In 2006, when he took a job as chief scientist for Wuxi Weifu Environmental Catalysts, a company in eastern Jiangsu Province, he also brought his wife and their two American-born children, in part, he says, because he wanted them to become steeped in Chinese language and culture.
His return coincided with a surge in domestic car production and government-led efforts to reduce tailpipe emissions. The company prospered, and so did Mr. Hu, who eventually became Wuxi Weifu’s president. It now provides catalytic converters for half of all Chinese-made cars.
Mr. Hu’s troubles began after his company refused to buy components from the Hysci Specialty Materials Company, which is based in Tianjin and once supplied Engelhard.
According to Mr. Hu and his lawyers, Hysci would not take no for an answer. They say Hysci’s well-connected chief executive, Dou Shihua, sent Tianjin public security agents to Wuxi Weifu to pressure Mr. Hu to change his mind.
The police raised allegations of stolen trade secrets but also suggested that the accusations would evaporate if the two companies did business together. Mr. Hu would not budge. “We have a system of quality control, and even one word from me could not change that,” he said.
In the end, the veiled threats gave way to an arrest, and Mr. Hu was put in a jail in Tianjin.
The patent infringement case that prosecutors eventually built against him cited technology that has been publicly available in the United States for decades, according to several scientists who rallied to his defense.
But even after prosecutors withdrew the case and Mr. Hu was freed, he found his return home blocked by immigration officials who claimed that he was still wanted by the Tianjin police. Each time he or his lawyer contacted the authorities there, however, they were told there were no such restrictions.
One of his lawyers, Wang Shou, said he believed that Mr. Dou, Hysci’s chief executive, was continuing to use his influence to exact revenge or get a deal yet.
Reached by telephone, a sales executive at Hysci refused to comment on the case. The Tianjin Public Security Bureau hung up before answering questions about Mr. Hu.
His family does not know what else to do. Although his daughter visited last summer, Mr. Hu’s wife and 16-year-old son are reluctant to come here, saying they fear they, too, could be prevented from leaving.
“I worry about my husband every hour of every day,” his wife, Hong Li, who is also an engineer, said by telephone from Los Angeles. “I don’t want my son to grow up without a father.”
The emotional anguish suffered by Mr. Hu has been compounded by pain from a herniated disc that worsened during the 17 months he slept on the floor of his jail cell.
Earlier this month, at a chemical engineering conference on the outskirts of Beijing, he lectured about ways to reduce emissions from heavy trucks in China.
As the conference wound down and his American colleagues headed to the airport, he made a joke about escaping across the border.
“If I could only invent something that would make me invisible,” he said.