A poppet valve (also sometimes called mushroom valve ) is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications.
85-536: Variable valve timing ( VVT ) is the process of altering the timing of a valve lift event in an internal combustion engine , and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems. Two-stroke engines use
170-480: A power valve system to get similar results to VVT. The valves within an internal combustion engine are used to control the flow of the intake and exhaust gases into and out of the combustion chamber . The timing, duration and lift of these valve events has a significant impact on engine performance. Without variable valve timing or variable valve lift , the valve timing is the same for all engine speeds and conditions, therefore compromises are necessary to achieve
255-411: A pushrod which transfers the motion to the top of the engine, where a rocker opens the intake/exhaust valve. Although largely replaced by SOHC and DOHC layouts in modern automobile engines, the older overhead valve layout is still used in many industrial engines, due to its smaller size and lower cost. As engine speeds increased through the 20th century, single overhead camshaft (SOHC) engines— where
340-561: A slide valve . Camshafts more like those seen later in internal combustion engines were used in some steam engines, most commonly where high pressure steam (such as that generated from a flash steam boiler ), required the use of poppet valves, or piston valves. For examples see the Uniflow steam engine , and the Gardner-Serpollet steam cars, which also included axially sliding the camshaft to achieve variable valve timing. Among
425-403: A two-stroke engine that uses a camshaft, each valve is opened once for every rotation of the crankshaft; in these engines, the camshaft rotates at the same speed as the crankshaft. In a four-stroke engine , the valves are opened only half as often, therefore the camshaft is geared to rotate at half the speed of the crankshaft. The camshaft's duration determines how long the intake/exhaust valve
510-477: A 4.4 L engine for a proposed replacement for the existing 30-98 model to be called the H-Type. In this engine the single overhead camshaft was to move longitudinally to allow different camshaft lobes to be engaged. It was in the 1920s that the first patents for variable duration valve opening started appearing – for example United States patent U.S. patent 1,527,456 . In 1958 Porsche made application for
595-478: A German Patent, also applied for and published as British Patent GB861369 in 1959. The Porsche patent used an oscillating cam to increase the valve lift and duration. The desmodromic cam driven via a push/pull rod from an eccentric shaft or swashplate . It is unknown if any working prototype was ever made. Fiat was the first auto manufacturer to patent a functional automotive variable valve timing system which included variable lift. Developed by Giovanni Torazza in
680-415: A VVT system requires a complex system, such as multiple cam profiles or oscillating cams. Late intake valve closing (LIVC) The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which
765-537: A cam phaser, controlled by the ECM, which continuously varies advancement or retardation of the camshaft timing. In 2007, Caterpillar developed the C13 and C15 Acert engines which used VVT technology to reduce NOx emissions, to avoid the use of EGR after 2002 EPA requirements. In 2010, Mitsubishi developed and started mass production of its 4N13 1.8 L DOHC I4, the world's first passenger car diesel engine that features
850-409: A camshaft are usually either: Many early internal combustion engines used a cam-in-block layout (such flathead , IOE or T-head layouts), whereby the camshaft is located within the engine block near the bottom of the engine. Early flathead engines locate the valves in the block and the cam acts directly on those valves. In an overhead valve engine, which came later, the cam follower presses on
935-520: A conventional cam lobe, while others use an eccentric cam lobe and a connecting rod. The principle is similar to steam engines, where the amount of steam entering the cylinder was regulated by the steam "cut-off" point. The advantage of this design is that adjustment of lift and duration is continuous. However, in these systems, lift is proportional to duration, so lift and duration cannot be separately adjusted. The BMW ( valvetronic ), Nissan ( VVEL ), and Toyota ( valvematic ) oscillating cam systems act on
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#17327870699351020-402: A different RPM range. Advancing the camshaft (shifting it to ahead of the crankshaft timing) increases low RPM torque, while retarding the camshaft (shifting it to after the crankshaft) increases high RPM power. The required changes are relatively small, often in the order of 5 degrees. Modern engines which have variable valve timing are often able to adjust the timing of the camshaft to suit
1105-503: A hammer used in forging or to pound grain. Evidence for these exists back to the Han dynasty in China, and they were widespread by the medieval period. Once the rotative version of the steam engine was developed in the late 18th century, the operation of the valve gear was usually by an eccentric , which turned the rotation of the crankshaft into reciprocating motion of the valve gear, normally
