Newsgroup: it.discussioni.auto
Titolo: Spiegazione V-TEC (in Inlgese)

Three Stage VTEC 
HO trovaot questo articolo leggendo una rivista On Line (TurboZine),
magari interessa a qualcuno :-)
First of all, despite being 16 valves, there is really only one cam
lobe for operation in the first two stages. This is supplemented by
the high lift center lobe (like in the normal DOHC VTEC). The second
intake valve rocker, however, simply rides along a zero lift ring in
place of the usual lobe on the cam shaft. To activate the second stage
at 2500 rpm(both valves opening), oil pressure is diverted into a
channel that drives a pin which connects intake valve rocker #1 with
#2. Now, both intake valves are being operated in unison. This is
perhaps not quite as ideal as a little valve stagger (as seen in the
DOHC VTEC apps), but sufficient enough for good power. 
To enter the third stage, at 6000 rpm, oil is now allowed into a
second channel connecting all the rocker arms. This oil drives a pin
which connects the linked #1 and #2 rockers to the center lobe
actuated #3 rocker which is, of course, the high lift and duration
mode. 
Frankly, the implementation, from an engineering standpoint, is
absolutely brilliant. You'll have to see the diagrams to see what I
mean. 
Power for the 1.5 peaks at about 130 hp@6900 (had to estimate from a
chart) and about 105-107 lb-ft @ 5200. The torque curve is very flat
down to about 2000 rpm and it still makes about 80 lb-ft at 1000 rpm.
The motor (not necessarily the car) is also apparently capable of
VTEC-E type economy. I'd have to say that it's pretty impressive to
take a motor that is 6.5% smaller than the 1.6 and have it make as
much torque and more hp with potentially better economy. Putting this
technology in the Accord 2.2 with some torque tuning would make for a
great everyday car (I figure 160+ hp and 150+ lb-ft).   
Two Stage VTEC 
VTEC is Honda's trademarked acronym for Variable valve Timing and
Electronic lift Control. Put simply, it's a method of directly
altering the cam profile that valves "see", so that the optimum grind
can be utilized at either high or low rpm. Honda currently has three
different VTEC systems for sale in the U.S., but the primary
differences are: level of complexity and design purpose. Since the
high performance version is the most oft discussed, I'll describe it: 
The high performance VTEC system, which made it's debut in the Acura
NSX, is also available on the Integra GS-R, Prelude VTEC and del Sol
VTEC. Using radical cam grinds to improve engine horsepower is
certainly nothing new, but the problem lies in driveability. The very
aspects of a cam grind that work so well for horsepower (high lift,
long overlap, etc.) do so by creating an ideal situation for a high
air flow at high engine speeds. Unfortunately, what works well for
that situation has the inverse affect on low speed torque and
driveability. Perhaps at one time or another we've all heard the V-8
hot rod that has a cam grind so radical that it can't even maintain an
idle, and the driver must constantly goose the accelerator to keep the
engine running. 
What to do? How about two different cam grinds, each optimized for a
different half of the rpm range. Honda achieves this with a rather
simple method. 
Picture, if you will, one cylinder of a DOHC, 4-valve per cylinder
engine. There are 4 cam lobes, each directly operating a valve (two
intake, two exhaust). The VTEC system has two more cam lobes, in
between each pair of the other respective sets. These two can then be
our high-rpm lobes, while the other four are the low-rpm lobes. 
The low-rpm lobes in this case then actuate the valves through a set
of rocker arms, so that the mechanical connection can be broken if
desired. The third, high-rpm lobe also has it's own follower, but it
is in a freewheeling state, flopping around and not contributing
anything. As our engine accelerates through it's rev range, it passes
through the power peak of the low-rpm lobes. Then, at the engine speed
and throttle position programmed into the computer's memory map a
signal is sent which electronically opens a spool valve, which then
directs oil pressure to a mechanical sliding pin. This pin locks the
rocker arms actuating the valves to the follower on the high-rpm cam
lobe. As this grind is steeper and higher then the other four cams it
will supersede them. In a few milliseconds you have completely altered
the valve timing and the engine's power band begins anew. 
The obvious benefits to this are the high-rpm power associated with a
radical cam grind, but with little or no negative affects on low speed
idle, driveability or torque. Just changing a fixed timing engine to a
cam grind equal to the high-rpm one used in the VTEC would produce an
engine which is utterly gutless below 5000rpm. 
Clearly, this system is intended to improve performance first, with
little effect elsewhere. But such a system can be used differently, as
that in the Civic VX. In this instance, the low-rpm lobes give a
staggered timing, where one valve opens fully but the second cracks
just a bit. This is to induce a high swirl rate into the chamber to
promote better combustion, which, when combined with a
computer-actuated lean burn helps to achieve high mileage. The
high-rpm lobes in this engine are a more conventional grind associated
with a sixteen valve 4-cylinder, to provide extra power in cases of
passing or merging. The VTEC system used in most Civics (EX, Si and
the del Sol Si but NOT del Sol VTEC) is a little closer to that in the
NSX, etc. The difference being that this system is vastly simplified
and operates on the intake valves ONLY. The exhaust valves are
actuated conventionally, which reduces the effect somewhat from the
full VTEC system. This is partly to reduce costs, and partly because
this engine is a SOHC, and the complex system of rocker arms to
actuate sixteen valves is prohibitive to the full VTEC system. The
VTEC on the Accord is close in design to that on the Civics as well,
optimized more for a smooth power delivery then high horsepower. 
The negative effects? Very few, really. Obviously it's very expensive,
with many complex parts involved. The biggest drawback is the
limitation to only two "modes" of valve timing. Most engineers are
still seeking ways to obtain unlimited variance of the valve timing,
so that it can be optimized to any engine speed, not just high or low
rpm. BMW's system approaches this method with a completely different
method of varying the valve timing. It is almost infinitely adjustable
*within it's range*, but alas it has a much smaller envelope between
the two extremes of it's variability than is possible with the Honda
system.     
Racing Engine      Conventional Engine       Fuel-Efficient Engine
-------------      -------------------       --------------------- 
High lift valve    Average power at low      Average power at low
timing             speed                     speed 
High power at      Average power at high     Poor performance at
high speed         speed                     high speed 
Undriveable at     Average fuel              Excellent fuel 
low speed          consumption               consumption 
High fuel          A "COMPROMISE" 
consumption 
    \                   |                   /
     \                  |                  / 
      \                 |                 / 
       \                |                / 
        \               |               / 
         \      VTEC                   / 
                ---- 
        Variable valve timing & lift
        Good power & economy at all speed.
Lorenzo "HUHI" Corsani 24, 147 
Fiat Coupè 20V & Clio16V Driver
ICQ: 19769872

