A new type of ultra-efficient engine has been attracting the attention of several big-name investors. The startup EtaGen, founded in 2010, announced recently that it has raised an $83 million Series C round of funding from investors including power company AEP, the venture arms of oil giant Statoil and energy company Centrica, Microsoft co-founder Bill Gates, and Vinod Khosla’s venture firm Khosla Ventures. The company has now raised $133 million, according to a press release. That a relatively young tech firm has been able to attract the interest of such high-profile investors is impressive.