The iconic Jeep Wrangler is a tough vehicle to make fuel-efficient.
Its separate frame and body add weight, and it has all the wind-cheating grace of a small municipal hospital.
The Wrangler SUV only gets redesigned every 10 years or so, so each new generation is a big deal to Jeep fans.
DON'T MISS: Turbo-4 to boost fuel economy in next Jeep Wrangler (May 2016)
With tougher fuel-economy requirements and more stringent safety standards, the next generation has likely been a tougher design challenge than ever before.
We expect the new Wrangler to be revealed sometime over the next year.
It's long been known that one of the routes the new Wrangler will take to comply with EPA fuel-economy rules is to offer a diesel engine as an option.
Alleged photo of the 2018 Jeep Wrangler Unlimited – Image via JL Wrangler Forums
Fiat Chrysler said so itself, in fact, more than a year ago. Now, it appears that engine is already being tested in public.
A post on fan site JL Wrangler Forums includes both new spy photos and details on the diesel Wrangler from an official Fiat Chrysler dealer meeting held recently at the company's headquarters in Auburn Hills, Michigan.
READ THIS: Jeep Wrangler Diesel To Come Well Before Wrangler Hybrid (May 2016)
According to the story, the Jeep Wrangler Diesel is on schedule to debut as a late 2019-model-year vehicle, likely sometime early in 2019.
One of the site's readers sent photos of a camouflaged 2019 Jeep Wrangler (the shape is much like the old one, surprise, surprise).
2014 Ram 1500 EcoDiesel
But as the story notes, the reader figured out that it was the diesel version under test—and even crawled under the vehicle to snap a photo of the tank for Diesel Emission Fluid.
That's the substance that allows the latest diesels from all makers to stay within EPA limits on emissions of nitrogen oxides (NOx).
Diesel passenger cars and light trucks have been under increased scrutiny from the EPA since the Volkswagen diesel scandal broke in September 2015, but Fiat Chrysler is having particular problems with its EcoDiesel Ram 1500 and Jeep Grand Cherokee at the moment.
CHECK OUT: EPA says Ram, Jeep diesel emission software violates Clean Air Act (Jan 2017)
The EPA said in January that those vehicles' emissions were over the limit, but Fiat Chrysler has fought back aggressively and the matter remains unresolved.
With a slow start on any form of electrification, Fiat Chrysler desperately needs diesel engines to keep its truck-heavy lineup within corporate average fuel economy guidelines for 2019 and beyond.
It's notable that a future hybrid version of the Jeep Wrangler is expected to appear, too, but only a considerable time after the diesel version goes on sale.