The Toyota Prius’s reign as the mpg king is over. When the Hyundai Ioniq Blue hybrid goes on sale in early 2017, it will carry a 58-mpg EPA combined rating, the highest of any non-plug-in vehicle. The car it dethrones, the Toyota Prius Eco, has an EPA combined rating of 56 mpg.
The Ioniq Blue hybrid’s city and highway ratings will read 57 and 59 mpg, respectively. Those figures apply only to the Ioniq Blue, a high-mpg model similar to the Prius’s Eco trim. The Ioniq Blue receives a special wheel-and-tire package among other small tweaks to squeeze an additional one or two mpg from the standard Ioniq hybrid.
Such high efficiency is a result of fanatical engineering of the naturally aspirated 1.6-liter four-cylinder engine, such as the unconventional use of a water-cooled exhaust-gas-recirculation system. With lower EGR temperatures, the Ioniq’s Kappa engine can fill a cylinder with as much as 20 percent exhaust gas during the intake stroke. The typical uncooled EGR system displaces 10 percent of the fresh-air charge. Hyundai claims this difference alone is good for a 3 percent fuel economy benefit by reducing the engine’s pumping losses.
The engine cooling system uses a split-circuit design to modulate temperatures of the head and block separately. The control logic opens the thermostat to the cylinder head at 190 degrees, while coolant starts flowing to the block at 221 degrees. The higher temperature in the block decreases the viscosity of the oil, reducing friction losses. Lower cylinder head temperatures help prevent catastrophic detonation, which allows more bandwidth in adjusting the ignition timing.
The engine runs a high 13.0:1 mechanical compression ratio with fuel sprayed directly into the combustion chamber via injectors with laser-cut holes of different sizes. These tailored holes reduce wall-wetting-when liquid fuel contacts the cylinder walls before it atomizes-to reduce particulate emissions. Continuously variable valve timing on both the intake and exhaust camshafts allows Atkinson-cycle operation. The net result is a claimed 40 percent thermal efficiency for the internal-combustion side of Hyundai’s gas-electric powertrain.
The Ioniq’s thrift isn’t all due to the engine, of course. It slices through the air with a low coefficient of drag of 0.24 thanks to its sleek shape, an active air shutter in the front grille, and BMW-style air curtains that direct air from the front fascia into the front wheel wells and around the tires. We also expect the Ioniq to match or beat the Prius Eco’s svelte 3033-pound curb weight. Like its Toyota rival, the Ioniq uses an aluminum hood and rear hatch. Hyundai also has eliminated the heavy lead-acid 12-volt battery. In its place, the Korean automaker installs a few extra lithium-ion cells in the battery pack under the rear seat. These cells are located close to the 1.56-kWH traction battery, but electrically they are only connected through a DC-to-DC voltage converter.
Hyundai hopes its Ioniq will capture the same squeaky-green image of the Prius by offering three alternative powertrain choices in the same basic bodyshell. The plug-in hybrid’s larger 8.9-kWh battery should provide 27 miles of electric range before the gas engine kicks on, while the Ioniq EV will cover 124 miles between charges with a combined city/highway efficiency rating of 136 MPGe. It should be noted, though, that these figures for the plug-in models are preliminary and may change slightly before production.
Buyers who need to travel beyond the EV’s full-charge range can opt for DC fast-charging capability that recharges the battery at up to 100 kW via SAE CCS connectors. That’s twice the power of every other EV save for the Tesla Model S, which charges at up to 120 kW using Tesla’s Supercharger network. For the time being, this extra capability is merely a way for Hyundai to future-proof its EV since existing CCS charging stations only support a maximum power output of 50 kW.