Diesel engines and GA: A good combination?

QUESTION: What is the state of art and the development of diesel engines for GA? Why is it better to put a diesel engine on a GA aircraft than a conventional one?

Alex Rougier
Turin, Italy

ANSWER: In the last issue, we started our discussion of this topic (Why is it better to put a diesel engine on a GA aircraft? June 9 issue). Let’s continue on the subject now from a little different approach. As I alluded to earlier, Lycoming is very capable of developing and certifying an engine of the type the general public keeps talking about, however there are some challenges that come along with the project for all parties involved.

First of all, as everyone knows, weight is a critical issue and every airframe manufacturer insists on weight reduction wherever possible. The characteristics of a diesel engine almost require it be built heavier than its gas engine counterpart. Most, if not all, are liquid cooled, so again we’ve got additional weight for the coolant radiators, plumbing, etc., not to mention the additional potential risk of leaks.

Another area that may present some real challenges is dealing with the vibrations produced by a diesel engine. This would require serious consideration from an engine mounting standpoint and, more challenging, for the propeller and its mounting to reduce vibrations greater than those we deal with in current configurations.

This reminds me of a story about a guy who went for a ride on a friend’s new Harley-Davidson motorcycle for the first time after riding a big Honda for many years. When he returned it to his friend he mentioned the fact that his feet kept vibrating off of the footboards. His friend quickly and bluntly advised him that it wasn’t a vibration that caused his feet to move, but rather the “”power pulse”” from that great Harley engine. I guess you could say our industry will have to learn new techniques to deal with the “”power pulse”” when and if we choose to go with a diesel.

If we look at fuel consumption alone, I think we can agree that a diesel is more fuel efficient, so that’s a given. Owners always want to know what the TBO (time between overhauls) is on their engine. I’m not certain that a new generation engine would offer as good as or better TBO than we currently have since TBO times are typically based on past experience, and the gathering and digesting of information over several years, in order to arrive at a realistic TBO time.

Please understand I’m not here to pooh-pooh any advancement in technology, but I’d like to think I’ve approached this subject from a practical, realistic angle to let you get some idea of how an airframe manufacturer might have to approach the subject in making any decision to proceed from current technologies. I really do believe that other powerplants will be used in our industry at some point in the future, but I don’t think we’ve reached that point just yet.

Remember, these are my personal thoughts on the subject and are not intended to reflect the opinion of anyone other than myself. For those of you who have broached this subject with me over the years at various shows, you’ll remember my comment that we’d never see a diesel engine in a series production general aviation aircraft in this country during my working career. I consider my working career ended when I retired from Lycoming nearly three years ago, so I guess I was safe in making that comment. However, I’m hopeful that sometime soon you’ll be able to tell me I was full of it and that there is, in fact, a series production aircraft being mass produced and selling in large quantities in North America and throughout the world. At that point, I’d say, we’ve all met the goal of witnessing a brighter future for the industry that we all love so much.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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