Development of an Electro-Hydraulic Camless VVA System 2007-24-0088
Among variable valve actuation systems, fully flexible systems such as camless devices are the most attractive valvetrains for near-future engines. This paper presents a research activity about an electro-hydraulic camless system for internal combustion engines. The Hydraulic Valve Control (HVC) system uses hydraulic forces to open the valve while a mechanical spring is used for the closure. The system is fed by an hydraulic pump and two pressure regulators which provide two different pressure levels: a high pressure level (approximately 100 bar) for the pilot stage and a low adjustable pressure level (from 20 to 90 bar) for the actuator power stage. The valve opening duration is controlled by varying the timing of the opening signal of the pilot stage; the valve lift is adjusted varying the oil pressure of the power stage. From a general point of view, the HVC system is an open loop device for engine valve actuation.
An electro-hydraulic model of the HVC system has been built in order to analyze the whole system and to provide an accurate design tool. The HVC key element is a spool valve, which operates as a three way / three position valve. This element is designed in order to ensure the synchronization of its own motion with that of the poppet valve mass-spring system. This mechanism, which will be studied in the paper, makes the HVC system very low power demanding, because the desired engine valve lift is achieved dynamically: in this way, its energy demand is about half of that needed if the engine valve was to be opened by a static equilibrium between hydraulic and mechanical forces.
Detailed experimental and numerical analyses have been carried out on the dynamic behavior of the HVC system, on its power consumption and on the valve landing phase (which is crucial for noise and durability issues).