Prime Number: 25

OK. 25 is not a prime number.* But it is the electric range, in miles, of the 2017 Prius Prime plug-in hybrid. (We couldn’t resist.)

First a bit of history to set things up.

Toyota brought out the first Prius Plug-in, based on the third-generation Prius, in 2012. The third-gen Prius came out in 2009 as a 2010 model year vehicle. In 2015, the fourth generation Prius—a whole new design, a whole new platform—was introduced as a model year 2016 car. At that point there was no plug-in version, just the main model that is called the “Prius Liftback.”

But now, there’s the 2017 Prius Prime, a plug-in, that’s based on the same TNGA architecture of the fourth-gen Prius. And although there are modifications to the Prime compared to the non-plug in model—as in being 4.2 inches longer—it also has a hatch configuration (but, as we’ll see, a significantly different one), but it is not called a “liftback,” as that is the nomenclature for the Prius Liftback. (I will admit to having been a little confused.)

Like the Liftback, the Prime has the same 1.8-liter, all-aluminum, DOHC, four-cylinder, Atkinson-cycle engine. It has the same suspension system, MacPherson struts in the front and a trailing arm-type double wishbone setup in the rear; however, the spring rates are optimized for the Prime, as it and the Liftback not only have that difference in length, but mass, as well (the Liftback curb weights range from 3,010 to 3,080 pounds, depending on trim, while the Prime ranges from 3,365 to 3,375 pounds).

But then things get different, whether it is obvious—like the addition of a charging port on the opposite side from the fuel filler opening (plug in on the passenger’s side; fuel on the driver’s side) or the fact that the Prime is a four-place vehicle rather than the five-passenger setup of the Liftback or the wholly new front and rear fascia and backlight or the available 11.6-inch touchscreen that’s located in the center of the instrument panel—or evident only from the engineering point of view.

A primary difference vis-à-vis the engineering is predicated on the ability to charge the 8.8-kWh lithium-ion battery pack from a standard outlet (Toyota recommends a dedicated GFI 15-amp outlet) or from a Level 2 charger (the battery charge time is <5.5 hours on the household plug and two hours, 10 minutes with the Level 2 charger). (The Prime has a new charging system: the previous plug-in has a charger output of 2 kW; the Prime is at 3.3 kW. And the volume of the charger is significantly smaller, 3.8 liters in the Prime compared with 8.0 liters). There was also the requirement to extend the electric vehicle (EV) driving range and speed at which it can occur compared with the previous-generation plug-in. The Prime is capable of traveling 25 miles on electricity alone (versus 11 miles for the 2015 model) and at up to 84 mph in EV mode (versus 62 mph).

Chief engineer Koji Toyoshima and his team recognized that they had to do what it took to optimize the powertrain system of the Prius Liftback for the Prius Prime.

For one thing, they had to consider the size of the battery. Obviously, the bigger the battery the greater mass it has and it takes more space. So they had to select a suitable range for the vehicle. Realize that the vehicle has an EPA-estimated fuel efficiency of 55 mpg city/53 highway/54 combined, so even without the plug-in capability it has quite a range (the fuel capacity is 11.3 gallons). The vehicle is rated at 124 MPGe from the standpoint of using the electrical power alone.

The Prime, like other Prius models, uses an engine (the aforementioned 1.8-liter) and dual motor generators, designated MG1 and MG2. For the 2016 Prius Liftback they made improvements to the transaxle, such as switching to a double-axis motor generator, boosting RPMs, employing a parallel axis reduction gear, and using a smaller-diameter motor.

For the Prius Liftback, only MG2 can be used as a drive motor. MG1 in a Prius has always been used to start the engine and to recharge the battery. But for the Prime, a one-way clutch between the engine and the transaxle, which fixes the engine axis in place, allows both MG2 and MG1 to be used as drive motors. The dual-motor system, consequently, reduces the need to start the engine while providing good EV acceleration.

Toyoshima emphasizes, however, that they also addressed other factors relating to efficiency, including aerodynamic efficiency (they’re using active grille shutters and aero stabilizing fins on the exterior), thermal efficiency (the HVAC system uses a heat-pump), and package efficiency (although the high-capacity lithium-ion battery, which is located in the trunk, as with the 2015 model, is twice the size of the previous battery, thanks to the use of high-density cells, the size of the battery pack is not twice as large: in terms of EPA cargo volume, the Prime is at 19.8-ft3 while the 2015 model is at 21.6-ft3, a reduction in capacity, but not much relative to the increase in battery performance—Toyoshima also points out that the battery case is now aluminum, so there is a weight save compared to the steel case of the previous model).

One main difference between the Prius Liftback and the Prius Prime is found in the back of the vehicle, a difference in aesthetic, mass and aero. While the taillamps on the Liftback are strongly vertical, on the Prime they are horizontal, traveling along the top of the decklid and tucking under. However, what is more notable, visible and unique is the “dual-wave” rear glass design, which Toyoshima says serves an aero function as it channels the air traveling across the surface of the backlight.

Was this done as a means to improve the aero count (the coefficient of drag for the Prime is 0.25; the Liftback is better, at 0.24) or was this something the design team created for purposes of appearance? Toyoshima admits that there was a little of both—but the dual wave as executed and the dual wave as originally proposed are different. Apparently, in the initial execution, the waves had such high crests and a low trough that it distorted the rear visibility.

Another notable aspect is that the hatch uses a carbon-fiber structure. Toyoshima says that it is produced in the Motomachi Plant in Toyota City—where the Lexus carbon-fiber-intensive LFA had been built—and that it is the most complicated structure that they’re producing with the material. By using the composite instead of aluminum, they’re cutting the weight by 8 pounds.

When you’re looking to achieve efficiency for range and performance, even ounces matter.

 *11 is a prime number. And that’s the all-electric range of the previous 2015 Prius Plug-in.