FRANKFURT, Germany — Ban the sale of gasoline and diesel cars by a deadline — 2040, 2030, even 2025. More and more governments are proposing just that.But how seriously can such deadlines be taken?The issue of how to phase out polluting traditional engines has been pushed to the forefront by scandals and crises. First Volkswagen's admission to cheating on U.S. diesel emissions tests, and more recently a push by cities in Germany and elsewhere to ban diesels to make the air cleaner.The political desire to switch to get rid of traditional engines, however, runs into a number hurdles in the real world.