Yamaha Technology 2007: YCC-I: Yamaha Chip Controlled Intake
The system changes the length of the intake funnel in order to maximize cylinder filling efficiency.
Normally the length of the intake funnel is a compromise. A long funnel is best for torque at low and mid rpm, and a short funnel is best for high rpm and maximum top power.
The YCC-I system makes it possible to use the high-rpm oriented system with the short funnel layout, while keeping the higher torque characteristics of the long funnel at low and mid rpm.
The principle of variable intake length is based on the phenomenon of reflected pressure waves in the intake duct. They reflect at the end of the intake duct and return to the intake valve.
At low rpm the intake duct is therefore longer, creating a longer time for the pressure waves to travel. This longer time is needed because the period the intake valves are closed is longer too, and the reflected pressure waves should arrive at the intake valve when it is opening.
At high rpm the intake duct is shorter, because the time the valves are closed is shorter too. In this way the arrival of the reflected pressure waves is again matching with the opening of the valves.
The length of the intake funnel is shortened by moving the upper part of the funnel away when the engine revs above a certain rpm range and a certain throttle opening angle.
With the upper part of the intake funnel moved away at high rpm, the pressure wave coming from the intake valve reflects at the opening at the end of the short lower funnel and returns to the intake valve.
The funnel is dimensioned exactly so, that the reflected pressure wave arrives at the intake valve precisely when the valve opens, in order to maximize cylinder filling efficiency.
At low rpm, there is more time between closing and opening of the valves. Therefore at low rpm the funnel has its maximum length, by connecting the upper and lower parts of the funnel. This way the pressure wave takes a longer time to travel, to match its return with the opening timing of the intake valve.
The YCC-I motor moves the upper part of the funnel by levers and a rod.
The movement takes 0.3 seconds.
Please see the videos for details about the pressure wave reflections.