Fracking for natural gas

In this Jan. 17, 2013, photo, Universal Well Services fracturing engineering team leader Mike Michaelson points to the 50 liter, 16 cylinder, Cummins diesel engine with four turbo-chargers that has been converted to run on a blend of diesel and natural gas at the Cummins Bridgeway facility in Gibsonia, Pa. Oil- and gas-field companies from Pennsylvania to Texas are experimenting with converting the huge diesel engines that operate pumps that propel millions of gallons of water, sand and chemicals down a well bore in the fracturing process to break apart rock or tight sands that trap natural gas.

Credit: AP Photo/Keith Srakocic