In order to lower the temperature at compression TDC, reducing the amount of hot exhaust gas remaining inside the combustion chamber is effective. For example, with a compression ratio of 10:1, a residual gas temperature of 750 deg. C, and an intake air temperature of 25 deg. C, if 10% of the exhaust gas remains, the temperature inside the cylinder before compression increases by roughly 70 deg. C, and the temperature at compression TDC is calculated to increase by roughly 160 deg. C. Therefore, it can be easily inferred that the amount of residual gas has an major impact on knocking.

These calculations are summarized in Fig.2, and as indicated, if the amount of residual gas is halved from 8% to 4%, the temperature at compression TDC is calculated to remain the same even when the compression ratio is increased from 11:1 to 14:1.

This reduction of residual gas was focused on for SKYACTIV-G, enabling the realization of a high compression ratio gasoline engine.