How reliable is the media report?
Why are you surprised?
The J-20 engines have been unknown. This means it can be either Chinese or Russian.
China's WS-10A engine has been around at least since 2005. The original WS-10 engine has been around even longer.
Clearly, China has the capability to build a next-generation turbofan engine.
The WS-10A is based on DD3 nickel-superalloy technology.
The WS-15 is based on DD6 next-generation superalloy technology.
The WS-15 is widely expected to debut around 2018. However, it should not be a surprise if the prototype(s) is available a few years earlier. If things go well, the WS-15 is ready earlier. If there are unexpected delays, the WS-15 would take longer.
Thus, 2018 +/- 3 years is very reasonable. We are three weeks away from 2016. For the WS-15 to appear now is perfectly within the expected time window.
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Let's review the known facts.
1. The WS-15 core was first tested in 2005.
2. China has prior experience in building the WS-10A turbofan engine.
Thus, after ten years of work (2005 --> 2015), China could have the WS-15 prototype(s) ready. This is not their first turbofan engine. This means China's experience with the WS-10A turbofan engine should hasten the development of the WS-15. China should know what to expect.
WS15
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Here's the probability tree.
Unknown J-20 engine -----> 50% probability Russian
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v
50% probability Chinese -----> 25% probability WS-15 engine
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v
25% probability improved WS-10 variant
On Chinese television, footage was shown of a Chinese WS-10A November 10, 2005 test (see third picture below).