Bear with me 1 minute with a brief side note. Someone said that my previous post did not answer the question of Diesel engines vs Turboprops. And while I was writing this to clarify my other post was deleted so this one has to stand alone. Okay now let me answer the question directly so that nobody has to use the info to make their own comparison.
The Diamond DA-62 is a twin engine with each of the 2.0L turbocharged 4 stroke inline 4 Diesel piston engines putting out 180HP (360 combined) maximum and 378 lbs/ft (756 combined) of torque. 3887 engine max rpm, Equipped with a reduction box of 1:1.69 gives a maximum prop speed of 2300 rpm. Rated at 16.6 mpg but the Austrian data doesn't specify weather that's US gallon or the all but abolished Imperial gallon. So I have fuel burn rates here. Each engine is said to burn 9.25 US gallon/hr @ 100% load on the ground, 12.83 US gallons per hour @ 60% load and 12,000 feet operation, 7.4 US gallon @ loiter. The plane is 5 - 7 passenger with 1 pilot. Maximum payload capacity including fuel 1600 lbs. And yes this information is required to answer the question poised. Each engine weighs 410 lbs dry. The manufacturer has listed 1800 hrs average time to overhaul. Manufacturer reccomends Diesel and/or Kerosene (Kerosene is Jet-A. everyone knows they are the same basic fuel) These engines are adapted from Mercedes automotive diesel road engines. Cheap cheap cheap. Solid proven engine design cars have been known to achieve 1,000,000 miles with the original engine.
A similar size turboprop plane is a bit of an oddity but I did locate specifications on a Cessna Denali which is said to have a full of fuel payload of 1100 lbs. It burns 60 GPH but does not specify at what specific engine load nor altitude. It is a single turboprop with 1240 horsepower, weighing about 600 lbs dry. No specs as to the prop shaft speed or torque are provided but this is not a diesel engine so one can safely assume it will spin faster and provide lower torque output. The manufacturer lists 4000-6000 mean hrs between overhaul. They do advertise 20% less fuel burned than the same size competitors engine, I take this to mean another brand of Turboprop engine. Rated for use with jet-a type fuel. These engines are newly developed and very expensive.
The Cessna has a slightly higher maximum speed (285 kt)
then the Diamond DA-62 (192 kt maximum cruise speed with an absolute maximum of 205 kt) which justifies a slightly higher fuel burn rate. However not 3 times the fuel consumption as indicated by the fuel burn rate. I would like to see them use a pair of 2.5L, 2.7L, or 3.0L 5 cylinder Diesels, the performance would be closer to that of the single turbine. Especially with use of bigger engines more aggressive props could be fitted to raise the top speed by 80kt. The service ceiling of the turboprop engine is rated higher. Key word being rated. Who actually knows the maximum altitude the little diesels would continue to give adequate, safe, & reliable performance.
Turboprop - <50% higher advertised time to overhaul.
- faster and higher rated altitude.
Diesel - <60% less total fuel burned plus the ability to burn actual Diesel and Kerosene not just Jet fuel as specified by the engine manufacturer.
Diesel engine is cheaper how much though ? Nobody knows. As the turboprop is very new on the market.
Two engines of any kind are always safer than 1 in the unlikely event that you lose an engine you can still remain airborne for a period of time. Best land quick.
I hope this is answer enough to the question.