Combustion engines are so fundamental, they go far beyond fossil fuels. They are one of the best ways of converting chemical energy to work. Fossil fuels just happen to be very high in chemical energy. You can burn hydrogen, ethanol, bio-diesel, paraffin, methanol....<p>IMO the better combustion engines become, the more options for fuel we have. For example, ethanol compares poorly to gasoline in power &amp; consumption, but with continued engine development, ethanol will be able to develop as much power &amp; economy as gasoline does today.