One commercially viable solution is that one of the Toyota Prius. The basic idea is the following (apparently there a few changes over the years since 1997 introduction):
Figure 1: Toyota prius hybrid system layout (source roperld)
Here you can see MG1 and MG2 which can act both as motors and as generators. From what I understand MG1 utilizes this additional speed of the engine to produce electricity by acting as a generator. The electricity is fed to MG2 which acts as an electric motor and provides torque to the wheels.
Additionally MG1 (Primary motor-generator), is used to start the ICE. By regulating the amount of electrical power generated (by varying MG1's mechanical torque and speed), MG1 effectively controls the transaxle's continuously variable transmission.
And MG2 (Secondary motor-generator): in electric only mode is used as a motor to drives the wheels and additionally is used in regenerative braking as a generator to produce power for the battery energy storage while braking the vehicle.
Figure 2: Toyota prius power split system (source nadascientific.com)
Finally , I agree with the opinion that this a bit too much as a project to undertake on its own.
There is a lot of literature on the web, just describing the toyota prius system (and I am guessing you can find a lot more for other cars)