Acoustic Apparatus for Determination of Mixture Ratio By Analysis of Engine Exhaust Gas E. F. WELLER General Motors Corp., Research Laboratories Division, Detroit 2, M i c h .
A mixture ratio analyzer was developed t o provide a fast a n d accurate means of indicating or recording t h e air-fuel ratio i n i n t e r n a l combuston engines. The analyzer converts t h e raw engine exhaust gas to a twogas mixture plus water vapor by t h e use of a n oxidation furnace. T h e water vapor is held t o a constant low value a n d t h e resultant binary mixture is analyzed in a n acoustic chamber. T h e acoustic gas analyzer operates by measuring t h e velocity of sound i n t h e gas using a simple feedback circuit i n which t h e acoustic chamber is t h e frequency controlling element. A frequency discriminator converts t h e signal t o a direct current voltage which is a function of t h e gas composition. The analyzer will indicate or record mixture ratios w-ith an error of &1%; t h e drift is 1% i n 24 hours. The accuracy achieved with t h i s u n i t is comparable to t h e accuracy obtained with more elaborate equipment, while t h e response t i m e is considerably shorter. I n this application t h e analyzer is more accurate a n d does not have t h e inherent errors found i n t h e Orsat, a n d therefore i t is more accurate t h a n devices depending o n t h e Orsat for dynamic calibration.
to the engine in a given time interval. The measurement of both air and fuel flow is subject to error and each of these errors is introduced into the air-fuel ratio calculated from these measurements. Therefore, this method of determining mixture ratio is subject to the combined error of the separate measurements. Another method of measuring mixture ratio is an inferential one in which a knowledge of the exhaust gas composition is required. The measurement of mixture ratio by the analysis of combustion products has become increasingly important in the past few years.
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X ENGINE testing, engine power and brake-specific-fuel consumption are of parimary concern, and both of these quantities are affected by changes in mixture ratio. In addition, the knocking tendencies of fuels vary with changes in mixture ratio, with some fuels being more sensitive than others. The fundamental method of determining mixture ratio in gasoline engines is to measure the weight of air and the weight of fuel supplied
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N o r m 1 Exhaust
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Figure 2. Effect of Air-Fuel Ratio o n Carbon Dioxide Content i n Normal and Oxidized Exhaust Gas 6
The Orsat gas analyzer in its most common form has been used extensively to measure mixture ratio (5) by the analysis of the combustion products. A typical analysis of the engine exhaust gas as a function of air-fuel ratio is shown in Figure 1. In using the Orsat analyzer on engine or raw exhaust gas, it is necessary to analyze for two and sometimes three of the exhaust gas components in order to determine which of the two ratios obtained from the carbon dioxide analysis is correct. This is because the carbon dioxide is absorbed first in the Orsat analyzer, oxygen second, and carbon monoxide third. The hydrogen concentration is not analyzed in the Orsat analyzer. The Orsat analysis is a slow process, and in some cases it is almost impossible to determine the mixture ratios in multicylinder engine work. The mixture ratio varies from cylinder to cylinder, resulting in a composite exhaust gas containing oxygen, carbon dioxide, and carbon monoxide. Furthermore, some of the fuel entering the carburetor is not completely burned, resulting in unburned hydrocarbons being present in the raw exhaust gas. These hydrocarbons are not detected in this analysis. To overcome the effect of unburned hydrocarbons in the exhaust gas, a method of,oxidixing the raw exhaust gas was developed ( 4 ) . This method not only simplifies the measurement
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489 ACOUSTIC GAS ANALYZER
The acoustic gas andyeer is a binary-type system well adapted
