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Desmodromic valves are those which are positively closed by
a cam and leverage system, rather than relying on the more conventional springs
to close the valves.
Image:Desmo2.jpg Image:Rockhopper.gif
This is
in the context of internal combustion engines. The valves in question are the
ones that allow air into the cylinder and (usually different ones) that allow
exhaust gases out. This refers, for example, to the valve control system used in
Ducati engines: both valve movements (opening and closing) are "operated". It is
usual to say that action on the valve is positive in both cases, in other words,
both strokes are controlled.
Desmodromic valve actuation has been applied
to all but a few Ducati motorcycles. Two primary mechanical methods have been
used to transfer timing information, from the crankshaft, to the camshaft and
ultimately the rocker arms and valves. Initially bevel-driven camshafts were
used. This involved transferring the timing information via several bevel (part
conical gears where the rotating axis' of the two lie on an angle - 90° for
example) gears and a shaft running on the outside of the engine block. Then at
around 1977, Chief Design Engineer Fabio Taglioni completed and tested an
actuation system that used rubberized metal belts with timing teeth. These teeth
would mesh with timing pulleys, also external to the main engine block, and
transmit the timing information to the valves.
The primary reason for
Desmodromic (Colloquial - "Desmo") systems is to improve valve timing at higher
engine revolutions. On very high performance valve spring engines, the spring
does not always have time to return to its pre-compressed position, causing the
camshaft to recompress the spring and valve prematurely. This is called "valve
float". The Desomodromic system also eliminates the extra "work" spent by the
motor to open spring actuated valves. Therefore giving more actual power at the
wheel rather than using it to work against the seat pressure on the
spring.
There is however a down side to this system. The timing of the
valve in regards to its opening and closing is governed by a belt-driven cam. If
this belt fails the valves will not close in turn causing the piston to "crash"
into the valve, a catastrophic failure. Because of this, Ducati recommends a
rigorous valve adjustment every 6,000 miles.
In general mechanical terms,
the word desmodromic is used to refer to mechanisms that have different controls
for their actuation in different directions. It is derived from two Greek roots,
desmos (controlled, linked) and dromos (course, track).
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Multi-valve
In automotive engineering, an engine is
referred to as multi-valve (or multivalve) when each cylinder has more than two
valves.
All tappet-valve, four-stroke internal combustion engines have at
least two valves per cylinder - one for intake of air and fuel, and another for
exhaust of combustion products. Adding more valves improves the flow of intake
and exhaust gases, potentially improving combustion efficiency, power, and
performance.
Most multivalve engines use an overhead camshaft to actuate
the valves, and many use double overhead camshafts (DOHC). However this is not
always the case: Chevrolet recently introduced a 3-valve version of its
Generation IV V8 which uses pushrods to actuate forked rockers, and Cummins
makes a 4-valve pushrod straight-6 Diesel, the Cummins
600.
[edit]
History
The first multivalve engine
was built by Peugeot in 1912 for Grand Prix racing. The technology was also
attempted by Bugatti, Bentley, and Stutz, but it was not until the 1970s that
this technology became widespread. The first was Jensen in the 1972 Jensen
Healey roadster. This used a Lotus developed version of a GM design which
resulted in a 1973 cc (2.0 litre) DOHC engine that delivered 140 bhp. Others,
including Cosworth (on the 1975 Chevrolet Vega's 2300 engine), Lotus Cars (on
the 1976 Esprit - which used a 160 bhp version of the same engine first seen in
the Jensen Healey), and BMW (on the 1979 M1's M88 engine). Triumph also
introduced a single overhead cam 16-valve head on the Slant-4 in their Dolomite
Sprint.
Ferrari followed Lotus and GM in to the multivalve designs with
their Quattrovalve 308. From there, Honda and Toyota rapidly spread the
technology to their mainstream models in the 1980s.
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Modern multivalve engines
Today, multivalve engines
are used by nearly all manufacturers. They are common among the Japanese and
European makers. The US manufacturers have lagged, though improvements to their
pushrod designs have caused some to question the benefits of multivalve
engines.
General Motors began using 4-valve DOHC heads with their Quad-4
and Northstar engines in the 1990s. The company worked with Lotus, a subsidiary
at the time, to adapt two pushrod engines for 4-valve DOHC cylinder heads: The
LT5 V8 from the Corvette ZR-1 and the 3.4 L LQ1 V6. Pushrod engines are still
the norm at GM, however.
Ford's DOHC success came with their
(multivalve-optional) Modular V8, SHO V6, and Mazda-developed B-family of I4
engines. Their Duratec family consists entirely of multivalve engines, and is
used across the product line.
