Toyota announced that its next Prius will be the most efficient yet, delivering up to 40 percent thermal efficiency.

This news came during last week’s Annual Spring Congress for Japan’s Society of Automotive Engineers where the automaker revealed more details about its class-leading gasoline Prius hybrid engine.

“The efficiency will be realized by improving the EGR (exhaust gas recirculation) limit from 21 percent (current model) to 28 percent,” reported Nikkei Technology Online. “The thermal efficiency of 40 percent would be the world’s highest for a mass-produced gasoline engine.”

Camouflaged 2016 Prius

Is this the next generation for Prius? A spy photo captured this heavily camouflaged hatchback at a gas station earlier this month.

This boost will make the next generation of Prius engine the most efficient yet for the brand. The first two generations, in comparison, posted 37 percent thermal efficiency. The 1.8-liter engine in the current third generation Prius is at 38.5 percent.

Last March, Toyota said that an upcoming Prius would get a 15-percent increase in fuel efficiency. It didn’t specify if this new engine would arrive with the 2016 model but previously Toyota had said its next-gen Prius would get its most thermally efficient engine to date.

It now appears as though the 2016 Prius will make good the promise.

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Though the 40 percent gain is significant, it isn’t quite as high as the carmaker originally hoped. In 2011, Toyota’s Koichi Nakata announced that the company hoped to reach 45 percent thermal efficiency for its hybrids.

Recirculating more exhaust gases, straightening the intake port and redesigning the shape of the piston are some of the ways Toyota was able to achieve an extra 1.5 percent thermal efficiency, as explained further by Nikkei Technology Online:

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 percent, 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.

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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 2,000 rpm, the mean turbulent velocity of the gas inside the cylinder can be increased from 2.5 m/s [meters per second] to 3.4 m/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 35 MJ [megajoule] to 100MJ.

(Photo credit: Yuriko Nakao/Bloomberg via Getty Images)