Tetra-Ethyl Lead: The End of an Era for a Well-Known Molecule

In the second NASCAR race of the 2007 season, mega-star Dale Earnhardt Jr. and his teammate dropped out early and finished at the back of the 43-car field at California Speedway. The reason, according to Earnhardt — the team’s engine builders had trouble with the new rule for 2007 mandating lead-free fuel for NASCAR’s premiere series.

Perhaps you are thinking, “Wait, wasn’t lead-free fuel mandatory for passenger cars in the 1970s?” It was, but the NASCAR formula preserves automotive technology in its 1950s form. The beam-axle, tube-frame 3,400-lb cars are powered by iron-block V-8 pushrod engines designed in the 1950s using carburetors — those complex, inefficient fuel-air mixing devices that used to sit atop every engine and now are only used in NASCAR and the smoky vehicles still made in former communist countries.

And in an irony only marketing goals could produce, Toyota fielded cars for the first time in 2007 in NASCAR’s top series, the Nextel Cup. To compete in this series, Toyota could not use the high-tech, aluminum-block, overhead-cam, fuel-injected engines that power most of their cars and trucks. Toyota developed and produced a cast-iron-block engine they now use in large pick-up trucks, just so they could have a NASCAR-legal engine.

Why was lead, specifically tetra-ethyl lead, in NASCAR engines until this year, and why would internal combustion engines burn leaded fuel anyway? The simple answer is the quest for high performance and longer engine life.

Yes, lead metal is heavy. No, it does not burn very well. Actually, a car burning leaded gas delivers slightly less power per gallon of gas consumed, because the lead does not, in fact, burn, so there is less combustible material per gallon.

What made tetra-ethyl lead vital to internal combustion engine designers is the need to slow down the rate the fuel burns inside an engine. In a four-stroke cycle engine that uses gasoline for fuel (diesels are different) fuel-air mixture is sucked into the engine as each piston travels down in the cylinder and then compressed as it travels up. At a pre-determined point, just before the piston reaches the top of its upward travel, a spark plug in the cylinder ignites the fuel-air mixture. The burning mixture propels the piston back down and creates power.

One method of making more power for a given size of engine is to increase the compression ratio — the ratio of the capacity of the cylinder to its fully compressed volume. Early engines used compression ratios in the range of six to one. Raising this ratio increased power, but had a huge drawback, when the fuel-air was compressed to less than a tenth of its original volume in a hot cylinder, the mixture might explode before it could be burned. The explosions reduce power and could wreck the engine.

Adding tetra-ethyl lead to gasoline slowed the burning and helped to prevent detonation or pre-ignition, popularly know as “knocking” — the sound you hear inside a car when an engine has this problem. Adding lead to gasoline allowed refineries of the 1930s to produce fuel for high-compression engines (known for various reasons as high octane fuel) that they could not produce easily using the refinery technology of the time.

So who wanted high-compression, high-performance engines? In most routine uses, cars use only a small fraction of their available power and would run well at compression ratios that allow the use of unleaded fuels. But in post-World War II America, when bigger was better in so many ways, marketers found that horsepower was sexy.

Horsepower became the number that a marketer could use in a simple “more is better” fashion to link their shiny sedan to the great speedways at Indianapolis, Monza, and LeMans. Higher compression was the route to higher horsepower, so the top of the line engines were made with higher compression ratios requiring higher octane premium fuel.

By the 1960s, the heyday of big, high-performance American-made engines, U.S. carmakers each had an engine tuned for maximum horsepower that was winning races at drag strips and the Daytona Speedway. But the street versions of these speedway monsters were expensive, required a lot of maintenance and were difficult to drive. They made gobs of power — more than 400 horsepower was the minimum in this league — but got gas mileage that was noticeably bad in an era of gas guzzlers and required a skilled pair of feet on the gas and clutch.

And then they were gone. By 1973, every new car sold in America could burn regular gas and tetra-ethyl lead was on its way out of gasoline formulations, except for propeller-driven aircraft and NASCAR stock cars. Of course, leaded gas was sold for many years for engines that could not run without it. By 2007, even NASCAR banned leaded gas and Dale Earnhardt, Jr., has not blown another engine.

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This article was originally published under the title "We're History" in the June 2007 edition of Chemical Engineering Progress magazine. This article was prepared by Neil Gussman, communications manager for the Chemical Heritage Foundation.