1190-515: A mechanical VVT system. The system was engineered by Ing Giampaolo Garcea in the 1970s. All Alfa Romeo Spider models from 1983 onward used electronic VVT. In 1989, Honda released the VTEC system. While the earlier Nissan NVCS alters the phasing of the camshaft, VTEC switches to a separate cam profile at high engine speeds to improve peak power. The first VTEC engine Honda produced was the B16A which
1275-483: A number of cams (discs with protruding cam lobes ) along its length, one for each valve. As the cam rotates, the lobe presses on the valve (or an intermediate mechanism), thus pushing it open. Typically, a valve spring is used to push the valve in the opposite direction, thus closing the valve once the cam rotates past the highest point of its lobe. Camshafts are made from metal and are usually solid, although hollow camshafts are sometimes used. The materials used for
1360-459: A plug, usually a disk shape on the end of a shaft known as a valve stem. The working end of this plug, the valve face, is typically ground at a 45° bevel to seal against a corresponding valve seat ground into the rim of the chamber being sealed. The shaft travels through a valve guide to maintain its alignment. A pressure differential on either side of the valve can assist or impair its performance. In exhaust applications higher pressure against
1445-414: A pressure differential for opening and closing while being inflated. Poppet valves are employed extensively in the launching of torpedoes from submarines . Many systems use compressed air to expel the torpedo from the tube , and the poppet valve recovers a large quantity of this air (along with a significant amount of seawater) in order to reduce the tell-tale cloud of bubbles that might otherwise betray
1530-476: A seat to uncover a port, the poppet valve lifts from the seat with a movement perpendicular to the plane of the port. The main advantage of the poppet valve is that it has no movement on the seat, thus requiring no lubrication. In most cases it is beneficial to have a "balanced poppet" in a direct-acting valve. Less force is needed to move the poppet because all forces on the poppet are nullified by equal and opposite forces. The solenoid coil has to counteract only
1615-420: A steeper camshaft profile is required, which increases the forces needed to open the valve. A related issue is valve float at high RPM, where the spring tension does not provide sufficient force to either keep the valve following the cam at its apex or prevent the valve from bouncing when it returns to the valve seat. This could be a result of a very steep rise of the lobe, where the cam follower separates from
1700-401: A variable valve timing system. Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. These names include: This method uses two cam profiles, with an actuator to swap between the profiles (usually at a specific engine speed). Cam switching can also provide variable valve lift and variable duration, however the adjustment
1785-414: A wider LSA to compensate for excessive duration can reduce power and torque outputs. In general, the optimal LSA for a given engine is related to the ratio of the cylinder volume to intake valve area. Camshafts are integral components of internal combustion engines, responsible for controlling the opening and closing of the engine's intake and exhaust valves. As the camshaft rotates, its lobes push against
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#17327870699351870-449: Is a diminutive of poupée . The use of the word poppet to describe a valve comes from the same word applied to marionettes , which, like the poppet valve, move bodily in response to remote motion transmitted linearly. In the past, "puppet valve" was a synonym for poppet valve ; however, this usage of "puppet" is now obsolete. The poppet valve is different from both slide and oscillating valves. Instead of sliding or rocking over
1955-471: Is acted on by two lobes simultaneously. Each camshaft has a phasing mechanism which allows its angular position relative to the engine's crankshaft to be adjusted. One lobe controls the opening of a valve and the other controls the closing of the same valve, therefore variable duration is achieved through the spacing of these two events. The drawbacks to this design include: This system is not known to be used in any production engines. The operating principle
2040-419: Is discrete rather than continuous. The first production use of this system was Honda's VTEC system. VTEC changes hydraulic pressure to actuate a pin that locks the high lift, high duration rocker arm to an adjacent low lift, low duration rocker arm(s). Many production VVT systems are the cam phasing type, using a device known as a variator which changes the phase (Phase refers to the relative timing between
2125-429: Is driven by the crankshaft through timing belts , gears or chains . An engine requires large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. Opening
