wartime repor - dticlogincycle in an effort to produce high indioated mean effective pressures...

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ARR No. E5K06

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25 MAR T94B

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

WARTIME REPOR ORIGINALLY ISSUED

December 1945 as Advance Restricted Report E5K06

CALCULATIONS OF THE PERFORMANCE OF A COMPRESSION-IGNITION

ENGINE-COMPRESSOR TURBINE COMBINATION

I - PERFORMANCE OF A HIGHLY SUPERCHARGED

COMPRESSION-IGNITION ENGINE

By J. C. Sanders and Alexander Mendelson

Aircraft Engine Research Laboratory- Cleveland, Ohio

WASHINGTON

NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite general distribution.

NACA LIBRARY MEMORIAL AEROJ

LABORATORY Laagley Fjjjkj, ya.

LANGLEY MEMORIAL AERONAUTICAL E "234 LABORATORY

3 1176 01354 1876 J

NACA ARB No. E5KD6

NATIONAL ADVISORY COMMITTEE FOB AERONAUTICS

ADVANCE RESTRICTED EEPORP

. CALCULATIONS OF TEE PERFORMANCE OF A COMEBESSION-IGNTTION i

ENGINE-COMPRESSOR TURBINE-COMBINATION

I - PERFORMANCE OF A HIGHLY SUPERCHARGED

COMPRESSION-IGNITION ENGINE

By J. C. Sanders and Alexander Mendelson

SUMMARY

Small high-speed slnr" -cylinder compression-ignition engines were tested to determine their performance oharaoteristics under high supercharging. Calculations were made on the energy available in the exhaust gas of the compression-ignition engines. The maximum power at any given maximum cylinder pressure was obtained when the compression pressure was equal to the mftT-iTrmTn cylinder pressure. Constant-pressure combustion was found possible at an engine speed of 2200 rpm. Exhaust pressures and temperatures were determined from an analysis of indicator cards. The analysis showed that, at rich mixtures with the exhaust baok pressure equal to the inlet-air pressure, there is excess energy available for driving a turbine over that required for supercharging. The presence of this excess energy indicates that a highly supercharged compression- Ignition engine might be desirable as a compressor and combustion chamber for a turbine.

INTRODUCTION

In view of the current interest in aircraft-propulsion systems that utilize gas turbines In the energy-conversion process (reference 1), the development of a means of supplying gas-at pressures and temperatures suitable for turbine operation is desirable.

The physical properties of materials available for turbine blades limit the ma-rinnim permissible gas temperature to approximately 1600° F. In a combustion-gas-turbine system consisting of a compressor, a combustion chamber, *»r^ a gas turbine, the temperature is maintained below the specified limit by supplying excess air. This limited temperature reduces the available energy in the gas that can be extracted

NACA ARR No. E5K06

by the turbine and for this reason highly efficient turbines and compressors are required to obtain over-all performance approaching that of the reciprocating engine (reference 2).

A dual power unit consisting of a highly supercharged compression- ignition engine geared to a turbine is proposed. The conditions of the working cycle would be so adjusted that the turbine, supplied with the exhaust gas from the engine, would produce a large propor-' tlon of the power. The oompression-ignition engine is proposed for use in this engine-turbine combination because it can operate at high pressures without the occurrence of knock and preignition. Some con- trol of the temperature of the gas entering the turbine can be effec- ted by varying the fuel-air ratio of the mixture burned in the engine. This plan for a dual power unit, which is not new, is mentioned In reference 1.

