Paul Lamar wrote:
I think the pipe line engine had BSFC and detonation problems with that huge combustion
chamber. I could be wrong on this. Perhaps a sphere's surface area to volume ratio may get
more favorable as the size increases.

Russell Kent replied:

surface area of a sphere = 4 pi r**2
volume of a sphere = 4/3 pi r**3
surface area/volume = (4 pi r**2) / (4/3 pi r**3) = 3/r

The bigger the radius, the lower the ratio.  Question is: does "lower" equate to "more
favorable" in this case?

Paul Lamar responded:

Heat is lost through the surface area.

Paul,
Combining this thread with the one about Diesel engines, perhaps a large displacement Wankel
could be built which was a spark-assisted Diesel engine?  Since Diesels generate less waste
heat, then the heat rejection problem caused by the low surface area to volume ratio would less
of an issue, correct?

Of course this is all pie-in-the-sky engineering...

Russell Kent

The trouble with a spark assisted low compression "diesel" is; it isn't really
a true diesel.

True diesels ignite from high heat and the high heat comes from the high
compression ratio as does the high efficiency and low heat rejection
to the coolant. A high mechanical compression ratio wankel rotary would have
a poorly shaped combustion chamber. Now if it was external compression
(a turbo charger) that could be another matter but unfortunately the high heat is
wasted in the intercooler. Now if we could only control the detonation
of a turbo rotary with out intercooler than maybe.....

Turbo compound is the simple answer.

Paul Lamar

The Diesel cycle is not constrained to compression ignition, but relates to a constant pressure heat
addition during expansion. So called Diesels using compression ignition, have less heat rejection
because they burn less fuel per charge, for a lower total heat input. A normally aspirated "Diesel"
engine will have less torque ( and less HP) at any given rpm than the same size gasoline (Otto
cycle) engine. Better efficiency is obtained because they are able to run a higher compression
(really expansion) ratio than a spark ignited engine. Also at light load, they run way leaner
than any spark ignited engine, and have less pumping loss. For the same compression (expansion)
ratio, the Otto cycle has a higher thermal efficiency than the Diesel cycle. In turn the Brayton
cycle
(gas turbine) has an even higher thermal efficiency. In the real world, they have to run where they
can
live, and the results are what you see. A direct injection stratified charge gasoline engine has many
of the same advantages and disadvantages as a "Diesel".
Vance

"So called Diesels using compression ignition"?????

Why do you say "So called" Vance? The vast majority of diesels (99.9%) we are familiar
with have compression ignition and compression ratios of 15:1 or higher.
 
Paul Lamar
 
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