DaimlerChrysler's Mercedes-Benz used
3-valve SOHC engines for many years, but recently switched to 4-valve designs.
Their American Chrysler operation has developed a number of successful
multivalve OHC I4 and V6 engines, but relies on pushrod V8s.
VAG
companies like Volkswagen and Audi now use 5-valve engines in many of their
vehicles after acquiring the technology from Bugatti who developed it for their
EB110 supercar.
Poppet valve
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 |
| Poppet valve |
A poppet valve is the type of valve system used in most piston engines, used
to seal the intake and exhaust ports. The valve is usually a flat disk of metal
with a long rod known as the valve stem out one end. The stem is used to push
down on the valve and o
pen it, with a spring generally used to close it when the stem is not being
pushed on. Desmodromic valves are closed by positive mechanical action instead
of by a spring, and are used in some high speed motorcycle and auto racing
engines, eliminating 'valve float' at high RPM.
For certain applications
the valve stem and disk are made of different steel alloys, or the valve stems
may be hollow and filled with sodium to improve heat transport and
transfer.
The engine normally operates the valves by pushing on the stems
with cams and cam followers. The shape and position of the cam determines the
valve lift and when and how quickly (or slowly) the valve is opened. The cams
are normally placed on a fixed camshaft which is then geared to the crankshaft,
running at half crankshaft speed in a four-stroke engine. On high performance
engines e.g. used in Ferrari cars, the camshaft is moveable and the cams have a
varying height, so by axially moving the camshaft in relation with the engine
RPM, also the valve lift varies. See variable valve timing.
In very early
engine designs the valves were 'upside down' in the block, parallel to the
cylinders - the so called L-head engine because of the shape of the cylinder and
combustion space, also called 'flathead engine' as the top of the cylinder head
is flat. Although this design makes for simplified and cheap construction, it
has two major drawbacks; the tortuous path followed by the intake charge
effectively prevents speeds greater than 2,000-2,500 RPM, and the travels of the
exhaust through the block lead to excessive overheating under sustained heavy
load. This design therefore evolved into 'Intake Over Exhaust', IOE or F-head,
where the intake valve was in the block and the exhaust valve was in the head;
later both valves moved to the head.
In most such designs the camshaft
remained relatively near the crankshaft, and the valves were operated through
pushrods and rocker arms. This led to significant energy losses in the engine,
but was simpler, especially in a V engine where one camshaft can actuate the
valves for both cylinder banks; for this reason, pushrod engine designs
persisted longer in these configurations than others.
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More modern designs have the camshaft on top of the cylinder head,
pushing directly on the valve stem (again through cam followers), a system known
as overhead camshaft; if there is just one camshaft, this is a single overhead
cam or SOHC engine. Often there are two camshafts, one for the intake and one
for exhaust valves, creating the dual overhead cam, or DOHC. The camshaft is
driven by the crankshaft - through gears, a chain or in modern engines with a
rubber belt.
In the early days of engine building, the poppet valve was a
major problem. Metallurgy was not what it is today, the rapid opening and
closing of the valves against the cylinder heads led to rapid wear. They would
need to be re-ground every two years or so, in an expensive and time consuming
process known as a valve job. Adding tetra-ethyl lead to the petrol reduced this
problem to some degree as the lead would coat the valve seats, hardening the
metal. Valve seats made of improved alloys such as stellite have generally made
this problem disappear completely and making leaded fuel unnecessary.
The
poppet valve was also used in a limited fashion in steam engines, particularly
steam locomotives. Most steam locomotives used slide valves or piston valves,
but these designs, although mechanically simpler and very rugged, were
significantly less efficient than the poppet valve. A number of designs of
locomotive poppet valve system were tried, the most popular being the Italian
Caprotti valve gear, the British Caprotti valve gear (an improvement of the
Italian one), the German Lentz rotary-cam valve gear, and two American versions
by Franklin, their oscillating-cam valve gear and rotary-cam valve gear. They
were used with some success, but they were less ruggedly reliable than
traditional valve gear and did not see widespread adoption.
Reed Valves
Reed valves consist of thin flexible metal or fiberglass strips fixed on one
end that open and close upon changing pressures across opposite sides of the
valve much like heart valves do. They are intended to restrict flow to a single
direction.
Two-stroke engines
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Reed valves are commonly used in two-stroke engines to control the
fuel-air mixture that is admitted to the engine crankcase. As the piston rises
in the cylinder the resulting vacuum opens the valve and admits the fuel-air
mixture. As the piston descends, it raises the crankcase pressure causing the
valve to close to retain the mixture and pressurize it for its eventual transfer
through to the combustion chamber.
