The engine of the current Prius
The engine of the current Prius
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The maximum thermal efficiency of the gasoline engine of the next-generation Prius that Toyota Motor Corp plans to release in 2015 is expected to reach 40%.

The efficiency will be realized by improving the EGR (exhaust gas recirculation) limit from 21% (current model) to 28%. The thermal efficiency of 40% would be the world's highest for a mass-produced gasoline engine. The maximum thermal efficiency of the "1NZ-FXE" engine, which is used for the first- and second-generation Prius, is 37% while that of the "2ZR-FXE," which is used for the third-generation (current model) Prius, is 38.5%.

This was announced at 2015 JSAE Annual Spring Congress, which is organized by Society of Automotive Engineers of Japan Inc from May 20 to 22, 2015, in Yokohama. When the EGR limit and, thus, the amount of cooled exhaust gas being circulated are increased, the heat capacity increases, enabling to lower the temperature of the gas inside the cylinder. As a result, knocking can be prevented, and cooling loss is expected to be reduced by 8%, compared with the current model.

To increase the EGR limit, Toyota will increase combustion speed. To realize the high-speed combustion, it is important to disturb the airflow inside the cylinder. The company increased tumble ratio (ratio between the swirl flow in the direction of the piston movement and that in the axial direction) from 0.8 (current model) to 2.8. Especially, the airflow near the top dead center of the piston can be easily disturbed with this method.

To increase the tumble ratio, the lower side of the suction port is straightened so that the airflow becomes a vertical vortex. According to Toyota's estimates, in the compression stroke with an engine speed of 2,000rpm, the mean turbulent velocity of the gas inside the cylinder can be increased from 2.5m/s to 3.4m/s.

In addition, Toyota changed the shape of the piston's top face by reducing the diameter of a cylindrical dent formed on the top face so that the airflow inside the cylinder can be easily disturbed. However, when the gas inside the cylinder is more disturbed to increase the flow velocity, it makes ignition difficult. To address this problem, the company plans to increase the ignition energy of the spark plug from 35mJ to 100mJ.