Remarks on the Variable Valve Actuation Systems of the I.C. Reciprocating Engines

Most of the current i.c. reciprocating 4-stroke engines are designed to operate with a fixed valve-timing diagram, which often results to be a compromise between torque and power performance. On the contrary, a variable valve actuation (VVA) system would allow to exploit different valve-lift diagrams, according to any operating conditions, in order to obtain high torque at low speeds and high power at top speeds, on the same engine, so improving its mechanic characteristic. Moreover, "VVA" systems operating on the intake-valve only, can provide the engine with a method to control its load without using the conventional throttle-valve, therefore avoiding the corresponding pumping losses. Experimental tests have also shown that adequate "VVA" strategies can exert positive influences on brake specific fuel consumption and exhaust-gas pollutant-emissions. More recent interesting "VVA" solutions have been exploited, concerning port deactivation for charge motion control, engine modular operation, more efficient engine brake system for diesel-powered vehicles, more advantageous management with turbocharging. This paper analyzes some of the most significant experiences and mechanisms, which have been worked out up to now, as far as "VVA" matters are concerned.