The Carlyle Group will purchase a significant minority stake in Quest Global Services, a leading engineering services firm, for an undisclosed sum, the two companies announced on Aug 8.

According to sources, the private equity firm will invest $500 million through funds managed by Carlyle Asia Partners to acquire a 28% stake in Quest Global Services at a valuation of $1.8 billion, displacing existing investors Bain Capital, Advent Capital, and GIC of Singapore.

Aside from Carlyle, Quest's founder Ajit Prabhu will invest $120 million to increase his stake in the company from 32% to 40%. According to insiders, the company will also invest $180-$200 million in a share buyback.

After two decades, The Carlyle Group is investing in Quest for the second time. The US fund invested $6 million in Quest Global in 2003 for a minority stake.

"The long association was one of the factors in choosing Carlyle," Ajit Prabhu, Chairman & CEO of Quest Global, said. "In the past, they assisted in making introductions when we were young." We now anticipate that they will assist us in propelling our next phase of growth. At Quest Global, we believe that engineering has a unique opportunity to solve today's problems."