A brief history of gasoline anti-knock agents

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In October 2002, the British newspaper The Guardian listed the “worst inventions of mankind,” and plastic bags were awarded this “honorable title.” A year later, China’s New World Publishing House also published a book titled \"The Worst Inventions of Mankind.\" This book was clearly written by a Chinese person, yet a fake English title is put on the cover; moreover, the author deliberately used two distinctly foreign-sounding pen names, “Aikuer” and “Mark”, in an attempt to make it seem as though it is a book translated from abroad, or at least a compiled work. The book criticizes 20 different inventions in one go; not only cigarettes and drugs, but also the Internet, cloning, genetically modified organisms, and even video games, monosodium glutamate, and chewing gum are included. This gives an idea of the quality of this book. 　　Among these 20 “worst inventions of humanity”, the automobile is, naturally, included. Indeed, in the eyes of today’s environmentalists, cars have almost become a target for criticism ； Even ordinary people, faced with increasingly congested traffic and polluted air that cannot be improved no matter what, inevitably start to complain about the growing number of cars in the city. However, perhaps out of reluctance, few people realize how much effort automotive engineers have put in to improve vehicle performance and avoid causing more environmental pollution! 　　Here I will only talk about one thing that is closely related to environmental protection – gasoline anti-knock agents. 　　Those who own cars know that the working principle of a gasoline engine is as follows: first, the gasoline is dispersed into fine droplets through a carburetor or fuel injector, and then mixed with air before entering the cylinder ； Then, this mixture is compressed by the piston, causing its temperature to rise; when ignited by the electric spark generated by the spark plug, it releases enormous energy that drives the engine’s crankshaft to rotate, thereby moving the wheels. 　　Unlike diesel engines, gasoline engines must be ignited by a spark plug to burn the mixture, rather than through the mixture’s self-ignition. However, if gasoline contains components such as n-heptane, which burn very easily, they often catch fire on their own inside the cylinder, generating temperatures much higher than those produced by the normal combustion of the fuel mixture. This results in extremely strong shock waves that cause the engine to vibrate and make noise; this phenomenon is known as detonation, also referred to as knocking, and is commonly called \"cylinder knocking\". 　　Detonation not only damages the engine but also leads to gasoline waste; therefore, how to improve the anti-knock properties of gasoline has always been an important research topic in the field of automotive engineering. 　　Engineers first identified several indicators for measuring a gasoline’s resistance to detonation, among which the octane number is the most commonly used. It is named this way because it was discovered that a component in gasoline called 2,2,4-trimethylpentane – commonly known as isooctane – has strong anti-knock properties. As a result, it and n-heptane were used as standards for measuring a gasoline’s anti-knock capacity; n-heptane was assigned an octane rating of 0, while isooctane was given an octane rating of 100. With such regulations, the octane rating of gasoline can be determined using specialized testing equipment. Octane number can be further divided into research octane number (abbreviated as RON) and motor octane number (abbreviated as MON), which are results obtained under different conditions. In our country, the RON value is now used uniformly to classify gasoline grades. For example, 90# gasoline refers to gasoline with an RON value of 90, while 93# gasoline has an RON value of 93; its anti-knock properties are superior to those of 90# gasoline. 　　However, straight-run gasoline obtained directly from petroleum distillation (whose main components are straight-chain alkanes) has a very low RON, ranging only from 40 to 60. Cracked gasoline produced from the cracking of heavy oil has a higher RON than straight-run gasoline, due to its relatively high content of aromatics; since the RON of aromatics is generally higher than that of straight-chain alkanes. Nevertheless, its RON remains below 90. If today’s cars were to use straight-run gasoline or cracked gasoline that has not been processed at all as fuel, the vibrations from driving them would be so intense as to shake the whole body. 　　How to improve the anti-knock properties of gasoline? An easy idea to consider is to add some special chemicals to it—these are gasoline anti-knock agents. The first person to make outstanding contributions in this area was the American chemist Thomas Midgley, Jr. (1889–1944). Actually, he initially studied mechanical engineering and graduated from Cornell University in 1911 ； After joining Dayton Engineering Company in 1916, he self-studied chemistry for a few years in order to research gasoline antiknock agents. At that time, chemists did not yet have a good understanding of the mechanism of combustion; therefore, Mikeli’s search for an effective anti-knock agent could truly be described as looking for a needle in a haystack. It’s hard for us to imagine these days that one of the theories guiding his search for blast-resistant materials was Mendeleev’s periodic law of elements! 　　Between 1918 and 1920, Mikieli discovered that benzene and ethanol could be used as anti-knock agents, and he filed patents for them. In 1921, a superior anti-knock agent was discovered by Mikieli, and that was tetraethyl lead (TEL), which was later widely used. Tetraethyl lead was first discovered by Germans in 1854; it is an oily liquid with a fruity odor. A small amount added to gasoline can **improve its anti-knock properties**, and it is easy to synthesize as well as inexpensive. In 1923, leaded gasoline for vehicles was introduced in the United States and quickly became widespread around the world. 　　