This has become a familiar storyline... During a series of trips to Africa last year, Tim Solso had a realization: China was beating him at his own game. So the chief executive of Cummins Inc., a maker of truck and machinery engines, vowed to catch up. He plans to quadruple the company's sales in Africa to about $1 billion within five years, investing $15 million annually to train staff and build sales offices from Johannesburg to Casablanca. The company recently installed in South Africa an executive to oversee Africa operations, previously supervised from Europe and Indiana. Cummins joins a growing number of U.S. companies vying for a stronger foothold on the continent. Caterpillar Inc., the giant maker of construction equipment, is selling more trucks to Mozambique and Zambia. Harley-Davidson Inc. is opening dealerships in Botswana and Mauritius. General Electric Co. has its first aircraft-leasing office in Ghana for Central and West African airlines. Google Inc., Archer Dan...