General Motors and ExxonMobil announced yesterday what they called a breakthrough in fuel-cell technology for automobiles. The companies have developed a new system that converts gasoline into hydrogen that is used to run a fuel cell, which in turn produces electricity to power a car or truck. According to the companies, the system is twice as efficient as conventional gasoline-powered engines and emits 50 percent less carbon dioxide and significantly less carbon monoxide and nitrogen oxides than standard engines. GM and ExxonMobil plan to put the system into a test car within 18 months and predict that it will be used in hundreds of thousands of cars, trucks, and buses within 10 years. “This technology is a bridge between today’s conventional gasoline-powered vehicles and where we believe we are eventually headed, which is hydrogen-powered vehicles that will be fueled by hydrogen refilling stations,” said GM Vice President Larry Burns.