Proceedings of ICES2005: ASME Internal Combustion Engine Division 2005 Spring Technical Conference April 5-7, 2005, Chicago, IL, USA
ICES2005-1007
A DIFFERENT COMBUSTION ENGINE
Alejandro Tort Oropeza Rogelio González Oropeza Félix Núñez Orozco
Universidad Nacional Autónoma de México
ABSTRACT
This paper investigates the possibilities of an EXTERNAL COMBUSTION ENGINE (ECE) capable of attaining high thermodynamic efficiency and low emission of noxious gases.
This ECE consists principally of an air compressor, a combustion chamber or combustor and an expansion or power cylinder. The compressed air is introduced into the combustor; the fuel is injected into the combustor, and a spark plug initiates the combustion. By performing the combustion in a combustor specially designed for that purpose, the combustion can be controlled and the generation of noxious gases can be reduced. The water that cools the jackets of the system is also injected into the combustor; by doing so, the temperature of the combustion products can be maintained at a value sufficiently low so as to minimize the formation of NOx,
and at the same time, a significant part of the heat transmitted to the cooling water can be recuperated instead of being dissipated in a radiator.
The water, already hot or evaporated, mixes with the combustion gases and expands in the power cylinder, participating in the generation of mechanical power. The efficiency of the cycle is increased.
INTRODUCTION
The idea of the ECE is an old one. In 1909 a Mr. G. B. Collier applied for a patent for an ECE; many others patents followed. The Stirling Engine is an ECE. The principle has not progressed because it does not offer significant advantages, or because it involves technical complications. It started to make sense when the emissions of the Internal Combustion Engines (ICE) became an issue of general preoccupation.
Fig 1 shows the ECE that we are presenting. The compressor and the power cylinder shown incorporate
only one cylinder each; both can of course be arranged with several cylinders, and the compressor can be designed with two or more stages of compression.
The power cylinder is actually the motor proper, the mechanism that generates mechanical power; it drives the
compressor and the water and fuel pumps (not shown). The admission and exhaust valves are operated by a cam
shaft, as in an ICE.
The compressed air is introduced into the combustor, the fuel is also injected into the combustor, a spark plug ignites the mixture and the combustion becomes self sustained.
The water that cools the jackets of the engine is also injected into the combustor, mixes with the combustion gases, and expands with the gases in the power cylinder.
Fig. 1 THE EXTERNAL COMBUSTION ENGINE
COOLING WATER IN EXHAUST AIR IN Proceedings of ICES2005 ASME Internal Combustion Engine Division 2005 Spring Technical Conference April 5-7, 2005, Chicago, IL, USA
WHY ANOTHER EXTERNAL COMBUSTION ENGINE
Boiler makers believe that in order to achieve an efficient combustion it is necessary to have an adequate space specially designed for that purpose: a furnace, an oven, or a combustor; from that perspective, the dead space of a cylinder is not an adequate combustion chamber. Countless efforts and resources have been invested to improve the combustion in the ICE, and much has been gained, but the problem persists. The best solution nowadays, is to eliminate the noxious gases in a converter after they have been already generated.
Just by performing the combustion in an appropriate combustor, the process can be controlled so it produces a reduced amount of contaminants; the gas turbine people do it with good results. To regulate the combustion, the combustion air can be introduced through several openings of the combustor, as shown in Fig. 2.
If a right amount of water is injected in strategic places of the combustor, the combustion temperature can be maintained at a level sufficiently low so as to minimize the generation of NOx; this temperature is still
high enough so the combustion process is not disturbed. The water injected already hot or evaporated by the absorption of heat in the jackets, mixes with the combustion gases, becomes high pressure superheated steam and expands with the gases in the power cylinder assisting in the generation of mechanical power and increasing the efficiency of the cycle.
We have fabricated several combustors and tried them with LPG, gasoline and Diesel Oil with promising results.
The last combustor tested has several ports for injecting air and water, and several openings to extract samples of the gases for analysis at several stages of the combustion. With this arrangement we have been able to reduce the formation of NOx and CO to around 5 ppm,
which gives some validity to the concept. The instruments used to measure CO2, CO, HC, O2 and NOx
are: an analyzer assembled in the UNAM with an Andros bank, and a Land instrument from England.
