ABSTRACT
The internal combustion reciprocating engines that are usually employed in automotive
applications need high torque at low revolution speeds to quickly increase car velocity
without shifting gears. However, high power at top speed is required to obtain an
adequate maximum speed as well as sufficient acceleration, through up-shifting.
Designers can decide, according to different valve-timing diagrams, whether to improve
torque at low speeds or power at top speeds. Unfortunately, both these improvements
cannot be carried out at the same time, if the valve-timing diagram does not change
with the engine revolution speed. Therefore, the current solutions for ordinary massproduced
automotive engines are often a compromise between torque and power
performance. A suitable choice of intake-valve and exhaust-valve timing, by means of
Variable Valve Actuation (VVA), could instead satisfy both these requirements at the
same time and even reduce fuel consumption and pollutant emissions. The aim of this
review is to outline some of the most significant proposals over the last forty years as far
as VVA systems are concerned.