precisely, some light on the problem is shown by the work of Buckley and Husa, in (1) who showed that the minimum ignition energy for an ammonia -air system was 680 millijoules in a condenser dis-. charge. A comparable figure for n-heptane in air was 0.3 millijoules. This probably explains why the ignition system modifications described in the paper markedly increased power output.
Another serious limitation on ammonia-air combustion involves the flammable limits, or range of concentrations that will sustain a flame. In the case of hydrocarbons, these limits are very broad. Thus, n-heptane, for example, will bum under ambient conditions in air at any concentration between 1.1-6.7 Vols % (2). This gives wide latitude to the engine designer in his quest for maximum power under some conditions (rich mixtures) or maximum economy under other conditions (lean mixtures). Further, the stoichiometric concentration occurs at 2.3% n-heptane. This allows more than a factor of 2 in fuel concentration on either side of the stoichiometric composition.
The ammonia-air system is quite,different. Here, the flammability limits are 15 and 28 Vol % ammonia (1), which in itself means that there is less than a twofold variation possible in ammonia concentration for a burnable mixture. The stoichiometric mixture occurs at 21.9%, with only a narrow latitude allowed on either side of this point. Thus, under ambient conditions, all flammable mixtures must be near stoichiometric. Undoubtedly; higher temperatures and pressures will somewhat broaden this range, so that a wider range of mixtures can be burned in an engine than indicated by these figures. An Interpretation of some of the exhaust composition data in the authors' Table 2 can be based on this approach. Thus, fuel mixture 2, containing ammonia plus 98.8% of the theoretical air, still burns only 68.8% of the ammonia inducted. Recognizing that some mixture in homogeneity probably exists in this system, we can theorize that the part of the charge in the center of the combustion chamber is well insulated from the walls, and thus probably burns completely because its temperature is high and its flammability limits fairly broad. However, the portion of the charge near the walls is cooler and has reduced flammability limits, so that quite likely a substantial portion of it is outside the combustible range and does not burn.
In addition to the flammability problem encountered in dry air, moist air would likely impose an additional penalty, since high humidity tends to substantially narrow the flammability limits, in the ammonia-air system (3).
To overcome this flammability limit problem, the authors have used several techniques with substantial success. . Supercharging and high compression ratios would both be expected to broaden flammability limits. The cracking of the ammonia prior to induction is another route, which is effective partially because of the very broad flammability limits, of hydrogen (4-74 Vol %). Another possible route is
available,, which involves the use of limit-broadening additives in the fuel. Such an approach is not needed in the case of hydrocarbons, with their wide flammability limits, but could be very useful in ammonia combustion. Some indication that flammability limits can be broadened in this way is found in the work of Egerton and Powling (4), who showed that additive amounts of ethyl nitrate had a significant effect in raising the upper limit of flammability of several light hydrocarbons.
Thereis also a second reason for anticipating that additives may improve the combustion, properties of ammonia-air mixtures. .This is based on some evidence indicating that the ammonia molecule must dissociate (at least partially) into nitrogen and hydrogen before it will burn. As the authors point out, a catalyst is required to make this reaction proceed measurably at temperatures below 900 F. This opens the possibility for an additive to be introduced for the purpose of promoting such dissociation during compression. An ammonia-soluble metal compound, decomposing in the engine to produce a fine dispersion of solid metal or metal oxide, might be one possible approach. If effective, it could facilitate, ignitability, broaden flammability limits, and increase flame speed. If this approach were successful technically, it might greatly add to the practicability of the military use of ammonia, since it would substantially reduce the need for major engine modifications.
In their work with hydrogen addition, the authors have shown how this approach is an attractive one for solution of some of the ammonia combustion problems. Their results may be somewhat optimistic in relation to practical approaches, since the cracking of ammonia would introduce nitrogen, as well as hydrogen, and the increased dilution would operate to reduce some of the gains shown when hydrogen is added. From an energy standpoint, the cracking of ammonia prior to induction into the engine has the effect of increasing the available energy by nearly 13% on a weight basis. However, Because the dissociation products occupy twice the volume of the ammonia, the energy would decrease on a volumetric basis by about 9%. Thus, an additional problem may be encountered in practice where it would be desired to maximize the mass of fuel and air inducted into the cylinder.
