Knock Intensity Research Articles

The Effects of Operating Conditions of the Engine on Antiknock Properties of Gasolines (2)

(1) The relationships between spark advance and power of the CFR engine under various conditions were studied. The maximum power spark advances at engine speed of 600rpm and 900rpm were compared with those specified for octane rating by Research and Motor method.(2) The comparison is made between the knock limited compression ratios by incipient knock and standard knock intensity of Research and Motor method. The possibility of estimating the Research octane number from the incipient knock limited compression ratio is deduced.(3) It is shown that spark advance and mixture ratio have more effects on antiknock properties as higher octane number fuels are used.

Effects of pressure and other variables on determinations of octane number

2showed that ratings decreased with decreasing charge density. They found that, at constant compression ratio, the knock intensity at 25 in. Hg with a %-in. venturi was equal to that at sea level with a % 6-in. venturi and throttle plate. The subsequent use of larger Venturis resulted in better agreement among ratings made at altitudes up to 5,000 ft. The problem became more serious in 1939, when the Research Method was adopted by the Cooperative Fuel Research Committee as a supplementary method of rating fuels. Tests showed that neither changing the size of Venturis nor varying the spark advance would compensate satisfactorily for altitude. In 1941 this Bureau, in cooperation with the CFR Motor Fuels Division, conducted tests which indicated [2] that, with the fuels available at that time, the effect of altitude on knock ratings could be eliminated by operating at constant knock intensity. Blends of secondary reference fuels (A-6 and C-12) were used in this work. A series of curves77 was established to indicate the proper compression ratios for maintaining standard knock intensity at all barometric pressures and octane numbers. These curves (one curve for each octane level) are parallel straight lines, one for each octane number, the slopes of which are 0.030 and 0.020 in./in. Hg for the Motor Method and for the Research Method, respectively. Application of this method in various laboratories gave ratings that were essentially independent of the altitude of the test engine. With the development of the Detonation Meter, Model 501, as an alternate to the bouncing pin, a second set of guide curves was established at this Bureau in 1948 for use with the %6-in. venturi. 1 This project was a joint undertaking of the American Petroleum Institute and the National Bureau of Standards. 2 Figures in brackets indicate the literature references at the end of this paper. More recently, two developments have led to the belief that the empirical method outlined above is not wholly satisfactory in insuring uniformity of ratings. These are changes in the composition of motor gasoline due to the method of manufacture and the more general use of the primary reference fuels, w-heptane and isooctane, instead of secondary reference fuels. Current gasolines are more sensitive to engine operating conditions than earlier fuels, and hence show greater differences in ratings by the Motor and Research Methods. The secondary reference fuels behave similarly with respect to the primary reference fuels. Hence the American Petroleum Institute and the National Bureau of Standards undertook a further investigation of the problem of altitude ratings in 1949. The objectives of the program were to establish the magnitudes of rating variation with pressure for several typical fuels, to check the guide curves, and to investigate the individual effects of certain operating variables.

THE OXIDATION, IGNITION, AND DETONATION OF FUEL VAPORS AND GASES: XV. THE CONCENTRATION OF FINELY DIVIDED CARBON IN TOWN GAS-AIR MIXTURES REQUIRED TO INDUCE SEVERE KNOCKING COMBUSTION

When using gaseous fuels for the C.F.R. engine, the lubricating oil decomposed to finely divided carbon when the gas was hydrogen and optimum spark advance nearly zero. When town gas was used, optimum spark advance varied from 85 to 15 degrees of crank angle, according to mixture strength, and the lubricant yielded carbon of the hard adherent graphitic variety. When using an L head engine having a combustion chamber with a large surface-to-volume ratio and with the spark plug so placed that optimum spark advance for town gas was approximately half that required with the C.F.R. engine, no appreciable carbon deposit of any variety was obtained. Conditions were therefore such that knocking combustion observed on adding finely divided carbon to the gas–air mixture was not caused to any appreciable degree by carbon derived from the lubricant. It was then found on adding carbon, as graphite dust, at a measured rate to the entering mixture, that approximately 0.3 mgm. in the end gas caused a knock intensity of the degree required to reduce brake horsepower by from 10 to 14%.