A preliminary evaluation of the desirability of a compression- Ignition engine-turbine combination requires knowledge of the engine performance with high inlet-air pressures and requires-Information on exhaust-gas temperatures and pressures. Information on exhaust-gas temperatures and pressures and a dlsoussion of the performance characteristics of a highly supercharged oompression-ignition engine are presented herein. No test data on performance at inlet-air pressures above 50 Inches of mercury absolute were available for the computa- tions, Tests were therefore conducted on two small high-speed compression-ignition engines to determine the operating characteristics at inlet-air pressures up to 90 Inches of mercury absolute. Particular emphasis was placed upon operation on the constant-prossure combustion cycle in an effort to produce high indioated mean effective pressures without excessive maximum cylinder pressure, Indicator cards were analyzed to obtain exhaust temperatures and pressures. These tests were run at the NACA Langley Field laboratory during the summer of 1941. No attempt is made herein to estimate the performance of the engine-turbine combination.

APPARATUS

Two four-stroke-cycle compression-ignition engines were used in these tests. One, which had a compression ratio of 13.1, consisted of a single water-cooled cylinder mounted on an NACA universal test engine crankcase. High turbulence in the combustion chamber was pro* ducod by the use of a displacer-type piston shown in figure 1 and more fully described in reference 3. The bore was 5 inches and the stroke 5g inches. The other englnu, which had a compression ratio

of 15, used an air-cooled cylinder with the same type of combustion chamber as the water-cooled cylinder and had a bore of 5j Inches and a stroke of 5y inches.

NACA ARB No. E5KD6

The simultaneous discharge from "both cylinders of a two- cylinder high pressure injection pump with plungers 11 millimeters in diameter metered fuel to a differential-area injection valve, vhich was spring-loaded to have an opening pressure of 2500 pounds per square inch. A multiple-orifice nozzle, designated K-4 in table I of reference 4, was used because it was found that this nozzle gave a good mixing of the fuel with the air.

The usual laboratory equipment was employed to measure power and fuel consumption. Combustion air was supplied at high pressure from a central supply system. Inlet-air pressure was measured at the surge tank and the air was unheated. Maximum cylinder pressures «Tifi indicator cards were obtained with a modified Farnboro engine- indicator described in reference 5.

PROCEDURE

Pour series of tests were run, three at a compression ratio of 13.1 and one at a compression ratio of 15. In the first series of tests inlet-air pressure, fuel flow, and injection advance angle for maximum power at- several maximum cylinder pressures were determined with the engine having a compression ratio of 13.1. A maximum- power best at a compression ratio of 15 and a maximum cylinder pressure of 1225 pounds per square Inch absolute was run in the second series of tests to show the effect of compression ratio on maximum power. In the third series the effect of injection advance angle, fuel-air ratio, and inlet-air pressure on fuel consumption nt a maximum cylinder pressure of 1400 pounds per square inch absolute was determined. Indicator cards to show the effects of inlet-air pressure and injection advance angle on combustion were obtained during the fourth series of tests. The last two series of tests were run on the engine having a compression ratio of 13.1'; All tests were run at an engine speed of 2200 rpm and atmospheric exhaust.

Maximum-power tests at low compression ratio. - At each of a number of fixed inlet-air pressures in the range from 45 to 80 inches of mercury absolute and with the fuel flow adjusted to give maximum power, the injection timing was advanced until an arbitrarily chosen maximum cylinder pressure was obtained. The inlet-air pressure, brake mean effective pressure, Injection advance angle, and maximum cylinder pressure were recorded. The tests were repeated at suc- cessively higher inlet-air pressures; the injection was retarded in each case, until the inlet-air pressure could not be further increased without increasing the maximum cylinder pressure, regardless of how much the injection was retarded. Similar tests were run at maximum cylinder pressures of 770, 955, 1135, 1225, 1315, and 1410 pounds per

NACA ABB No. E5K06

square Inch absolute. The engine having a compression ratio of 13.1 was used for these tests.

The indicated mean effective pressure vas calculated from the indicated power obtained by adding the measured brake pover and the motoring power required to drive tho engine at test speed and at test inlet-air pressure when no fuel was injected. Friction tests were made with a cold engine because ignition of the lubricating oil Interfered with motoring measurements when the engine was warm.

