A review of variable valve actuations and their effects on s.i. engine performance

Most current i.c. 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 timing (VVT) system would allow the exploiting of different valve-lift diagrams according to any of 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, VVT system operating on the intake-valve only, can provide the engine with a method to control its load without using the conventional throttle-valve. Experimental tests have also shown that adequate VVT strategies can exert positive influences on brake specific fuel consumption and exhaust-gas pollutant-emissions. This paper analyzes some of the most significant experiences and mechanisms, which have been worked out up to now, as far as VV is concerned.