As usual, George hit the nail squarely on the head.  In order to get power up on a diesel the peak cylinder pressures are much higher than on a spark-ignition engine - something like 2,000 psi, compared to 1,000.  The structure of the engine is proportional to the peak pressure, so the engine almost has to be heavier.  Most of the combustion chamber is in the piston, so piston cooling becomes the limiting factor and a great deal of heat is rejected to the oil as a result.  Heat rejection during combustion is roughly proportional to the pressure TIMES the temperature difference, so the heat rejection into the cylinder head is also very high.  This means large heat exchangers and more cooling drag.  A diesel at full power requires about 10% excess air, so the filter (if any) and muffler (if any) has to be larger, as does all the associated plumbing.  There was a comment about the power of the engine rolling off above 15,000 ft.  The reason is that the engine is running very high manifold pressure - I'm guess at least 60 inches and more likely 90 inches or even more - in order to get the desired power.  Turbocharger technology just isn't up to the task of producing the required pressure ratios above a fairly low altitude.  You could go to series turbochargers, but that would add more weight.  It would certainly produce plenty of cabin pressure, except that the critical altitude would be even lower if some air is taken off for pressurization.  And then there is the fuel - it produces more BTU's per gallon, but fewer BTU's per pound than gasoline (about 5% in both cases).  Depends on whether you are buying the fuel (per gallon) or carrying the fuel (per pound).  Another little detail - many diesel fuel system really, really don't like air in the fuel and some don't self-prime.  And if you're flying in the flight levels, don't let the fuel get too cold (of course, that's why jet-A is different from diesel fuel).  I like diesels, but for a 300-hp aircraft application the big-bore air-cooled gasoline engines are remarkably good.

Gary Casey

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