Knocking in an engine

Fig. 7.11. Cinefilm sequence leading to weak knock in an engine. Time measured from spark ignition. The flame propagates from left to right and extensive end gas reaction has occurred at 3.50 ms. White spots are seeding particles introduced to measure gas...

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... 

Gasoline with too low an octane number can cause knocking in the engine of a car, when the fuel ignites too easily and burns in an uncontrolled manner. Knocking lowers fuel efficiency, and it can damage the engine. 

J.S. Cowart, M. Haghgooie, C.E. Newman, G.C. Davis, W.J. Pitz and C.K. Westbrook, The Intensity of Knock in an Internal Combustion Engine An Experimental and Modeling Study, SAE Technical Paper 922327 (1992). 

At one time, very large amounts of lead were used in the production of tetraethyl lead, a compound that reduces the amount of knocking in an internal combnstion engine. The problem with tetraethyl lead, however, is that it tends to break down within an engine, releasing free lead to the environment. Because of the health hazards that lead poses for humans and other animals, tetraethyl lead has been banned for use as a gasoline additive. 

The various types of hydrocarbons in gasoline behave differently in their preflame reactions and thus, their tendency to knock. It is difficult to find any precise relationship between chemical structure and antiknock performance in an engine. Members of the same hydrocarbon series may show very different antiknock effects. For example, normal heptane and normal pentane, both paraffins, have antiknock ratings (octane numbers) of 0 and 61.9, respectively (Table 5.5). Very generally, aromatic hydrocarbons (e.g., benzene and toluene), highly branched iso-paraffins (e.g., iso-octane), and... 

Woodbury, Lewis and Canby110 were not able to detonate ethyl ether-air in their small bomb apparatus, although ether is recognized to have bad knocking characteristics when used in an engine. However, they observed... 

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. 

The idea that the processes, which occur during the combustion of a fuel in an engine cylinder, take place by a chain reaction mechanism comparable to that associated with the liquid phase autoxidation of benzal-dehyde, has led to the proposal that knock suppressors act by destroying the chains and reducing the rate at which the flame front accelerates. It is known that surfaces or solid bodies suppress chain reactions, in fact one of the criteria for a chain reaction in gaseous combustion processes is the decrease in rate of reaction caused by the increase in surface exposed to the gases. However, the mode by which the chains are broken or... 

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