Some time ago, Messrs. Kefley, Felt, and Adams in Ethyl Corporation's Detroit Research Laboratories briefly ex-amined the engine performance of ammonia in a single cyl-inder Waukesha engine with variable compression ratio and removable dome head. They maintained intake air temperature at 130 F, jacket temperature at 230 F, compression ratio at 8.5, and manifold pressure essentially • atmospheric. Spark energy, supplied by the engine's Bendix CR 4-1 magneto, was about 80 millijoules. Even at the low speed of 600 rpm, they encountered poor combustion, as indicated by a brake thermal efficiency of 13.75%. This compares with a value of 22.2% for isooctane containing 3 ml TEL/gal under the same conditions. When two spark plugs were fired simultaneously, the value ammonia increased to 18.5%. By the additional technique of increasing the compression ratio to 12.65, the brake thermal efficiency was raised to 21%. In general, their conclusions from this work support those of the authors.
Through their study, the authors have shown that there would be no incentive to use ammonia as a fuel in the civilian market as long as hydrocarbons are available.
Even in the military context; much further work needs to be done to clearly define the potentiality of the overall concept. The present work is an excellent beginning on the usefulness of ammonia in gasoline engines. Additional work needs to be done on diesel-cycle engines and on multifuel engines. These engines will encounter other .problems, due to the probable' low cetane number of ammonia and the difficulty of keeping a stratified charge within the flammability limits. Detailed consideration needs to be given to the economics of the entire concept, including not only the energy depot aspects, but also the complex task of modifying engines without rendering them so cumbersome as to be ineffective. Finally, the overall evaluation of the system in terms of competitive cost-effectiveness, in relation to present methods of wartime fuel supply, will probably be the factor that determines whether it ever becomes a reality.
1. Buckley and Husa, Chem. Eng. Prog. 58, 81 (1962).
2. Lewis and von Elbe, "Combustion, Flames and Explosions, " New York, 1951.
3. Perry, "Chemical Engineers' Handbook, " Third Edition, p. 1587.
4. Egerton and Powling, Proc. Roy. Soc. A 193, 172, 190 (1949).
American Oil Co.
MESSRS. CORNELIUS, HUELLMANTEL AND MITCHELL have conducted a very sensible and logical program to explore the suitability of ammonia as an engine fuel. We commenc them for their work.
During 1963, C.J. Domke and I at the American Oil Company Laboratories investigated the performance of ammonia in engines. The approach and scope of our work was much like that of the. General Motors work except that we did not investigate the effects of dissociation of ammonia, and we extended our work to compression-ignition engines. We agree completely with the results the authors-present. We would like to offer a few observations on the performance of compression-ignition engines operated on ammonia.
The Armed Forces inventory includes a high fraction of vehicles with compression-ignition engines and they, as well as spark -ignition, must be accommodated in the energy de-pot concept. We were able to operate a CFR cetane method engine on pure ammonia and ammonia with several additives . Ammonia (or ammonia plus additives) was injected in conventional fashion, although it was necessary to advance injection timing greatly and to use a plunger and bushing assembly in the injection pump larger in displacement than the one normally used, in the cetane method engine. The engine was started on kerosene, and then switched to ammonia. The engine would not restart on pure ammonia at 35:1-compression ratio with normal coolant and inlet air temperatures. However, we were able to attain ignition and regular combustion by raising coolant temperature to 370 F and air temperature to 270 F. Power output at these conditions was slightly less than that obtained at normal conditions with kerosene. We did not analyze the exhaust, but it had a very strong odor of ammonia. Our investigation of additives was limited to only a few choices. Several were effective; they permitted ignition and regular combustion to be attained with somewhat -lower compression ratio and coolant and air temperatures.