Utilizing the same method of calculation we figured out the thermodynamic performance of both, an ECE and a Diesel Engine operating under similar conditions. The outcome encouraged us to pursue the investigation because the calculated indicated efficiency of the ECE resulted more than 5 % over the efficiency of the Diesel Machine. This, in our opinion, is important because the largest loss in a thermal power engine is the thermodynamic loss. Since the efficiencies run at about 50 %, an increase in efficiency of 5 % results in a reduction of fuel consumption of the order of 10 %.
OTHER ADVENTAGES AND CHALLENGES
Besides the reduction of pollution and the increase in efficiency we see the following advantages of this engine:
On the other hand, This ECE represents some technical challenges that have to be dealt with, the outstanding being:
CALCULATED PERFORMANCE
When we started studying this cycle, we realized that it was going to be difficult to arrive to precise results. We had just rough ideas for instance about what would be: the required amount of water to be injected, the most convenient percentage of theoretical air (TA), the amount of heat lost by the hot surfaces including the surface of the combustor (radiation), the best expansion factor, or the amount of heat transmitted to the water.
Fig. 2 THE COMBUSTOR
•The compression and the expansion cylinders are two different mechanisms; consequently, the compression and the expansion ratios are completely independent. It is possible to select the most convenient combination of compression and expansion ratios. The fuel/air ratio can be selected with the sole purpose of obtaining the most convenient operation.
•There is no knocking effect to worry about, nor high excess air required to improve the efficiency.
•A combustor permits to accomplish an efficient combustion with a wider range of fuels, including less refined hydrocarbons.
•The fuels are not required to meet specific octane or cetane specifications.
•Since the combustion gases are admitted at high pressure to the power cylinder, the size of the admission valve is not critical.
•The ECE requires a tank of deionized water. The deionized water is not expensive and it is easy to produce. The water requirement is of about 2 gallons per gallon of fuel.
•The admission valve is subject to high temperatures and has to be cooled.
TABLE 1: VARIABLES CONSIDERED IN THE CALCULATIONS AIR ADMITTED % OF TA HEAT TRANSMITTED TO THE WATER % OF THE LHV HEAT RECUPERATED BY THE WATER % OF THE LHV COMPRESSION RATE STAGES OF COMPRESSION EXPANSION
RATE INJECTED WATER
LBS. PER LB. OF FUEL 110 0 0 16 1 8 0 150 5 5 18 2 12 1 200 10 10 16 2 300 15 15 18 3 400 20 20 4 25 25 5 30 20 6 35 35 40
We did not have and still do not have a design for the engine, so we could not calculate with a good degree of precision some of these parameters.
Table No.1 shows the variables considered in the calculations. A combination of one figure of each column corresponds to a case. To define a case, it is necessary to calculate pressure, temperature and volume for each state of the cycle, the energy available or required by the several processes, and the efficiency of the cycle. The quantity of possible combinations of the variables is very large. We did not run them all, but we calculated sufficient number of cases so as to arrive at meaningful results.
The calculations involve: a long nomenclature, simplifications and assumptions that require explanations, more than 30 equations, the preparation of tables for the properties of the mixture of the combustion products and water vapor, and the design of empirical formulas for those properties. We do not think that all paper work fits into the scope of this presentation, but we can provide it if required.
Our objective was not to arrive at a very precise set of numbers, but to compare the calculated performance of this ECE with the performance of a representative known engine; in this case a Diesel Engine operating under similar conditions. If the calculations for the two engines are made with the same method, the comparison is valid even if the absolute numbers are not very precise. The fuel considered in these calculations is Diesel Fuel Oil with a low heating value (LHV) of 18250 BTU/lb, the TA is 15 lbs per lb of fuel (lbs/lbf), the compression and expansion processes are isentropic, and the constant pressure combustion process 4-6 (Fig. 3) includes the heat lost due to radiation, and the heat absorbed by the cooling water and not recuperated.
Table 2 shows how the water temperature and the efficiency of the cycle vary as the amount of water is increased. The air admitted to the combustor, the
compression and expansion ratios, the heat transmitted to the water and the radiation lost are the same for all the cases. With no water injection (first column) there is no heat recuperation, all the heat absorbed by the water has to be lost by radiation or dissipated. The indicated efficiency of the cycle in this case results of 46%. As the injection of water is increased from cero, to 1 lb of water per lb of fuel (lbs/lbf), (2nd column), 20% of LHV is recuperated, and the efficiency of the cycle jumps from 46% to 55.4%, but the water temperature reaches the unacceptable value of more than 3000°F. As the quantity of water injected is further increased, the water temperature, the temperature of the gases at the combustor outlet, the pressure and temperature at the end of the expansion, and the efficiency of the cycle are reduced. An injection of 2.5 lbs/lbf results in a water temperature of 860°F which is quite manageable, and the efficiency is 51.6%.
