點解汽油要加鉛？

實質非直接加鉛
而是加入'烷基鉛' eg, 四乙基鉛 (C2H5)4Pb
可提升汽油燃燒時的質素 (提升'抗爆能力', 即與氧氣混合壓縮時較不會自燃)
提高內燃機的效率, 減少對引擎的損傷.
據我所知, 只有以前的汽油才有這類的添加劑,
不過現時的法例已經禁止了.

資料嗎? 自己找.
可用以下關鍵字:
leaded petrol, unleaded petrol, anti-knocking agent, alkyl lead, tetraethyllead, octane rating, autoignition.

"The elimination of lead from gas is one of the great environmental achievements of all time," Browner said. "Thousands of tons of lead have been removed from the air, and blood levels of lead in our children are down 70 percent. This means that millions of children will be spared the painful consequences of lead poisoning, such as permanent nerve damage, anemia or mental retardation." The actions taken today, although procedural, mark the end of a quarter-of-a-century of work to keep Americans safe from exposure to lead from gas.

Adverse health effects from elevated levels of lead in blood range from behavior disorders and anemia to mental retardation and permanent nerve damage. Fetuses and children are especially susceptible to low doses of lead, often suffering central nervous system damage or slowed growth.

Lead has been blended with gasoline, primarily to boost octane levels, since the early 1920s. EPA began working to reduce lead emissions soon after its inception, issuing the first reduction standards in 1973, which called for a gradual phasedown of lead to one tenth of a gram per gallon by 1986. The average lead content in gasoline in 1973 was 2-3 grams per gallon or about 200,000 tons of lead a year. In 1975, passenger cars and light trucks were manufactured with a more elaborate emission control system which included a catalytic converter that required lead-free fuel. In 1995 leaded fuel accounted for only 0.6 percent of total gasoline sales and less than 2,000 tons of lead per year. Effective January 1, 1996, the Clean Air Act banned the sale of the small amount of leaded fuel that was still available in some parts of the country for use in on-road vehicles. EPA said fuel containing lead may continue to be sold for off-road uses, including aircraft, racing cars, farm equipment, and marine engines.

In addition to increasing the octane of gasoline, leaded gasoline also protected exhaust valve seats (in vehicles designed to operate on leaded gasoline) from excessive wear. Both of these objectives are now accomplished without the use of leaded gasoline. Owners of older vehicles with engines designed for leaded fuel may use an unleaded gasoline of comparable octane. For vehicles operating under higher loads, a lead substitute additive may be used, but owners should check with vehicle manufacturers as to which lead substitute additives are appropriate.