From these results and ideas they generated,, we believe that with some engine modifications and with proper additive treatment, it may be possible to mate compression-ignition engines operate satisfactorily on ammonia-based fuel. The alternate, of course, is to convert all compression-ignition engines to spark ignition. More work is necessary to determine which would be the best solution.
S. MYERS AND O. A. UYEHARA
University of Wisconsin
THE ENERGY. DEPOT concept has been of considerable interest to the discussors. In 1962 the discussors and other coworkers, presented a paper entitled "Portable Power From Nonportable Energy Sources." This paper was presented at the National SAE Powerplant meeting in Philadelphia and published in the 1963 SAE Transactions. The basic purpose of this paper was to see what solutions might be found to the problem of providing an energy supply suitable for portable power plants when our present petroleum supplies were exhausted. While this is a slightly different problem than the one under discussion today, there are marry similarities and the conclusions are remarkably similar.
Inasmuch, as two different groups considering two different, .but related, problems reached similar conclusions, it would appear that these conclusions were fundamental and should be emphasized. The first of these common conclusions is that, barring unexpected breakthroughs, chemical storage of energy is the only practical engineering technique. All other possible techniques are all too bulky, either be-cause of high shielding requirements or because of the large bulk of the energy storage material and system.
The second common conclusion is that only materials available on a large scale, that is, water and air, can be considered for production of fuel because of the large quantities of material involved. The implication here, is that hydrogen is going to be the basic means for storing the energy originally obtained, from nuclear or some other stationary power source.
The third common conclusion is that it will be necessary to store the fuel in the liquid form or its equivalent. Hy drogen can, of course, be combined with other compounds such as nitrogen to form easily liquifiable fuels. If this procedure is followed, one of the essential requirements is that the resulting fuel be stable both chemically and with respect to shock.
The fourth conclusion is that it was undesirable to manufacture a fuel containing oxygen inasmuch as oxygen is readily available in the air free of charge.
It is interesting to note that the papers by Mr. Rosenthal and Mr. Grimes, as well as our earlier paper, reached these common conclusions. It should be clear that these conclusions are applicable when considering portable, power plants. Different conclusions might be reached when considering stationary power plants.
It is pointed out in the papers that the number of potential fuels containing hydrogen and other readily available compounds, are very limited. As the paper by Cornelius and co authors points out, the fuel finally chosen, ammonia, has certain combustion problems. These combustion problems seem to arise primarily because of the low energy content per cubic foot of mixture. In this respect the behavior of the ammonia-air mixture reminds one of the lean mixtures of conventional fuels which also have comparatively low heating values per cubic foot of mixture. Thus, it is very interesting to note, with the exception of turbulence, all of the steps taken to improve combustion performance of ammonia as an engine fuel increased the energy content per cubic foot of mixture at the time of spark, that is, in-creased compression ratio, turbocharging, and so forth.
The thought of improving combustion by enriching the ammonia-air mixture with hydrogen is intriguing and ingenious. We do have some questions, however, regarding the way in which this was accomplished, particularly, in view of the comments just made regarding the low heating value per-cubic foot of the ammonia-air mixture. If we interpret properly the experiments conducted by Cornelius and his associates, the tests were run by adding hydrogen only to the ammonia-air mixture. The dissociation of ammonia, however, will produce both hydrogen and nitrogen. Is it proposed to separate the hydrogen and nitrogen, discarding the nitrogen and adding the hydrogen to the ammonia-air mixture? If not, should not the experiments have been run with the addition of both hydrogen and nitrogen to the ammonia-air mixture? Would the heating value of the mixture have been significantly less if hydrogen and nitrogen rather than just hydrogen had been added? Would it have not been just as easy, experimentally, to have added hy--drogen and nitrogen in the proper proportions?
The discussors would also like to raise the question of whether or not the thermal inertia of the dissociator, which decomposes the ammonia to produce the hydrogen, would be a complicating factor in the design of the engine-dissociator system? For example, if the engine were operating at low load with consequent low dissociator temperatures and the throttle were suddenly opened, requiring increased quantities of ammonia to be dissociated, would the dissociator be able to supply these increased quantities?
