There is a wide variety of engines available to produce the required thrust.  The best propulsive efficiency and range are provided by jet engines which are the preferred method of thrust production at this time. Electric brushless motors have demonstrated proven performance but are still seriously limited by the low energy density of today's battery technology.  Furthermore, serious heat and internal battery resistance issues have arisen when power levels are run at the sustained high power output needed by proposed eVTOL aircraft during taking off. These battery capacity and cooling issues have limited the few flying examples of actual maned eVTOL aircraft to less than 30 minutes of flight time. However, a hybrid approach of using jet-fueled or hydrogen-powered turbojets for takeoff and landing along with the option of electric power for forward flight propulsion allows for an ultra low emissions aircraft without sacrificing range. Additionally, standard turbojet engines can run interchangeably on diesel, kerosene, Jet-A or hydrogen and can even get a sizable boost in thrust from water injection.  When considering low emissions fuel sources another advantage of hydrogen is its energy density. Diesel has an energy density of 45.5 megajoules per kilogram (MJ/kg), slightly lower than gasoline, which has an energy density of 45.8 MJ/kg. By contrast, hydrogen has an energy density of approximately 120 MJ/kg, almost three times more than diesel or gasoline. In electrical terms, the energy density of hydrogen is equal to 33.6 kWh of usable energy per kg, versus diesel which only holds about 12–14 kWh per kg. What this means is that 1 kg of hydrogen, used in a fuel cell to power an electric motor, contains approximately the same energy as a gallon of diesel.  Hydrogen used in fuel cells has an energy-to-weight ratio ten times greater than lithium-ion batteries. Consequently, it offers much greater range compared to batteries while being lighter and occupying smaller volumes.