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THIS paper represents a method by which the knocking characteristics of automotive engines may be compared in relation to the Research Method and Motor Method engines. The effects of many engine variables on the ratings of sensitive fuels in passenger-car engines are illustrated. These variables include compression ratio, engine speed, air density, distributor tolerances, and temperature. Direct comparisons are made of the manner in which 1955 passenger cars utilize fuel antiknock quality. It is indicated that two knock test methods must be used to achieve fuel quality control as fuel quality is recognized by engines operated in passenger cars.

A NEW research tool, the crank angle-time recorder, is described. This instrument conveniently obtains a permanent record of the time at which various combustion phenomena occur in the engine cycle. Use of the recorder in studies of normal and abnormal combustion (deposit-ignited surface ignition) has provided information of interest to both engine designer and petroleum refiner. Studies include determinations of those deposit-ignited flame fronts which result in knock, effect of fuel antiknock quality and additives on surface ignition, and resistance of fuels to surface ignition. The records obtained show that considerable variability exists in the time at which normal flame fronts arrive at an ionization gap. Some factors affecting magnitude of this variability are ignition timing, fuel-air ratio, engine throttling, changes in manifolding, and fuel type.

THIS paper analyzes ignition-delay data and knocking characteristics of fuels. An approach to the problems of fuel sensitivity and engine severity has been made by attempting to relate the properties of the fuel-air mixture as shown from ignition-delay data and the temperatures and pressures reached by the compressed gases in an engine. The relation between octane numbers and ignition-delay characteristics of the fuels is examined. Antiknock properties of tel are investigated. It is shown that the amount of antiknock effectiveness is related to the amount of tel decomposed.

Major engine-design features of the 17.6 cu in. engine are described and engine development is traced by photographs and sectional drawings. Fuel testing with the 17.6 engine produced these results: ratings were obtained of many API-NACA pure hydrocarbons, which permitted relating variable compression-ratio results with supercharged results; Army-Navy performance numbers above 100 were established; the most sensitive fuels were indicated to be most prone to failure by preignition. The engine also contributed greatly to the development of spark plugs. The catalytic effects of spark plug electrode materials on the ignition of methyl alcohol and unleaded benzene are discussed.

SEVERAL factors are involved in the answer to the question, “How do atmospheric conditions affect the ability of a fuel to satisfy the antiknock requirement of automotive engines?” As is well known, an increase in atmospheric temperature increases the octane-number requirement of engines. This paper points out, however, that this causes little change in the road octane-number ratings of commercial fuels. Increasing the absolute humidity has the opposite effect to increasing the temperature and tends to counteract the undesirable effects of changing temperature throughout the various seasons of the year. Increasing the barometric pressure or decreasing the wind velocity both increase the tendency of commercial fuels to knock. Factors indirectly related to weather conditions, such as the coolant or thermostat used in an engine, also affect the knocking tendency of a fuel.

RESULTS of an investigation directed toward determining why deposits increase antiknock requirement are discussed here. Data are presented which indicate that substantially 100% of the increase in octane-number requirement caused by deposits results from a combination of thermal and volume effects. An analysis procedure is given which indicates that deposit-thermal effects may result entirely from the heat that is stored in the deposits. Thus, the deposits absorb heat during the combustion process in one cycle and transfer it to the fresh charge during the intake and compression portions of the next cycle. The findings reported in this paper show that those engines with the smallest area of combustion-chamber surface, for a given displacement, would be expected to have the smallest thermal effects and hence should have minimum deposit effects.

DEPOSIT-induced ignition (the erratic ignition of the fuel-air mixture by combustion chamber deposits) is one of the problems hindering the development of higher compression, more efficient engines. Deposit-induced ignition results in uncontrolled combustion, which often is followed by knock. In some modern engines, the suppression of knock originating through this mechanism may require higher fuel antiknock quality than that required to suppress ordinary knock. Fuel composition and volatility have been found to affect the amount of deposit ignition. Reduction in fuel end point reduces deposit ignition. Among individual leaded hydrocarbons, aromatics produce by far the most deposit ignition, but the differences among full-boiling gasoline stocks of similar volatility do not appear to be related to their hydrocarbon-type proportions. Engine operating conditions favorable to carbon formation tend to increase deposit ignition and magnify differences among fuels.

DATA obtained with a cylinder of passenger-car-engine size are discussed. Compression ratios of from 5.7 to 15 to 1 have been explored rather completely with four types of combustion chamber. The investigation of this compression ratio range has included the determination of fuel economy at 1200 and 3000 rpm, with particular emphasis on part-load economy. In order to make it possible to compare results over a wide range of compression ratios, fuel economy data are presented in terms of relative thermal efficiency. Knocking data are presented in terms of air density in the combustion chamber and in terms of an empirical equivalent of air density. It is shown that, over a considerable compression ratio range, the knock-limited combustion-chamber air density on isooctane or 80 octane-20 heptane is independent of compression ratio. It is shown that turbulence has rather considerable effects in improving part-load economy and knock-limited performance.