It is also interesting to note in Fig. 14 of the Cornelius paper that 3% hydrogen addition does not produce any significant improvement over 2% except at the very highest speed of 4000 rpm. Can the authors give any explanation of this "leveling off" with increased hydrogen addition?
United Nuclear Corp.
THE PAPERS PRESENTED in this session on the Energy Depot concept deal primarily, with the production fuel in the field and the. utilization of this fuel to power army vehicles. In all of the concepts discussed, an inherent part of the energy depot is a mobile nuclear power plant which generates the electric power required:as input for the fuel production process. I would like to offer some-brief remarks about the nuclear power generating portion of the energy depot.
This power source is being developed under the Military Compact Reactor (MCR) program, for which Allison Division, General Motors Corporation is the prime contractor. United Nuclear Corporation, whom I represent, has the responsibility as subcontractor to Allison for design and development of the nuclear reactor portion of the MCR.
One of the accomplishments of this effort has been the design of an MCR unit for an electric output in the same. 3000 kw range for which the Allison and Allis-Chalmers energy depot conceptual designs were evaluated .
To meet the size and weight requirements of mobility, the MCR is packaged in modules comparable to those described for the various energy depot fuel processing units. These packages are trailer or truck mounted for operation in the field, and are transportable overland on their trailers or alternatively by air or sea.
The nuclear reactor, surrounded by its biological shield, which must meet extreme low weight requirements compared with ordinary reactor shields, is the basic source of thermal energy. Its heat is transferred to a liquid metal coolant, which ultimately heats air in a heat exchanger. The heated air drives an open Brayton cycle gas turbine engine, which drives an alternator whose electric output is the power source for the fuel manufacturing units. Auxiliary electrical equipment and controls for the nuclear plant are housed in separate units connected to the reactor and engine by electric cabling.
The energy depot constitutes an extremely attractive and logical application for nuclear power. This is an example of where the demand for long time operation without additional fuel supplies is uniquely supplied by a nuclear energy source.
Mr. Rosenthal's paper pointed out the weight attractiveness of the energy depot in comparison with the continued supply of gasoline fuel, and the cost studies that have now been initiated. There are, of course, uses of the nuclear powered energy depot where independence from external supplies is an asset which cannot be measured in dollars, for instance, the holding of a key spot which would otherwise be lost. Nevertheless, It is of some interest to note, that although the cost of uranium fuel and its associated plant are generally considered to be high under military circumstances, the cost of delivered gasoline can also be surprisingly high.
A simplified analysis of several, special wartime situations shows that the predominant cost of supplying a major defense position is often that of the replacement value of aircraft and ships lost in bringing in the supplies; On this basis, considering such instances as Tobruk and Malta in World War II (for which the necessary statistics are described by Winston Churchill in his books), delivered gasoline is found to cost $50-$80/gal. In different circumstances, then, gasoline may range in value from the $0.30 or so, per gal. available at the local filling station up to numbers hundreds of times as great under front line fighting conditions.
THE MOBILE ENERGY depot modules present several logistical problems, which I think might be illustrated by Figs. B-N.
B. This is China on the old Burma-China Road between Kunming and. Chungking. This section of the road is called the "Ladder." It has no guard rails and steep grades, with a 1/4 in of slime. When raining the Goer just wouldn't go! Only a 6 x 6 or 8 x 8 would do the job.
C. This is' another view of the old Burma-China Road. D. The photograph shows typical terrain between China and northeast India. We flew petroleum products over this area until the pipelines were built.
E. This is up front on the Ledo Road near Myitkyina; you needed snowshoes here.
F. The Ledo Road where a temporary wooden bridge with a load limit of 4 to 5 tons and a 4 in. pipeline on each side was built.
G. Two of the 4 in. -pipelines - ahead of the finished road. These 4 in. coupled pipelines handled 1500 gal/hr/ line. We shipped aviation gas - motor gasoline and diesel -fuel via these lines. There was a pumping station every 8 miles. One of these lines could refuel seven of the old M-4 tanks/hr.