The knock rating of motor fuels

The knock rating of a fuel is essentially a direct comparison of the intensity of the knock produced by i t with that of a standard fuel, these intensities being measured according to a specified procedure. Other methods, such as chemical analysis or bomb explosions, have been found to be inadequate for predicting the knock characteristics of a fuel when in actual service. The fuels used as a standard of comparison are normal heptane and isooctane (2,2,4-tr imethylpentane) , first proposed by Edgar? These fuels are pure hydrocarbons and thus can always be duplicated; they are similar to each other and to gasolines in their physical and chemical properties, and when blended can duplicate the range of antiknock qualities of gasolines likely to be used for motor cars, since isooctane is considerably better and normal heptane poorer in this respect than ordinary motor fuels. Thus blends of these substances establish a standard scale for comparison of any motor gasoline. In order that the results of different laboratories may correlate, not only is a standard reference fuel necessary, but the engine and the operating procedure must be standardized, because operating conditions affect the relative knock intensities of fuels. Therefore, the Cooperative Fuel Research, single-cylinder, variable compression, vapor-cooled engine was designed specifically for this purpose. In order to obtain the knock rating of a fuel this engine is operated at 900 rpm with a mixture temperature of 300~ and the knock intensity of the fuel measured? The blend of normal heptane and isooctane which matches the fuel in knock intensity under the same operating conditions is then determined, and the fuel is said to have an octane number which is the percentage of isooctane in the isooctane-heptane mixture which it matched. A specified knock intensity is used which is obtained by varying the compression ratio and is measured by means of a bouncing pin. Secondary reference fuels which have been suitably calibrated against normal heptane and isooctane blends are generally used instead of thc expensive primary reference fuels. This method was developed by the Cooperative Fuel Research Committee, on which are represented the Automotive Industry, the Petroleum Industry, and the National Bureau of Standards, and is the result of a great deal of co6perative work done by individual laboratories throughout the world. In 1932, the Committee had developed the octane number scale, the knock-testing engine, and a procedure. The procedure at that time differed from the present procedure in that the engine was operated at 600 rpm instead of 900 rpm and the mixture temperature was approximately room temperature instead of 300~ Also the spark timing was retarded 3.5 degrees with respect to that now in use. This method, now called the C.F.R. research method, was modified because results obtained with it did not correlate with results obtained in service.

High-Octane Fuels

As INTERNAL combustion engines have been improved and as the quality of gasoline has increased there has been a great development in measurement of antiknock rating. Originally the unknown fuel was run in a special single-cylinder engine, and the knock intensity matched against a blend of two standard fuels, one of the best known, isooctane or 2,2,4-trimethylpentane, and the other a very poor fuel, normal heptane. Octane number is the percentage of isooctane in the matching blend. Since different components of a fuel react differently to variations in test conditions, two different standard methods are used in testing automotive fuels, the CFR motor method and the CFR research method. Both are run in a special variable compression engine equipped to measure mechanical knock. The motor method is run at higher engine speed and generally gives lower octane values than the research method on commercial automotive fuels. Needless to say, the figures used in advertising are ...

The Knock Rating of Motor Fuels.

The knock rating of a fuel is essentially a direct comparison of the intensity of the knock produced by i t with that of a standard fuel, these intensities being measured according to a specified procedure. Other methods, such as chemical analysis or bomb explosions, have been found to be inadequate for predicting the knock characteristics of a fuel when in actual service. The fuels used as a standard of comparison are normal heptane and isooctane (2,2,4-tr imethylpentane) , first proposed by Edgar? These fuels are pure hydrocarbons and thus can always be duplicated; they are similar to each other and to gasolines in their physical and chemical properties, and when blended can duplicate the range of antiknock qualities of gasolines likely to be used for motor cars, since isooctane is considerably better and normal heptane poorer in this respect than ordinary motor fuels. Thus blends of these substances establish a standard scale for comparison of any motor gasoline. In order that the results of different laboratories may correlate, not only is a standard reference fuel necessary, but the engine and the operating procedure must be standardized, because operating conditions affect the relative knock intensities of fuels. Therefore, the Cooperative Fuel Research, single-cylinder, variable compression, vapor-cooled engine was designed specifically for this purpose. In order to obtain the knock rating of a fuel this engine is operated at 900 rpm with a mixture temperature of 300~ and the knock intensity of the fuel measured? The blend of normal heptane and isooctane which matches the fuel in knock intensity under the same operating conditions is then determined, and the fuel is said to have an octane number which is the percentage of isooctane in the isooctane-heptane mixture which it matched. A specified knock intensity is used which is obtained by varying the compression ratio and is measured by means of a bouncing pin. Secondary reference fuels which have been suitably calibrated against normal heptane and isooctane blends are generally used instead of thc expensive primary reference fuels. This method was developed by the Cooperative Fuel Research Committee, on which are represented the Automotive Industry, the Petroleum Industry, and the National Bureau of Standards, and is the result of a great deal of co6perative work done by individual laboratories throughout the world. In 1932, the Committee had developed the octane number scale, the knock-testing engine, and a procedure. The procedure at that time differed from the present procedure in that the engine was operated at 600 rpm instead of 900 rpm and the mixture temperature was approximately room temperature instead of 300~ Also the spark timing was retarded 3.5 degrees with respect to that now in use. This method, now called the C.F.R. research method, was modified because results obtained with it did not correlate with results obtained in service.