Starting with the 2014 Formula 1 season, the engines have changed from a 2.4-litre naturally aspirated V8 to turbocharged 1.6-litre V6 "power-units". These get a significant amount of their power from electric motors. In addition they include a lot of energy recovery technology. Engines run on unleaded fuel closely resembling publicly available petrol. The oil which lubricates and protects the engine from overheating is very similar in viscosity to water. The 2006 generation of engines spun up to 20,000 rpm and produced up to . For , engines were restricted to 19,000 rpm with limited development areas allowed, following the engine specification freeze since the end of . For the 2009 Formula One season the engines were further restricted to 18,000 rpm.