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Given that the traditional four-stroke Otto-cycle engine piston engine only has a thermal efficiency of 25-30 percent, there is clearly still plenty of room for improvement. While most of the green automobile attention in recent years has been focused on electrification, liquid fuels still have about 100 times the energy density of today's best lithium-ion batteries, a difference that probably won't change significantly any time in the near future.

Volvo announced last week a prototype flexible fuel engine that can burn five different types of fuel: E85, methane (made from either natural gas or plant sources, so Volvo is counting this as two fuels), regular petrol gasoline or hythane, (which is 10 percent hydrogen and 90 percent methane). The vehicle, called the Multi-Fuel, was unveiled at the Michelin Challenge Bibendum, the "international sustainable mobility event".