From the 1920s until the 1970s, most gasoline cars in the USA were using fuel that had lead mixed into it. The reason for this was to reduce the engine knocking effect from abnormal combustion in internal combustion engines of the time. While lead — in the form of tetraethyllead — was effective at this, even the 1920s saw both the existence of alternative antiknock agents and an uncomfortable awareness of the health implications of lead exposure.
We’ll look at what drove the adoption of tetraethyllead, and why it was phased out once the environmental and health-related issues came into focus. But what about its antiknock effects? We’ll also be looking at the alternative antiknock agents that took its place and how this engine knocking issue is handled these days.
It’s a Matter of Octane
In an internal combustion engine (ICE), ideally the air-fuel mixture that gets injected into a cylinder will detonate at the perfect moment where the flame front will travel outwards from the point of ignition, with every bit of the air-fuel mixture burning up fully. This will allow for maximum use of the energy in the fuel mixture, while causing a clean stroke of the piston.
In reality, however, pockets of this fuel-air mixture will detonate before the flame front reaches them. These so-called ‘cool flames’ occur because of the compression by the piston combined with slight unevenness in the mixture, causing additional pressure waves in the cylinder. This raises the cylinder pressure and causes the typical metallic pinging noise that is indicative of engine knocking. Depending on how many of these pockets detonate outside of the spark plug’s flame front, the result may be increased wear on components, or even their outright destruction.
Hereby the octane rating of the fuel is crucial, as it essentially determines at which compression level the fuel will detonate (without spark). High octane fuels thus do burn less easily, but allow for far higher levels of compression, which effectively produces more power. In contrast, diesel engines require lower octane fuels, as they only compress air, with the fuel being injected at the end of the compression cycle, with the heat from the compressed air detonating the fuel.
Time to Knock It Off
There are fortunately a number of ways to prevent this premature detonation effect. These include:
- Using a fuel with a higher octane rating.
- Adding more fuel to the air-fuel mixture.
- Reducing the compression level in the cylinder.
- Reducing the load on the engine.
You can choose the first point by using a so-called antiknock agent, a chemical that raises the octane rating of the fuel by raising the temperature and pressure at which auto-ignition occurs. Tetraethyl lead (TEL) is one example of such an agent. Its chemical formula is (CH3CH2)4Pb.
Inside the engine’s cylinders, the function of TEL is to quench the spontaneous ignitions that occur outside of the flame front by dealing with the pyrolized radicals that would otherwise sustain the chain reaction of the cool flame. Here the lead is the actual reactive agent, while the rest of the TEL serves to allow it to dissolve into gasoline (courtesy of its alkyl groups).
As the TEL is burned, it produces carbon dioxide, water and lead:
(CH3CH2)4Pb + 13 O2 → 8 CO2 + 10 H2O + Pb
The lead can further react with oxygen to form lead(II) oxide:
2 Pb + O2 → 2 PbO
Left alone, the lead and lead(II) oxide would accumulate inside the engine and destroy it. To prevent this, lead-scavengers such as 1,2-dibromoethane and 1,2-dichloroethane are added to form lead(II) bromide and lead(II) chloride respectively (unfortunately neither are as pretty as lead(II) iodide). These compounds are easily removed from the engine during normal operation, from where they’d be released into the environment.
The Competition
In addition to lead, two other substances were known to increase the octane rating of gasoline fuel: ethanol (C2H6O) and benzene (C6H6). For ethanol this octane rating raising property is due to ethanol being suitable as a complete (albeit more expensive) replacement for gasoline fuel. As ethanol has by default a higher octane rating than most gasoline fuels, mixing a percentage of ethanol into gasoline fuel causes the latter to have a higher octane, which achieves the desired antiknocking effect.
Benzene is a hydrocarbon which appears naturally in crude oil. It’s present in gasoline as a result, where it’s also responsible for the characteristic sweet smell around gasoline refueling stations. Although now usually kept at less than 1% in gasoline due to the carcinogenic properties of benzene, before TEL’s introduction in the 1920s as a fuel additive, benzene was regarded as a good antiknock agent as it too raised the octane rating. By the 1950s TEL had virtually replaced benzene as antiknock agent.
