Thanks to you all on your feedback. Here my 2 cents on those based on experience with the design and integration with known Aero Diesel Engines brands:
- Personally no issues with 2, 4 strokes and turboshaft engines; Each has its Pros and Cons, it is just a matter of which Pros you need for your application and, what Cons you are ready to cope with. The PPD 2 stroke 100 HP engine is at about 0.75 Kg/HP and an SFC of around 0.40 lb/HP/hr on a narrow power range around cruise power. What I was saying is that the technolgy exists and is proven with direct drive 4 strokes diesel at 0.75 Kg/HP but with an SFC of 0.32 lb/HP/hr (20% less) over essentially the whole power range from idle to T/O power. However, it is more suited for 150 HP up to about say 600 HP making the PPD 100 HP engine more adapted for the smaller a/c (but not excluding it from the higher power outputs) due to its simplicity, engine-aircraft cost ratio and, lower usage applications. The 4 stroke aero diesel engines are more suited for the higher value a/c engaged in high usage (commercial, utility applications) making them good turbine competitors since the latter has a near twice SFC and higher initial acquisition and maintenance costs (reason for the ball parked 600 HP upper limit).
- Totally concurr with the remarks about a good support network for any newcommer...actually that is the challenge (the latest aero diesel "casualty" unfortunately lacked in the understanding of that concept). Unfortunately worldwide those sponserors of Aero Diesel engines that have a solid support network, deep pockets and the true will of promoting Aero Diesel engines (indeed the current AVGAS engine majors are potentially confronted with canibalism if they promote aero diesel engines) can be counted on the fingers of one hand that has been releived of a few (read plural with a capital "S") of them.
- The problem with gears is the cyclic nature of the torque signature of a recip engine where as with a turboprop engine it is for all intents purposes a constant. For the same number of pulses per rev diesels have the positive and negative peaks at essentially twice what they are with spark ignition engines. Car gearboxes are indeed reliable but are subjected to substantially much smaller torque densities and peak torque extremes than in aero applications. Car gearboxes are also not subjected to the gyroscopic effects of the propeller by virtue of their design. Though gearboxes could be better designed and beefed up (read mass), personally by experience I would rather trade the mass of a gearbox to the mass of additional pistons (comonality of parts) and have an engine with a lower peak torque signature due to the increased pulses per rev and a resulting lower volumetric power density (i.e. reliability).
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