The Toyota Prius hasn’t just been synonymous with hybrid technology; it has been the most fuel-efficient model in the U.S. market for years. And since its 2016 redesign, the Prius Eco has been the most fuel-efficient gasoline-powered car without a charging port, ever. So how does the Hyundai Ioniq hybrid’s Blue model, right out of the gate, jump to a spot ahead of the Prius with a combined 58 mpg, making it the most fuel-efficient hybrid available in the United States?
First, Hyundai has more development years invested than you might think. A dedicated team within Hyundai began working on hybrid powertrains nearly 12 years ago, and developing its so-called TMED (transmission-mounted electrical device) hybrid system 10 years ago. TMED made its debut, as a system, at the 2008 Los Angeles auto show and then in production form in the 2011 Hyundai Sonata hybrid.
As we concluded on our first drive of the Ioniq hybrid, this is a car with reasonably good drivability, and while we won’t call it downright sporty, it has a perkiness that’s missing from the Prius’ powertrain. It’s hard to believe the Ioniq’s hybrid kit is derived from the system in that 2011 model, in which a lack of finesse in that hybrid powertrain made drivability downright clumsy. But with reworked software Hyundai has made tremendous strides since then. The Ioniq makes the most of the efficiency potential in this hybrid layout, with a different engine and transmission than the Sonata hybrid, and a body structure and platform that were conceived from the start to be hybrid specific.
At the Ioniq’s U.S. media launch this past week, we asked several key project members how they managed to best the Prius with the automaker’s first attempt at a dedicated hybrid. And here’s what they pointed to:
An engine that runs lean. According to Yong-Seok Kim, the team leader for eco-technology performance development, thermal management and this engine’s claimed 40 percent thermal efficiency (same as that of the Prius) are two of the most significant factors, as are some fundamental design choices, such as the engine’s long 1.35 stroke-to-bore ratio. The 1.6-liter Atkinson-cycle Kappa four-cylinder uses the engine-block dimensions otherwise reserved for engines in the 1.0- to 1.4-liter class, keeping its thermal mass to a minimum. Split cooling circuits for the head and block as well as a cooled EGR system help keep the air charge consistent and cool. A 2900-psi direct-injection system (the high end of today’s gasoline DI) aids combustion efficiency.
Battery and power controls. The second most important thing, according to Kim, is the Hyundai’s battery-control strategy. Compared with Toyota and other automakers, Hyundai has more experience using lithium-ion batteries, he says, and much more (higher resolution) data on how to use the battery at particular temperatures and conditions. “So even if GM, in the Volt or Bolt EV, uses the same chemistry and battery, we guarantee it with more warranty, and our efficiency is better because our control algorithm is better,” said Kim.
Very little lost in translation. Hyundai’s choice of transmissions also works toward maximizing efficiency; the dual-clutch automatic in the Ioniq has a torque-transfer efficiency of 95.7 percent—better than a planetary automatic transmission, a CVT, or the Prius’s two-motor power-split arrangement. The other advantage, as we’ve already found out with some driving time, is that you feel more connected to the powertrain while accelerating; there’s none of the detached, motorboat-like sounds you get in the Prius—or in the Honda Accord hybrid, where Honda has shown how it can eliminate the transmission (and those mechanical losses) completely.
Cutting drag. The Ioniq team attacked all aspects of drag, starting with carefully tweaked aerodynamics. All versions of the Ioniq have a super-slippery 0.24 coefficient of drag (and we’re impressed that the team didn’t have to skimp on side mirrors to get there). All Ioniq models with an internal-combustion engine (hybrid and plug-in-hybrid models) have automatic grille shutters. Underbody shielding is used across the lineup. The brake calipers’ pistons retract slightly to avoid any unwanted pad-on-disc drag when you’re not stepping on the pedal, while Blue models have special aero-optimized wheels. Getting very low rolling resistance is the other crucial piece. Hyundai is using Michelin tires—Michelin Energy Saver A/S tires on Blue and SEL models and, on Limited models, Primacy MXM4 tires (priced at $166 each by Tire Rack), which Kim said are the most expensive tires he has ever used.
Putting the 58-mpg Blue on a special diet. Obviously, reducing weight is an important step, too. Hyundai didn’t just work to keep mass at a minimum in major components—aluminum hood, liftgate, and suspension components, 54 percent high-strength steel for the core body structure—it also spent a lot of effort micromanaging weight on the features level. That’s especially true in the base Blue model—the one that gets a 58-mpg combined rating, while the rest of the lineup gets 55 mpg.
This bears a little more explanation, though. The U.S. EPA requires vehicles to be grouped into 125-pound weight classes—inertia classes, actually. If an option—leather seats, for example—is applied on more than 33 percent of vehicles, then the weight of the seats has to be accounted for in the vehicle’s ETW. Sunroofs are a good example, because they weigh about 28 pounds—an amount that can easily push a vehicle into the next test class—according to John Shon, the manager of product planning at Hyundai Motor America.
While there’s no difference in powertrain or core functionality from Ioniq Blue to mid-level Ioniq SEL and the top-of-the-range Limited, Hyundai whittled the Ioniq Blue, which has no major options, to a lower test class than the rest of the lineup by keeping a tight rein on equipment (and actually, there are just five build combinations of the entire Ioniq hybrid lineup, aside from color choices). The Blue omits the other models’ rear center fold-down armrest, power driver’s seat, heated front seats, lower door molding, and blind-spot monitor, all of which add weight, as well as LED headlamps, which don’t. In the noise-isolation area, the Blue model gets its own (lighter) dash pad, trunk mat, and carpet. A wheel design specific to the Blue model saves two-thirds of a pound per wheel.
It’s all part of some careful planning made by Shon and his U.S. team to manage expectations and make sure that buyers of the most fuel-efficient Ioniq get what they want—and that buyers of more lavishly equipped versions aren’t disappointed with their real-world mileage. “You see quite a bit of that throughout the industry,” explained Shon. “Ultimately, you want to give those customers that choice, because as you add weight and features, you’re going to get lower fuel economy.”
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