Table No.3 shows the comparison of the performance of the ECE with the performance of a Diesel engine. Both engines have a compression ratio of 18. The air admitted to the Diesel goes from 150% of TA to 350%. The expansion ratio of the Diesel is determined by the percentage of TA air admitted. For the ECE we chose a water injection of 2.5 lbs/lbf of fuel because as we saw in table 2, with that injection and a heat recuperation of 20% of the LHV, the temperature of the water (it actually is superheated steam) is 860°F. We also chose 150% of TA for all the cases shown, because as can be seen in this table, with that amount of air, the temperature of the gases at the combustor outlet has the sufficiently low value of 2610°F. The expansion ratios go from 10 to 18. The heat transmitted to the cooling water in a Diesel engine varies largely depending on the size of the engine and the application: 15% to 20% for large engines and 20% to 35% for automotive engines (L.C. Lichty and Neil Mac Coul). We chose the figure of 20% because we consider it representative of the Diesel engine we want
AIR TO THE COMBUSTOR
% OF THEORETICAL AIR 150 150 150 150 150 COMPRESSION RATIO 18 18 18 18 18 18 EXPANSION RATIO 16 16 16 16 16 16 WATER INJECTED lbs /lbf 0 1 1.5 2.0 2.5 3.0 HEAT TRANSMITTED TO THE WATER % of LHV 25 25 25 25 25 25 RADIATION LOSS % of LHV 5 5 5 5 5 5 HEAT RECUPERATED BY THE WATER % of LHV 0 20 20 20 20 20 HEAT DISSIPATED % of LHV 20 0 0 0 0 0 WATER TEMPERATURE AS INJECTED ° F - > 3000 2700 1623 860 594 TEMPERATURE OF THE GASES AT
THE COMBUSTOR OUTLET ° F 2934 3053 2895 2748 2610 2480 TEMPERATURE OF THE GASES AT
THE END OF EXPANSION ° F 1513 1590 1511 1437 1371 1303 PRESSURE OF THE GASES AT THE
END OF EXPANSION psia 22.13 22.47 22.35 22.24 22.16 22.0 INDICATED EFFICIENCY % 46 55.4 54.2 52.9 51.6 50.4
TABLE 2: THE EFFECT OF INJECTING WATER IN THE COMBUSTOR
150 F IG . 3 P V D IAG R AM O F T H E E C E C YC L E 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 V O L . f t.3 /lb . fu e l P R ES S. p si a 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 E ff. % 8 1 4 1 0 1 2 1 6 1 8 7 7 7 7 7 7 1 4 6 F IG . 3 P V D IAG R AM O F T H E E C E C YC L E 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 V O L . f t.3 /lb . fu e l P R ES S. p si a 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 E ff. % 8 1 4 1 0 1 2 1 6 1 8 7 7 7 7 7 7 1 4 6
DIESEL ENGINE EXTERNAL COMBUSTION ENGINE
% OF THEORETICAL AIR 150 200 300 350 150 150 150 150 150
COMPRESSION RATIO 18 18 18 18 18 18 18 18 18
EXPANSION RATIO 8.05 9.21 10.86 11.48 10 12 14 16 18
WATER INJECTED lbs/lbf - - - - 2.5 2.5 2.5 2.5 2.5 HEAT TRANSMITTED TO WATER
% LHV 20 20 20 20 25 25 25 25 25 HEAT RECUPERATED % LHV - - - - 20 20 20 20 20 RADIATION LOSS % LHV 3 3 3 3 5 5 5 5 5
COMBUSTION LOSS % LHV 2 2 2 2 - - - - -
PRESSURE AT COMPRESSOR INLET
psia 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 VOLUME AT COMPRESSOR INLET
Ft3/ lbf 300 401 601 701 300 300 300 300 300
TEMPERATURE AT COMPRESSOR
INLET ° F 70 70 70 70 70 70 70 70 70 PRESSURE AT COMPRESSOR OUTLET
psia 794 794 794 794 794 794 794 794 794 VOLUME AT COMPRESSOR OUTLET
Ft3/ lbf 16.7 22.3 33.4 39.0 16.7 16.7 16.7 16.7 16.7
TEMPERATURE AT COMPRESSOR