H. - A portion of the Ledo Road that was near completion and included a Bailey bridge and mules!
I. The picture shows a finished portion of the Ledo Road. Grades in this area were as high as 14 to 16% There was one short stretch at 25%.
J. This is a Korean river with a bridge out.
K. Korea - just back of the combat line. This dirt road with up to 12% grades was the only line of communication. A pipeline came within 15 miles of this road.
L. Typical Korean terrain,
M. This photograph was taken in Korea and shows fuel storage for the tent stoves. Kerosene was used. What do we do when we use ammonia?
N. Here are 6 in. pipelines and a pumping station in Korea. These 6 in. lines with a pumping station every. 16.5 miles would handle 3300- gal/hr. They would refuel 10 new M-60 tanks per hour. The newer welded 6 in. and 8 in. pipelines operating at higher-pressures will handle several times as much fuel.
There is every reason to believe that South Vietnam (and the territories north and west of South Vietnam) has equally difficult terrain. Certainly, my recent visit to Thailand would confirm this.
The problems I visualise can be summarized by the fol-lowing comments:
Apparently, these Mobile Energy Depot modules weigh about 30, 000 lb and a complete unit about 100, 000 lb. It takes a plane nearly the size of the 707 to carry 30, 000, pounds.. There are not many military air strips in the area where we might engage in combat that will take the 707.
A 16-ton Goer Weighing 39,000 lb without a payload will be needed to carry the 30,000 lb modules. The 8-ton Goer suggested for transporting the hydrogen or ammonia weighs 28,200 lb without a payload. These two vehicles will also need aircraft of large payload capacity.
In wartime, energy is needed for many things besides moving vehicles. Cooking and space heating are two major items. Fifty per cent of the fuel supply for Korea, during the winter months, was for space heating.
At no time during World War II or the Korean War was there an acute shortage of fuel. There were some, close calls however! There was fuel in Sicily before there was food. General Patton ran away from his fuel supply. When told to collect his 5-gal gasoline cans, he replied he was not a gar-bage collector. He was a great general, but not good at logistics.
We blew up the rail tank cars and fuel dumps at Gafsa in North Africa when we thought we were going to lose the town. We had to refuel and regather our equipment after the kicking around we got at Kasserine Pass. We did the refueling at night from conventional 5,000 gal tractor-trailers (with no lights) directly into vehicles and into 5-gal cans.
The Army's performance specifications for vehicles require that they operate satisfactorily from +115 F to -25 F without the use of starting aids other than manifold heaters or glow plugs. From -25 F to -65 F, starting and heating aids may be used. Fuel temperatures of + 145 F have been measured in the desert. There is no reason to believe that the storage vessels for hydrogen and/or ammonia would not be exposed to the same temperatures.
Our M-60 tank is capable of a 24 hr battlefield day without refueling. Newer concepts talk of a 36-hr battlefield day. To achieve this with ammonia, would require 2.8 times the volume of fuel, which would make an impossible sized tank; or would reduce the battlefield day by the same amount, for example, from 24 hr to 8.6 hr.
For many years, our top military command has failed to recognize the need for maximum effort in studying the protection of our supply lines by sea in time of war. Preoccupation with the missile may have been the cause. I am glad to note from recent published data that antisubmarine warfare planning is now being emphasized!
COL. KERMIT O. LINDELL
United States Army
AS A RESULT of the discussions presented, I would like to amplify and perhaps clarify a few of the points presented during the session. First of all, the cost-effectiveness of the concept has not been evaluated. The Stanford Research Institute has just recently been awarded a government contract to perform an operational analysis of the Nuclear Powered Energy Depot concept to include an evaluation of the cost-effectiveness. As the study is just getting underway, even a preliminary estimate is not available at this time, as to whether or not the concept is economically attractive.