2210-448: Is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure. Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide ( NOx ) emissions by 24%. Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged. Early intake valve closing (EIVC) Another way to decrease
2295-413: Is freed from this constraint, allowing performance to be improved over the engine operating range. Piston engines normally use valves which are driven by camshafts . The cams open ( lift ) the valves kind for a certain amount of time ( duration ) during each intake and exhaust cycle. The timing of the valve opening and closing, relative to the position of the crankshaft, is important. The camshaft
2380-441: Is of this type. Also known as "combined two shaft coaxial combined profile with helical movement", this system is not known to be used in any production engines. It has a similar principle to the previous type, and can use the same base duration lobe profile. However instead of rotation in a single plane, the adjustment is both axial and rotational giving a helical or three-dimensional aspect to its movement. This movement overcomes
2465-582: Is often used as a standard measurement procedure, since this is considered most representative of the lift range that defines the RPM range in which the engine produces peak power. The power and idle characteristics of a camshaft with the same duration rating that has been determined using different lift points (for example 0.006 or 0.002 inches) could be much different to a camshaft with a duration rated using lift points of 0.05 inches. A secondary effect of increased duration can be increased overlap , which determines
2550-430: Is open for, therefore it is a key factor in the amount of power that an engine produces. A longer duration can increase power at high engine speeds (RPM), however this can come with the trade-off of less torque being produced at low RPM. The duration measurement for a camshaft is affected by the amount of lift that is chosen as the start and finish point of the measurement. A lift value of 0.050 in (1.3 mm)
2635-525: Is present around the valve stem, therefore a valve stem oil seal is used to prevent oil being drawn into the intake manifold and combustion chamber. Typically, a rubber lip-type seal is used. A common symptom of worn valve guides and/or defective oil seals is a puff of blue smoke from the exhaust pipe at times of increased intake manifold vacuum , such as when the throttle is abruptly closed. Historically, valves had two major issues, both of which have been solved by improvements in modern metallurgy . The first
Variable valve timing - Misplaced Pages Continue
2720-404: Is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions. Early intake valve opening Early intake valve opening is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature. By opening the intake valve early, some of
2805-433: Is that the cam and follower profiles must be carefully designed to minimise contact stress (due to the varying profile). Ferrari is commonly associated with this system, however it is unknown whether any production models to date have used this system. This system is not known to be used in any production engines. It consists of two (closely spaced) parallel camshafts, with a pivoting follower that spans both camshafts and
2890-408: Is that the one follower spans the pair of closely spaced lobes. Up to the angular limit of the nose radius the follower 'sees' the combined surface of the two lobes as a continuous, smooth surface. When the lobes are exactly aligned the duration is at a minimum (and equal to that of each lobe alone) and when at the extreme extent of their misalignment the duration is at a maximum. The basic limitation of
2975-412: Is two eccentric drives and controllers are needed for each cylinder (one for the intake valves and one for the exhaust valves), which increases complexity and cost. MG Rover is the only manufacturer that has released engines using this system. This system consists of a cam lobe that varies along its length (similar to a cone shape). One end of the cam lobe has a short duration/reduced lift profile, and
3060-527: Is usually by pressing on the end of the valve stem, with a spring generally being used to return the valve to the closed position. At high engine speeds ( RPM ), the weight of the valvetrain means the valve spring cannot close the valve as quickly enough, leading to valve float or valve bounce . Desmodromic valves use a second rocker arm to mechanically close the valves (instead of using valve springs) and are sometimes used to avoid valve float in engines that operate at high RPM. In most mass-produced engines,
3145-440: Is usually referred to as a double overhead camshaft engine (although colloquially they are sometimes referred to as "quad-cam" engines). Accurate control of the position and speed of the camshaft is critically important in allowing the engine to operate correctly. The camshaft is usually driven either directly, via a toothed rubber "timing belt"' or via a steel roller "timing chain". Gears have also occasionally been used to drive
3230-416: The camshaft 25 times per second, so the valve timing events have to occur at precise times to offer performance benefits. Electromagnetic and pneumatic camless valve actuators offer the greatest control of precise valve timing, but, in 2016, are not cost-effective for production vehicles. The history of the search for a method of variable valve opening duration goes back to the age of steam engines when
