In the 1920’s the US Navy was experimenting with the concept of aircraft carriers (Figure 1). However, post the First World War the US was also trying to create an aviation industry since they had used predominantly French planes during the war (Figure 2). The aero engines available to the US Navy were not very powerful and in conjunction with the low octane aviation fuel that was available at the time (75 at best), they had problems getting aircraft airborne before the end of the carrier flight deck. In order to avoid the resulting accidents without adding capacity (and hence weight) to the engine it was decided to increase the compression ratio of the engines. Unfortunately, using a low octane fuel with a high compression engine results in knocking or pre-ignition. If nothing is done about this the engine is destroyed, with the added bonus that it looses power just before it lets go. The US Navy started to look for suitable anti-knock agents.

Figure 1: Photograph of the United States Navy's first aircraft carrier, USS Langley, converted in 1920 from the collier USS Jupiter.

Figure 2: This SPAD XIII is typical of the French sourced aircraft used by the National Museum of the United States Army Air Service during WWI (U.S. Air Force photo).

Early efforts at using additives for anti-knock included the use of iodine (I₂) as far back as 1916 when it was added to kerosene which was the fuel being tested at the time. The rational for using iodine was a strange one, in that it was thought that pre-ignition was related to autumn (fall) colors, and hence its choice was original due to its color rather than a chemical rational. However, irrespective of the rational, iodine does work. The US Navy looked for alternative anti-knock agents including ones that would raise the octane number. Tetraethyl lead or TEL (Figure 3) was first used by Thomas Midgley, Jr. (Figure 4). The use of TEL in automotive fuel started in the US, while Europe used alcohol, which is equally good as an anti-knock agent. The advantages of leaded gasoline, its higher energy content and storage quality, eventually led to a universal switch to leaded fuel.

Figure 3: The molecular structure of tetraethyl lead [Pb(C₂H₅)₄].

Figure 4: American mechanical engineer and chemist Thomas Midgley, Jr. (1889 -1944).

One of the advantages of TEL was that the addition of about 0.1% increases the octane of a fuel of about 10-15 points; aviation spirits used in WWII reached 150 octane. In fact the anti-knock properties of TEL were so good that one of its biggest unwanted side effects was treated by further additives rather than search for an alternative.

While TEL showed increased octane rating and good anti-knock properties, one of its inherent chemical properties caused another problem. TEL actually corrodes the metal used for valves, valve guides and valve seats (Figure 5). This effect was overcome by the use of harder alloys for these components. However, their introduction was slow and it was not until the 1940’s that cheaper cars benefited from up-rated components. In fact, even in the 1970’s many manufacturers would save money by only using the better alloys for the exhaust valves, etc. So while the corrosive properties of TEL could be overcome with better alloys, its other drawback required the addition of further chemicals.

Figure 5: Diagram of the effects of valve recession.

The combustion of TEL results in the formation of lead oxide (PbO), Equation 1 and Equation 2, which built-up in the combustion chamber and on the valve seats. The presence of lead oxide particles within the combustion chamber results in the presence of hot spots that lead to pre-ignition, the very thing the TEL was added for in the first place. Furthermore, the presence of lead oxide build-up on the valve seats resulted in pore seating of the valves that resulted in power loss; again the very thing that TEL was designed to overcome.

(1)

(2)

It was found that the addition of 1,2-dibromoethane, also known as ethylene dibromide (EDB, C₂H₄Br₂) removed the unwanted lead oxide. The EDB reacted with the lead oxide to generate lead bromide (PbBr₂), that is relatively volatile (boiling point = 916 °C) and hence is swept out of the engine through the exhaust. An alternative additive was 1,2-dichloroethane, also known as ethylene dichloride (DCE, C₂H₄Cl₂). The DCE reacts in a similar manner to form lead chloride (PbCl₂) whose boiling point is 950 °C.

So in order to overcome knocking, TEL was added, but to overcome the problems of TEL, ethylene dibromide was added. Thus, the concept of a fuel additive package was first exploited. There was one advantage of the generation of lead bromide, in that it was found to act as an excellent lubricant, and it dramatically reduces valve seat wear. Thus, the supposed valve seat protection property of TEL is actually a result of trying to overcome one of its major disadvantages.

The knock on effect of the TEL/EDB package is that valve seats can be made of softer, cheaper, material and automakers adopted this approach in many cases. Thus, there is a concern for older vehicles that the use of unleaded fuel will cause valve seat recession because of the extra wear that occurs in the absence of the TEL/EDB lubrication package. However, while unleaded fuel does not have the corrosive properties of leaded fuel, older engines run on unleaded only suffer valve seat recession if they are routinely revved to high rpm (such as racing or highway driving) or used top haul heavy loads (such as a trailer or caravan). Modern engines with alloy heads (rather than iron) heads are not subject to the valve recession.

Better refining processes have allowed the octane level of unleaded fuel to be raised to 98 RON, which removes the need for lead additives, since at that level can be used for modern engines under normal uses. For extreme use such as motorsports alternative additive packages have been developed that provide better wear resistance than the TEL/EDB package, while at the same time providing a better anti-knock performance as well.