Hydrogen is being researched as a fuel for vehicles because it produces no carbon dioxide emissions. If hydrogen fuel becomes more popular, it has the potential to be less expensive than other fuels, if low-cost production via electrolysis can be implemented. Hydrogen can be used and created in fuel cells to power electric motors or burnt directly in combustion engines. BMW has developed a bivalent internal combustion engine that can switch between petroleum and liquid hydrogen fuels. In gaseous form, hydrogen is difficult to store and has a low volumetric energy density. It can be produced in many different ways, but many of the methods produce carbon dioxide. The most promising method is electrolysis. Safety of the hydrogen storage tanks in the event of an accident has been investigated, and various tests show that they do not present any problems. When used as a fuel, hydrogen has a wide range of flammability and low ignition energy. These properties allow hydrogen to be burned using a wide range of air-fuel mixtures, but problems arise with premature ignition. Crankcase ventilation is very important when burning hydrogen, because of the low ignition energy. Proper ventilation is needed to prevent ignition in the crankcase and the formation of water in the engine oil. Hydrogen and natural gas are very similar as fuels, so the differences between the components needed to burn them are trivial, and interoperable systems are easily made.