3315-415: The camshaft (s) control the opening of the valves, via several intermediate mechanisms (such as pushrods , roller rockers and valve lifters ). The shape of the cams on the camshaft influence the valve lift and determine the timing of when the valves open. Early flathead engines (also called L-head engines ) saw the valves located beside to the cylinder(s), in an "upside down" orientation parallel to
3400-477: The 1890s and 1900s used an "automatic" intake valve, which was opened by the vacuum in the combustion chamber and closed by a light spring. The exhaust valve had to be mechanically driven to open it against the pressure in the cylinder. Use of automatic valves simplified the mechanism, but valve float limited the speed at which the engine could run, and by about 1905 mechanically operated inlet valves were increasingly adopted for vehicle engines. Mechanical operation
3485-547: The 1920s when maximum allowable RPM limits were generally starting to rise. Until about this time an engine's idle RPM and its operating RPM were very similar, meaning that there was little need for variable valve duration. The first use of variable valve timing was on the 1903 Cadillac Runabout and Tonneau created by Alanson Partridge Brush Patent 767,794 “INLET VALVE GEAR FOR INTERNAL COMBUSTION ENGINES” filed August 3, 1903, and granted August 16, 1904. Some time prior to 1919 Lawrence Pomeroy, Vauxhall's Chief Designer, had designed
Variable valve timing - Misplaced Pages Continue
3570-487: The Lobe Separation Angle is increased to compensate. A lay person can readily spot a long duration camshaft by observing the broad surface of the lobe where the cam pushes the valve open for a large number of degrees of crankshaft rotation. This will be visibly greater than the more pointed camshaft lobe bump that is observed on lower duration camshafts. The camshaft's lift determines the distance between
3655-412: The RPM of the engine at any given time. This avoids the above compromise required when choosing a fixed cam timing for use at both high and low RPM. The lobe separation angle (LSA, also called lobe centreline angle ) is the angle between the centreline of the intake lobes and the centreline of the exhaust lobes. A higher LSA reduces overlap, which improves idle quality and intake vacuum, however using
3740-423: The boat's submerged position. Poppet valves are used in most piston engines to control the flow of intake and exhaust gasses through the cylinder head and into the combustion chamber . The side of the poppet valve which sits inside the combustion chamber is a flat disk, while the other side tapers from the disk shape to a thin cylindrical rod called a "valve stem". In a typical modern mass-production engines,
3825-407: The cam lobe (due to the valvetrain inertia being greater than the closing force of the valve spring), leaving the valve open for longer than intended. Valve float causes a loss of power at high RPM and in extreme situations can result in a bent valve if it gets struck by the piston. The timing (phase angle) of the camshaft relative to the crankshaft can be adjusted to shift an engine's power band to
3910-497: The camshaft by the governor. The Serpollet steamcars produced very hot high pressure steam, requiring poppet valves, and these used a patented sliding camshaft mechanism, which not only varied the inlet valve cut-off but allowed the engine to be reversed. An early experimental 200 hp Clerget V-8 from the 1910s used a sliding camshaft to change the valve timing. Some versions of the Bristol Jupiter radial engine of
3995-458: The camshaft is located within the cylinder head near the top of the engine— became increasingly common, followed by double overhead camshaft (DOHC) engines in more recent years. For OHC and DOHC engines, the camshaft operates the valve directly or via a short rocker arm. The valvetrain layout is defined according to the number of camshafts per cylinder bank. Therefore, a V6 engine with a total of four camshafts - two camshafts per cylinder bank -
4080-496: The camshaft located at the bottom of the engine). In turn, OHV engines were largely replaced by the overhead camshaft (OHC) engines between 1950s until 1980s. The location of the valves is broadly the same between OHV and OHC engines, however OHC engines saw the camshaft located to the top of the engine with the valves and OHC engines often have more valves per cylinder. Most OHC engines have an extra intake and an extra exhaust valve per cylinder (four-valve cylinder head), compared with
4165-540: The camshaft. In some designs the camshaft also drives the distributor , oil pump , fuel pump and occasionally the power steering pump. Alternative drive systems used in the past include a vertical shaft with bevel gears at each end (e.g. pre-World War I Peugeot and Mercedes Grand Prix Cars and the Kawasaki W800 motorcycle) or a triple eccentric with connecting rods (e.g. the Leyland Eight car). In