OUTLET ° F 1131 1131 1131 1131 1131 1131 1131 1131 1131 TEMPERATURE OF THE WATER
INJECTED ° F - - - - 860 860 860 860 860 TEMPERATURE AT THE COMBUSTOR
OUTLET °F 2931 2539 2113 1984 2610 2610 2610 2610 2610 VOLUME AT THE BEGINNING OF THE
EXPANSION Ft3/lbf 37.3 43.5 55.3 61.1 39.6 39.6 39.6 39.6 39.6
PRESSURE AT THE END OF THE
EXPANSION psia 54.5 43.9 33.8 30.8 41.2 32.4 26.5 22.2 18.9 VOLUME AT THE END OF THE
EXPANSION Ft3/ lbf 300.5 400.7 601.1 701.3 395.5 474.6 553.7 632.8 711.9
TEMPERATURE AT THE END OF THE
EXPANSION °F 1414 1067 729 628 1132 1043 971 911 859 EFFICIENCY OF THE CYCLE % 40.0 41.8 43.9 44.4 47.7 49.5 50.7 51.5 52.0
VOLUME Ft3/lbf
F IG 4 : T EM PER A T U R E OF T HE GA SES A T T HE C OM B U ST OR OU T LET
18 0 0 2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0 2 8 0 0 3 0 0 0 3 0 3 5 4 0 4 5 50 55 6 0 V OL [ F t3/ lb f ] F I G . 5 : P E R F O R M A N C E O F T H E E C E 1 5 2 0 2 5 3 0 3 5 4 0 4 5 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 R E LE A S E PR ES S . p si a 1 0 1 2 1 4 1 0 1 0 1 0 1 6 1 8 1 2 1 4 1 6 1 8 1 2 1 4 1 8 1 2 1 6 1 8 1 6 1 4 3 0 0 % T A 2 0 0 % T A 1 50 % TA 11 0% TA F I G . 5 : P E R F O R M A N C E O F T H E E C E 1 5 2 0 2 5 3 0 3 5 4 0 4 5 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 R E LE A S E PR ES S . p si a 1 0 1 2 1 4 1 0 1 0 1 0 1 6 1 8 1 2 1 4 1 6 1 8 1 2 1 4 1 8 1 2 1 6 1 8 1 6 1 4 F I G . 5 : P E R F O R M A N C E O F T H E E C E 1 5 2 0 2 5 3 0 3 5 4 0 4 5 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 R E LE A S E PR ES S . p si a 1 0 1 2 1 4 1 0 1 0 1 0 1 6 1 8 1 2 1 4 1 6 1 8 1 2 1 4 1 8 1 2 1 6 1 8 1 6 1 4 3 0 0 % T A 2 0 0 % T A 1 50 % TA 11 0% TA 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0 1 4 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 RE L E A S E T E M P . °F 1 0 1 2 1 4 1 6 1 0 1 2 1 4 1 6 1 8 1 0 1 0 1 8 1 4 1 6 1 8 1 2 1 4 1 2 1 8 1 6 3 00 % T A 2 0 0 % TA 1 50 % TA 1 10% T A 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0 1 4 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 RE L E A S E T E M P . °F 1 0 1 2 1 4 1 6 1 0 1 2 1 4 1 6 1 8 1 0 1 0 1 8 1 4 1 6 1 8 1 2 1 4 1 2 1 8 1 6 3 00 % T A 2 0 0 % TA 1 50 % TA 1 10% T A 4 7 4 8 4 9 5 0 5 1 5 2 5 3 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 R E L E A S E V O L . F t3/ l b f E FF % 3 0 0 % T A 2 00% TA 1 50 % TA 1 10 % TA 1 0 1 2 1 0 1 1 1 0 1 2 1 2 1 4 1 2 1 4 1 4 1 4 1 6 1 6 1 6 1 6 1 8 1 8 1 8 1 8 4 7 4 8 4 9 5 0 5 1 5 2 5 3 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 R E L E A S E V O L . F t3/ l b f E FF % 3 0 0 % T A 2 00% TA 1 50 % TA 1 10 % TA 1 0 1 2 1 0 1 1 1 0 1 2 1 2 1 4 1 2 1 4 1 4 1 4 1 6 1 6 1 6 1 6 1 8 1 8 1 8 1 8
FIG. 5 PERFORMANCE OF THE ECE FOR VARIOUS % OF TA AND EXPANSION RATIOS FIG. 4 GAS TEMPERATURE AT THE COMBUSTOR OUTLET
REL A S E PR E S S URE p si a REL A S E T E M P E R A T URE °F EFF IC E NC Y % T E MP E R A T URE ° F 110 150 300 200
VOLUME AT THE COMBUSTOR OUTLET Ft3/lbf
to compare. Our principal objective of these calculations is to compare the efficiencies of the two engines, besides, we did not want the calculations to favor the efficiency of the ECE; for that reason we selected the value of 25% for the ECE, which compared with the 20% of the Diesel is probably high.
