improvement in fuel economy, and current engine designs can be adapted rather inexpensively.
Keep in mind, however, that this is a technology that would go well with a series hybrid (aka range extended EV), where the engine "just" needs to do is be able to smoothly and reliably start and reach an efficient operating RPM to generate
electricity. If that were to happen it could increase fuel economy by a lot more. Perhaps we'll see it in the second generation Volt?
GM hopes to reach production with this technology in about 5 years.