4250-402: The cylinder. Although this design made for simplified and cheap construction, the twisting path of the intake and exhaust gasses had major drawbacks for the airflow, which limited engine RPM and could cause the engine block to overheat under sustained heavy load. The flathead design evolved into intake over exhaust (IOE) engine , used in many early motorcycles and several cars. In an IOE engine,
4335-487: The design of two valves per cylinder used by most OHV engines. However some OHC engines have used three or five valves per cylinder. James Watt was using poppet valves to control the flow of steam into the cylinders of his beam engines in the 1770s. A sectional illustration of Watt's beam engine of 1774 using the device is found in Thurston 1878:98, and Lardner (1840) provides an illustrated description of Watt's use of
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#17327870699354420-429: The desired result in intake and exhaust efficiency . This has been described in simulations. Practical results will vary based on available ambient combustion cycle gases in a naturally aspirated system, or forced air geometry as well as fuel pulse width timing and other factors which may or may not be available on vehicles equipped with variable valve timing. An engine equipped with a variable valve timing actuation system
4505-484: The development of the Corliss valve . These were widely used in constant speed variable load stationary engines, with admission cutoff, and therefore torque, mechanically controlled by a centrifugal governor and trip valves . As poppet valves came into use, a simplified valve gear using a camshaft came into use. With such engines, variable cutoff could be achieved with variable profile cams that were shifted along
4590-436: The distance that the valve opens (the valve lift ) is greater than the distance from the peak of the camshaft's lobe to the base circle (the camshaft lift ). There are several factors which limit the maximum amount of lift possible for a given engine. Firstly, increasing lift brings the valves closer to the piston, so excessive lift could cause the valves to get struck and damaged by the piston. Secondly, increased lift means
4675-469: The early 1920s incorporated variable valve timing gear, mainly to vary the inlet valve timing in connection with higher compression ratios. The Lycoming R-7755 engine had a Variable Valve Timing system consisting of two cams that can be selected by the pilot. One for take off, pursuit and escape, the other for economical cruising. The desirability of being able to vary the valve opening duration to match an engine's rotational speed first became apparent in
4760-421: The exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward. By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke. By closing
4845-522: The first cars to utilize engines with single overhead camshafts were the Maudslay, designed by Alexander Craig and introduced in 1902 and the Marr Auto Car designed by Michigan native Walter Lorenzo Marr in 1903. In piston engines , the camshaft is used to operate the intake and exhaust valves . The camshaft consists of a cylindrical rod running the length of the cylinder bank with
4930-549: The inert/combusted exhaust gas will back flow out of the cylinder via the intake valve, where it cools momentarily in the intake manifold. This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the temperature of the cylinder and nitric oxide emissions. It also improves volumetric efficiency, because there is less exhaust gas to be expelled on the exhaust stroke. Early/late exhaust valve closing Early and late exhaust valve closing timing can be manipulated to reduce emissions. Traditionally,
5015-436: The inlet and exhaust camshafts, expressed as an angular measure.) of the camshaft and valves. This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however the duration and lift cannot be adjusted. These designs use an oscillating or rocking motion in a part cam lobe, which acts on a follower. This follower then opens and closes the valve. Some oscillating cam systems use
5100-411: The intake and exhaust valves), mechanically controlled ignition systems and early electric motor speed controllers . Camshafts in piston engines are usually made from steel or cast iron, and the shape of the cams greatly affects the engine's characteristics. Trip hammers are one of the early uses of a form of cam to convert rotating motion, e.g. from a waterwheel, into the reciprocating motion of
5185-687: The intake valve while the exhaust valve is still open may cause unburnt fuel to exit the engine, leading to lower engine performance and increased emissions. According to engineer David Vizard's book "Building Horsepower", when both intake & exhaust are open simultaneously, the much-higher-pressure exhaust pushes the intake-charge back, out from the cylinder, polluting the intake-manifold with exhaust, in worst cases. Early variable valve timing systems used discrete (stepped) adjustment. For example, one timing would be used below 3500 rpm and another used above 3500 rpm. More advanced "continuous variable valve timing" systems offer continuous (infinite) adjustment of
