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 los...