Why is it better to put a diesel engine on a GA aircraft?

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: This is a frequently asked question, always popping up at the big shows, like Arlington and Oshkosh. I thought I’d answer it from my personal perspective. Please keep in mind the following comments may not be accurate from an engineering standpoint nor a true representation of current market surveys, but simply my personal thoughts regarding the subject.

My answer to the basic question: I’m not convinced that it is better to put a diesel engine on an aircraft than a conventional engine that is currently on the market.

First, let me make it clear that I do believe at some point in the future there will be an alternative fuels engine that will find its place in the general aviation aircraft fleet. For many years when the question would come up at EAA AirVenture, Sun ‘n Fun and other popular trade shows as to why Lycoming wasn’t certifying a diesel or other alternative fuels engine, my answer was somewhat harsh but realistic. Lycoming is capable of developing and certifying such a product, but someone has to want to buy it. It was at this point that the eyebrows would rise from those asking the question and further explanation was required.

If we allow logic to creep into the equation at this point, I believe it’ll be rather easy to understand where I’m coming from. Let’s say Lycoming were to develop and certify an engine that would use something other than Avgas. You can imagine the cost of accomplishing a challenge of this kind in today’s dollars, so this begs the question: where will the payback from this tremendous investment come from and how quickly will the company recapture this expenditure? What we’re asking here is, who will our customer be and how many units will they require in a reasonable period of time? This now causes a trickledown effect and means some airframe manufacturer has to want to purchase this new product. Who might this be? For conversation purposes, let’s take any well established airframe manufacturer from the industry today. For them to consider a radical move to reengine and/or introduce a completely new model they too must satisfy themselves that there is a viable market out there and it presents an opportunity for them to make a profit. We must never lose sight of the fact that money drives nearly everything that we choose to do, and putting a product on the market that will not generate a reasonable return isn’t going anywhere.

While we all can understand the popularity of a diesel type engine because of fuel availability and cost outside of North America, I don’t think the overall market demand is at a point to convince the big airframe manufacturers to lunge into a big push to bring to market a product that may not generate adequate sales for a reasonable return on their investment at this time. Maybe I can clarify what I’m trying to say this way. If you were one of the big airframe manufacturers I’m certain you’d recognize the fact that, let’s say, 70% of your annual sales comes from North America. If the remainder of your sales comes from the rest of the world, would you be confident that enough sales of this new generation aircraft would justify the costs of certification and production?

If the point is the cost of fuel, I’ll admit diesel outside of North America is more reasonable, but that reasoning doesn’t play in the U.S. at this time. With the cost of diesel now exceeding that of Mogas, who will our customer be, how many units will they require and where does that leave us? I understand that Avgas is still more costly, but let’s be honest — would you buy a new aircraft tomorrow if it burned something other than Avgas?

So what would you do if you owned an airframe manufacturing company and had to convince the shareholders why you wanted to make a huge investment of millions of dollars because everyone keeps asking why you don’t have a product that uses something other than Avgas?

Next issue: Our engines expert continues this discussion.

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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