Engine knock, octane number

Product characterization aims at defining their end-use properties by means of conventional standard measurements related as well as possible — and in any case, being the object of a large consensus— to end-use properties. We cite for example that octane numbers are supposed to represent the resistance of gasoline to knocking in ignition engines. 

The octane value of an unknown fuel sample is determined by comparing its knocking tendency to various primary reference fuels. Its measured octane is equal to the octane of the PRE which has the same knocking intensity. Knock intensity is controlled to an average value by varying the compression ratio of the CER engine. In practice, the exact value of a fuel s octane number is determined to the nearest 0.1 octane number by interpolation from two PREs that are no more than two octane numbers apart. 

The octane numbers of many pure compounds have been measured and reported in the Hterature. Probably the most comprehensive project was carried out under the auspices of the American Petroleum Institute (18). Table 2 Hsts RON and MON values for a number of representative compounds. Some aromatic compounds cannot be tested neat in the knock engine, so these are evaluated at levels of 20%, and the equivalent octane number is calculated. The values for oxygenates in Table 2 have been reported elsewhere (19). 

Octane number is a measure of a fuel s abiUty to avoid knocking. The octane number of a gasoline is deterrnined in a special single-cylinder engine where various combustion conditions can be controlled. The test engine is adjusted to give trace knock from the fuel to be rated. Various mixtures of isooctane (2,2,4-trimethyl pentane) and normal heptane are then used to find the ratio of the two reference fuels that produce the same intensity of knock as that by the unknown fuel. 

Research Octane Number (RON) is a quantitative measure of a fuel to knocking, simulating the fuel s performance under low engine severity (at 600 rpm and 120°F). 

Gasoline with too low an octane number can cause knocking in the engine of a car, when the fuel ignites too easily and burns in an uncontrolled manner. Knocking lowers fuel efficiency, and it can damage the engine. 

Research Octane Number is a measure of the knocking characteristics of gasoline in a laboratory engine and can be used to characterize gasoline quality. The best linear regression model to predict RON for catalyst A has the form ... 

Engine knock is measured by two ASTM methods, ASTM D-2699 and D-2700. Method ASTM D-2699 is identified as the research octane number (RON) and method ASTM D-2700 is identified as the motor octane number (MON). The primary differences between these two methods are summarized in TABLE 3-3. 

The term octane number requirement (ONR) is used to describe the octane number of the fuel which will provide initial knock or trace knock, under highspeed and high-load conditions. Engines vary widely in their ONR. This is due to both engine design and environmental conditions. The following conditions can affect the ONR ... 

In some circumstances, the octane number of the gasoline used in the engine must be increased to prevent knock caused by accumulated deposits. This phenomenon is called Octane Requirement Increase (ORI). Both prevention and reversal of ORI are difficult to achieve. 

Problems related to engine knock can be readily diagnosed by measuring the octane number of gasoline. However, it is also possible to look at the distillation profile of gasoline and anticipate problems linked to engine knock. 

For example, the T-10 through T-40 fractions of available gasolines may possess significantly different octane numbers. This difference may be manifested in various levels of knock resistance under low-temperature engine starting and warmup operation. 

Aromatic compounds are formed to some extent in most combustion processes in addition they are added in considerable quantities to unleaded gasolines in order to increase the octane number and prevent knock in engines. Aromatic species are of environmental concern, both because they are harmful to the environment and because they are important precursors to dioxins and soot. 

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