The cost-effectiveness of a concept such as this is extremely difficult to evaluate. With respect to Mr. Gay's remarks concerning the Ledo Road, some 300 planes and many lives were lost during World War II delivering supplies over "The Hump" into China. If a concept may save lives and equipment, how do you factor this element into a cost-effectiveness analysis? Is a life worth ten thousand dollars, a hundred thousand dollars, or a million dollars? As a matter of fact, I might suggest that if a Nuclear Powered Energy Depot had been available during World War II, it may not have been necessary to construct the Ledo Road to provide a means of supplying troops in China; a substitute for gasoline, the major item of supply involved, probably could have been manufactured in China. This type of operation, I believe, is an excellent example of an actual situation where an Energy Depot could have been profitably employed.
Also, it is extremely difficult to forecast the nature of future warfare, nuclear warfare. World War II and Korea can no longer be considered as valid historical examples. The exact nature of the conditions on a nuclear battlefield are still a matter of speculation. To quote, in substance, a general officer I heard speak several years ago, "You probably won't know what type of tactics you will employ until you become actively engaged in a nuclear war." It is recognized, however, that the Army must seek advanced concepts of supply to supplement or replace the present concepts of logistical operations.
Secondly, I would like to point out that the Energy Depot concept is not, at this stage in the development, fixed in concrete. If there is a means of producing, in quantity, a more suitable fuel under the same constraints, we would be extremely interested; for an example, a process for producing methanol.
Thirdly, let me reiterate that during the evolutionary phase of development, it is intended that ammonia be used as a supplement to normal petroleum supplies. Not all units would necessarily be equipped with vehicles capable of burning ammonia. The capability to use ammonia as well as petroleum products in vehicles and Army aircraft would be limited to designated units and/or special situations such as airhead operations or deep penetrations. In these particular instances, it might be extremely difficult to supply petroleum fuels utilizing present distribution procedures.
In conclusion, to insure that there is no misunderstanding about the basic problem, let me illustrate it in a different manner. You have, on one hand, the tremendous energy density available in a nuclear reactor; and, on the other hand, a large, growing requirement for energy to operate vehicles, aircraft and other equipment. How do you convert the energy of the reactor into a form which will be usable as fuel by vehicles and aircraft? If, at, the same time, you can eliminate the requirement for an extensive distribution system and relatively fixed and vulnerable fuel supply depots, a major military advantage may accrue. This is the basic problem and the Nuclear Powered Energy Depot concept offers one possible means of solving this problem,. Direct trans -mission of electrical energy, without wires, offers, perhaps, another means of solving the basic problem.
AUTHOR'S CLOSURE TO DISCUSSION by W. CORNELIUS
THE DISCUSSION OF Dr. R. Flannery raises several significant points. Some of these are covered in greater detail in Mr. Rosenthal's paper (p. 274).
The major use of fuel in the Army is in vehicles; this more difficult case was treated first. Energy depot fuels can be used in modified space heaters and to power the equivalent of motor-generator sets.
Direct consumption of ammonia in a fuel cell would be a more ideal solution if adequate performance could be obtained . R and D should be directed toward this end. More recent studies have indicated that 250 fuel consumption in the dissociator may be a conservative estimate for fuel uti -lization.
The State of the art of fuel cells has advanced rapidly Fuel cell people today talk of the possibility of building fuel cell modules within a factor of 2 of the projected fuel cell modules.
The air purification chemical requirements were projected on the maximum theoretical requirements to remove all of the acidic gases from three times the air requirements of the cell. Continued, experimental studies have indicated that marked reductions can be made and air purification chemicals would be a small requirement in the future.
AUTHORS CLOSURE TO DISCUSSION by L. W. HUELLMANTEL, AND H. R. MITCHELL
THE AUTHORS WISH to express their appreciation for the complimentary and constructive remarks regarding the paper by the several discussors. The prepared discussions have aided in clarifying certain important aspects of the energy depot concept.
With regard to Dr. Rifkin's comments, we agree that it would be worthwhile to investigate the use of fuel additives other than hydrogen to promote the combustion of ammonia. In our investigation, we used only hydrogen to conform with the-initial ground rules that all fuel constituents be produced from readily available materials. It might prove to be more feasible to use an ammonia-soluble metal compound as suggested by Dr. Rifkin rather than hydrogen if the addition of only a small amount of the compound to the fuel is required and, the provision of this compound does not invalidate the Mobile Energy Depot concept.
