Octane requirement increase

The additives capable of controlling the octane requirement increase, have as far as they are concerned, a complex structure and are closely guarded industrial secrets. 

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). 

In some circumstances, the octane number of the gasoline used in the engine must be increased to prevent knock caused by accumulated deposits. This phenomenon is called Octane Requirement Increase (ORI). Both prevention and reversal of ORI are difficult to achieve. 

Alkyl lead compounds are extremely effective gasoline antiknock agents. By decomposing to form lead oxide compounds during the gasoline combustion process, lead alkyls interrupt the rapid chain scission reactions which lead to combustion knock. Also, lead alkyls help to prevent exhaust valve seat wear and may minimize octane requirement increase. However, unless utilized in conjunction with lead scavengers such as 1,2-dichloromethane, lead deposits can accumulate within the gasoline combustion chamber. 

FIGURE 6-15. Engine Deposit Locations Impacting Octane Requirement Increase... 

Only a few additives have been proposed as general-purpose deposit modifiers or preventers. Boron compounds have already been mentioned (46). Use of metal chelates of pentadione (PD) as a combustion catalyst, said to remove deposits and keep clean combustion-chamber surfaces, was reported in 1949 (7) and was disclosed in patents as far back as 1937. More work on octane-requirement increase depressants may be expected. 

Autoignition inhibitors 2 Octane-requirement increase depressants 171... 

Because of its large number of empirical parameters, this model can be fitted to a variety of fuels and can reproduce the important autoignition phenomena. This is accomplished with only a modest computational requirement and, consequently, it has been used in a number of engine modelling studies over the past 20 years. Its main deficiencies are the difficulty of extending it to new fuels (which has been done only by the original authors) and its still limited representation of the real chemistry. Because of the generic formulation, the species are rather ill-defined. It would be difficult to use, for instance, in a study of chemical octane requirement increase, discussed in Section 7.5.2. 

The most important effect of combustion chamber deposits is that they make the engine more knock-prone as they build up. This is quantified as an octane requirement increase (ORI), and it typically reaches 5-10 octane numbers. Several mechanisms probably play a part in the phenomenon. Principally these are volumetric, thermal and chemical. The volumetric effect is the increase in the compression ratio caused by the volume of the deposits. This has been estimated [147] to be responsible for only about... 

Fig. 7.18. Comparison of measured knock occurrence with that predicted using the Hu and Keck model for various coolant temperatures. 90 primary reference fuel 1900 rpm. (a) clean engine, showing good agreement, (b) engine with deposits knock occurs earlier than predicted, (c) engine with deposits, as (b) but modelled with fitted amounts of hydrogen peroxide (1-5 ppm) in the end gas agreement restored, providing evidence for chemical octane requirement increase. From [76].

J.D. Benson, Some Factors which Affect Octane Requirement Increase, SAE Technical Paper 750933 (1975). 

PFI and DIG injector deposits, if allowed to form, can increase onissions, cause engines to lose power and driveability, and decrease fuel economy [2,5,27]. IVDs can result in octane requirement increase (ORI) in some engines, increase NOx emissions, and also cause power loss CCDs can produce ORI in almost all engines, increase certain emissions, like hydrocarbon (HC) and NOx, and change ignition timing [2]. Thus these types of deposits seriously affect the engine operation and need to be carefully controlled. 

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