It is often asserted that series hybrids are not as efficient as parallel hybrids (citation needed) but this is only because they are being built with inefficient reciprocating internal combustion engines. The obvious solution is to use microturbines. Chrysler was able to make tolerably reliable and functional turbine vehicles starting in 1963, whose primary faults were short drivetrain life, high drivetrain noise though not high turbine noise, manual starting sequence, and high NO (nitrogen oxide) output, though all other emissions are very low.
These incredibly heavy vehicles nonetheless had excellent performance and efficiency characteristics for their day, and if some of these problems could be solved they would be excellent for use in small vehicles, with a correspondingly smaller turbine. One possible solution is a miniaturized version of the Chrysler Turbine Engine with an integrated generator rather than any output drive shaft. One of the major missing pieces for such a project is a high-temperature magnetic bearing such as those now being developed in a NASA/Army program. If this project bears fruit, it may eliminate the most serious barrier to the use of turbines in street applications. The military's interest is based on their desire for more fuel-efficient military vehicles, both in combat and non-combat roles.