Valve timing is the precise timing of the opening and closing of valves in an internal combustion engine. It is controlled by the camshaft and can be varied by modifying the camshaft or using variable valve timing. With traditional fixed valve timing, engines experience a period of valve overlap when both intake and exhaust valves are open simultaneously. Variable valve timing uses computer control and oil pressure to advance or retard cam timing while the engine is running, changing valve duration, overlap, and sometimes lift. It has been implemented in many Japanese and European engines since the 1980s-1990s and more recently in some American engines.

1. Title:-Valve Timing Diagram
Mechanical Engineering Department
Submitted By -
Name : S. Avinash
Section : B
Roll No. : 27
Sem : 6th
Faculty Advisor -
Kishor S. Rambhad
(Asst. Prof. Mech. Engg. Dept.)
Session 2016-17
Subject – Energy conversion-II
TAE II: Technical Presentation

2. VALVE TIMING
In a piston engine, the valve timing is the precise timing of the opening and closing
of the valves. In an internal combustion engine those are usually poppet valves and
in a steam engine they are usually slide valves or piston valves.
In four-stroke cycle engines and some two-stroke cycle engines, the valve timing is
controlled by the camshaft. It can be varied by modifying the camshaft, or it can be
varied during engine operation by variable valve timing. It is also affected by the
adjustment of the valve mechanism, and particularly by the tappet clearance.
However, this variation is normally unwanted.

4. Valve overlap
With traditional fixed valve timing, an engine will have a period of
"valve overlap" at the end of the exhaust stroke, when both the intake
and exhaust valves are open. The intake valve is opened before the
exhaust gases have completely left the cylinder, and their considerable
velocity assists in drawing in the fresh charge. Engine designers aim to
close the exhaust valve just as the fresh charge from the intake valve
reaches it, to prevent either loss of fresh charge or unscavenged exhaust
gas. In the diagram, the valve overlap periods are indicated by the
overlap of the red and blue arcs.
Key:
TDC = Top dead centre
BDC = Bottom dead centre
IO = Inlet valve opens
IC = Inlet valve closes
EO = Exhaust valve opens
EC = Exhaust valve closes

5. The Inner Workings of Variable Valve Timing
Variable Valve Timing (VVT) is a way to advance/retard valve timing, and
change duration, overlap and even lift in some applications while the engine
is running.
VVT is computer-controlled and typically uses oil pressure to change the
position of a phaser mechanism on the end of the camshaft to advance or
retard cam timing.
VvT has been used on numerous Japanese (Honda, Nissan and Toyota) and
European (Audi, BMW, Mercedes and VW) engines since the late 1980s and
early 1990s, but only in the last decade or so on domestic engines (such as
Ford 4.6L V8s, Chrysler 2.4L and 3.6L VVT engines, Chevy 2.4L Ecotec,
etc.).
On overhead cam engines where there are separate cams for the intake and
exhaust valves, changing the timing of one cam also changes the effective
duration and overlap, and on engines where VVT also involves activating
extra cam lobes or changing the rocker arm fulcrum point, VVT can also
change total valve lift.

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