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#17327870699355270-411: The intake valves only. Eccentric cam drive systems operates through an eccentric disc mechanism which slows and speeds up the angular speed of the cam lobe during its rotation. Arranging the lobe to slow during its open period is equivalent to lengthening its duration. The advantage of this system is that duration can be varied independent of lift (however this system does not vary lift). The drawback
5355-400: The intake valves were located directly above the cylinder (like the later overhead valve engines ), however the exhaust valve remains beside the cylinder in an upside down orientation. These designs were largely replaced by the overhead valve (OHV) engine between 1904 until late-1960s/early-to-mid 1970s, whereby the intake and exhaust valves are both located directly above the cylinder (with
5440-419: The late 1960s, the system used hydraulic pressure to vary the fulcrum of the cam followers (US Patent 3,641,988). The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%. Alfa Romeo was the first manufacturer to use a variable valve timing system in production cars (US Patent 4,231,330). The fuel injected models of the 1980 Alfa Romeo Spider 2000 had
5525-400: The length of time that both the intake and exhaust valves are open. It is overlap which most affects idle quality, in as much as the "blow-through" of the intake charge immediately back out through the exhaust valve which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation. In general, increasing a camshaft's duration typically increases the overlap, unless
5610-462: The mid-1990s. Exhaust valves are subject to very high temperatures and in extreme high performance applications may be sodium cooled. The valve is hollow and filled with sodium, which melts at a relatively low temperature and, in its liquid state, convects heat away from the hot valve head to the stem where it may be conducted to the cylinder head. Common in second world war piston engines, now only found in high performance engines. Early engines in
5695-483: The other end has a longer duration/greater lift profile. In between, the lobe provides a smooth transition between these two profiles. By shifting area of the cam lobe which is in contact with the follower, the lift and duration can be continuously altered. This is achieved by moving the camshaft axially (sliding it across the engine) so a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration. The downside to this arrangement
5780-402: The poppet valve. When used in high-pressure applications, for example, as admission valves on steam engines, the same pressure that helps seal poppet valves also contributes significantly to the force required to open them. This has led to the development of the balanced poppet or double beat valve , in which two valve plugs ride on a common stem, with the pressure on one plug largely balancing
5865-723: The pressure on the other. In these valves, the force needed to open the valve is determined by the pressure and the difference between the areas of the two valve openings. Sickels patented a valve gear for double-beat poppet valves in 1842. Criticism was reported in the journal Science in 1889 of equilibrium poppet valves (called by the article the "double or balanced or American puppet valve") in use for paddle steamer engines, that by its nature it must leak 15 percent. Poppet valves have been used on steam locomotives , often in conjunction with Lentz or Caprotti valve gear . British examples include: Sentinel Waggon Works used poppet valves in their steam wagons and steam locomotives. Reversing
5950-615: The pumping losses associated with low engine speed, high vacuum conditions is by closing the intake valve earlier than normal. This involves closing the intake valve midway through the intake stroke. Air/fuel demands are so low at low-load conditions and the work required to fill the cylinder is relatively high, so Early intake valve closing greatly reduces pumping losses. Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%. It also reduced nitric oxide emissions by 24% at partial load conditions. A possible downside to early intake valve closing
6035-452: The restricted duration range in the previous type. The duration range is theoretically unlimited but typically would be of the order of one hundred crankshaft degrees, which is sufficient to cover most situations. The cam is reportedly difficult and expensive to produce, requiring very accurate helical machining and careful assembly. Poppet valve It consists of a hole or open-ended chamber, usually round or oval in cross-section, and
6120-497: The scheme is that only a duration variation equal to that of the lobe nose true radius (in camshaft degrees or double this value in crankshaft degrees) is possible. In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees. This is the principle behind what seems to be the very first variable cam suggestion appearing in the USPTO patent files in 1925 (1527456). The "Clemson camshaft"
6205-435: The spring force. Poppet valves are best known for their use in internal combustion and steam engines, but are used in general pneumatic and hydraulic circuits where a pulsed flow control is wanted. The pulse can be controlled by a combination of differential pressure and spring load as required. Presta and Schrader valves used on pneumatic tyres are examples of poppet valves. The Presta valve has no spring and relies on
