Perhaps the most debated feature of the Volt is its fourth mode of operation, called "Extended Range Combined 2 Motor Driving"
Through all of the Volt-related stories on here, there is sometimes a vocal minority that declare the Volt is not an EREV. They claim they were lied to, cheated, and robbed of the promise of a pure series hybrid experience. They reference articles from third party websites that "prove" the Volt is merely a parallel hybrid, and they continue to criticize its design as a result of this mode.
Back to Basics: "EREV" Defined First, let's take a step back for all of our casual readers that may not be familiar with this terminology. The term "EREV" was invented by GM. It stands for "Extended Range Electric Vehicle" and was coined specifically for the Volt. The term describes a vehicle that is capable of some amount of electric range regardless of speed and acceleration - as long as the battery has charge remaining - without the use of any gasoline. Following depletion of the battery, a gas engine turns on to seamlessly provide additional power to take the vehicle as far as it needs to travel, as long as there is gasoline in the tank. Proponents of the technology love that they can perform their daily driving without using any gasoline, while also having the ability to take a long trip using gasoline, without needing to stop and recharge. The term EREV has more recently become synonymous with a "series hybrid" which I also describe below.
The Toyota Prius, the best selling parallel hybrid on the market (without a plug), requires use of the engine for heat, fast acceleration, and high speeds.
"Parallel Hybrid" Another term is "parallel hybrid" which describes the typical hybrid vehicles available on the market today. A parallel hybrid typically needs the engine at least some of the time to perform under various accelerations and speeds. The most common example of a parallel hybrid is the Toyota Prius. The electric motor operates "in parallel" with the engine, which is very efficient, but requires use of the engine the majority of the time. Under certain speeds and accelerations, the engine is not required, but when higher speeds or acceleration is needed, the vehicle can not provide these demands without use of the engine. Here, the electric motor is not the only source of kinetic power; the engine also directly provides kinetic power, much like traditional vehicles.
"Series Hybrid" And the final term I want to touch on here is a "series hybrid," which describes a hybrid vehicle that has its engine "in series" with the electric motor. Additionally, the engine is "behind" the motor, in that it only creates electricity, and the electric motor(s) are solely responsible for propelling the vehicle. Examples of series hybrids include the BMW i3 (with range extender option), as well as the Via Motors lineup of vehicles.
Which one is the Volt? So the big point of contention is, where does the Volt fit in? Is it a parallel or series hybrid?
Well, the answer is really a little of both. For this reason, as I started out saying, GM coined the term "Extended Range Electric Vehicle" and since they defined it to describe the Volt, the Volt is by definition an EREV.
However, first and foremost, the Volt is a series hybrid, and here's why: When the battery has charge remaining, the Volt provides full power via its electric motors regardless of acceleration or speed. Floor the accelerator to 100mph, and the engine never turns on. Additionally, once the battery is depleted, the Volt always operates in series mode when under approximately 37mph. In this mode, one motor works in unison with the engine to create electricity, and the other electric motor uses this electricity to propel the vehicle. At speeds above 37mph - when maximum power is needed - the Volt still operates as a series hybrid. As such, the vehicle is mechanically capable of operating as a series hybrid at all speeds, and in fact does so whenever power demand at higher speeds is at its highest.
The engineers designing the Volt could have stopped there, but purely to increase fuel efficiency (think cylinder deactivation in a V8), they designed in a virtual parallel-like mode that some EV aficionados take exception to. While the Volt is 100% capable of being propelled in series mode at all speeds and accelerations, at speeds above 37mph and under reduced power loads, the Volt can indirectly link the engine to the wheels through one of the electric motors. By doing so, some inefficiencies of conversion are eliminated. Rather than converting the mechanical energy of the engine to electricity, then converting the electrical energy back to mechanical energy to propel the car, the engineers provide this indirect mechanical link to the wheels: the engine is coupled to one of the electric motors (which normally acts as a generator), and that electric motor instead turns the drive shaft. Doing this provides the same "electric" feel, but results in a 10-15% efficiency improvement under these load conditions, by avoiding the conversion losses from mechanical to electrical energy and back. Again, thinking back to a V8 with cylinder deactivation, nobody would ever claim such a vehicle is anything other than a V8 despite 4 cylinders being deactivated at higher speeds and lower power demands to save fuel. In the same way, the Volt is first a serial hybrid; it doesn't perform this virtual "parallel" operation because it needs to, only because it saves fuel... and if you demand more power by flooring the accelerator, it resorts back to series operation, just like that V8 would reactivate its remaining 4 cylinders under the same scenario. The video below is one of the best I've seen describing the various modes of operation that comprise the Voltec drive train, which is a great animation of the modes I've tried to summarize.
