Hyundai’s top research and development official, Vice Chairman Lee Hyun-soon, has left the auto maker.

Lee resigned for “personal reasons,” a Hyundai U.S. spokesman says.

Lee championed Hyundai’s move in recent years to more advanced engines, including the newly launched 2.0L direct-injected and turbocharged 4-cyl. found in the Hyundai Sonata, as well as sister brand Kia’s Optima and Sportage and Hyundai-Kia’s first hybrid system, also in the Sonata and Optima.

Lee additionally heralded the development of the auto maker’s Tau V-8, a Ward’s 10 Best Engines winner for three consecutive years.

As a young designer, Lee dreamed of building a world-class V-8 to replace the dated 270-hp 4.5L Hyundai sourced from Mitsubishi, he told Ward’s in a January interview.

Lee fought to have the Tau built, as he thought it was important for Hyundai to have a prestigious flagship V-8. But, as he said in the January interview, “not everyone agreed.”

Lee is credited with developing Hyundai’s first-ever in-house engine, the Alpha 4-cyl., in 1991. Prior to Lee’s arrival Hyundai sourced all its engines from Mitsubishi.

He joined Hyundai in 1984, after being wooed away from the General Motors Institute, now Kettering University, in Flint, MI, by Hyundai founder Chung Ju-yung. “Lee was a lecturer at the GM Institute (when) he got a call from Chung Ju Yung,” then-Hyundai spokesman Oles Gadacz told the American media in Seoul in 2007.

“(Chung) personally asked him to come work for the motherland and said, ‘I’ll give you a blank check, do what it takes to develop an engine.’ And Lee was all of maybe 30 years old (at the time).”

Lee asked GMI for a year’s sabbatical to return to South Korea and explore an “opportunity,” Gadacz related.

“He went to work for Hyundai and started the (Alpha) program, (and) after a year, he went back to GMI and still found his name on the door, his desk there and everything still waiting for him,” he adds. “But he had to sadly tell GMI he was not coming back and saw a great future here.”

Yang Woong-chul, current president of Hyundai’s Namyang R&D center outside Seoul, reportedly will replace Lee but keep his current title of president.