Engine knock, wherein fuel ignites unevenly along the cylinder wall resulting in damaging percussive shockwaves, is an issue that automakers have struggled to mitigate since the days of the Model T. The industry's initial attempts to solve the problem — namely tetraethyl lead — were, in hindsight, a huge mistake, having endumbened and stupefied an entire generation of Americans with their neurotoxic byproducts.

Dr. Vaclav Smil, Professor Emeritus at the University of Manitoba in Winnipeg, examines the short-sighted economic reasoning that lead to leaded gas rather than a nationwide network of ethanol stations in his new book Invention and Innovation: A Brief History of Hype and Failure. Lead gas is far from the only presumed advance to go over like a lead balloon. Invention and Innovation is packed with tales of humanity's best-intentioned, most ill-conceived and generally half-cocked ideas — from airships and hyperloops to DDT and CFCs. 

MIT Press

Excerpted from Invention and Innovation: A Brief History of Hype and Failure by Professor Vaclav Smil. Reprinted with permission from The MIT Press. Copyright 2023.

Just seven years later Henry Ford began to sell his Model T, the first mass-produced affordable and durable passenger car, and in 1911 Charles Kettering, who later played a key role in developing leaded gasoline, designed the first practical electric starter, which obviated dangerous hand cranking. And although hard-topped roads were still in short supply even in the eastern part of the US, their construction began to accelerate, with the country’s paved highway length more than doubling between 1905 and 1920. No less important, decades of crude oil discoveries accompanied by advances in refining provided the liquid fuels needed for the expansion of the new transportation, and in 1913 Standard Oil of Indiana introduced William Burton’s thermal cracking of crude oil, the process that increased gasoline yield while reducing the share of volatile compounds that make up the bulk of natural gasolines.

But having more affordable and more reliable cars, more paved roads, and a dependable supply of appropriate fuel still left a problem inherent in the combustion cycle used by car engines: the propensity to violent knocking (pinging). In a perfectly operating gasoline engine, gas combustion is initiated solely by a timed spark at the top of the combustion chamber and the resulting flame front moves uniformly across the cylinder volume. Knocking is caused by spontaneous ignitions (small explosions, mini-detonations) taking place in the remaining gases before they are reached by the flame front initiated by sparking. Knocking creates high pressures (up to 18 MPa, or nearly up to 180 times the normal atmospheric level), and the resulting shock waves, traveling at speeds greater than sound, vibrate the combustion chamber walls and produce the telling sounds of a knocking, malfunctioning engine.

Knocking sounds alarming at any speed, but when an engine operates at a high load it can be very destructive. Severe knocking can cause brutal irreparable engine damage, including cylinder head erosion, broken piston rings, and melted pistons; and any knocking reduces an engine’s efficiency and releases more pollutants; in particular, it results in higher nitrogen oxide emissions. The capacity to resist knocking— that is, fuel’s stability— is based on the pressure at which fuel will spontaneously ignite and has been universally measured in octane numbers, which are usually displayed by filling stations in bold black numbers on a yellow background.

Octane (C8H18) is one of the alkanes (hydrocarbons with the general formula CnH2n + 2) that form anywhere between 10 to 40 percent of light crude oils, and one of its isomers (compounds with the same number of carbon and hydrogen atoms but with a different molecular structure), 2,2,4-trimethypentane (iso-octane), was taken as the maximum (100 percent) on the octane rating scale because the compound completely prevents any knocking. The higher the octane rating of gasoline, the more resistant the fuel is to knocking, and engines can operate more efficiently with higher compression ratios. North American refiners now offer three octane grades, regular gasoline (87), midgrade fuel (89), and premium fuel mixes (91– 93).

During the first two decades of the twentieth century, the earliest phase of automotive expansion, there were three options to minimize or eliminate destructive knocking. The first one was to keep the compression ratios of internal combustion engines relatively low, below 4.3:1: Ford’s best-selling Model T, rolled out in 1908, had a compression ratio of 3.98:1. The second one was to develop smaller but more efficient engines running on better fuel, and the third one was to use additives that would prevent the uncontrolled ignition. Keeping compression ratios low meant wasting fuel, and the reduced engine efficiency was of a particular concern during the years of rapid post–World War I economic expansion as rising car ownership of more powerful and more spacious cars led to concerns about the long-term adequacy of domestic crude oil supplies and the growing dependence on imports. Consequently, additives offered the easiest way out: they would allow using lower-quality fuel in more powerful engines operating more efficiently with higher compression ratios.