Maxlmum-power tests at high compression ratio. - The maximum- power tests were conducted at a compression ratio of 15 in the same manner as the maximum-power tests at a compression ratio of 13.1, except that the only value of maximum cylinder pressure tested was 1225 pounds per square inch absolute.

Fuel-consumption test. - Fuel-consumption characteristics were obtained at Inlet-air pressures of 68.2, 73, 76.5, 81.4, and 84.4 inches of mercury absolute. At each inlet-air pressure, suf- ficient fuel was injected to produce maximum power and the injection timing was adjusted to give a maximum cylinder pressure of 1400 pounds per square inch absolute. Observations were made of fuol consumption at various loads at the Inlet-air pressure and the injection timing thus obtained.

Indicator-card tests. - Two series of indicator cards were obtained. All the cards in one series were taken on cycles having peak pressures of 1415 pounds per square Inch absolute. At Inlet- air pressures of 45, 60, 70, and 80 inches of mercury absolute, the Injection was advanced until a meximuo cylinder pressure of 1415 pounds per square inch absolute was obtained and Indicator cards were taken. The fuel flow was that for maximum power at each inlet-air pressure.

The other series of indicator cards was obtained with the injection-advance angle so adjusted that combustion was as near con- stint pressure.as possible. These Indicator cards were obtained at inlet-clr pressures of 48, 58, 70, 73.6, and 80 inches of mercury absolute and maximum-power fuel flow.

POVER AND FUEL-CONSUMPTION GHABACTERISTICS

Maximum-power tests. - Figure 2 shows that at any given maximum cylinder pressure between 770 and 1410 pounds per square inch absolute, the maximum power was attained when the compression pressure was approximately equal to the maximum cylinder pressure. For example, at a maximum cylinder pressuro of 1225 pounds per square Inch absolute, the maximum power was obtained when the Inlet-air pressure

NASA AKR Nc. E5E06

was 75 inches of mercury absolute and the .corresponding compression pressure without fuel was Ü70 pounds ppr .square inch absolute, which indicates that the'engine was operating on approximately a constant-pressure combustion cycle.• Because the compression pressure in the oylinder while the engine is under power is- slightly higher, than the compression pressure while the engine is being motored, the real agreement between the maximum cylinder pressure and the compression pressure is closer than is shown by figure 2.

The limit of supercharging for a given maximum cylinder pressure is reached when the compression pressure equals tho maximum cylinder pressure. In all these tests the highest power was reached at the limit of supercharging as shown In figure 2,

Evaluation of the power characteristics of the compression- ignition engine by comparison with a spark-ignition engine may bo obtained from figure 3. For the same power the spark-ignition engine requires an inlet-air pressure about 8 percent lower than the compression-ignition engine when the injection is advanced to give a constant-volume combustion cycle. When the injection is retarded to simulate a constant-pressure combustion cycle, however, the Inlet-air prossure required for the same powor by the compression- lngition engine is 25 percent greater than that required by the spark-ignition engine. Jjfbon a maximum cylinder pressure is achieved in the compression-ignition engine as low as In the spark-Ignition engine, this loss must be incurrod. As can be seen from figure 3, the compression-Ignition engine with a compression ratio of 13.1 has about the same ratio of maximum cylinder pressure to indicated mean effective pressure as a spark-ignition engine with a compression ratio cf 6.4. The maximum-power fuel-air ratio is used in each case.

Effect of compression ratio on pressure-limited power. - The effect of compression ratio on the maximum power obtainable with a given maximum cylinder pressure is illustrated In figure 4. The maximum indicated mean effective pressure attainable with a maximum cylinder prossure of 1225 pounds per square inch absolute is 264 pounds per square Inch at a compression ratio of 15 and 300 pounds per square inch at a compression ratio of 13.1. The corresponding Inlet-air pressures were 62 and 75 inches of mercury absolute, respectively.

