Diesel engines will never be the best choice of engine for helicopters, unless the objective is to have a helicopter whose sole purpose is long range flight.
If any of the manufacturers were to design a new helicopter today they would not choose a diesel engine over a Ly-Con, Bombardier or turbine. For the simple reason that the weight of a diesel engine in a helicopter would marginalise the performance of the machine to such an extent that it would not be worth bothering! Yes, Avgas is expensive and yes, helicopters burn rather a lot of it, but even in 'rip-off-Britain' the fuel cost is typically only ~25% of the true operating cost of the machine. Further, if we all switch to Jet fuel then how long do you think it will be before the government bump up the tax on jet fuel for GA customers!
Helicopters designed and built today will be in service for at least 2-3 decades and so critical design decisions like the one relating to the powerplant must be made to create the best helicopter possible and not pander to current tax laws for an apparent 'easy-saving' on DOC. Imagine what would happen if the government increased the tax on Jet fuel for GA users and a manufacturer had just built a new machine with an over-worked aero-diesel engine (to get a sensible power-to-weight). The machine would be more expensive than a Gasoline/Avgas equivalent, would have less payload, perform worse, require more maintenance and would likely be less reliable (for a number of reasons)...then the tax goes up and all of a sudden its no cheaper to operate either! Just food for thought…
Also if a manufacturer was going to design a new engine for a GA helicopter it would not be a diesel engine - as they will always be much heavier than any of the serious alternatives. It is possible to design modern gasoline engines that are very efficient (in comparison to existing aero 2-stoke diesels and in particular Ly-Con engines) and there are many other propulsion technologies that provide interesting opportunities.
Diesel cycle engines are not the way forward for helicopters, as we will all see in due time.
Hope this helps
Christmas CRAN
