Knocking fuels

A few cases occur in which hot-corrosion and wet corrosion are interdependent, the wet corrosion arising from the condensation of liquids generated during a period at elevated temperatures. The formation of condensates of hydrobromic acid in engines burning anti-knock fuels containing ethylene dibromide is important in this context. Such cases are properly considered as aqueous corrosion. 

Fig. 7.13. Net heat release rate, Q, and unburned mass fractions at different crank angles for knock intensities of (1) 1490, (2) 111 and (3) 6.5 kPa. Asterisks denote onset of knock. Fuel PRF, stoichiometric mixture. From [106].

The aromatics are excellent non-knocking fuels and vary very little among individual members in this respect. 

It has been shown that alcohols like benzene have ignition temperatures higher than corresponding normal paraffin hydrocarbons and that they are also excellent non-knocking fuels. 

Alcohols do not form peroxides when undergoing oxidation, oxidize at relatively high temperatures, and comprise non-knocking fuels.45 Aromatic compounds as benzene and toluene have relatively high ignition temperatures and are excellent non-knocking fuels. 

In order to characterize the behavior of motor fuels or their components with regard to knocking resistance but without involving chemical composition criteria which are complex and not easy to quantify, the traditional method that has been universally employed for more than 50 years consists of introducing the concept of octane number. 

Eor a considerable period, >90% of the new cars in Brazil operated on E96 fuel, or a mixture of 96% ethanol and 4% water (82). The engines have high compression ratios (ca 12 1) to utilize the high knock resistance of ethanol and deUver optimum fuel economy. In 1989 more than one-third of Brazil s 10 million automobiles operated on 96% ethanol/4% water fuel. The remainder ran on gasoline blends containing up to 20% ethanol (5). 

Knock is caused by unwanted chemical reactions in the combustion chamber. These reactions are a function of the specific chemical species which make up the fuel and the environmental conditions to which the fuel is subjected during the compression and power stroke in the engine. Therefore, both the chemical makeup of the fuel and the engine design parameters must be considered when trying to understand knock. 

Vehicle Fa.ctors. Because knock is a chemical reaction, it is sensitive to temperature and reaction time. Temperature can in turn be affected either by external factors such as the wall temperature or by the amount of heat released in the combustion process itself, which is directiy related to the density of the fuel—air mixture. A vehicle factor which increases charge density, combustion chamber temperatures, or available reaction time promotes the tendency to knock. Engine operating and design factors which affect the tendency to produce knocking are... 

Measuring Octane. Two different values need to be considered when discussing octane measurements. One is the knocking tendency of the fuel, called the fuel octane number. The other is the knocking tendency of the vehicle, called octane number requirement. 

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 number requirement (ONR) of a car is the octane number which causes barely audible, ie, trace knock when driven by a trained rater. The Coordinating Research Council (CRC), a research organi2ation funded joindy by the American Petroleum Institute (API) and the American Automobile Manufacturers Association (AAMA), has defined test procedures for measuring ONR. Each car is driven under a set of light and heavy accelerations until the most sensitive driving mode is determined. Then a series of fuels is mn in the car until trace knock is determined. Each year, CRC members measure ONR of more than 100 cars and pubHsh the results. 

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. 

By defining isooctane as having an octane number of 100 and / -heptane as having an octane number of 0, the volumetric percentage of isooctane in heptane that matches the knock from the unknown fuel can be calculated as the octane number of the fuel. For example, 90 vol % isooctane and 10 vol % normal heptane produce a 90-octane-number reference fuel. 

The air/fuel ratio can be varied, usually in the range 0.8 (rich) to 1.1 (lean), and a typical limit of the compression ratio is set at about 10 1 by the tendency of the engine to knock at higher ratios, leading to severe mechanical stresses in the engine components. 

In the Diesel engine, a higher compression ratio can be achieved, up to about 18, before knock presents a problem, and the fuel is injected into the combustion chamber near tire end of the compression stroke as a spray. The... 

ETHYL FLUID see MOTOR FUEL ANTI-KNOCK COMPOUND ... 

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