Hello all, I am Larry from Colorado -- thanks for the heads up Automotivebreath!
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Why did the piston not go back down in the video?
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I suspect that was a "oneshot combustion event" solely for the purposes of the photography, as the video was used to show off the capabilities of the camera.
This sort of photography is very difficult to setup on a normally running engine. When NACA did similar ultra high speed photos at 40,000 and 200,000 frames per second in the 1940's they would motor the engine at operational speed, while preheating the engine to normal operational temps and then fire the plug once to avoid shooting 10's of millions of frames of useless film. Not quite as big a deal with digital photography I guess but still a consideration.
That said if David contacted the camera manufacter he might be able to get more detail about the setup configuration. I am not sure us mere mortals would have enough clout to get past their normal media relations folks.
Although I am interested in what is going on at the piston crown boundary layer in that video the other thing that caught my eye is how the flame kernel shoots to the top of the cylinder at over 70 mph by my calculations and them blooms out against the cylinder head with combustion then moving down back into the main combustion chamber volume from the top rather than from the center. I am curious if that flame kernal motion is driven by boyancy effects (rising like a hot air balloon) or by general mixture motion as the fuel air mix column is compressed by the piston. If the first case it would have interesting effects depending on engine layout with the flame kernal moving towards the high side of the combustion chamber. In a V8 and a flat opposed that means combustion would move down across the piston face from the intake side where on an inline vertical engine it would do as pictured above.
My guess is that the above density difference is due to thermal density changes as the relatively cooler fuel air mix is heated by the piston. If it was due to the piston motion I would think you would see it decay away and move off the piston crown farther as the piston motion goes to zero at TDC.
Larry