Journal of the Society of Motion Picture Engineers (1930-1949)

absorption is neglected, n is the refractive index of the optical glass, and k is the number of glass-air surfaces. Since the refractive indexes of optical glass in common use lie between 1.5 and 1.7, the average value of (n — l)2/(w ~h I)2 is about 5% and the transmittance can be computed with reasonable accuracy3 as / = (0.95)*. The high-speed camera optical system consisted of 16 glass-air surfaces since both the field lens and the moving lenses were multi component lenses. The amount of transmitted light, therefore, through the lens system and prisms amounted to 44% of the incident light not considering that lost at the stellite mirror. Therefore, since about 50% of the light was lost at the stellite mirror, the approximate total transmitted light was about 22% of the incident light. If all the glass-air surfaces had been coated, as is now current practice, the approximate total transmittance would have been about twice as much or 44%. The present-day Eastman HighSpeed Camera Model III, a rotatingprism type camera, having a coated Ektar lens and an uncoated prism can be used without the large stellite mirror mounted over the engine head and the transmittance of this optical system is approximately 83%. The older ERPI Camera (Electrical Research Products, Inc.) with uncoated optics, another rotating-prism type of camera, without the engine stellite mirror had a transmittance of about 73%. Since the ERPI exposure time amounted to about 25% of the time between frames and the engine camera to about 90% of the time between frames, the total amount of light reaching the film per frame at the same frame speed in the two cameras was about the same due to the total transmittance difference of the two cameras. Experimentation with an ERPI camera since the engine camera was built has shown that even though the flame front moved about three-and-one-half times as far per exposure with the engine camera as it did with the ERPI camera, photographs of about equal definition were obtained with the two cameras. This apparent anomaly may be at least partially explained by the distortion of the images in the rotating-prism cameras due to the motion of the prisms. Returning to the combustion photographs obtained with the engine highspeed camera, unrestricted views of nonknocking and knocking explosions were taken. The knocking explosion pictures revealed in a most striking manner the occurrence of spontaneous ignition in sections of the unburned charge well ahead of, and completely separated from, the advancing flame front. Figure 6 illustrates this difference between normal or nonknocking combustion and knocking combustion. From similar pictures and corresponding pressure records Drs. Withrow and Rassweiler4 were able to sort out pressure changes due to combustion from those due to piston motion on the pressure records and found very important relations between per cent of pressure rise and per cent of fuel-air mixture burned. They were also able to determine the effects of changes in mixture ratio, spark position and throttle opening upon the combustion process — all very important to the operation of our modern automobile engines. In 1939 interest was revived in a combustion phenomenon known as "after-running" (tendency for engines to continue running at very low speeds after ignition has been shut off). At this time a 1939 V-8 Cadillac engine was made available to these Laboratories and quartz windows were mounted on the left bank affording an unrestricted view of the combustion chambers in these four cylinders. This engine with the quartz windows in place is shown in Fig. 7. This engine may be familiar to the reader since it was later exhibited at the New York World's Fair. In this investigation it was necessary F. W. Bowditch: Research Photography 479