During the first two decades of the twentieth century there was considerable interest in ethanol (ethyl alcohol, C2H6O or CH3CH2OH), both as a car fuel and as a gasoline additive. Numerous tests proved that engines using pure ethanol would never knock, and ethanol blends with kerosene and gasoline were tried in Europe and in the US. Ethanol’s well-known proponents included Alexander Graham Bell, Elihu Thomson, and Henry Ford (although Ford did not, as many sources erroneously claim, design the Model T to run on ethanol or to be a dual-fuel vehicle; it was to be fueled by gasoline); Charles Kettering considered it to be the fuel of the future.

But three disadvantages complicated ethanol’s large-scale adoption: it was more expensive than gasoline, it was not available in volumes sufficient to meet the rising demand for automotive fuel, and increasing its supply, even only if it were used as the dominant additive, would have claimed significant shares of crop production. At that time there were no affordable, direct ways to produce the fuel on a large scale from abundant cellulosic waste such as wood or straw: cellulose had first to be hydrolyzed by sulfuric acid and the resulting sugars were then fermented. That is why the fuel ethanol was made mostly from the same food crops that were used to make (in much smaller volumes) alcohol for drinking and medicinal and industrial uses.

The search for a new, effective additive began in 1916 in Charles Kettering’s Dayton Research Laboratories with Thomas Midgley, a young (born in 1889) mechanical engineer, in charge of this effort. In July 1918 a report prepared in collaboration with the US Army and the US Bureau of Mines listed ethyl alcohol, benzene, and a cyclohexane as the compounds that did not produce any knocking in high-compression engines. In 1919, when Kettering was hired by GM to head its new research division, he defined the challenge as one of averting a looming fuel shortage: the US domestic crude oil supply was expected to be gone in fifteen years, and “if we could successfully raise the compression of our motors . . . we could double the mileage and thereby lengthen this period to 30 years.” Kettering saw two routes toward that goal, by using a high-volume additive (ethanol or, as tests showed, fuel with 40 percent benzene that eliminated any knocking) or a low-percentage alternative, akin to but better than the 1 percent iodine solution that was accidentally discovered in 1919 to have the same effect.

In early 1921 Kettering learned about Victor Lehner’s synthesis of selenium oxychloride at the University of Wisconsin. Tests showed it to be a highly effective but, as expected, also a highly corrosive anti-knocking compound, but they led directly to considering compounds of other elements in group 16 of the periodic table: both diethyl selenide and diethyl telluride showed even better anti-knocking properties, but the latter compound was poisonous when inhaled or absorbed through skin and had a powerful garlicky smell. Tetraethyl tin was the next compound found to be modestly effective, and on December 9, 1921, a solution of 1 percent tetraethyl lead (TEL) — (C2H5)4 Pb — produced no knock in the test engine, and soon was found to be effective even when added in concentrations as low as 0.04 percent by volume.

TEL was originally synthesized in Germany by Karl Jacob Löwig in 1853 and had no previous commercial use. In January 1922, DuPont and Standard Oil of New Jersey were contracted to produce TEL, and by February 1923 the new fuel (with the additive mixed into the gasoline at pumps by means of simple devices called ethylizers) became available to the public in a small number of filling stations. Even as the commitment to TEL was going ahead, Midgley and Kettering conceded that “unquestionably alcohol is the fuel of the future,” and estimates showed that a 20 percent blend of ethanol and gasoline needed in 1920 could be supplied by using only about 9 percent of the country’s grain and sugar crops while providing an additional market for US farmers. And during the interwar period many European and some tropical countries used blends of 10– 25 percent ethanol (made from surplus food crops and paper mill wastes) and gasoline, admittedly for relatively small markets as the pre–World War II ownership of family cars in Europe was only a fraction of the US mean.

Other known alternatives included vapor-phase cracked refinery liquids, benzene blends, and gasoline from naphthenic crudes (containing little or no wax). Why did GM, well aware of these realities, decide not only to pursue just the TEL route but also to claim (despite its own correct understanding) that there were no available alternatives: “So far as we know at the present time, tetraethyl lead is the only material available which can bring about these results”? Several factors help to explain the choice. The ethanol route would have required a mass-scale development of a new industry dedicated to an automotive fuel additive that could not be controlled by GM. Moreover, as already noted, the preferable option, producing ethanol from cellulosic waste (crop residues, wood), rather than from food crops, was too expensive to be practical. In fact, the large-scale production of cellulosic ethanol by new enzymatic conversions, promised to be of epoch-making importance in the twenty-first century, has failed its expectations, and by 2020 high-volume US production of ethanol (used as an anti-knocking additive) continued to be based on fermenting corn: in 2020 it claimed almost exactly one-third of the country’s corn harvest.