05/07/2004

Adiabatic Engines

In the army's adiabatic engine experiments, a nearly perfect adiabatic
engine showed a bfsc improvement of typically less than 5% and an at
best of 7%.

A semi adiabatic engine ( heads, valves, piston - but water cooled walls
) showed about 2% improvement in bfsc.

But the purpose was not to improve native bfsc, but to provide driving
power for a compound stage.  The conclusion in evidence matched the
predication that significantly more energy would be available to drive
the bottoming cycle expander.  

Enough so the compound unit provided 15 to 25% power of the total engine
power available.

Separate data showed that decreasing the exhaust port area and most
importantly the initial pipe run to about 60 percent of the valve
curtain area continually improved both power and bfsc.  The reduction of
irreversible heat losses due to sudden expansion allowed the greater
recovery of energy at both the turbocharger turbine and the down stream
expander.

An inverted lyscholm screw recovered the most lower end power, and a
turbine tended to recover power in the normal diesel power band and was
not optimal for army diesel engines.

For those that can think outside the factory approved technical box,
this makes for an extremely interesting idea - a high speed turbine.
Most turbos are in wastegate well before wot and about 50% of the gas is
surplus.  A second stage high speed turbine would nicely recover up to
about 25% of the engines total power or more - if the engine approaches
adiabatic.

Looking to find that book ( around here in a Daves World pile of books
), because what Tom experienced sounds familiar.  Was the head and
valves coated as well as the piston?

Partial coating would significant reduce the heat losses and reduce the
heat in the chamber on compression.  This would seriously reduce the
charge temperature near TDC and result in significant ignition delay.

As I recall, they had to remove the intercoolers on some of the engines
to get the charge heat at TDC high enough for reliable ignition.  A
production engine could use significantly higher compression ( and thus
expansion ) to recover this heat loss.

Again, based on an old lechers memory, I suspect that the carbon marking
may have been the direct result of etching and erosion of the coating
rather than porosity.  Ceramic has high heat resistance - but it quite
fragile with regards to impact and direct desiel spray impact virtually
ice cold at 20,000 psi on superheated ceramics would probably do weird
things.

Possibly the lower compression temperatures ( loss of combustion chamber
heat retaining surfaces ) resulted in a slower burning charge all the
way through and accounts for the delayed combustion.

Not arguing the results or the conclusions - obviously based on actual
experimentation.  Just tossing out what the army found similar results
with different explanations.

But on a SI engine, the charge temperature reductions for knock control
are worth it.  Dave Vizard claims that simply coating the intake valve
will knock 60F off the inlet temperature and that's good for an
effective 2 units increase in octane.  Plus he reports that a piston
coated properly will knock loudly for minutes without damage - up to an
hour in one case.