Given the fact that they operate via
air pressure alone, reed valves are not as precise as rotary valves since
physical inertia causes them to open later than the optimum time. Manufacturers
have attempted to address this in part by creating multi-stage reeds with
smaller, more responsive reeds within larger ones that provide more volume later
in the cycle.
The repeated flexing of the valve material eventually
causes metal versions to fatigue and fail to seat properly while fiberglass ones
will merely snap off and be digested by the engine.
Pulse
jets
Reed valves are also used in valved pulse jet engines, such
as the Argus engine in the German V-1 flying bomb. Their function is much the
same as in a piston engine. They are pulled open by a partial vacuum created by
an overexpansion of combustion gasses. The open valves allow a charge of fuel
and air into the engine, which explodes, increasing the internal pressure and
closing the valves. The cycle then repeats.
Sleeve Valve
The Sleeve valve is a type of valve for piston engines that has a number of
advantages over the more common poppet valve, used in most engines, as well as
disadvantages that have precluded its widespread adoption to date. Sleeve valves
were used in some pre-World War II luxury and sports cars and saw substantial
use in 1940s aircraft engines, but fell from use quickly owing to advances in
poppet-valve technology and to their tendency to burn considerable amounts of
lubricating oil.
In a normal engine using poppet valves, the valves are
opened by the camshaft pushing down on the top of the valve, sometimes via a
long pushrod and rocker taking the power from the crankshaft area to the top of
the cylinders. A spring wrapped around the valve stem closes the valve when the
cam stops pushing on it.
The problem with this system is that as the RPM
of the engine increases, the speed at which the valve moves also increases,
which increases the loads involved due to the inertia of the valve, which has to
be opened quickly, brought to a stop, then reverse direction and close and
brought to a stop again. Large valves that allow good flow have considerable
mass, which requires a strong spring to overcome the inertia from this mass. At
some point, the valve inertia overwhelms the spring and stops following the cam
profile, closing well after the cam lobe has moved away. This "valve float" can
eventually cause the valve to not close at all before the cam comes round to
open it again. In some engines, the piston may not be able to travel its full
stroke without colliding with an open valve, which does the piston and the valve
no good at all. Even in "non-interference" engines, at some point the valve head
can simply part from the valve stem due to the inertia effects. Very strong
springs increase friction loads caused by the rubbing of the cam lobe against
the parts that open the valve. Some claim the spring loads also cause simple
mechanical losses (the cam has to push against the spring to open the valve),
but the cam "regains" much of this energy when the spring closes, the valve, as
it helps to push the cam around, as well. Thus, these losses are minor.
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The sleeve valve avoids all this. As the name implies, the valve is
constructed as a sleeve, typically one that fits around the piston inside the
cylinder. Several ports (holes) in the side of the cylinder replace the more
normal intake and exhaust ports on the head. Similar holes in the sleeve open
and close the ports like a poppet valve would, but do so by being rotated into
position. The sleeve has a gear ring on the bottom that runs in a channel, and a
small cut in the cylinder wall exposes the gear so that the sleeve can be
turned.
Another design is more "traditional" in that the sleeve is placed
under the cylinder head. This has the advantage of being easier to build, as
construction of a sleeve strong enough to bear the loads of the piston riding on
it is not all that easy. A similar design rotates the entire cylinder head
instead. However the advantages of this design compared to traditional valve
systems is somewhat limited, and the rotating head version of the sleeve valve
system did not see widespread use.
There is no need for a spring in the
sleeve valve, and the power needed to operate the valve remains largely constant
with RPM – so the system can be used at very high RPM, and with no penalty for
doing so. Furthermore it does away entirely with the camshaft, pushrods and
rockers, replacing them all with a single gear running directly off the
driveshaft. For an aero engine this sort of simplification and weight savings is
an engineer's dream.
Another advantage of the system is that the actual
size of the ports can be easily controlled. This is important when the engine
runs over a wide range of RPM, because the speed at which the air can move into
and out of the cylinder is defined by the size of the leading to the cylinder,
and does not vary linearly with RPM. In other words at high RPM the engine
typically wants larger ports that remain open longer in terms of one cycle,
something that is fairly easy to arrange with a sleeve at the cost of a more
complex gearing system.