to the measurement of gas compositions which contain only two gases, or three gases providing the concentration of the third remains constant. It utiliaes the principle that the velocity of sound in a gas is proportional to the molecular weight. If the gas is made up of two unlike gases, then the velocity of sound is a function of the proportions of the molecular weights of each of the gases. The heart of the acoustic gas analyzer is the acoustically resonant chamber or sound cell shown in Figure 5 . This cell conterinn two electroacoustical transducers, one loczted a t each end of the chamber. Provisions are made to adjust the length of the cell by moving B plunger on which one of the transducers is mounted. This adjustment changes the sound path length. The cell is also provided with a heat,er and thermmtat to maintain the cell a t a constant temperature. The gas enters the cell and passes through a heat exchanger before tangentidly entering the sound chamher, from which it is exhausted to the atmosphere. The electronic circuit associated with the cell is shown in block diagram form in Figure 6 and the circuit diagram in Figure 7. The output from one of the transducers is transformer coupled to the input of VI, which is an amplifier stage. The output of VI is transformer coupled to the other trsnsducer. This is simple feedback amplifier with sufficient gain to cause it to oscillate, the sound cell acting as the frequency controlling element. .4 portion of the signal from the oscillator is fed to a limiting amplifier, V2. The output of this amplifier is coupled t,o a frequency discrimina-
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with the Orsat analyzer to some extent, hut more important, it eliminates the unhurned hydrocarbons. The raw exhaust gas is passed over hot copper and cupric oxide wire. Figure 2 shows a typical analysis of the oxidized exhaust gas. Such an analysis gives the true mixture ratio and in addition eliminates the double valued carbon dioxide curve. A new type of mixture ratio analyzer which uses the oxidised gas in conjunction with Bin acoustically resonant cas column has been developed. Acoustic methods of measuring gas compositim are not new and several forms of analyzing equipment have been d e v i d ( I , 8, 6). This analyzer, shown in Figure 3, will continuously indicate and record both rich and lean mixture ratios with an error of +1% or less. This analyzer has a response time of 30 seconds and presents the information in percentage of carbon monoxide in nitrogen which can he easily converted t o mixture ratio with the aid of a suitable conversion chart such as shown in Figure 4.
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Oxidized Exhaust Gas Composition for Average Commercial Gasoline
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Aeoustio Test Cell
ANALYTICAL CHEMISTRY
490 tor, T4, Ca, V 3 ,and V4. The output of the frequency discriminator is a direct current voltage proportional to frequency. The discriminator output is at an impedance level too high to be used conveniently, so a cathode follower stage, Vg and VG, is used to drive the recorder or meter. A simple voltageregulated power supply furnishes the necessary power to operate the electronic unit. Precautions in the construction of the frequency discriminator cricuit have eliminated v a r b tions caused by ambient temperature changes. EXHAUST GAS MIXTURE RATIO ANA LY ZFB
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The acoustic gas analyzer is well adapted to the measurement of engine exhaust gas after it has been passed through the oxidation furnace. This oxidized exhaust gas contains only carbon dioxide, nitrogen, and water VERNIER SENSITIVITY vapor. By properly controlling the TWO CONDUCTOR water vapor content, a suitable twoSHIELDED COARSE SENSITIVITY gas mixture is obtained. A block diagram of the mixture ratio analyzer iACOUSTIC IOV A( is shown in Figure 8. Exhaust gas is CHAMBER pumped at a high rate through a ceramic filter and three-way valve by a small diaphragm pump. This high pumping rate scavenges the sample Figure 7. Circuit Diagram for Acoustic Gas Analyzer line rapidly, with the excess gas being exhausted to the atmosphere at the second three-way valve. The gas to The oxidized gas from the furnace contains a varying amount of be analyzed passes through a pressure regulator, which eliminates water vapor which must be controlled. A small heat exchanger engine back pressure surges, and into the electric oxidizing furmaintained at 35' F. by a Freon 12 refrigerator system condenses na.ce charged with copper and cupric oxide. most of the water and holds the humidity quite constant. The gas leaving the heat exchanger enters the analyzing cell and is then exhausted to the atmosphere. A pressure gage measures the pressure drop across the cell and is calibrated in flow rate; the flow rate is adjusted with the second three-way valve. The cupric oxide is reduced after being used on rich mixtures for several hours. During this time the unburned hydrocarbons, hydrogen, and carbon monoxide are being oxidized, the oxygen being supplied by the cupric oxide. The cupric oxide is conveniently regenerated by operating the first three-way valve, shutting off the sample gas, and opening the intake of the pump to the atmosphere. The oxygen will quickly oxidize the copper to the cupric oxide state. To operate on lean mixtures, it is first necessary to condition the furnace charge. Lean mixtures contain an excess of oxygen which must be reI moved. The furnace charge is conL_ _ _ - - - 1 ditioned for this use by passing city gas through the analyzer which conFigure 8. Block Diagram of Mixture Ratio Analyzer
