General Motors and Ford Motor Company plan to add gas-electric hybrid sport-utility vehicles by 2004 as the world's two largest automakers strive to improve fuel economy. GM plans to start selling a hybrid SUV in 2004 that would boost fuel economy by 20 percent. Ford in 2004 plans a version of its Explorer SUV with a higher-voltage electrical system, with a technology that shuts the engine off when the vehicle is stopped. Ford's goal is 27 miles per gallon, compared to 16 to 22 mpg for the conventional 2002 Explorer. Automakers have been looking at the gas-electric technology as a way to improve fuel economy and lower emissions, and are under pressure from tightening federal and state regulations, especially in California. Ford last year said it would increase the fuel economy of its SUV fleet by 25 percent in five years; General Motors has responded it will top any of Ford's improvements. GM said its gas-electric technology, which it calls ParadiGM, will be applied to its global Epsilon mid-size platform, which will be used on sedans, minivans, crossover vehicles and light trucks as well as SUVs. The hybrid SUV that GM plans to start marketing in 2004 will combine a V-6 or a four-cylinder engine with a pair of electric motors and a battery pack. GM will use a 42-volt battery system. Ford's start-stop engine system that will be used on Explorers is known as an integrated starter-generator. The system replaces a conventional vehicle's starter and alternator. Though the Escape hybrid SUV that debuts in 2003 will be powered by both an electric motor and four-cylinder gasoline engine, Ford's gas-electric Explorer will have a six-cylinder gasoline engine assisted by a 42-volt battery. The new Explorer also will have a system that recharges the battery while the vehicle is braking. The 42-volt battery will provide the vehicle with a small acceleration boost. The Explorer also will have a 12-volt battery -- the current standard in vehicles -- to operate electronic systems such as emergency flashers and the radio. The start-stop feature debuted on Ford's P2000 HEV concept car and will be used in production for the first time on the Escape hybrid.