Hydrogen fuel cells are a more efficient way of powering cars than traditional gasoline.  Hydrogen fuel cells still use fuel but they are similar to a mix of combustion engine and electric engine.  The fuel cells use pressurized hydrogen to power an electric motor instead of using gasoline as fuel.  The only problem is that combustion engines are made cheaper and safer.  Another problem is that not many people know how to service these types of cars.  It may be a more efficient method of fuel, but it will take a while until prices come down that they are common.

This relates to engineering because engineers had to think of a way to have a reliable fuel source and they are trying to make them cheaper.