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Combustion improvement - Efforts were concentrated on the development of a combustion chamber which offers high thermal efficiency over the entire operation range with lowered emissions.
First, an optimum intake air throat diameter was selected to maintain volumetric efficiency in the high-speed range and maximize intake air flow velocity in the low- and mid-speed ranges, thus enhancing volumetric efficiency in all speed ranges with the output performance in high speed range being ensured. The valve angle was then narrowed for optimization and the combustion chamber made more compact (reduction of surface/volumetric ratio) to reduce cooling loss for higher thermal efficiency, without relinquishing adequate throat diameter. (Fig. 3) Squish area, which creates turbulence of the air/fuel mixture (squish) during the compression stroke, was given around the valves to ensure high volumetric efficiency and high combustion speed. (Fig. 4) (1) As a result, the combustion chamber is a compact pent roof with intake and exhaust valve angles of 27 degrees. Squish area to bore area ratio is 17.3 percent and the squish clearance is 0.68 mm. The throat diameter is 28.5 mm on the intake side; and 25 mm on the exhaust side. (Fig. 5)
Fig. 3 Effect of throat diameter on volumetric efficiency
Fig. 4 Effect of the adoption of squish area on volumetric
and thermal efficiencies
Fig. 5 Combustion Chamber Design
The emission of hydrocarbons has been greatly reduced by eliminating the crevice volume, the space around the piston top land, valves, and spark plug that extinguishes the flame. (Fig. 6) shows the locations of these changes and the hydrocarbon reduction effect.