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over a period of 97 hours and the average deviation between the Orsat and the mixture ratio analyzer was found to be o.13Y0carbon dioxide in nitrogen at the point tested. The analyzer exhibited a drift of about 0.1% carbon dioxide in nitrogen in 2-1 hours of continuous operation. The mixture ratio analyzer has been used on many different types of engines. Figure 9 shows selected portions of a continuous recording of an engine mixture ratio analysis. The engine speed, throttle setting, and mixture ratio were held constant for this test. Orsat analysis of the exhaust gas was in close agreement with the mixture ratio analyzer results except for a few points. It is difficult to obtain an Orsat sample which is representative of a particular point on the recording. Figure 10 shows a typical recording of the analysis of the exhaust gas from a single cylinder laboratory engine. The large variations in mixture ratio were not detected or suspected before a recording was made of the exhaust gas. The fault lay in the carburetor design and a slight modification greatly improved the mixture ratio. ACKNOWLEDGMENT
The author wishes to express his appreciation and thanks t o
E. J. Martin, F. W. Chapman, and the late W. S. Erwin for their helpful and stimulating discussions. LITERATURE CITED
R. W., and Barlow, G. E., Australian J . Sci. Research, 1 , NO. 2, 176-89 (1948). (2) Crouthamel, C . E., and Diehl, Harvey, ANAL.CHEW,20, 515 (1) Abbey,
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Figure 10. Single-Cylinder Laboratory Engine
(3) D’iilleva, B. A., and Lovell, W. G., S A E J o v m Z , 38, No. 3, 90-6 (1936). (4) Gerrish,‘ H. C., and Meem, J . L., Jr., S a t l . Advisory C m m . Aeronaut. Rept. 757 (1943). (5) Mikelson, W., U. S. Patent 2,283,750 (May 19, 1942).
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RECEIVED for review July 15, 1953. Accepted November 16,1963.
verts the cupric oxide to copper. After the analyzer has operated a short period on either rich or lean mixtures it is then possible to analyze exhaust gases which are at or near theoretical optimum mixture ratios. METHOD OF CALIBRATION
The most reliable method of calibrating the analyzer is to use a com ressed gas mixture of known composition obtained from Ohio 8hemical Co., Cleveland, Ohio. This gas, of approximate specified composition, is furnished in convenient containers and must be analyzed with an Orsat analyzer before use. At least five analyses should be made in order to obtain a good average of the gas. Two tanks of gas, one to check low concentrations and one for high concentrations of carbon monoxide in nitrogen, have been found sufficient. When the analyzer has been warmed up with a fresh charge of cupric oxide, the calibrated gas is passed through the analyzer at the same flow rate that is used in analyzing exhaust gas. The gas is allowed to flow long enough to give a steady reading before making adjustments. The zero set and sensitivity are adjusted in order to bring the analyzer into calibration. After calibration, the unit is ready for operation. I t has been found that the zero setting must be checked each time the analyzer is to be used. This requires the use of only one test gas. The sensitivity or spread adjustment needs checking about once each week of continual operation.
Spectrophotometric Determination of Cholesterol And Triterpene Alcohols in Wool Wax -Correction In our recent publication of the above title [+&SAL. CHEY.,25, 149i-9 (1953)l the wrong equations were given for Methods I and I1 under the heading “Calculation of Results” on page 1499. The correct equations are as follows. Method I. Free Alcohols. Cholesterol, weight
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FRAXCIS E. LUDDY ARTHURTURNER, JR. JOHN T. SCANLAN
RESULTS
An Orsat analyzer was used to check the gas from the mixture ratio analyzer and the results from the Orsat were compared with the readings obtained from the mixture ratio analyzer recorder During an extensive and thorough test, 116 analyses were made
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- UC’UL’
Eastern Regional Research Laboratory U.S. Department of Agriculture Philadelphia 18, Pa.
