Self-Study Programme 246 Variable valve timing.pdf

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Service.
Self-study programme 246
Variable Valve Timing
with fluted variator
Design and Function
The demands on combustion engines continue to
grow. On one hand, customers want more power
and torque, while on the other, one cannot lose
sight of fuel economy and increasingly stringent
emissions laws. In regard to valve timing, this
means that engine-speed and load dependent
adjustment of the inlet and exhaust camshafts is
indispensable.
Therefore, of course, adjustment systems are
continuously being improved in technical design
as well as in means of adjustment.
In this self-study programme, we would like to
introduce to you the design and operation of the
new camshaft adjuster with fluted variators. At
Volkswagen, first the 2.8 litre V6 and the 2.3 litre
V5 engines will be equipped with this system.
Later other engines, such as the W8 and W12
engines will be equipped with this variable valve
timing system.
246_148
NEW
Attention
Note
The self-study programme presents the design and
operation of new developments.
The contents are not updated.
Please refer to the intended Service literature for current instructions for testing,
settings and repairs.
2
Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Variable valve timing . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Advancing the inlet camshaft. . . . . . . . . . . . . . . . . . . . .10
Retarding the inlet camshaft . . . . . . . . . . . . . . . . . . . . . 11
Inlet camshaft regulation . . . . . . . . . . . . . . . . . . . . . . . .12
Exhaust camshaft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Exhaust camshaft, basic setting . . . . . . . . . . . . . . . . . . . 14
Exhaust camshaft, idle . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Oil system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Engine management. . . . . . . . . . . . . . . . . . . . . . . . . . . 17
System overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Circuit diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Self-diagnosis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Test your knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . .27
3
Introduction
The task of variable valve timing
Variable valve timing has the task of setting the most advantageous valve timing for the particular
engine for the operating modes idle, maximum power and torque as well as exhaust gas recirculation.
TDC
Idle
At idle, the camshafts are set so that the inlet
camshaft opens late and, consequently, closes
late as well. The exhaust camshaft is set so that it
closes well before TDC. Due to the minimal gas
residue from combustion, this leads to smooth
idling.
Ec
Io
Ic
Eo
BDC
246_001
Power
TDC
To achieve good power at high engine speeds,
the exhaust valves are opened late. In this way,
the expansion of the burned gases can act
against the pistons longer.
The inlet valves open after TDC and close well
after BDC. In this way, the dynamic self-charging
effect of the entering air is used to increase
power.
Ic
Ec
Io
Eo
BDC
246_002
Intake
Compression
Power
Exhaust
Io:
Ic:
Eo:
Ec:
Inlet opens
Inlet closes
Exhaust opens
Exhaust closes
4
Torque
To achieve maximum torque, a high degree of
volumetric efficiency must be attained. This
requires that the inlet valves be opened early.
Because they open early, they close early as well,
which avoids pressing out the fresh gases.
The exhaust camshaft closes just before TDC.
Ic
Ec
TDC
Io
Eo
BDC
246_003
Exhaust gas recirculation
Internal exhaust gas recirculation can be
achieved by adjusting the inlet and exhaust
camshafts. In this process, exhaust gas flows from
the exhaust port into the inlet port while the
valves overlap (inlet and exhaust valves are both
open). The amount of overlap determines the
amount of recirculated exhaust gas. The inlet
camshaft is set so that it opens well before TDC
and the exhaust camshaft does not close until just
before TDC. As a result, both valves are open
and exhaust gas is recirculated. The advantage
of internal exhaust gas recirculation over
external exhaust gas recirculation is the fast
reaction of the system and very even distribution
of the recirculated exhaust gases.
Ec
TDC
Io
Ic
Eo
BDC
246_004
The illustrated valve timing is intended to demonstrate the basic principle and the effects of
variable camshaft timing. Of course every engine has valve timing adapted to its mechanics
and its engine management.
5

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