This was worth a read:
Honda Worldwide | Civic Hybrid | 1st Generation Civic Hybrid
Quote:
The New Honda IMA System employs the VTEC System to stop operation of valves in three of the four cylinders, reducing engine friction by 50%, and making possible highly efficient electrical regeneration.
VTEC Cylinder Idling System
When the throttle is open during acceleration or cruising, the valve liftmode rocker arm and the idle-mode rocker arm are engaged via a synchronizing piston. During deceleration, as soon as the throttle is closed and regeneration begins, the synchro piston is positioned inside the idle-mode rocker arm, disengaging the lift-mode rocker arm so that the valve remains at rest. Since the cylinder is sealed off, pumping losses (resistance caused by engine aspiration) that result in engine friction are reduced, and the wheels' rotational energy is effectively transmitted to the motor. Moreover, cylinder idling can be maintained at engine speeds as low as 1,000rpm, for even greater regenerative efficiency.
|
So, Honda chose to keep the valves closed, rather than open, the logic being that the air inside will act as a spring, and the energy used in compression would be mostly recovered during expansion. Leaving them open would force the engine to pump air in and out of the cylinder, resulting in lost energy.
So, it's better to deactivate cylinders by closing all valves, rather than leaving them open.
Honda's system is also extremely reliable. You basically never hear about failures with their variable cam system, which has been in most of their production cars since the early 90's.