Knocking combustion

The conventional Cl-engine operates with a relatively high compression ratio and in an unthrottled manner. Delayed direct injection of fuel in the compression process prevents knocking combustion and enables load... 

Various fuel grades are designed to guarantee knock-free operation. Knocking combustion can damage the engine and can result in serious power loss if allowed to persist. 

The importance of hydrogen-containing species in knock reactions is strikingly shown in knocking combustion studies of carbon monoxide by Anzilotti and Tomsic (6). The presence of 1.4 mole % of water markedly lowered the knock resistance of nearly anhydrous carbon monoxide. Other studies show that water enters into the combustion reactions of carbon monoxide and contributes hydrogen to the intermediate combustion products 125). 

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, ) ... 

Cinefilms of knocking combustion by Withrow and Rassweiler [49] showed, as early as 1936, that in a knocking cycle the end gas autoignites... 

H. Schapertons and W. Lee, Multidimensional Modelling of Knocking Combustion in SI Engines, SAE Technical Paper 850502 (1985). 

D. Bradley, J. Pan and C.G.W. Sheppard, Turbulence and Flow Field Effects, in Gas/Surface Interactions and Damaging Mechanisms in Knocking Combustion, ed R.R. Maly, Final Report Contract JOUE-0028-D-(MB) (1993) p. 57. 

Radiation and ionization. The work with bombs and engines has indicated that small amounts of antiknock dopes eliminated the shock wave in combustions of explosive mixtures and induce a continuous combustion in the wake of the flame front. It has also shown that during knocking combustion a partial burning outruns the remainder and causes a violent liberation of energy during some portion of the cycle. ... 

Figure 5.3 gives an example of a combustion diagram recorded during knocking conditions. This is manifested by intense pressure oscillations which continue during a part of the expansion phase. 

The Octane Requirement Increase, ORI, is a phenomenon manifested by the appearance of knocking and is due to the increase in engine octane demand with time. This phenomenon is correlated with the increase of solid deposits in the combustion chamber. Although the causes have not been determined with certainty, some companies have patented additives which modify the deposits. The effect is to limit the increase in octane demand (Bert et al., 1983 Chevron, 1988 Nelson et al., 1989). 

Knock is caused by unwanted chemical reactions in the combustion chamber. These reactions are a function of the specific chemical species which make up the fuel and the environmental conditions to which the fuel is subjected during the compression and power stroke in the engine. Therefore, both the chemical makeup of the fuel and the engine design parameters must be considered when trying to understand knock. 

Vehicle Fa.ctors. Because knock is a chemical reaction, it is sensitive to temperature and reaction time. Temperature can in turn be affected either by external factors such as the wall temperature or by the amount of heat released in the combustion process itself, which is directiy related to the density of the fuel—air mixture. A vehicle factor which increases charge density, combustion chamber temperatures, or available reaction time promotes the tendency to knock. Engine operating and design factors which affect the tendency to produce knocking are... 

Octane number is a measure of a fuel s abiUty to avoid knocking. The octane number of a gasoline is deterrnined in a special single-cylinder engine where various combustion conditions can be controlled. The test engine is adjusted to give trace knock from the fuel to be rated. Various mixtures of isooctane (2,2,4-trimethyl pentane) and normal heptane are then used to find the ratio of the two reference fuels that produce the same intensity of knock as that by the unknown fuel. 

In the Diesel engine, a higher compression ratio can be achieved, up to about 18, before knock presents a problem, and the fuel is injected into the combustion chamber near tire end of the compression stroke as a spray. The... 

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