Among the R&D projects Toyota Motor is exploring to further lower fuel consumption and emissions are two concepts on a pathway to deliver gasoline engines featuring more than 45% thermal efficiency for application in its future hybrid vehicles, according to Koichi Nakata of Toyota in his presentation today at SAE 2011 High Efficiency IC Engines Symposium in Detroit.
The engine used in the first- and second-generation Prius (the 1.5L 1NZ-FXE) had a thermal efficiency of about 37%; the thermal efficiency of the new 1.8L unit in the third-generation Prius (2ZR-FXE) has a thermal efficiency of about 38%. Toyota is targeting a thermal efficiency of more than 40% with what Nakata called its Future Concept 1, followed by thermal efficiency of more than 45% in Future Concept 2 (which is based on concept 1).
In the 2ZR engine (third-generation Prius), some of the main technologies Toyota applied are the Atkinson cycle with variable valve timing to control intake valve timing, cooled EGR, and lowered friction. (In the 1NZ engine, Toyota reduced friction 21.1% compared to an engine for a comparable conventional vehicle; the 2ZR engine in the newest Prius drops that another 26.8%, in large part by the removal of parasitic loads via the hybrid system (e.g., no alternator).
Concept 1 is a cooled EGR stoichiometric spark-ignited direct-injection concept, featuring a long stroke design (stroke/bore=1.5) and cooled EGR with an EGR ratio of more than 30%. The long-stroke design (lengthening the stroke while maintaining displacement), reduces heat loss and also increases piston speed, creating more turbulence. A high tumble ratio intake port (TTR=3.0) and a high-energy ignition system (100 mJ) also contribute to improved combustion. Toyota is continuing to reduce friction.
Concept 2 is a turbocharged lean burn concept, built on the base of concept 1. It also uses the long stroke design, with a high tumble ratio and a higher-energy ignition system (150 mJ).
The high tumble ratio intake port extends the lean limit from 19 to 23, Nakata said. In addition, the lean limit is also increased by using a spherical face on the piston. Furthermore, the high discharge current in the ignition system also gives a higher lean limit.
Nakata said that the engine team has currently delivered a 42.4% thermal efficiency in concept 1 and 43.7% thermal efficiency in concept 2.
Work is ongoing, focused on increasing the expansion ration and decreasing pumping loses. Toyota is also considering a variable super high expansion ratio cycle for further improvements. Nakata suggested that such an engine applied in a hybrid would result in total lifecycle greenhouse gas emissions comparable to that of an electric vehicle.