Fuel Economy Factors
 
The electric motors and battery don’t get all the credit for the high fuel economy in the Prius. The internal combustion engine (I.C.E.) does its part, too. The 2ZR-FXE 1.8-liter 4-cylinder gasoline engine achieves more than 40 percent thermal efficiency, thanks to reduced internal friction and improved combustion. It’s also quieter than the previous generation’s engine. Toyota found extra efficiency by cooling the Exhaust Gas Recirculation (EGR) system, and by using exhaust heat recirculation to quickly bring engine coolant up to optimal temperature.
 
Because the Prius, under certain circumstances, can operate in battery mode alone, it incorporates a Vehicle Proximity Notification System (VPNS) to help alert pedestrians and cyclists. A small in-vehicle speaker emits a low but noticeable warning tone when traveling below approximately 15 mph, and the sound changes with vehicle speed.