Variable cam timing

[Edward Hernandez R]

Sorry guys, I haven't been able to access email for a while. Apparently, I left some of you slobering:

"Ed, are you going to say anything else about this, or are you going to let your remark just lie there????  :-)"

"Come on Ed, don't tease us like that.
I remember a mechanical type described in Motor Trend a while back.
The chain end of the camshaft was threaded, and the sprocket screwed
onto the thread.  There was an oil driven piston that pressed against
the sprocket. As the piston moved the sprocket back and forth, it
caused the camshaft to be advanced or retarded via the threads.  It
was a very coarse thread."

Nope, you missed my hint completely. You describe the "common" way of
phasing cams that most OEMs have played with. Some have made it to
production(Nissan, M-B, Ford, et al).

"Sounds like you guys are cookin sumpin' up over there at Ford..."

You are right, except that the beanies told us to can it and put it
up on the shelf for later use. Happens too often over here. No more
hints, except to say the first post was very, very close to what we
tried and succeeded in achieving.

Anything that controls the camshaft as a whole component cannot be
described as having infinitely variable duration on an engine where
the cam controls the valves on more than one cylinder. Changing the duration during a lift event for one cylinder will compromise the
valve events on all other cylinders, unless perhaps you have a two
cylinder engine which the K-series aren't.

Hydraulic bleed systems(and systems that phase the entire cam) can be
used to achieve real BSFC improvements, not penalties. The way to do
this is to delay intake closing at the low rpm. This kills torque, so
the throttle must be opened further which reduces pumping losses and
improves BSFC. Well designed strategies will base cam phasing on
throttle position as well as rpm. In this way, cam phasing can give
you the best of both worlds, not just performance improvements.

"This would change only the timing, not the shape of the lobe. Controlling the lift and duration of the valve would change the performance much more dramatically. Why not use a solenoid valve to
bleed off pressure from hydraulic lifters? The cam could be ground
much more radical and the computer could easily bleed."

Not necessarily true. Of the four events(EVO, EVC, IVO, IVC, where the
initials are Exhaust or Intake, Valve and Closing or Opening), IVC
is the most dominant. Substantial gains have been achieved with cam
phasing alone. In order to bleed lifters, the solenoid you mention
must reside inside the lifter/lash adjuster. If you put it in the oil
galley where the lifters reside, the lifters must be designed "loose"
in order to respond to varying galley pressures which makes them very
noisy(okay, so that's really an OEM concern, not a DIY concern).
Still, the computer cannot easily control this as you mentioned unless
there is some form of feedback on EACH valve. Variable bleed lifter
technology isn't to the point yet where you can count on it without
feedback. The manufacturing tolerance required for REPEATABLE bleed
rates needed for a system like this are not achievable(for mass production OEMs) yet. And no one has invented true constant viscosity
oil yet. The computer would have a tough time, not an easy one.

Cam phasing systems just need a cam position sensor in conjuction with
a crank position sensor, and these already exist on SEFI engine
management systems. Sure, you CAN get more out of a variable profile
system, but I'm not sure I would call it dramatically better, espec-
ially if your cam phasing device can swing the cam 30 or 40 
degrees already. I would imagine the DIY crowd would be interested in 
trying this in it's simplest form. Oops, I babbled again.

Ed Hernandez
Ford Motor Company
ehernan3 at ed8719.pto.ford.com

PS I hope I didn't forget any carriage returns, dn.