PHOTOGRAPHING "KNOCK."

Ellesmere Guardian, Volume LII, Issue 37, 8 May 1931, Page 2

PHOTOGRAPHING "KNOCK."

Moving pictures of combustion in a motor car engine, which reveal the nature of engine knocking were recently made in the United States. The performance of every engine is influenced greatly by differences in chemical constitution between fuels, and if the composition of the fuel is not what it ought to be, the engine announces the fact decisively in the form of the complaint called "knock." In studying the nature of knock, or detonation, an. American expert photographed the inside of a gasoline engine during satisfactory performance and during knocking. A narrow quartz window, extending across the combustion chamber, for the full length of it, was placed in the top of the cylinder head. The length and location of this window were such that the whole sweep of combustion, starting from the spark plug and ending at the far side of the combustion chamber, was visible in the camera. The camera contained a very fast lens, with means for focussing it, and a rotating drum with a film wound once around it. This drum rotated at a constant speed, and in a direction at right angles to the axis of the window. What was obtained when the shutter was oepned was a moving picture of the flame as it dashed across from the spark plug to the opposite end of the combustion chamber. The picture differed from a regular one, however, in that the shutter remained open all the time and in that the film moved at a constant speed, so that the image obtained was all in one piece and spread out on the film. From the pictures of normal or nonknocking combustion, it could be seen that the flame moved at a finite, but not at a uniform velocity; and, second, that it took about 40 degrees of engine revolution to cross the chamber. Thus, for a car running 30 miles per hour, the combustion of the charge takes only about four onethousandths of a second to complete itself.