The previous article received lots of very good comments, and since my reply to one comment became too long, I decided to post a new article about it.
One reader proposed either push-pull hybrid where one engine would be diesel and the other would be electric motor. There was another possibility also considered, with coaxial propellers the same thing. This is a valid point and would work. There are some challenges on it therefore here is some cons I considered and hereby listed for this setup:
I may post this as a separate article also because otherwise it possibly does not get read by that many:
This is reply to a commenter for the earlier article:
There is a little incompatibility here that I don't see how to overcome:
- the diesel engine operates at medium rpm which requires reduction drive
- the electric motor can designed to be direct drive and low rpm without need for reduction unit
Having series hybrid there is weight penalty of two brushless DC motors and the engine and the battery, but no other systems. The engine runs the brushless DC motor without reduction gear and the motor that is used as generator can be designed to operate at the rpm the engine operates. The other motor which drives the prop can be made to operate at low rpm.
-> this sytem has NO:
- weight penalty of reduction gear unit
- reliability penalty of reduction gear unit
- need for propeller clutch and the associated reliability penalty and weight penalty
- need for drive shaft to achieve aerodynamic cowling shape
You already listed the most of the pros for the diesel direct drive. I list the cons:
The diesel direct drive cons:
- would not work without clutch, the power pulses would make the prop come off in flight if it did not fail on ground testing already
- does not get necessary power to weight ratio from the engine because of the need to run it at low rpm because of the prop requires low rpm
- weight penalty of the additional gear reduction unit
- reliability penalty of the additional gear reduction unit
- weight penalty of the clutch
- reliability penalty of the clutch (in Thielert engines they have failed now and then, especially in the original design, the latest engine models might have addressed this issue but I am not sure)
- added complexity for the conversion, this is a major consideration in homebuilt experimental since added complexity can add lots of cost in terms of labor if it goes very much beyond "I can do that myself".
- aerodynamic cowling shape may require drive shaft, and reliable drive shaft has been proven to be hard to design and manufacture such way that it would be 100% reliable
- the diesel engine is harder for the prop than a electric motor because of power pulses (even with clutch) and more expensive propeller is needed than would be needed with the electric motor alone.
There is however a case what has not been talked about for your case:
- planetary gear system for driving the electric motor and the diesel engine at the same time - Toyota Prius hybrid synergy drive thing. That is about bullet proof and single point of failure will not stop the prop, one motor is enough to continue driving the prop.
- This of course has associated weight penalty. On Toyota Prius it does not matter, but on aircraft it does matter.
Case for push-pull:
- To avoid drive shaft, the diesel engine would need to be the front engine.
- case for achieving any kind of laminar flow to the fuselage would be pretty much lost
- inefficiency problems on the rear prop because of the front prop. I have not quantified this on the other hand, apparently nobody is able to answer how much is the penalty, it is not even exact in literature.