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Lots of "new" diesel engine lines appearing, one particularly
QUESTION:Let me preface by stating I have absolutely no credentials. Although I
did replace a head on a Kabota Diesel in a skid steer I used to own.
By the way Rick, the Kabotas are water cooled in that application.
Karen I believe you are reading the charts wrong. The "constant"
pressure part of the cycle in Rick's engine is from ~tdc to the point
when the pressure drops below the peak pressure from compression
alone. Past this point is referred to as "Isentropic expansion" and
in Rick's example considerable work is still being done at 90 degrees
atdc which is the last data point he writed.
ANSWER: The compression stroke is Isentropic (heat being generated by
the compression of the air but also being lost into the enclosure
etc)
(ii) At about tdc ignition occurs, so heat is being put into the
system which means the cyl pressure is higher than the energy
expending in the compression stroke.
(a) As you can see from the numbers the constant pressure part of
the cycle is completely consistent with heat being added at as small
a rate as possible over as long a period as possible.
(b) That's the biggest advantage a diesel has over a spark engine,
it can keep adding heat to the system over a longer period so
holding a constant pressure rise as the volume of the chamber gets
bigger (NB not constant psi readings but a constant increase over
the energy lost in the compression stroke, it's reversible ?
yes?).
(c) The quantum of the numbers don't matter much in this
discussion, it's the fact that the chamber is still receiving heat
even as the piston goes down the bore, such that the power stroke
stays under constant pressure even though the volume is increasing.
(d) Again in the figures provided it's clear heat must still be
getting added to the system at 90 atdc because even at that point
where the piston is speeding down at it's fastest rate the
heat/pressure is being maintained even increased.
(e) So even at 90 atdc the injection & burning is continuing so the
engine can convert more energy into work. Yes the actual pressure is
lower but when you consider that the crank is at 90 degs, then even
that lesser number is much more useful for power (making torque at
any given revs).
(iii) After the injection stops & in this case may I suggest we
don't really know when that is, because despite Rick's words it's
very clear the injection is maintained to 90 atdc. However when the
injection does end & the after burn is finished then no further heat
is being added to the system, then there is some isentropic heat
expansion as the piston finishes going to the bottom of the bore.
(just as there was heat generated & lost on the compression stroke
so to the bottom of the power stroke)
(iv) At bdc or there abouts there is constant volume heat rejection.
The piston is parked at bdc & the exhaust is purged along with the
unused heat.
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