The same criteria applies to the heat loss due to radiation: 3% for The Diesel engine and 5% for the ECE. The lower lines of the table show the pressure, volume and temperature for the states of: compressor inlet, compressor outlet, beginning of the expansion, completion of the expansion; and the last line shows the thermodynamic efficiency of the cycle.
For a probable economic selection of 300% of TA for the Diesel and a ratio of expansion of 16 for the ECE the efficiencies are 43.9% and 51.5% respectively.
Fig. 3 shows the PV diagram and the efficiency of an ECE operating with 150 % of TA, a compression ratio of 18, and an expansion ratio of 8 to 18. The heat transmitted to the water is 25 % of the LHV, the heat recuperated is 20% of the LHV, and the water injected is 2.5 lbs/lbf.
The numbers close to the PV diagram refer to the states of the cycle: 1 air at the entrance of the compressor, 4 air at the end of the compression, 6 combustion products at the beginning of the expansion, and the several numbers 7 correspond to the end of the expansion for different expansion ratios. The expansion ratios are the numbers close to the efficiency curve. This diagram actually is the superposition of the diagrams of the compression cycle and the combustion-expansion cycle which actually are independent diagrams. The superposition results in a figure similar to the diagram of a Diesel cycle.
The figure 4 shows the variation of the volume and temperature of the gases at the combustor outlet as the %TA varies (numbers bellow de curve).
The families of curves of the figure 5 show the performance of the ECE for different percentages of TA admitted and different expansion ratios. Release pressure, release temperature and efficiency are shown as function of the release volume. The numbers close to the curves correspond to the expansion ratios.
CONCLUSIONS
We have pursued this project during several years. More theoretical work is required. Table 2 shows that the efficiency can be increased significant amounts by decreasing the quantity of water injected. If the heat transmitted to the water could be reduced, the water injected could also be reduced; a reduction of water from 2.5 to 2lbs/ lbf would result in an efficiency increase of 1%.
No formal economic evaluation has been made; we think that this ECE should have a cost similar to the cost of an equivalent ICE.
It is also necessary to test the combustor at higher pressure and to build and test complete prototypes. The study, so far, tell us that the project can work, and that it has potential advantages over the ICE and its alternatives.
Some engineers with whom we have discussed this idea think that it could be specially suited for medium and large stationary units, power plants, ships and maybe locomotives. We think that it has advantages even over the hybrid-electric. The ECE requires an additional tank of water; the deionized water is not important in the cost of the operation. The hybrid on the contrary requires an electric motor, an electric generator, a special drive train and a pack of batteries all of which are more expensive than the water tank. In any case, the ECE is no competition to the hybrid; the hybrid can be equipped with an ECE and gain additional fuel savings.
ACKNOWLEDGMENTS
The authors express their appreciation to: Jorge Guiza from the Instituto de Investigaciones Eléctricas, to Pedro Rincón from the Laboratorio de Control de Emisiones de la UNAM, to Vicente Lopez from the Mechanical Engineering Laboratory of the UNAM, and to Edmundo Hurtado for their valuable suggestions during the development of this work and their assistance in testing the performance of the combustors.
REFERENCES
The Internal Combustion Engine in Theory and Practice: Charles Fayette Taylor.
Internal Combustion Engines: M.I.T. Press Edward F. Obert.
Gas Tables: Joseph H. Keenan, Jing Chao and Joseph Kaye.
Steam Tables: Joseph H. Keenan and Frederick G. Keyes.
Internal Combustion Engines: L.C. Lichty and Neil Mac Coull.
Thermodynamics: Faires/Simmang. Heat and Thermodynamics: Zemansky.
Guy B. Collier, United States Patent Office. Patent No. 1,130,148.