magari interessa a qualcuno :-)
[supermegacut]
Lorenzo, sei veramente unico! :-))
Poi, quando ci si vede, mi devi raccontare che lavoro fai o cosa
studi, ma soprattutto dove trovi tutto 'sto tempo per soddisfare le
nostre più recondite curiosutà motoristiche!! :-DD
Ciao ciao!
Zerocento 26,90 (forse...)
Tigrottadriver
"Finestrini socchiusi su strade indifese dai nostri pesanti HP"
L. Ligabue

nostre pių recondite curiosutā motoristiche!! :-DD
Ok :-D Comunque per ora non faccio niente, ho finito da poco il
servizio civile,  ma da Gennaio vedrai i post in meno :-)) 
Lorenzo "HUHI" Corsani 24, 147 
Fiat Coupč 20V & Clio16V Driver
ICQ: 19769872

ICQ: 19769872
 Saluti, Matteo                     
 
VW Passat Variant driver 19,90    
e-mail : 
ida-d...@libero.it
 
alternative e-mail : 
fab...@iii.it
 

Now, when I've got some time, I'll trie to traslate all! :-))
It's a good idea, but I think that this is a big work :-) I'll hope
you'll take some days at work to do all :-DD
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ICQ: 19769872
 Saluti, Matteo                     
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Grazie per l'articolo!! :-)) opss....Thanks for the article :-)
Of nothing !! (di niente :-DD)
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:-DDD Good, your Inglish is very good. 
English please ;)))
Saluti
Flex
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 Saluti, Matteo                     
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