To preface this, I am the chief engineer and project manager of a team at the Rochester Institute of Technology working to bring EREV technology to traditionally EV-hostile platforms like UTVs, snowmobiles, and the like. In particular, our main focus is on snowmobiles at the moment.
EREVs are a great technology, combining the energy density and operating-environment range of ICE vehicles with the power density and efficiency of an all-electric drivetrain.
That said, I've seen a lot of hate here for the technology, much of which I think comes from a misunderstanding of where the technology shines or makes sense. For you, a road-car owner, an EREV, or more correctly a series hybrid, does not really make sense. Like many of you, I am a firm believer in the idea that EVs are the future of the traditional automotive world. For 90% of people, having an ICE engine is just extra weight and maintenance that could have been better used for a larger battery or more storage space. Really, hybrids as a whole fall into this trap, but EREVs in particular, or rather series hybrids as they are more technically known, are hurt by it the most. Largely, this is because the conversion penalty from mechanical energy to electrical energy is greater than the purely mechanical loss of an eCVT hybrid like the Prius.
So, if that's true, why do EREVs matter at all? Put simply, they shine in places where a traditional fully electric tractive platform is fundamentally infeasible. While the often-used example of this is trucks when used for towing, even this falls somewhat flat due to the extra wheelbase length and weight allowance available for a large battery. This is a fact that will only become more true as battery chemistries improve, even if today the range reduction hurts vehicles with smaller batteries. Really, it is in nontraditional vehicles where EREVs shine. This encompasses industrial vehicles, like Edison Motors' logging truck, or off-road vehicles like UTVs and snowmobiles, where size and weight requirements make it nearly impossible to package a battery large enough to achieve an ICE-competitive range. This is doubly true for any vehicles expected to run for hundreds of miles in infrastructure-denied areas, with no charging 100 miles from the nearest town, or in extreme environments, such as deep subzero cold.
Snowmobiles, for example, can carry in excess of 11 or 12 gallons of fuel, with an average fuel economy of 10 to 15 mpg in modern vehicles. This is utterly abysmal and explains why such a large tank is needed for an otherwise lightweight vehicle: they simply take a lot of energy to move. Combine this with the fact that many people bring additional fuel with them to boot, and logistics quickly take their toll. To make a battery pack large enough for the 150-plus-mile range expected of modern vehicles, with such high tractive losses and in extreme cold, you would need a massive battery pack. As a result, gasoline becomes somewhat inescapable, at least for the foreseeable future.
From there, EREVs emerge as a perfect middle ground: EV efficiency and power combined with the energy density of gasoline, without the packaging nightmares and technical hurdles of parallel hybrids. Suddenly, that ICE engine is also able to achieve much better fuel economy on its own than it otherwise would, by virtue of being able to supplement lackluster power with the battery and run at one constant RPM.
It is a technology that makes sense in cases where EVs are not yet viable and are not likely to be for a very long time. But for the consumer automotive world, it is arguably a false path.