MOTORING

Hawera Star, Volume XLVII, 16 June 1928, Page 15

MOTORING

FUEL KNOCK. RESULT OF RAPID COMBUSTION. Fuel .knock or detonation of the charge in an automobile engine is caused by abnormally rapid combustion as compared with normal combustion. Just what is the cause of this speed-ing-up of the burning or the charge has not been determined, although several ingenious hypotheses liar e been advanced, according to a report made by Thomas Midgely, jun., chairman of a sub-committee of the Research Committee of the Society of Automotive Engineers, at the annual meeting of the and published in that organisation’s monthly journal. . It is known that detonation is increased by increasing the temperature or the pressure of the charge during the time of the explosion, and that local hot-spots dr an increase in the length of flame travel also tend to increase the phenomenon. Detonation varies greatly with the air and fuel ratio or the mixtuie. The effects of detonation are, first, a disagreeable .noise, varying from a slight “ping’’ to a very loud clatter; loss of powejf if the knocking is severe, and a tendency to overheat the engine and the cooling water. Second, this over-heating frequently results in pre-ignition; and third, in extreme cases, breaking of the spark-plug porcelains’- actual rupture of the piston heads, and damage to bearings or other structural parts may result. Probably the most important effect of the tendency towards detonation is that it limits the compression ,pressure that can be used in an engine to give satisfactory performance with ordinary commercial fuels. This seriously limits the fuel economy. Chemical composition of the fuel probably is the greatest single factor that influences detonation, according to the report. The paraffin hydrocarbons that occur in straight-run gasolines apparently tend to knock badIv, whereas certain other paraffin hydro-ca rbons are good anti-knock fuels. As a class, however, the nap-t-hene, aromatic, and olefine hydrocarbons which occur in many straightrun or cracked gasolines tend to knock distinctly less than the normal paraffin hydro-carbons. One of tlie most difficult problems in connection with detonation is that of comparing accurately the detonation characteristics of the different fuels. Many, hut not all, compounds of nitrogen have a moderate anti-knock effect and practically all volatile compounds of iodine, selenium, tellurium, nickel, cohalt, tin, and lead have marked effects in preventing detonation. Some are enective even when present in extremely small quantities. It is recommended that future research he directed toward the development of a standard and reproducible method of testing and rating fuels as to their detonating properties, modification of both engines and fuels so that higher compression pressures can be used without detonation, and determination of the mechanism of detonation and its prevention.