This paper presents a study of fuel performance in spark ignition engines in relation to abnormal combustion phenomena such as surface ignition and rumble. This was accomplished by observation of the distribution of flame arrivals at ion gaps located within the engine cylinders, or peak pressure position, in terms of crank angle degrees.
By this means, the surface ignition behavior and mass burning rates of LIB (leaded isooctane-benzene) and commercial fuel components were studied in multi-cylinder engines at 10:1 and 12:1 C.R. Additional information was obtained in two instrumented CFR engines operated at 10:1 and 6.75:1 C.R.
It is concluded that the abnormally high peak cylinder pressures giving rise to rumble result from cycles in which combustion occurs early. Early combustion was caused by deposit-ignition of the charge but was also strongly affected by mass burning rate of the charge. Differences in mass burning rate between fuels also affected rumble differently for this reason. Furthermore, the mass burning rate for any fuel tested was greatly increased by the presence of combustion chamber deposits or by finely divided leaded deposits aspirated into the engine.
A large part of the rumble intensity difference noted between the 0 and 100 LIB reference fuels (benzene + 3 cc TEL and iso-octane + 3 cc TEL) may be accounted for by the difference in their burning rates (10-14%). Whether or not this will be a factor in rating rumble by audible means depends on where the ignition timing is set. However, even when allowance is made for burning rate, the 0 LIB fuel shows more early ignitions than 100 LIB in the presence of either accumulated or aspirated deposits. It is, therefore, concluded that these fuels differ also in their tendency to be ignited by deposits.
This information suggests that when rating the rumble tendencies of fuels and engines, a distinction should be made between rumble caused by fuel burning rate differences and that caused by deposit ignition tendency.
Phosphorus added to the fuel in sufficient concentration was found to change the nature of the deposits and thus minimize rumble by reducing deposit-ignition. However, it did not prevent rumble due to early combustion caused by an increased burning rate of the charge.
In a 10:1 C.R. multicylinder engine, significant differences in rumble tendency were found between cylinders. These differences resulted apparently from variations in fuel-air ratio and other factors that could influence the deposits formed and the mass burning rate of the fuel charge.