Modern cars control engine knock by cylinder-fuel injection and by computers to retard the spark. This is a mechanical solution.
This letter, addressed to the scientific community, proposes a chemical origin for knock and a chemical class of compounds for study.
Triplet oxygen is responsible for free radical combustion of hydrocarbons. Triplet oxygen is paramagnetic, and its chemistry is typified by rapid radical reactivity. Singlet oxygen is responsible for the precombustion knock of hydrocarbons. Singlet oxygen is very reactive and diamagnetic. Its chemistry, which is very different from that of triplet oxygen, is typified by nonradical participation in "ene" reactions and in Diels-Alder reactions. Investigation of singlet oxygen quenchers leads to a class of ashless antiknock candidates.
Excitation of triplet oxygen to singlet oxygen is unfavorable and involves different excited states. Singlet oxygen concentration level in the engine compartment of cars is unknown and may vary depending on placement of generator, distributor, and carburetor. A charge transfer between oxygen and an electron donor in the fuel could lead to "O2-minus." Finally, the participation of trioxidane, H2O3, is important:
H2O + H2O3 ⇒2H2O + O=O (singlet oxygen)
Singlet oxygen can persist for over an hour at room temperature.
All the antiknock evidence is consistent with participation of singlet oxygen:
• In the n-heptane and isooctane scale, singlet oxygen is more reactive with n-heptane but less reactive with isooctane than free radical intermediates.
• Organometallic compounds are antiknock compounds: tetraethyl lead, ferrocene, iron pentacarbonyl, and others. All are singlet oxygen quenchers.
• There are ashless compounds belonging to the class of singlet oxygen quenchers: hydrazines, furans, etc., that have not been thoroughly examined or tested.
The reversible transformation of triplet oxygen to singlet oxygen is important. Sea mammals dive very deep, stay for hours, and surface without effect. These mammals may have a mechanism to convert myoglobin from proteinx-Heme-Fe-O-O* (radical and reactive) to proteinx-Heme-Fe-O=O (neutral and stable). The understanding of this triplet-singlet interconversion, when fully understood, may extend to mammals containing hemoglobin (humans).
The writer thanks Emeritus Professor J.D. Roberts, Caltech, for helpful suggestions; Professor Sydney A. Cameron, University of Illinois, Urbana; and Supervisor Chris Uzelmeir, Shell Development Co. (retired).
Edgar J. Smutny
Senior staff, Shell Development Co. (retired)
Houston
