The slant-shaped squish chamber recently developed by our company improves charging efficiency and raises compression ratio of its gasoline-engines. We clarified its knock suppression mechanism using rapid compression-and-expansion machine, autoignition simulation, and 3-dimensional turbulent flame propagation simulation. The results indicate the following: 1. Timing of flame acceleration is a key of knock suppression, that is, optimum timing of flame acceleration exists in the latter half of combustion, because expansion stroke suppresses the temperature rise in the end gas. This universal mechanism improves knock-limited ignition timing and compression ratio of the above engines. 2. A cause of the flame acceleration in the slant-shaped squish chamber is strengthened turbulence occurring in front of the flame and in reverse squish flow which separates at the squish lip. 3. The reverse squish flow, which is well known to be induced by the suction due to the larger expansion ratio of squish region than the other region of combustion chamber, is enhanced by the thermal expansion of burning mixture.
