— a barrier to higher compression ratios?
A NEW BARRIER to higher compression ratios has recently become apparent — engine rumble! This phenomenon will prevent further increases in compression ratio unless corrective measures are taken.
This paper describes the phenomena of engine rumble not only in terms of the noise and vibrations that emanate from the engine but also in terms of the pressure development inside the cylinder. Rumble is the result of abnormally rapid pressure buildup in the combustion chamber due to multiple ignition of the fuel-air mixture by glowing deposits.
Since deposits are responsible for the occurrence of engine rumble, studies have been made to determine the contribution of various gasolines and oils to the rumble tendency of the deposits formed. Results from dynamometer and road tests show that combustion-chamber deposits formed by the use of some oils and fuels are considerably less likely to cause rumble than deposits from others. A phosphorus gasoline additive was tested at concentrations from ½ to 1 theory to determine its effectiveness in reducing rumble in 12/1 compression ratio cars.
Another aspect of the rumble problem is the inherent resistance of the fuel itself to ignition by deposits. Deposit ignition resistance ratings of several fuels were made in a car on the road.
Several engine design and operating variables were studied to determine their effect on the occurrence of rumble. These included compression ratio, air/fuel ratio, inlet air temperature and humidity, coolant temperature, engine load, and engine speed.
Evidence is shown that engines having compression ratios as high as 12/1 can be operated satisfactorily with respect to rumble if fuels and oils are carefully selected.*