The 6% decrease in energy content of the fuel charge on a volumetric basis as mentioned by Dr. Rifkin caused by the dissociation of the ammonia is incurred only if all of the ammonia is dissociated . However, in order to provide the desired 2.5% by weight concentration of hydrogen in the fuel mixture, only about 12.5% of the ammonia must be dissociated, and hence the decrease in energy content of the fuel is only about 1.7%. This decrease is small when compared to the large increase in combustion efficiency that is realized when hydrogen is added to the ammonia.
We agree with Dr. Rifkin that there is presently no incentive to use ammonia as a fuel for commercial vehicles. The current cost of producing ammonia would have to be reduced drastically to make it competitive in cost with gasoline. As mentioned in the paper, an engine would have to be modified and certain auxiliary systems and controls would have to be incorporated. Any significant emission of ammonia from the engine tailpipe could not be tolerated because of its irritating odor. Regarding the military use of this fuel, we agree with Dr. Rifkin that a much more thorough evaluation of the entire concept will be required before its feasibility can be ascertained.
Drs. Myers and Uyehara raised the question as to whether it would have been more appropriate during our engine testing program to have added a mixture of nitrogen and hydrogen to the ammonia rather than hydrogen alone to simulate the ammonia dissociation products. We considered doing this when we initiated our hydrogen addition studies. However our calculations indicated that the decrease in energy content of the fuel mixture due to this additional nitrogen would amount to only about 1.7%. Also, we did not wish to further complicate the fuel supply system by adding a third gas supply
Regarding the thermal inertia of the dissociator, we believe that a full scale dissociator will have to be constructed and tested on an engine in order to determine the magni- tude of this problem. We appreciate that some lag in engine acceleration will result through the use of a dissociator. This may be compensated partially by the fact that less hydrogen (as a percent of the total fuel supplied) is required when operating at full throttle with a supercharger than when running at part load . In fact, we have mentioned in our paper that satisfactory full throttle engine power may be developed solely by supercharging without any recourse to hydrogen addition. In addition to this questionable thermal intertia of a dissociator, the possibility exists that the engine exhaust temperature may not be high enough at low engine part load conditions to insure adequate hydrogen emission from a thermally heated dissociator. It was for this reason that an electrically heated type of dissociator was investigated by the authors.
The question raised by Drs. Myers and Uyehara regarding the general "leveling off" of engine power with increasing hydrogen addition above 2% can be explained by comparing the relative energy contents of stoichiometric ammonia air and hydrogen air mixtures on an equal volume basis. The heating value of the hydrogen air mixture is only about 4% greater than that of the ammonia air mixture. Therefore, only a relatively small increase in power can be expected as the concentration of hydrogen in an ammonia-air mixture is increased above the amount required as a combustion promoter for ammonia. We found also that the spark advance required for maximum engine power differed appreciably for ammonia air mixtures and the investigated ammonia hydrogen air mixtures. We conjectured that some loss of power occurred in trying to compromise these incompatible spark advance requirements.
Mr. Wagner's comments regarding his investigation of ammonia as a fuel for compression ignition engines are appropriate since our work was confined to spark ignition engines . We concur with him that the combusion of ammonia in a compression ignition engine should also be investigated, since a large number of military vehicles are powered presently by diesel engines.
In summary, we wish to emphasize that the work described -in our paper was performed during a relatively short period of time. . Engine testing was started in March of 1963 and was concluded in January of 1964. Basic ammonia dissociation studies were conducted through the summer of 1964. Engine tests were performed only at steady, state operating conditions while using manual controls. No transient engine conditions were investigated. In view of the constructive criticism of the paper by the several discussors as well as our own thoughts on the matter, we believe that additional studies are needed to provide a more exhaustive evaluation of ammonia as a fuel for military vehicles powered by spark-ignited engines.