Antiknock action theories

Accomplishments to date in the utilization of antiknock agents have been largely made by cut and try methods, rather than by the application of theory. It is logical to expect that continuing basic research will provide a better understanding of knock and antiknock action, and of the tetraethyllead-fuel relationship, and this should lead to de-... 

Several theories exist on the mechanism for antiknock action, such as outlined in Refs 1 5. Demougin(Ref 6, p 139), H.Moureu C.Dufresse attribute the action of... 

Early experiments of Berl and coworkers (15, 16), together with the findings of Withrow and Rassweiler (136), support the metallic theory of antiknock action. In Berl s work colloidal lead exhibited antiknock action, while colloidal lead oxide had no effect. Spectra of engine knocking combustion obtained by Withrow and Rassweiler showed the presence of metallic lead but not lead oxide or other lead oxides. The metallic theory is also supported by other investigators (96, 101, ) ... 

The nature and mechanism of the antiknock effect is still little understood. Considerable effort has been devoted to a study of the mechanism since the time tetraethyllead first became a commercial antiknock agent but direct proof of the mechanism is still lacking. Very possibly, both homogeneous and heterogeneous mechanisms maybe competing in antiknock systems. Reviews of the various theories that have been proposed to explain antiknock action have been made byBaruschandMacpherson 31>, Lewis and von Elbe 206), Ross and Rifkin 267>, Chamberlain, Hoare and Walsh 88>, and Beatty and Edgar 44>. 

The theory in common with a number of the others does not explain the antiknock action of the organic amines nor does it explain the action of knock inducers. It is not clear why trietliylchlorolead and diethyl-dichlorolead should be, respectively, 75 per cent and 50 per cent as effective as tetraethyl lead. 

As much controversy has existed regarding the cause of the actual knock in an engine as exists in regard to the mechanism of the combustion and action of antiknock dopes during engine operation, and a number of theories lias been proposed, the very diversity of which indicates their Inadequacy. Just as in the case of oxidation mechanism studies, a large numlier of experiments have been performed at other than conditions... 

Molecular collision theory. The molecular collision theory proposes that undecomposed hydrocarbon fuel molecules directly in front of the explosion wave will be bombarded and thus activated by the highly active molecules from the explosion wave itself. Garner and Saunders 85 studied this phenomena by means of the spectra of acetylene air detonations. They explain the formation of carbon in such detonations by the decomposition of acetylene into 2C and Ha by the action of molecular collisions. The application of this theory to the mechanism of the action of antiknocks in internal combustion engine operation is not clear. 

The theories that have been advanced to the effect that the decomposition products of the antiknock dopes and not the compounds themselves are the effective centers of the action has been tested by the use of metallic colloids, prepared in various ways, in the fuels by which engines were operated. The work on colloidal metal sols has been based on the theory that knocking is due to the spontaneous ignition of the unburned charge in an engine cylinder. By acting as catalysts for combustion these substances insure a slow, homogeneous combustion rather than a detonation. 

A somewhat similar theory10 postulates the formation of colloidal lead by the decomposition of lead tetraethyl, which deposits on sharp points, edges, and projections in the cylinder which would otherwise aid reaction to an extent that a detonation wave would result. The theory fails to explain the action of organic amines, of di- and tetravalent. selenium, of the colloidal metal sols, and fails to account for the immediate recurrence of knocking when the antiknock dope is discontinued in the gasoline feed. However, tubes coated with lead oxide have been found to reduce the oxidation of hexane more than lead tetraethyl vapors.1158 On the basis of this finding the hypothesis was advanced that oxidation occurs to some extent prior to compression and at the first contact of the gaseous mixture with the hot walls, aud that in the presence of the lead... 

Radiation screen theory. Midgley 188 adopted Perrin s theory, that radiations from the initial flame activate and accelerate reaction by splitting the hydrocarbons into a more reactive condition, in explaining knocking and assumed that antiknocks acted as screens for absorbing the radiation and controlling the velocity of flame movement. Experimental proof in support of this theory has not been obtained and its merit will be difficult to demonstrate. The theory is not adequate, however, to explain the action of knock inducers. 

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