The Measurement of Engine Knock by Electro-Acoustic Instruments 360104
THIS is another chapter in the development of a quantitative knock-measuring apparatus. The first part of the paper describes tests in which the knock sound was picked up by a microphone located a few inches from the cylinder head of a test engine.
Oscillograms were taken of the knock disturbance, and measurements were made of the energy in various frequency bands. The latter study included the effects of L-head and overhead-valve design, as well as some work with cylinder heads of iron, aluminum and bronze.
It was found that the C.F.R. standard engine develops a pronounced peak in knock energy at frequencies between about 6000 and 7000 cycles per sec., while the L-head design of this same engine had an additional peak between 3000 and 4000 cycles per sec.
It was found also that a pronounced background of engine noise develops at frequencies below about 2500 cycles per sec.
A brief account is given of the application of these principles to the measurement of knock in cars on road tests.
The second part of the paper deals with a special microphone installed so as to be in direct contact with the gases in the combustion chamber which are at the seat of the detonation disturbance.
It has been found that the use of filters which cut off low frequencies to varying degrees has a pronounced influence on knock rating, and may be of considerable importance in the development of a routine knock-measuring procedure which will give better correlation than the method now in use of individual fuels with values obtained in cars on the road.