Volkswagen rigged emission tests on about 2.8 million diesel vehicles in Germany, the country’s transport minister said on Friday, nearly six times as many as it has admitted to falsifying in the United States. The board is widely expected to name Matthias Mueller, the head of its Porsche sports-car division, as chief executive to replace Martin Winterkorn, who quit on Wednesday, and at least four senior executives are expected to be purged.