Those data illustrate that,because a higher inlet-air pressure can be usod,more power can be obtained by using a low compression ratio for a. highly supercharged compression-ignition engine deliv- ering exhaust gas to a turbine than by using a high compression ratio. The minimum compression ratio of a compression-ignition engine is limited by tho ignltibllity of the cylinder charge. Starting becomes difficult In warm climates when the compression ratio is less than 12

NACA ARK No. E5X06

and in cold climates when the compression ratio is below 14 unless an air preheater is used. For increased power output for the compression-ignition engine-turbine combination, lower compression ratios seem desirable and may be practical if a hot plug is used to ignite the fuel.

Fuel consumption. - The fuel-air ratios were computed, assuming a volumetrio efficiency of 0.90, and were plotted with fuel consumption «rid inject Ion-advance angle in figure 5. The reciprocal of the indicated specific fuel consumption was used because the reciprocal is proportional to the indicated efficiency. This figure clearly shows that, when ^he injection is retarded, the indicated efficiency is reduced. Figure 6 shows the volumetric efficiency for a similar type engine.

Indicator-card tests. - An example of an original record obtained with the Farnboro indicator is shown in figure 7; records presented in such form are indistinct. The curves on the indicator cards obtained (figs. 8 and 9) were therefore drawn by hand to represent the mean of the points generated for each test condition by the Farnboro indicator. The pressure scales are not lioear in the low ranges.

Figure 6 shows that, when the inlet-air pressure is increased and the injection retarded, the cycle changes from an approximation' of the constant-volume combustion cycle to a constant-pressure com- . bustion cycle. It is not always possible, however, to obtain a constant-pressure combustion cycle with a retarded injection, as shown in figure 9. At low maximum cylinder pressure, a large delay in ignition of the fuel injected resulted in most of the fuel ignit- ing at one time and causing a sharp rise in pressure. The injection consequently was considerably retarded after top center to prevent the wigf*"»• cylinder pressure from exceeding the compression pressure. A test at a maximum cylinder pressure lower than 770 pounds per square inch absolute was attempted but it was found necessary to retard the injection to Buch an extent that ignition was irregular as a result of the low compression temperature during injection.

The data from the Farnboro indicator for a constant-pressure combustion cycle were transferred to a pressure-volume basis and are presented in figure 10. A true Diesel cycle was not attained because the expansion after cut-off was isothermal (the expansion exponent being approximately 1.0) resulting from the continuation of combustion during the oxponslon stroke. The isothermal expansion is undesirable buccuse the economy and power at a given maximum cylin- dor pressure are loss than those obtained with a true Diesel cycle having adlabatlc expansion. The close approach to the Diesel cycle shown by the indicator diagram in figure 10, however, is considered

NACA ARB No. E5K06

remarkable for a small compression-ignition engine operating at a high engine speed of 2200 rpm.

Energy available in the exhaust gas. - The energy available in the exhaust gas was obtained by analysis of an indicator card. Observations of exhaust-gas temperatures as measured by a thermo- couple could not be used for this purpose because temperature measured In this manner. Is much lover than the true value, as is explained in reference 6.

A description of the method of analyzing the indicator cards is given in appendix A and the results of this analysis are shown in figures 11 and 12. From figure 11 the release temperature and pressure may be obtained for computation of exhaust temperature and energy available at various operating conditions. Such com- putations were made for the special case of a fuel-air ratio of 0.065 and sea-level ambient pressure with lntake-manlfold and exhaust-manifold pressures equal. The method of making this computation is described in appendix B and the results are shown in figure 12 together with the energy required for supercharging. The energy available in the exhaust at high inlet-air pressures exceeds the required supercharger power. The combined efficiency of a representative supercharger and turbine is 50 percent. The required energy to the turbine, computed by dividing the energy required for supercharging by 0.5, is shown to be less than the available energy, thereby indicating the possibility that the turbine can develop excess energy, which could be used to augment the power of the engine, particularly at high inlet-air pressure.