Less important advantages include leaving the
cylinder head empty so the spark plug can be placed wherever is best, the valve
is not being continually "hammered" into the port leading to rapid wear, and the
exhaust's heat is spread evenly around the cylinder, rather than generating a
hot spot on the exhaust valves. Hot spots in engines must be avoided, they can
often lead to the destructive problem of knock. In the sleeve valve engine this
is not an issue, so they can be run at higher compression.
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The sleeve has one major disadvantage though, and that is that it can't
be sealed well. In a normal engine the piston is sealed into the cylinder with
rings, and after a "breaking in" period any imperfections in one is scraped into
the other. The result is a tight fit. This sort of fit is not possible on the
sleeve valve however, because the piston and sleeve are moving in different
directions. In the 1940s this was not a major concern because the poppet valves
typically leaked anyway.
The sleeve valve principle was invented in 1903
by an American, Charles Yale Knight. Although he was not able to sell the idea
in the US directly, a trip to Europe got several luxury car firms to sign up and
pay his expensive premiums. He first patented the design in Britain in 1908.
Gabriel Voisin built nearly all of his cars to this design, and contrary to
public opinion, they were fast; many won races. Daimler of England used the
principle in its V-12 which it brand-named the 'Double Six'); another top-level
firm was the Minerva of Belgium. Upon his return to America he was able to get
some firms to use his design; here his brand name was 'Silent Knight' -- the
selling point was that his engines were quieter than those with valves. The most
well-known of these were the Stearns Company of Cleveland, which sold a car
named the Stearns-Knight, and the Willys firm offered a car called the
Willys-Knight.
A number of sleeve valve engines were developed starting
with a seminal research paper by the RAE, published in 1927 by Harry Ricardo.
This paper outlined the advantages of the sleeve valve, and seemed to suggest
that poppet valve engines would not be able to evolve much beyond 1500 hp (1,100
kW). Napier and Bristol started developments of sleeve valve engines that would
eventually result in the two most powerful piston engines in the world, the
Napier Sabre and Bristol Centaurus.
After the war the sleeve valve
rapidly disappeared. As it turned out the problems with sealing and wear on
poppet valves were remedied by better materials, and soon the poppets were
sealing very well indeed. Oil leakage dropped almost to zero, and the power used
by the springs and camshaft was a small price to pay for such a tight seal. The
problem with oil leakage in the sleeve is much more "built into" the system.
Also, the inertia problems of large valves were solved to some extent by using
several smaller valves rather than one large valve. This increases flow area and
reduces mass, while not significantly reducing strength. These multi-valve
engines are now nearly ubiquitous.
Recently the sleeve valve has started
to make something of a comeback, owing largely to the same type of improvement
that led to its demise. Newer materials, and more notably newer and dramatically
better construction techniques, can make a sleeve valve engine that is so
"tight" that it leaks very little oil. However most advanced engine research
continues to look at entirely different designs, like the rotary engine, as
opposed to more conventional improvements like the sleeve.
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Tappet
In mechanical engineering, a tappet is a
projection which imparts a linear motion to some other component within an
assembly. In automotive mechanics, a 'tappet' is a somewhat archaic term which
is falling into disuse being largely replaced by other terms such as rocker-arm.
Properly speaking, a tappet is only that part of a rocker-arm which makes
contact with an intake or exhaust valve stem above the cylinder head of a
gasoline or diesel engine. In an overhead cam engine, the rocker-arm pivots on a
fixed shaft while one projection of the rocker-arm rides on an eccentric cam
lobe of the rotating cam shaft. This creates an oscillating linear motion at the
'tappet' side of the rocker-arm, opening and closing a valve as the tappet
'taps' on the valve stem. The closing of the valve is typically accomplished by
an compression spring placed between the valve collet (or retainer) and the
cylinder head above the combustion chamber.
Traditionally, the nominal
distance (clearance) between the tappet surface and the valve's contact surface
was maintained by means of an adjustment screw on the tappet. Today, this is
typically accomplished by introducing shims into this space to create the
necessary clearance or by hydraulic adjusters.
This traditional mechanism
for opening and closing valves, while tried and true, has its drawbacks. Valve
clearances periodically require adjustment as the contact surfaces of both
tappet and valve stem wear. Also the problem of valve float has for decades
plagued high-rpm engines. This problem occurs when a valve spring cannot close
the valve quickly enough at high engine rpm's. While this has largely been
solved with modern metallurgy, engines in the 20,000 plus rpm range can still
exhibit valve float.
The Ducati motorcycle company partially solved these
problems with their desmodromic valve train. The desmodromic principle attempts
to minimize wear by minimizing clearances between contacting surfaces, and
eliminating the 'tapping' action of valve actuation. The real advantage to a
desmodromic valve however, is that it's positively closed by the mechanism
instead of allowed to close by spring action. Mercedes successfully built and
raced desmodromic Grand Prix engines in the 1950s, but never put them into
production.
