Originally Posted by Lou Crane

I've re-read the thing again, and at least glanced at the images to refresh...

About the combustion pressure on cylinder interior surfaces - of course it is ideally equal everywhere and perpendicular to the surfaces it meets. If the piston does not move, by definition, no work is done. I use the word ideally because the events happen in such brief time that we may need to consider flame - or is it more the combustion - progression.

In a spark or hot wire catalyzed 'burn' we have a fair idea of when combustion starts and where. There have been studies of flame progression done by the US Society of Automotive Engineers many years ago. Glass cylinder head or some such. They included studies of flame front progression. At the time, these were for modest RPM, low compression, gasoline (petrol) burning engines with valves, and probably the spark plug, outside the cylinder diameter in a side chamber.

They recorded flame propagation speed and characteristics. Clearly, ignition began at the 'trigger' point, the spark plug gap. In a 'glow plug' engine. I'd presume much the same applies. The catalyst wire is hot enough to act as trigger location, Gasoline and kerosene (paraffin) flame progression were both slower than alcohols. I seem to recall they tested what we refer to as wood alcohol - methanol. Burning ideal proportions of alcohol and oxygen, the equipment used then could not measure the propagation speed. Speeds for petrol and paraffin were measured. (Our fuels are quite unlike the tested fuels - we blend in oil and do not run to maximum potential.)

Consider: The time available to initiate the most useful portion of the 'burn' is extremely brief. E.g., at 18,000 RPM, that's 300 full rotations per second 108,000 degrees per second. If the most effective portion of the 'burn' lasts from 15° before TDC to about 25° after TDC, about 40°, the actual time involved would be 0.00037 seconds. Even in the small distances in our combustion chambers, it may still take significant time for the pressure to equalize to all exposed surfaces...

Initiation, propagation and early decline of pressure all occur in that duration. Strobe photography can capture the initiation, progression and waning of the event. (I assume not during one event, but at sequential iterations delayed by defined degrees of shaft rotation. Not sure, but I doubt that other adequate methods existed when SAE did these studies in the 1920's and 1930's. We have advanced, since, but the basics of the combustion progress remain.)

40° may be a generous guess at the shaft degrees involved. The pressure falls as combustion completes and as the piston moves down increasing the trapped volume from its optimums during the burn. Piston motion is, of course, the work combustion produces..

Do we actually know if paraffin burns during our model diesel combustion? Specifically, does the process include actual flame? Petrol and methanol we must presume to burn with a definite flame. In din light, the hot orange light in the exhaust is undeniable. Does that also apply to our diesels?

Re: the baffled piston OS 15D - the projection was not a reciprocating piece, it was an alignment peg that slid into the recess in the mating part only during reassembly. That emerged clearly over several subsequent posts.