SUMMARY OF RESULTS

From the tests made on small high-speed compression-ignition engines to determine the suitability of this type of engine for use In an engine-gas turbine combination, the following results were obtained:

1. The maximum power attainable in a cylinder with maximum pressure limited by structural strength was obtained by supercharging until the compression pressure equaled the maximum cylinder pressure. The lowest ratio of maximum cylinder pressure to indicated mean effective pressure was 4 when the compression ratio was 13.1.

2. Engine operation with approximately constant-pressure combustion was found possible with the displacer-type chamber at on engine speed of 2200 rpm.

NAß A ABB No. E5B06

3. The exhaust energy available with rloh mixtures at sea level with the exhaust hack pressure equal to the Inlet-air pressure exceeded the energy required to drive a turbosupercharger.

Aircraft Engine Research Laboratory, National Advisory Committee for Aeronautics,

Cleveland, Ohio.

NACA ABE No. B5K06

APH5NDU A

INDICATOR-CARD ANALYSIS

In order to find, the temperatures and pressures "before release, an analysis of Indicator cards was made. The release pressure can- not he direotly read from the indicator cards ohtained with a modified Farhboro engine Indicator because the pressure scale is inaccu- rate below 300 pounds per square inch.

An analysis was made of the cycle shown in figure 10 using the following symbols:

n exponent for compression stroke in equation FT11 = k

ne exponent for expansion stroke

vco volume above piston at cut-off

v„ clearance volume

v^ displacement volume Tco rc cut-off ratio,

c Tc + vd r compression ratio, vc

T^ temperature at release, Tl

P. pressure at release, lb/sq ft absolute

T]_ inlet-air temperature, °R

P, inlet-air pressure, lb/sq ft absolute

Experimental values of ne, nQ, and r0 for rich mixtures were obtained from Indicator cards taken in the tests reported herein. The cards used for lean mixtures were taken from figure 4 of reference 7. These cards were taken with an optical indicator with the engine under power for only one cycle. The compression ratio was 13.9. The exponent nc was found to be approximately 1.35, as shown In figure 13. Figure 13 also shows n for a range of fuel-air ratios from 0.058i to 0.0027. The value of the cut-off ratio was available only for rich mixtures from the indicator oards obtained in the present test. It was assumed that the cut-off ratio is constant for rich mixtures and Is proportional to the fuel-air ratio up to the theoretically correct mixture. The variation of nQ, nc, and rQ with fuel-air ratios are shown in figure 14.

10 NAD A ABR No. E5K06

Release temperatures and pressures for most operating conditions at e compression ratio of 13.1 are plotted in figure 11. These values were obtained hy substituting the experimental values of the constants from flgure--14, in the following equations:

T4 = Tir^-^rc"0 (1)

P* = ?! § (2)

With this information, the exhaust energy can he calculated for almost any condition.

NACA ARE No. E5K06 11'

APPENDIX B

CALCULATION OF EXHAUST-GAS TEMPERATURE AND AVAILABLE ENERGY

Tho temperature of the exhaust gas can he calculated from the conditions "before release. During the release process two actions occur. Eich small volume of gas escaping from the cylinder Is assumed to undergo a free expansion with practically no change of temperature. The gases remaining In the cylinder undergo an approximately adiahatic expansion. The relation between the temperature and the density will then be approximately as shown in the following figure:

T

Both tho temporature T and tho density P decrease until the pressure becomes equal to the oxhaust pressure. The temperature remains constant while the remainder of the gases are exhausted from tho cylinder. The cver?go terapercturo cf the exhaust gas can therefore be obtained by intogr.iting tho area under tho curve and dividing by the '-.bsrisgr.

The equation for the average temperature Is derived as follows:

T.