In internal combustion engines of the future, the use of
tappets in the mechanical opening and closing of valves may disappear completely
in favor of electronically controlled linear actuators. Motors utilizing this
technology already exist (though production costs are high). The advantages here
are many; valves can be opened or closed dynamically, maximizing power output
and fuel consumption based on changing conditions, and without respect to the
mechanical limitations of a camshaft; valves can be opened or closed almost
instantaneously, eliminating valve float; engines can be used as very effective
downhill-brakes, much like the engine brake of a semi-trailer truck; the seals
and bushings of a traditional valve train could potentially be eliminated,
reducing the overall complexity of the motor.
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VTEC (standing for Variable valve Timing and lift Electronic Control) is a
system developed by Honda Motor Co., Ltd. to improve the combustion efficiency
of its internal combustion engines throughout the RPM
range.
Introduction to VTEC
In the regular
four-stroke automobile engine, the intake and exhaust valves are actuated by
lobes on a camshaft. The shape of the lobes' determine both the timing and the
lift of each valve. Timing refers to when a valve is opened or closed with
respect to the combustion cycle. Lift refers to how much the valve is opened.
Due to the behavior of the gases (air and fuel mixture) before and after
combustion, which have physical limitations on their flow, as well as their
interaction with the ignition spark, the optimal valve timing and lift settings
under low RPM engine operations are very different from those under high RPM.
Optimal low RPM valve timing and lift settings would result in insufficient fuel
and air at high rpm, thus greatly limiting engine power output. Conversely,
optimal high rpm valve timing and lift settings would result in very rough low
RPM operation and difficult idling. The ideal engine would have fully variable
valve timing and lift, in which the valves would always open at exactly the
right point and lift high enough for the engine speed in use.
In
practice, such a perfectly adjustable timing and lift system is complex and
expensive to implement and is therefore found only in costly experimental and
limited production engines. The vast majority of modern automobile engines
operate with a fixed camshaft profile that represents a compromise between low
RPM smoothness and high RPM power output. And since the average automobile
engine spends most of its time running in the low RPM region, there is typically
more emphasis on low RPM smoothness at the expense of high RPM output.
Performance-tuned engines have cam profiles that are optimized more towards high
RPM operation, where the greatest power can be obtained. But this means that low
speed operation is compromised. Anyone who has heard a racing car or a
highly-tuned hot rod sitting at idle will note that the engine sounds like it is
barely capable of running at that speed.
DOHC
VTEC
Honda's VTEC system is a simple and fairly elegant method of
endowing the engine with multiple camshaft profiles optimized for low and high
RPM operations. Instead of only one cam lobe actuating each valve, there are two
- one optimised for low RPM smoothness and one to maximize high RPM power
output. Switching between the two cam lobes is controlled by the engine's
management computer. As engine RPM increases, a locking pin is pushed by oil
pressure to bind the high RPM cam follower for operation. From this point on,
the valve opens and closes according to the high-speed profile, which opens the
valve further and for a longer time. The VTEC system was originally introduced
as a DOHC system in the 1989 Honda Integra sold in Japan, which used a 160 hp
(105 kW) variant of the B16A engine. The US market saw the first VTEC system
with the introduction of the 1990 Acura NSX, which used a DOHC V6. The DOHC VTEC
system has high and low RPM cam lobe profiles on both the intake and exhaust
valve camshafts. This resulted in the most power gain at high RPMs and DOHC VTEC
engines were thus used in the highest performance Honda automobiles. In contrast
to the SOHC implementation which switches between cam profiles seamlessly, when
the DOHC version switches cams there is a definite change in the engine
note.
SOHC VTEC
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As popularity and marketing value of the VTEC system grew, Honda applied
the system to SOHC engines, which shares a common camshaft for both intake and
exhaust valves. The trade-off is that SOHC engines only benefit from the VTEC
mechanism on the intake valves while the exhaust valves are still actuated by a
single cam profile.
SOHC VTEC-E
Honda's next version of
VTEC, VTEC-E, was used in a slightly different way; instead of optimising
performance at high RPMs, it was used to increase efficiency at low RPMs. At low
RPMs, only one of the two intake valves is allowed to open, increasing the
fuel/air mixture's swirl in the cylinder and thus allowing a very lean mixture
to be used. As the engine's speed increases, both valves are needed to supply
sufficient mixture, and thus a sliding pin as in the regular VTEC is used to
connect both valves together and start the second one moving too.
