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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.
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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)

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.

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

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

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.