Now for some unexplained reason, there are a few EV advocates that take great exception to this additional mode that is purely there to increase efficiency and minimize gasoline use. Somewhat ironically, they would apparently rather the Volt operate as a pure series hybrid all the time and burn more gasoline, rather than providing the same electric drive and smooth torque with greater efficiency at higher speeds and lower engine power requirements. This continues to baffle me and many others, who recognize the potential the Volt has to eliminate daily gasoline use while also maximizing efficiency for longer trips at higher constant speeds.
Some hate the Volt because Obama likes it. Others hate the Volt because of the bailout, and still others hate it because its not "always" a serial hybrid. Can't we all instead like the vehicle for what it is? Clearly owners love the car, and they should know best right?
The fact that the Volt is 100% capable of providing full acceleration and top speed using only its electric motors makes it just like an electric vehicle for the first 40 or so miles. (See Footnote 1 below) This is untrue of many other plug-in hybrids such as the C-Max Energi, Fusion Energi, and Plug-in Prius. Unlike the Volt, they will require the use of the gas engine to avoid sacrificing acceleration and/or to reach top speed.
Additionally, the Volt is 100% capable of providing full acceleration in series mode once the battery is depleted, with very few exceptions. The Volt doesn't stop there though, and implements a virtual "parallel" mode at higher speeds and lower power demands... not because it NEEDS to do so, but because it saves even more gasoline without sacrificing performance. The only other main competitor to the Volt, the BMW i3 with range extender, does not have the same capability to provide the same performance under extended range operation. The engine is simply not sized for the majority of conditions, and the "pure series" configuration further prevents some efficiency gains under higher speeds.
In Summary... As a result, the Volt (and its sister, the Cadillac ELR) are really the only two nationally available and commercially viable "Extended Range Electric Vehicles" on the market today. They provide meaningful all-electric range, and no sacrifice in performance once the battery has been depleted and the gasoline engine kicks in let you travel as far as you desire. No other mainstream vehicles on the market can make this aggregate claim, and that's what makes the Voltec technology so promising. (As noted above, the BMW i3 with range extender comes close, but its engine is too undersized to provide the same performance over the majority of driving situations once the battery is drained). The Voltec technology is the "gateway drug" that will allow mainstream consumers to fall in love with pure electric drive, and eventually purchase full electric vehicles once ranges are sufficient and fast charging is prolific. This capability has led to over 72% of Volt miles being traveled purely on electricity alone without a drop of gasoline, and an average of over 120 miles traveled for every gallon of gasoline used (source: VoltStats.net).
in its segment.
TL;DR for the Mass Market Consumers For the average consumer that doesn't care to learn the terminology, simply know that the Volt is arguably the only affordable solution to a pure electric vehicle with sufficient gas-free range for the majority of American commuters and their daily driving, with the ability to continue driving beyond that range with no stopping or performance degradation as long as you have gasoline in the tank.
So to the few vocal EV advocates, let's stop criticizing the Volt's virtual "parallel" mode like it is as an Achilles heal. It provides greater efficiency with the same electric feel, winning more awards (and some more listed here) in the last 5 years than I can count. Instead, take the time to understand the technology, and educate potential consumers on the potential it holds: gas free daily driving, and the ability to take a 1,000 mile road trip without a second thought. There's a reason why the Volt continues to be one of the most loved vehicles, and as such, the Volt is one of the best "gateway" technologies to help mainstream consumers begin to transition to purchasing full electric vehicles.
Footnote 1: In sub-freezing temperatures, the Volt will occasionally cycle its engine on and off to provide added cabin heat when the battery still has remaining charge. This requirement is apparently an artifact of outdated rules requiring engine use to help defrost the cabin more quickly. The Volt is still capable of heating the cabin without the engine using its resistive heater, but the regulations for defrosting have required engine use despite it not being strictly necessary for typical operation. Even so, this still results in much less gasoline use than other plug in hybrids like the Plug in Prius, which require the engine to be running whenever heat is requested, regardless of outside temperature.
Footnote 2: The author of this story has no affiliation with GM, professional or otherwise. He has no relatives, friends, or acquaintances that work at GM. Additionally, he does not have any financial investments in GM. He does own a Volt though, and it is hands-down the best vehicle he has ever owned. As a result of the Volt, the author will never go back to a vehicle that doesn't have a plug.