I I I I

The formula above shows that isooctane is technically pentane in which three hydrogens have been replaced each by a methyl (CH3) radical. Two of the substitutions occur in position 2 and one in position 4 of the molecule. Hence isooctane is 2,2,4-trimethylpentane.

The effective octane rating of a fuel depends on the conditions under which it operates. For this reason, more than one octane rating can be associated with any given fuel. The rating displayed on the gas station pump is usually an average of two differently measured values. A more complete discussion of this topic can be found in a book by J. B. Heywood.

Additives increase the octane rating of gasoline. Iodine can be used but is expensive. Up to a few years ago, tetraethyl lead was the standard additive in leaded gasolines. Environmental concerns have eliminated this type of fuel. High octane rating is now achieved by increasing the percentage of cyclic (benzene series) hydrocarbons. Thus, one avoids poisoning by increasing the risk of cancer and, incidentally, paying more for fuel.

The presence of ethanol in gasoline increases its resistance to detonation as indicated in Figure 3.10. It can be seen that the addition of 30% ethanol to low-grade gasoline raises its octane rating from 72 to 84. The

Alcool (Brazil)

70 I AVR

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Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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