The primary variable valve actuation strategies for diesel engines are variable late or early intake valve closing for control of effective compression ratio for Miller cycle and part-time HCCI, PCCI, or LTC; variable early exhaust valve opening for exhaust temperature control for after-treatment regeneration and improved engine transient response; on/off control of intake pre-bump and/or exhaust post-bump for IEGR and control of residual fraction; and on/off control of compression release and brake gas recirculation events for engine braking. Lost-motion hydraulic VVA is well suited to diesel engines due to the capability of on-off control of secondary events for IEGR and engine braking, high load capacity for early exhaust opening and engine braking, and inherent protection against valve-to-piston contact.
Production requirements for VVA systems include proven reliability/durability, cost effectiveness, compact packaging, cold start capability, acceptable valve seating velocity over the full operating range, and convenient lash setting. Several production-intent lost-motion VVA systems are described for variable late intake valve closing and IEGR intake pre-bump and for early exhaust valve opening and engine braking. Features include a lost-motion cam profile, a collapsing tappet located either between the rocker and the bridge or between the push-tube and the rocker, and a hydraulic circuit with a high-speed solenoid valve. Partial main event lift is provided with the tappet collapsed for failsafe operation and cold start. A valve catch to control valve seating during early intake valve closing is incorporated in the tappet assembly. A valve control unit, communicating with the fuel control, actuates the high-speed solenoid valves. The variable valve lift capabilities of several VVA systems and the valve-train parasitic loss over the range of operation are discussed, showing both simulation and test.