6290-404: The valve and the valve seat (i.e. how far open the valve is). The farther the valve rises from its seat the more airflow can be provided, thus increasing the power produced. Higher valve lift can have the same effect of increasing peak power as increased duration, without the downsides caused by increased valve overlap. Most overhead valve engines have a rocker ratio of greater than one, therefore
6375-628: The valve helps to seal it, and in intake applications lower pressure helps open it. The poppet valve was invented in 1833 by American E.A.G. Young of the Newcastle and Frenchtown Railroad . Young had patented his idea, but the Patent Office fire of 1836 destroyed all records of it. The word poppet shares etymology with " puppet ": it is from the Middle English popet ("youth" or "doll"), from Middle French poupette , which
6460-435: The valve opening duration was referred to as "steam cut-off ”. The Stephenson valve gear , as used on early steam locomotives, supported variable cutoff , that is, changes to the time at which the admission of steam to the cylinders is cut off during the power stroke. Early approaches to variable cutoff coupled variations in admission cutoff with variations in exhaust cutoff. Admission and exhaust cutoff were decoupled with
6545-436: The valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions. The main factor preventing this technology from wide use in production automobiles is the ability to produce a cost-effective means of controlling the valve timing under the conditions internal to an engine. An engine operating at 3000 revolutions per minute will rotate
6630-420: The valve timing. Therefore, the timing can be optimized to suit all engine speeds and conditions. The simplest form of VVT is cam-phasing , whereby the phase angle of the camshaft is rotated forwards or backwards relative to the crankshaft. Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered solely with a cam-phasing system. Achieving variable duration on
6715-426: The valves are solid and made from steel alloys . However some engines use hollow valves filled with sodium , to improve heat transfer . Many modern engines use an aluminium cylinder head. Although this provides better heat transfer, it requires steel valve seat inserts to be used; in older cast iron cylinder heads, the valve seats are often part of the cylinder head. A gap of 0.4–0.6 mm (0.016–0.024 in)
6800-456: The valves commonly failed because the locomotives were commonly operated in excess of 160 km/h (100 mph), and the valves were not meant for the stresses of such speeds. The poppet valves also gave the locomotive a distinctive "chuffing" sound. Camshaft A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion . Camshafts are used in piston engines (to operate
6885-483: The valves, allowing the intake of air and fuel and the expulsion of exhaust gases. This synchronized process is crucial for optimizing engine performance, fuel efficiency, and emissions control. Without precisely engineered camshafts, the smooth and efficient operation of an engine would be compromised. The most common methods of valve actuation involve camshafts and valve springs, however alternate systems have occasionally been used on internal combustion engines: Before
6970-660: Was achieved by a simple sliding camshaft system. Many locomotives in France, particularly those rebuilt to the designs of Andre Chapelon, such as the SNCF 240P , used Lentz oscillating-cam poppet valves, which were operated by the Walschaert valve gear the locomotives were already equipped with. The poppet valve was also used on the American Pennsylvania Railroad 's T1 duplex locomotives , although
7055-1044: Was considered a non-useful "technological showpiece" as late as 2004 due to the system's weight penalty. Since then, motorcycles including VVT have included the Kawasaki 1400GTR/Concours 14 (2007), the Ducati Multistrada 1200 (2015), the BMW R1250GS (2019) and the Yamaha YZF-R15 V3.0 (2017), the Suzuki GSX-R1000R 2017 L7, the Moto Guzzi V85TT, the Harley Davidson Milwaukee-Eight, the KTM 1390 Super Duke. Variable valve timing has begun to trickle down to marine engines. Volvo Penta 's VVT marine engine uses
7140-560: Was installed in the Integra , CRX , and Civic hatchback available in Japan and Europe. In 1992, Porsche first introduced VarioCam , which was the first system to provide continuous adjustment (all previous systems used discrete adjustment). The system was released in the Porsche 968 and operated on the intake valves only. Variable valve timing has been applied to motorcycle engines but
7225-476: Was that in early internal combustion engines, high wear rates of valves meant that a valve job to regrind the valves was required at regular intervals. Secondly, lead additives had been used in petrol (gasoline) since the 1920s, to prevent engine knocking and provide lubrication for the valves. Modern materials for the valves (such as stainless steel) and valve seats (such as stellite ) allowed for leaded petrol to be phased out in many industrialised countries by
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