In
North American markets, VTEC-E can be found in Honda's most fuel efficient cars,
including the 1992-1995 Civic VX and 1996-2000 Civic HX.
3-Stage
VTEC
Honda also introduced a 3-stage VTEC system in select markets,
which combines the features of both DOHC VTEC and SOHC VTEC-E. At low speeds,
only one intake valve is used. At medium speeds, two are used. At high speeds,
the engine switches to a high-speed cam profile as in regular VTEC. Thus, both
low-speed economy and high-speed efficiency and power are
improved.
i-VTEC
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i-VTEC introduced continuously variable camshaft phasing on the intake
cam of DOHC VTEC engines. The technology first appeared on Honda's K-series four
cylinder engine family in 2002. Valve lift and duration are still limited to
distinct low and high rpm profiles, but the intake camshaft is now capable of
advancing between 25 and 50 degrees (depending upon engine configuration) during
operation. Phase changes are implemented by a computer controlled, oil driven
adjustable cam gear. Phasing is determined by a combination of engine load and
rpm, ranging from fully retarded at idle to maximum advance at full throttle and
low rpms. The effect is further optimization of torque output, especially at low
and midrange RPMs.
In 2004, Honda introduced an i-VTEC V6 (an update of
the venerable J-series), but in this case, i-VTEC had nothing do to with cam
phasing. Instead, i-VTEC referred to Honda's cylinder deactivation technology
which closes the valves on one bank of (3) cylinders during light load and low
speed (below 80 mph) operation. The technology was originally introduced to the
US on the Honda Odyssey, and can now be found on the Honda Accord Hybrid and the
2006 Honda Pilot. An additional version of i-VTEC was introduced on the 2006
Honda Civic's R-series four cylinder engine. This implementation uses very small
valve lifts at low rpm and light loads, in combination with large throttle
openings (modulated by a drive-by-wire throttle system), to improve fuel economy
by reducing pumping losses.
With the continued introduction of vastly
different i-VTEC systems, one may assume that the term is now a catch all for
creative valve control technologies from Honda.
VTEC in
motorcycles
Apart from the Japanese market-only Honda CB400 Super
Four Hyper VTEC, introduced in 1999, the first worldwide implementation of VTEC
technology in a motorcycle occurred with the introduction of Honda's VFR800
sportbike in 2002. Similar to the SOHC VTEC-E style, one intake valve remains
closed until a threshold of 7000 rpm is reached, then the second valve is opened
by an oil-pressure actuated pin. The dwell of the valves remains unchanged, as
in the automobile VTEC-E, and little extra power is produced but with a
smoothing-out of the torque curve. Critics maintain that VTEC adds little to the
VFR experience while increasing the engine's complexity. Drivability is a
concern for some who are wary of the fact that the VTEC may activate in the
middle of an aggressive corner, upsetting the stability and throttle response of
the bike.
References
Honda Motor Co., Ltd. (2004).
Technology Close-up. Retrieved Sep. 16, 2004.
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Driving with VTEC
The original VTEC technology did
not do all that much to improve engine power or efficiency at low speeds, though
it did mean that Honda did not need to consider high-speed operation at all for
its low-speed cam profile. Thus, this has led some to accuse VTEC of being more
hype than actual improvement for the average driver. The counter-argument is
that with VTEC the higher-speed power is there if the driver needs it. Unlike a
higher displacement or force induced engine of similar power output, VTEC allows
a smaller and more efficient engine. The ability of the VTEC engines to develop
higher RPMs, however, allowed Honda to deliver them with transmissions having
lower gearing, which served to increase the acceleration.
A vehicle
achives its greatest acceleration by keeping the engine RPM as close to the peak
power output as possible, delivering maximum power. For VTEC engines this means
keeping the needle at some rather lofty RPMs, and more frequent shifting to
maintain high RPM. To some people this is a desirable trait: lots of driver
involvement in the process of extracting excellent performance. To others,
especially those accustomed to the Kansas-flat HP curves of muscle cars, the
high RPM and frequent shifts become bothersome. The sound at 8000 (and higher)
rpms, however, is intoxicating to some.
VVT-i
VVT-i, or Variable Valve Timing with Intelligence,
is an automobile variable valve timing technology developed by Toyota. The
latest version of VVT-i varies the timing of the intake valves by adjusting the
timing chain connecting the intake and exhaust camshafts. A pump applies
hydraulic pressure to adjust the gear driving the timing chain.