Ethanol can be produced from oil (ethylene), as well as from biomass (sugar cane, corn, etc.). It is however a fuel type that has only seen widespread popularity since the 1970s. TEL had the benefit over ethanol as an antiknock agent that only a small amount would be needed to have the same effect, yet at similar cost. TEL however also had the additional benefit that its use as fuel-additive could be patented.
TEL Fallout
Ultimately history shows us that TEL would prevail over benzene and ethanol, with ethanol only making a resurgence in the 1970s during the phase-out of TEL. As information uncovered over the past decades shows, the reason for this was a deliberate strategy by the companies behind the Ethyl partnership (General Motors, ESSO and DuPont) to bury the science about the well-known harmful effects of lead, the expected blood serum lead levels from adding TEL to gasoline and the expected effects on the environment.
As this 2005 paper by William Kovarik (PDF) summarizes, the use of ethanol as an antiknock agent was commonplace by the time that TEL was introduced, but over the decades, the misinformation campaign by Ethyl was so effective that people came to believe that TEL was the only antiknock agent available. In the end it would take fifty years of research, as well as scientific, court and regulatory challenges to produce evidence about the harmful effects of TEL that were so damning that leaded gasoline was phased out in the 1970s in the US, though not without Ethyl first suing the Environmental Protection Agency (EPA).
Among one of the effects noted by researchers of the effects of increased lead levels in blood serum was that of a sharp negative effect on the developing brain, leading to a lower IQ, poor impulse control and troubles at school. Later studies introduced the lead-crime hypothesis, which links the rise in violent crime since the 1930s and the sharp drop-off in the early 1990s with the exposure of children to high blood serum lead levels, which would have impaired brain development.
Although the Ethyl corporation still exists today, the use of TEL in gasoline has been essentially reduced to zero, aside from use in avionics fuel, antique cars, and so on. As the use of TEL is incompatible with catalytic converters due to lead being a catalyst poison, the requirement of adding a catalytic converter to new cars in the late 1970s US made the demise of TEL for cars a certainty. Europe, Asian nations and so on also phased out TEL until today only one plant in the world still (legally) produces leaded gasoline.
Antiknock Strategies Today
Even though modern ICEs have hardened components that can withstand engine knocking without damage, and the mixing of ethanol into the gasoline fuel is becoming ever more commonplace, other antiknock agents are still around, with methylcyclopentadienyl manganese tricarbonyl (MMT, (C5H4CH3)Mn(CO)3) having been used for years in a number of countries.
Ferrocene (Fe(C5H5)2) is also used as a fuel additive, used as an alternative to TEL, such as for use in antique cars. Increasing the amount of 2,2,4-trimethylpentane (iso-octane, also a petroleum product) in gasoline serves to reduce the knocking, as was originally discovered by Graham Edgar in 1926. Iso-octane forms the 100 point on the octane rating scale.
In addition to fuel additives, modern digitally controlled gasoline engines have built-in mechanisms that detect and control engine knocking, by adjusting the ignition timing and pressure. This allows for the engine to automatically adjust itself to fuels with different octane ratings. This of course comes with its own set of challenges, as for example this 2017 paper by Petyon Jones et al. titled “Stochastic Simulation and Performance Analysis of Classical Knock Control Algorithms” details.
Just A Historical Footnote
It’s interesting to consider these revelations and new innovations in light of the transition of the car industry from the internal combustion engine to electric motors, which do not have any of these issues. Free from the stigma of leaded gasoline and its combustion products, it will be interesting to see how we will regard this chapter in human history fifty years from now.
By then not having regenerative braking would probably seem beyond quaint, as would be the ritual of weekly (or daily) refueling or recharging. Maybe the issue of fine particulate dust from tires and brake disks will have become the next environmental issue.
[Main image source: Tetraethyl Lead by David Brodbeck CC-BY 2.0]
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