Schlieren combustion

[automotivebreath]

To start this thread heres a quote from Larry from Colorado. On another
forum we are discussing this clip. I am hoping to get more opinions on
what this tells us.

"This video of combustion taken at 10,000 frames per second, with the
Photron Ultima APX-RS camera Schlieren photography records density
differences so you can clearly see the high temperature areas generated
by combustion process. Schlieren photography only sees changes in density,
much like you see mirage and heat waves on a hot highway. All you can
say with certainty is that the area just above the piston crown is distinctly
different in local density than the fuel air mix a centimeter or so above the
piston crown."

"That said my guess is that the activity that we see in that clip is heat transfer
by conduction from the hot piston crown to the relatively cooler fuel air mix."

Schlieren combustion

In the combustion clip, focus on the activity above the piston surface as it
approaches the cylinder head. First thoughts are this is related to heat
transfer from the piston crown. The cylinder head temperature should be near
the same (possibly lower with heat transfer to the water jackets) yet the
activity is only seen above the piston. Could there be more? Is it possible
that this activity is kinetic energy that is being transmitted into the
mixture as the piston displaces the fluid during compression?

Here's some screen shots from the clip.

[DavidVizard-GFN]

Randy,
This looks neat stuff,
Can you give us the link to the video?

DV

[automotivebreath]

The link is in the original quote from Larry. Looking back it is sort of hidden so
this time I put it in bold. David I'm very interested in your opinion on this, it ties
directly into some of the the R&D work I'm currently involved with.

Schlieren combustion

"This video of combustion taken at 10,000 frames per second, with the
Photron Ultima APX-RS camera Schlieren photography records density
differences so you can clearly see the high temperature areas generated
by combustion process. Schlieren photography only sees changes in density,
much like you see mirage and heat waves on a hot highway. All you can
say with certainty is that the area just above the piston crown is distinctly
different in local density than the fuel air mix a centimeter or so above the
piston crown."

"That said my guess is that the activity that we see in that clip is heat transfer
by conduction from the hot piston crown to the relatively cooler fuel air mix."

Schlieren combustion

[Pinhead]

Why did the piston not go back down in the video?

[automotivebreath]

Pinhead that is a very good question, this type of detail puts doubts in
the accuracy of the entire clip.

[Stan Weiss]

Also it looked like the piston spent was much time ATDC as it took to travel up. Now that is some real dwell time.

[hotrod]

Hello all, I am Larry from Colorado -- thanks for the heads up Automotivebreath!

Why did the piston not go back down in the video?

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

[automotivebreath]

Welcome to GoFastNews Larry.
Thanks for sharing you thoughts on the interesting video.

Here's a link to a higher quality version of the same video with a frame counter:

Schlieren Imaging

Early flame development is determined by large scale in-cylinder flow, in this
example the mixture at the plug is very quiescent, thats just a fancy word
the educated use for still or inactive. I suppose this was staged to
photograph laminar burning which is much more organized than turbulent
flame. Perhaps squish was left out for the same reason.

With no movement to stretch the flame kernel, the result is flame kernel
development in the vincenty of the spark plug. If swirl were present the
flame kernel would stretch in the direction of the flow developing a larger
flame area during the early stages and faster transition to turbulent flame.

The earliest frame I'm able to capture is 83, showing what you describe.

Two things I would like to point out. First the flame develops faster on the
left side of the chamber, the the early flame kernel that shoots to the top of
the cylinder results in development on the left side a a rate over 2X the
right side. My thinking is the ground electrode cools the flame kernel
delaying development in the area.

Secondly watch just below the spark plug when the flame front reaches
the activity at the piston surface after frame 133, the flame explodes,
the difference between frame 133 and 175 is enormous. This I would think
is what is described as a wrinkled laminar flame front, the key to elevated
flame speed. Your thoughts?
















.

[Cammer]

IMHO we are witnessing a clever fake.

[automotivebreath]

Cammer please define "fake". This does appear to be staged to slow
the combustion process down to show off the photography equipment,
is this what you mean?