From the gas equations:

05 Tap + Tgpg (1)

av

T = T,

T5

P4 -*

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12 NACA ABB No. E5E06

Substituting the ezpreasion for T, Tg,' and pg in equation (l) and integrating

T o — ] »• 7 (_

1 + (7-D rr\ (2)

When the release conditions and the exhaust-hack pressure are known, the exhaust-gas temperature can he calculated from equation (2). The values of 7 were obtained from figures 2 and 4 of reference 8.

After the exhaust-gas temperature was calculated from equation (2), tht« energy available for a turbine cycle was obtained from figure 9 of reference 8. The results are shown plotted in figure 12.

REFERENCES

1. Smith, G. Geoffrey: Aircraft Propulsion Systems. An Examina- tion of the Possibilities of the Combustion Gas Turbine: Tur- bines in Conjunction with Airscrews. Flight, vol. XLII, no. 1764, Oct. 15, 1942, pp. 417-421.

2. Pioning, Werner: The Efficiency of Combustion Turbines with Constant-Pressure Ctmbuation. NACA TM No. 975, 1941.

3. Moore, C. S., and Poster, H. H.: Performance Tests of a Single- Cylinder Compression-Ignition Engine with a Displacer Piston. NACA TN No. 518, 1935. .,

4. Foster, H. HY: The Quiescent-Chamber Type Compression-Ignition Engine. NASA Rep. No. 568, 1936.

5. Collins, John H., Jr.: Alterations and Tests of the "Farnboro" Engine Indicator. NACA TN No. 343, 1930.

6. King. W. J.: Measurement of High Temperatures in High-Velocity Gas Streams. A.S.M.E. Trans., vol. 65, no. 5, July 1943, pp. 421-428.

7. Rothrock, A. M., and Waldron, CD.: Effects of Air-Fuel Ratio on Fuel Spray and Flame Formation in a Compression-Ignition Engine. NASA Rep. No. 545, 1935.

8. Pinkel, Benjamin, and Turner, L. Richard: Thermodynamic Data for the Computation of the Performance of Exhaust-Gas Turbines. NACA ARR No. 4B25, 1944.

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NACA ARR No. E5K06

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Fig NACA ARR No. E5K06

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Fig. 10 NACA ARR No. E5K06

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estimated from analysis of indicator cards obtained on a compression- ignition engine. Compression ratio, 13.1; fuel-air ratio. 0.067- inlet-air temperature, 95° p; engine speed, 2200 rpm; exhaust back pressure is equal to the inlet-air pressure.

MACA ARR No. E5K06 Fig. 13

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Fig. 14 HACA ARR No*. E5K06

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.02 ,ok .06 Puel-alr ratio

.08 .10 .12

Figure ll|.. - Effect of fuel-air ratio on cycle exponents and cut-off ratio of compression-ignition engine.

Nattoal Adteäsos? Committee for Aeronautics, Lewis FSSgM Propulsion Lab., Cleveland, O.

THE PERFORMANCE OF HIGHLY SUPERCHARGED COMP'iESSBDN-IGNITEDN ENGINES AMD THE EX- HAUST ENERGY AVAILABLE FOR A TURBINE - AND APPENDIX A AND B, by 7. C. Sanders and Alexander Eäcsrflslson. Jan MM5. 26 pp. RESTRICTED

DIVX3I0N: POCTS? Pteäsis, FscSprccalSag (8) SECTION: Porformaaca (13) DXSTMBUTXON: U. S. ESBitäry Orgamteattas request copies from ASTKA-BSC. CJhess rov&e requests to ASTXA-BSC »too AMC, TOTSgfcMfottamsa Air Force Bass, Baytea,, O, Atta: NACA,

ocaanr EnroaciaTHEi QIZBTOICTED

L Sanders, 7. C. H, Mendelson, Alesander

cm scats TBCXM. CHOKHD eiav 8Sh.V,illl SüiiCE SOTEQ

DOOTOCCTED

wartime repor - dticlogincycle in an effort to produce high indioated mean effective pressures...

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