However, as early as the 1st century BC, the ancient Greeks already knew that lead was a poison ； Modern medicine tells us that lead accumulates in the human body and can cause significant damage to the circulatory, nervous, and digestive systems. In 1887, it was discovered that lead posed an even greater threat to children. By 1904, lead-based pigments were identified as the main cause of lead poisoning in children. Five years later, France, Belgium, and Austria were among the first countries to ban the use of lead white (whose chemical composition is lead carbonate) as a pigment. Even a year before the introduction of leaded gasoline for use in vehicles, the U.S. Public Health Service had already issued a public warning about the high toxicity of leaded fuels ； In 1925, leaded gasoline for vehicles was also taken off the market for a time. All this shows that people were not completely unaware of the hazards of tetraethyl lead from the very beginning; it’s just that, after weighing its advantages and disadvantages, they decided that the benefits outweighed the risks. We might wonder how many engines would have to be scrapped early on if tetraethyl lead had not been used How much waste will this result in, and what harm will it cause to the environment? 　　By the 1960s, this judgment of pros and cons had reversed. On the one hand, environmentalists have found that tetraethyl lead is an important air pollutant and the main source of lead in the air ； On the other hand, people can no longer tolerate the increasingly severe air pollution. This doomed tetraethyl lead to fade from the historical stage. In the last two decades of the 20th century, the major developed countries **successively switched to lead-free gasoline. Our country also stopped the production of leaded gasoline for vehicles on January 1, 2000. 　　Meanwhile, automotive engineers are also looking for alternatives to tetraethyl lead. In 1959, an anti-knock agent named MMT was developed in the United States; like tetraethyl lead, it is an organometallic compound. However, MMT was not widely used; the United States banned its use in 1978 (it is still used in small amounts in our country today). In the 1970s, oxygen-containing organic compounds attracted considerable attention, and several anti-knock agents with excellent properties were developed. Among them, methyl tertial-butyl ether (MTBE) was the most widely used, and its production began officially in Italy in 1973. MTBE has very low toxicity, and there is currently no conclusive evidence of its carcinogenicity (it is classified as Group 3 by IARC). It also has a high RON value of 117; thanks to the oxygen atoms in its molecule, it can improve the combustion efficiency of gasoline and reduce the emission of toxic exhaust gases. Therefore, gasoline containing MTBE is referred to as \"clean gasoline\". 　　By 1996, two areas in Saint Monica, California, USA, shut down 50% of their water supply facilities simply because MTBE was detected in the groundwater, which ultimately led to several states in the United States banning the use of MTBE as an anti-knock agent. It gives the impression that scientists have once again underestimated the hazards of a chemical, as if the tragedy of DDT is repeating itself. I have noticed that some \"environmentalists\" in our country have begun to use the example of the United States banning MTBE as a new weapon to attack the construction of chemical plants ； Moreover, just as they spread rumors that PX is highly carcinogenic, these people also claim that MTBE is a \"powerful carcinogen,\" using sensationalism to incite uninformed members of the public. 　　What is the truth of the matter? Investigations revealed that the MTBE in Santa Monica’s groundwater did not originate from cars, but rather from a leak at an old gas station. It can be said that this was entirely a coincidence. Therefore, despite the loud calls in the United States for a nationwide ban on MTBE, the EU and Japan remain unmoved and continue to use it to this day. Of course, it is true that MTBE is relatively difficult to degrade naturally; therefore, the EU and Japan now prefer another antiknock agent that is easier to degrade—ethyl tertial-butyl ester (ETBE), which has performance as good as that of MTBE. 　　Another alternative to MTBE is ethanol. As mentioned above, before the excellent anti-knock properties of tetraethyl lead were discovered, Mikiyuki had already found that ethanol also has anti-knock effects. The main reason for not developing ethanol anti-knock processes at that time was that ethanol is a hydrophilic substance; even a small amount of water in gasoline would cause ethanol to transfer into the water, thereby reducing the anti-knock properties of the gasoline ； Moreover, anhydrous ethanol itself has water-absorbing properties; ethanol gasoline that originally contains no water will separate into layers as it is exposed to air due to water absorption. At that time, it was difficult to overcome this technical bottleneck, which is why ethanol-blended gasoline did not see development; even today, such gasoline does not withstand long-term storage or transportation. Ethanol is more expensive than gasoline, and if ethanol-blended gasoline is used extensively, oil prices will inevitably rise. Therefore, despite the strong calls in the United States for the use of ethanol-blended gasoline, its actual supply is limited—although overall, the widespread use of ethanol is likely to be the trend in the future. 　　Having talked about the history of the development of gasoline anti-knock agents, I can’t help but share a few thoughts. Admittedly, in the history of human technological development, there have been tragedies such as DDT and Thalidomide, as well as disasters like the London smog and Minamata disease, but these are after all extreme examples. People have had at least some awareness of the hazards of more chemicals since the day they were first put into use ； The long-term use of many chemicals known to be harmful is often a last resort taken after carefully weighing the pros and cons. Mankind has not been destroyed by its own hands; on the contrary, thanks to increasingly extensive scientific knowledge, we are living better and better. 　　Yes, what else besides science can save humanity? Should it rely on impulsive sentimentality, or ignorant fear?