4-2-1 Exhaust System
One option to significantly reduce residual gas is the adoption of a 4-2-1 exhaust system. As shown in Fig.3, when the exhaust manifold is short, the high pressure wave from the gas emerging immediately after cylinder No. 3’s exhaust valves open, for example, arrives at cylinder No.1 as it finishes its exhaust stroke and enters its intake stroke. As a result, exhaust gas which has just moved out of the cylinder is forced back inside the combustion chamber, increasing the amount of hot residual gas. With a short exhaust manifold, the high pressure wave arrives at the next cylinder within a short amount of time, causing this adverse effect to continue from low to high engine speeds. However, with a long 4-2-1 exhaust system, since it takes time for the high pressure wave to reach the next cylinder, the effect mentioned is limited to extra-low engine speeds, making the reduction of residual gas at almost all engine speeds possible.
In addition, a pipe length of over 600mm was necessary to elevate actual-use torque, but space was saved by adopting a loop shape.
The major issue with the 4-2-1 exhaust system is that the long distance cools the exhaust gas before it reaches the catalyst, delaying the catalyst’s activation. Exhaust gas temperature can be increased by delaying the ignition timing, but too much retardation causes unstable combustion. For SKYACTIV-G, stable combustion was realized even when the ignition timing after engine-start is considerably delayed.
This was made possible by adopting a piston cavity (Fig.4) and optimizing fuel injection in order to formulate a stratified air-fuel mixture around the spark plug. Furthermore, the piston cavity resolved the issue of the initial flame coming in contact with the piston head and generating a cooling loss.