Toyota
recently started offering a new technology, VVTL-i, which can alter valve lift
(and duration) as well as valve timing. This is accomplished differently than
Honda's VTEC. Instead of switching between two different cam shafts, there are
spacers that are slid between a single cam shaft and the valves. At higher RPM,
controlled by the ECU, oil pressure is directed to push the spacers between the
camshaft lobes and valves, effectively simulating a much "hotter cam" and
creating increased valve lift. When the engine falls in RPM the spacers are
pulled out of action and the engine operates more economically. This system is
used in the Yamaha designed 1.8 liter 4 cylinder 2ZZ-GE which is used in the
Toyota Celica GT-S, Toyota Corolla XRS and Lotus Elise 111R. This variable lift
technology allows the engine to be quite fuel efficient at lower RPMs and, when
called upon, able to produce much more power at the expense of fuel
economy.
For 2006, the company will add dual VVT-i, which varies timing
independently on both the intake and exhaust cams. This will debut on the 2006
Toyota Avalon's 2GR-FE V6.
Variable Valve Timing
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Variable valve timing, or VVT, is a generic term for an automobile piston
engine technology. VVT allows the lift or duration or timing (some or all) of
the intake or exhaust valves (or both) to be changed while the engine is in
operation. Two stroke engines use a Power valve system to get similar results to
VVT.
Overview
Valve timing gears on a Ford Taunus V4
engine — the small gear is on the crankshaft, the larger gear is on the
camshaft. Since the camshaft gear is twice the diameter of the crankshaft gear,
it runs at half the crankshaft RPM. See gear ratio. (The small gear left is on
the balance shaft)
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|
Valve timing gears on a Ford Taunus V4 engine
— the small gear is on the crankshaft, the larger
gear is on the camshaft. Since the camshaft gear
is twice the diameter of the crankshaft gear,
it
runs at half the crankshaft RPM. See gear ratio.
(The small gear left is on the balance
shaft) |
Piston engines normally use poppet valves for intake and exhaust. These are
driven (directly or indirectly) by cams on a camshaft. The cams open the valves
(lift) for a certain amount of time (duration) during each intake and exhaust
cycle. The timing of the valve opening and closing is also important. The
camshaft is driven by the crankshaft through timing belts, gears or
chains.
The profile of these cams is optimized for a certain engine rpm,
and this tradeoff normally limits low-end torque or high-end power. VVT allows
the cam profile to change, which results in greater efficiency and
power.
At high engine speeds, an engine requires large amounts of air.
However, the intake valves may close before all the air has been given a chance
to flow in, reducing performance.
On the other hand, if the cam keeps the
valves open for longer periods of time, like with a racing cam, problems start
to occur at the lower engine speeds. This will cause unburnt fuel to exit the
engine since the valves are still open. This leads to lower engine performance
and increased emissions.
Presure to meet environmental goals and fuel
efficiency standards is forcing car manufacturers to turn to VVT as a solution.
Most simple VVT systems (like Mazda's S-VT) advance or retard the timing of the
intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of
cams at a certain engine rpm. Still others can alter duration and lift
continuously.
History
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The first experimentation with variable valve timing and lift was performed
by General Motors. GM was actually interested in throttling the intake valves in
order to reduce emissions. This was done by minimizing the amount of lift at low
load to keep the intake velocity higher, thereby atomizing the intake charge. GM
encountered problems running at very low lift, and abandoned the
project.
The first functional variable valve timing system, including
variable lift, was developed at Fiat. Developed by Giovanni Torazza in the
1970s, the system used hydraulic pressure to vary the fulcrum of the cam
followers. The hydraulic pressure changed according to engine speed and intake
pressure. The typical opening variation was 37%.
The next big step was
taken by Honda in the late 1980s and 90s, where Honda began by experimenting
with variable valve lift. Pleased with the results, engineers took the knowledge
and applied it to the B16A engine, fitted to the 1989 EF9 Honda Civic. From
there it has been used in a variety of applications, from sport to utility, by
many different auto makers.
In the year 1992, BMW introduced VANOS, their
version of a variable valve timing system, on the BMW M50 engine used in the 3
Series. VANOS significantly enhances emission management, increases output and
torque, and offers better idling quality and fuel economy. The latest version of
VANOS is double-VANOS, used in the new M3. Double-VANOS adds an adjustment of
the intake and outlet camshafts.
Variable valve timing was the sole
domain of overhead cam engines until the 2005, when General Motors began
offering the LZE and LZ4, pushrod V6 engines with VVT. For the 2006 model year,
General Motors will introduce the Vortec 6200, the first mass-produced pushrod
engine with variable valve timing.
