Variable cam timing

[atsakiri at ford.com]

A description of some variable valve timing actuators already
in production may be helpful.  The following information comes
directly from an article by W. Demmelbauer-Ebner, A. Dachs, and
H. P. Lenz, published in the October 1991 issue of Automotive
Engineering.


Alfa Romeo, 2.0 l DOHC 2V
-------------------------

"Actuation of the device begins with the electronic control unit
(ECU), determining timing advance based on throttle position, 
speed, and load.  A solenoid mounted on the cover at the sprocket
end of the camshaft closes a pintle valve, forcing oil pressure
to build up within the device.  This oil pressure forces a ring 
gear, which acts as an hydraulic piston, to travel axially toward
the rear end of the camshaft.  The outer diameter of the annular
piston is mated to the sprocket by a straight spline, while the
inner diameter is mated to the intake camshaft by a helical spline.
Forcing the annular piston toward the rear end of the camshaft
produces an angular displacement of the intake camshaft with
respect to the sprocket, effectively phasing the camshaft."

"When the ECU determines that retarded timing is necessary, the
solenoid valve at the front end of the camshaft is opened and oil
pressure on the piston is relieved."


Nissan 3.0 l  (also intake cam phasing)
------------

"A solenoid valve operated by the ECU controls pressurized oil
flow to the hydraulic piston to actuate phasing.  Unlike Alfa
Romeo's design, the annular piston has helical splines on both
the inner and outer diameters, allowing more angular phasing
for a given axial travel and a more compact device."


Mercedes-Benz 3.0 l I-6 and 5.0 l V-8  (also intake cam phasing)
-------------------------------------

"The Mercedes-Benz system employs a double-splined hydraulic
piston to alter phasing between the sprocket and camshaft ..."

"The solenoid-control actuator operates a 3-way shuttle valve to
minimize parasitic pressure losses, a significant difference 
compared to the 2-position valves of the Alfa and Nissan.  As
with the other two production cam phasers, timing advance
is achieved by supplying solenoid-controlled pressurized oil to
the annular hydraulic piston.  However, retardation is achieved by 
applying oil pressure to the opposite side of the annular piston
rather than by a return spring."



Sorry for the long length.  Any spelling errors are mine, not the
authors'.  

Anthony Tsakiris 

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The opinions expressed are my own and not necessarily those of my employer.