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

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flame moving from the bottom to the top of the photographs. The upper sloping edge of the light portion was therefore a time-distance plot of the flame front movement across the combustion chamber. The region marked afterglow in the photographs was produced by the burned but luminous gas behind the flame front. It was from these researches that it was definitely determined that normal combustion in a gasoline engine was initiated at a single point — the spark discharge — and spread from this point like a grass fire at a rate which could be determined from the slope of the pictures similar to the flame records of Fig. 3. It was also found that during the combustion process there is a relatively narrow zone in which all of the combustion process takes place and which moves progressively through the charge. From this type of record it was found that flame speed is proportional to engine speed and that knock in a gasoline engine consists of the last part of the charge burning at a rate many times that occurring under normal combustion conditions. This is illustrated by comparing the flame records of Fig. 3. So it was that the first photographs of the burning of gasoline and air in a spark ignition engine furnished many of the basic facts which are now used daily in the design of better automobile engines. This type of photography was limited, however, particularly in regard to determining the shape, structure and movements of the flame fronts and the nature of knock. In particular it was hoped that flame photographs would supply information regarding the effect of engine speed, chamber shape, fuel types, etc., on the rate of flame travel. These considerations led to the development of another engine which allowed an unrestricted view of the entire combustion chamber. The head and cylinder block are shown in Fig. 4. The problem of sealing the quartz window in the head frame required many hours of research. The problem facing the investigators at the time the original plans for the engine were drawn up was what type of camera could best be used for taking photographs of the combustion process through the full quartz head. • At this time a number of high-speed camera designs were available in the literature; however, careful consideration of the following aspects of the photographic problem showed that many of these cameras would be unsuitable: 1. The subject to be photographed was self-luminous; that is, the light was emitted from the flames themselves. This imposed a severe limitation upon camera design from an exposure standpoint and ruled out those cameras which depended upon a separate high intensity source of illumination. 2. The picture frequency had to be relatively high since the flame front moved at a high velocity from the spark plug across the combustion chamber. Under normal operating conditions at an engine speed of 2000 rpm a frame speed of 5000 frame/sec was required in a combustion chamber 5 in. long to obtain 20 pictures of the combustion process. At 400 rpm, however, a frame speed of only 1000 frame/sec was required for the same number of pictures of the combustion process. 3. The available light was fixed by the engine conditions and could not be varied at will. Experiments with the original flame camera showed that satisfactory results could be obtained with an exposure of 0.0002 sec and an engine speed of 1200 rpm, with an //1. 5 lens using a hypersensitive panchromatic film. Consequently, the light emitted by the combustion process was sufficient for frame speeds of 5000 frame/sec at an engine speed of 2000 rpm provided the duration of exposure was comparable with the time interval between frames and provided the lens speed was //1. 5 or greater throughout most of the exposure. In order to fulfill these condi 474 December 1952 Journal of the SMPTE Vol. 59