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Plumbing Valves
Ball valve
A ball valve (like the butterfly valve, one
of a family of valves called quarter turn valves) is a valve that opens by
turning a handle attached to a ball inside the valve. The ball has a hole, or
port, through the middle so that when the port is in line with both ends of the
valve, flow will occur. When the valve is closed, the hole is perpendicular to
the ends of the valve, and flow is blocked. The handle position lets you "see"
the valve's position.
The body of ball valves may be made of metal,
ceramic, and/or plastic. The ball may be chrome plated to make it more
durable.
There are three general types of ball valves: full port,
standard port, and reduced port.
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|
schematic 3 way ball valve |
* A full port ball valve has an oversized ball so that the hole in the ball
is the same size as the pipeline resulting in lower
friction loss. Flow is unrestricted.
* A standard port ball valve is
usually less expensive, but has a smaller ball and a correspondingly smaller
port. Flow through this valve is one pipe size smaller than the valve's pipe
size resulting in slightly restricted flow.
* In reduced port ball
valves, flow through the valve is two pipe sizes smaller than the valve's pipe
size resulting in restricted flow.
* A "trunnion" ball valve has a
mechanical means of anchoring the ball at the top and the
bottom.
Manually operated ball valves can often be closed quickly and
thus there is a danger of water hammer. Some ball valves are equipped with an
actuator that may be pneumatically or motor operated. These valves can be used
either for on/off or flow control. A pneumatic flow control valve is also
equipped with a positioner which transforms the control signal into actuator
position and valve opening accordingly.
There are also three-way ball
valves, with a T-shaped hole through the middle. With such a valve the flow can
be directed to either one or the other or both sides or be closed off
completely.
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Check valve
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| this siamese clappered inlet allows
one or two inputs into a deluge gun |
A check valve is a mechanical device, a valve, that normally allows fluid to
flow through it in only one direction. A double check valve is often used as a
backflow prevention device to keep potentially contaminated water from siphoning
back into municipal water supply lines. A clapper valve is a type of check valve
used in or with firefighting, and has a hinged gate (often with a spring urging
it shut) that will only remain open in the outflowing direction.
Some
types of irrigation sprinklers and drip irrigation emitters have small check
valves built into them to keep the lines from draining when the system is shut
off.
Nikola Tesla invented a deceptively simple one-way valve for fluids
in 1920 (U.S. patent # 1,329,559).
In electronics, a diode functions in
the same manner.
Chemigation valve
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A chemigation valve is an apparatus designed to protect water supplies
from agricultural chemicals used during chemigation, the application of
chemicals such as fertilizers and pesticides through irrigation water. Most
chemigation valves consist of a spring-loaded check valve, a low pressure drain,
an air and vacuum relief valve, and an injection port for introducing the
chemicals downstream of the check valve. Many chemigation valves also have a 4
inch inspection port so that a person can reach inside and feel if the check
valve is still functional. Some governments require the use of two chemigation
valves installed in series if hazardous chemicals are to be injected.
Gate valve
A gate valve is a valve that opens by lifting
a round or rectangular gate out of the path of the fluid. Gate valves are
sometimes
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| Gate valve |
used for regulating flow, but many are not suited for that purpose,
having been designed to be fully opened or closed. When fully open, the typical
gate valve has no obstruction in the flow path, resulting in very low friction
loss.
All gate valves have a rising or a nonrising stem. Rising stems
provide a visual indication of valve position. Nonrising stems are used where
vertical space is limited or underground.
Bonnets provide leakproof
closure for the valve body. Gate valves may have a screw-in, union, or bolted
bonnet. Screw-in bonnet is the simplest, offering a durable, pressure-tight
seal. Union bonnet is suitable for applications requiring frequent inspection
and cleaning. It also gives the body added strength. Bolted bonnet is used for
larger valves and higher pressure applications.
Globe valve
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Globe valves are named for their spherical body shape. The two
halves of the valve body are separated by a baffle with a disc i
n the center. Globe valves operate by screw action of the handwheel. They are
used for applications requiring throttling and frequent operation. Since the
baffle restricts flow, they're not recommended where full, unobstructed flow is
required.
A bonnet provides leakproof closure for the valve body. Globe
valves may have a screw-in, union, or bolted bonnet. Screw-in bonnet is the
simplest bonnet, offering a durable, pressure-tight seal. Union bonnet is
suitable for applications requiring frequent inspection or cleaning. It also
gives the body added strength. Bolted bonnet is used for larger or higher
pressure applications.
Many globe valves have a class rating that
corresponds to the pressure specifications of ANSI 16.34.
