NEW KNOCK-TESTING METHODS NEEDED TO MATCH ENGINE AND FUEL PROGRESS 610200
Basic specifications of the Research method for evaluating the antiknock quality of motor gasolines were established more than thirty years ago, and those for the Motor method in 1932. The objective in the development of the Motor method was duplication of the behavior of gasolines that had been observed in automobiles during the 1932 Uniontown cooperative test program. Car engines of that era included such design features as short-length cylinder jackets, hot valves and manifold stoves, and mufflers with large flow resistance. These items were responsible for high metal temperature in the cylinders and much mixture dilution by exhaust gas. Similar effects on knocking were obtainable in test engines by extreme heating of the intake manifold or cylinder jacket. The former was adopted for the Motor method. The Research method was retained for evaluation of fuels under “mild” conditions which it was hoped might be achieved in future car engines.
When the present laboratory test methods were adopted, the maximum-knock speed of automobiles operated on the usual commercial gasolines ordinarily was below 15 miles per hour (less than 750 engine rpm).
Subsequent experience proved that high metal temperatures in combustion chambers of cars decreased engine durability and permissible compression ratio. Alteration of design which lowered the temperatures also markedly reduced severity toward sensitive fuels at low speeds and lessened the significance of Motor-method ratings. Then two trends of automobile design began which were destined to have additional important effects on the knocking of gasolines:
Commercialization of automatic transmissions having relatively high stall speeds; and
Drastic boost of engine power at high speeds through alteration of valve timing and opening up of fuel-induction systems.
The modifications have boosted the median speed of maximum knock of U.S. automobile engines to about 1700 rpm.
Refining methods now in general use but unknown in 1932 have greatly altered the chemical composition of gasolines. New antiknock compounds have come into use, whose volatility and combustion characteristics differ from those of tetraethyllead.
All of these engine and refining advances have caused Research and Motor ratings to become less reliable indices of the antiknock quality of gasolines in automobiles. Heating of the intake manifold or cylinder jacket of test engines has proved to be an unsatisfactory substitute for higher speed in simulation of knocking conditions in currently produced automobile engines.
Also, a large need exists for an instrument with response to knocking which is parallel to that of the human ear.
Better knock-testing methods, it appears, can be developed around an engine which includes the CFR high-speed crankcase and a new cylinder.
Modern refining processes and new antiknock compounds often produce patterns of distribution of antiknock quality over the distillation range of gasolines, which were not visualized 30 years ago. Imported foreign cars, as well as U.S. cars with manual transmissions and maximum-knock speeds near 1000 rpm, are highly sensitive to these altered fuel characteristics. A knock-testing method intended to reflect fuel behavior at low speed in cars of these types must provide for simulation of the pattern of fuel vaporization and distribution that occurs during acceleration.
A wide variety of skills is represented by the scientists and engineers who, during the past three decades, have been responsible for the advances in automobile design, petroleum refining, instrumentation, and understanding of the combustion process. A major task ahead is organized application of these diverse talents to the development of new knock-testing methods. The technical and economic incentives are high. At the beginning of the sixties they provide a challenge equal to that which, three decades ago, motivated the pioneers in the creation of the Research and Motor methods.