TABLE OF CONTENTS Valve timing is normally a compromise in design that aims to achieve good performance across the operating speed range of the engine. Inlet valves open a few degrees of crank rotation before top dead centre and close several degrees after bottom dead centre, as shown in Figs 1.14 and 1.15. The period, at the piston top dead centre position, when both valves are open together, is known as valve overlap. It is normally fixed. However, early opening and late closing of the inlet valve has an unhelpful effect on low engine speed emissions and design developments, such as variable valve timing, have been introduced to improve emissions and other aspects of engine performance.
Variable valve timing is a development that has been enabled by the use of electronic control which permits valve timing to be changed while the engine is operating, to suit low speed, intermediate speed and high speed operation. The variations in inlet valve timing are approximately as follows:
Low speed inlet valves opened later to improve idling performance;
Intermediate speed inlet valves opened a few degrees earlier to take advantage of manifold design and thus improve cylinder filling and performance;
High speed a larger degree of early opening of the inlet valves.
The amount of variation in the timing may be limited to approximately 10
The aim of the system is to reduce harmful exhaust emissions and improve engine performance.
