Control valve

A valve controller or a valve drive is a mechanism for a reciprocating engine , the valves and thus the charge exchange controlled by opening and closing the air intake and exhaust gas outlet ducts.

He is in almost all four-stroke engines used – exceptions like the valve engine are rare. In two-stroke engines by contrast, mainly no valves are used, except for large two-stroke diesel engines , which have an outlet.

In operation valves several times during each second – at nearly twice marine diesels, but about a hundred times in racing engines – opened and closed, so accelerated and brought to a standstill. The movements of the Vertile and the piston in the cylinder must be carefully matched.

Usually a valve is a camshaft , by the crankshaft is driven by a plunger opens. The few exceptions are pneumatic – hydraulic – or solenoid valves . Valves are closed coil springs , rare by torsion springs , pneumatic or Desmodromic .

Types

While the valves are predominantly uniformly, there are significant differences in design of the valve assembly, the valve actuator, the camshaft and the camshaft drive arrangement common designs.
Basic Types

Fundamental distinction is according to the arrangement of valves and camshafts to the cylinder.
Lever and linkage at IOE
Side valve controller
OHV with pushrods and rocker arms
DOHC with bucket tappets
poppet

At the first motor only the exhaust valve has been controlled, the inlet valve is “automatically” is opened by the negative pressure of the priming piston. The poppet valve closes at the end of the intake stroke by a spring, the exhaust valve has been actuated by means of a bumper cam. Motors with sniffer valves can reach their design very low speeds. The Daimler riding car engine reached 700 min -1 and the De Dion-Bouton engine at the end of its development 1500-1800 min -1.
IOE

When IOE (inlet over exhaust) “hangs” the intake valve in the cylinder head and is driven by a camshaft via bumper and rocker arms. Including the vertically arranged outlet valve is controlled directly by the camshaft. IOE is also known as “change” or “unchecked”. The valve control IOE has mostly exposed rocker arms, so that the lubrication could be a problem. With IOE in Anglican speaking also the poppet is called, which is above the exhaust valve.
side valve

A very simple, common today only at power generators and lawnmowers construction is SV (Side Valves, “lateral valves”, even Standing Valves, “standing valve”): The camshaft is the crankshaft and knocking over bars in addition to the cylinder “standing “valves in the cylinder head into it on. The design of the cylinder head is called “Flathead” and is known since 1904. [1] The cylinder head is when only a simple side valve cover. The cylinder block is in turn complex because in addition to the piston bore includes the gas channels and the valve guides. The combustion chamber has a large unfavorable surface. Accordingly, this type of valve control is only for low compression ratios and low speeds suitable. Advantages are the flat design of the motor and the simple casting of the cylinder head.

A special form of side-valve engine, the T-head engine , are mounted at the inlet and outlet on the opposite side. This design with cross flow cylinder head was in the early 20 Century for powerful engines (such as Mercedes , Laurin & Klement , Stutz , American LaFrance spread), but has no relevance today.
OHV

In OHV “hang” (Overhead Valve) all the valves in the cylinder head, the camshaft is in the crankcase, in V engines over the crankshaft. One also speaks of “camshaft in”. The valves are bumpers and rocker actuated. The combustion chamber can be molded low. This design was the beginning of mass motorization usual and is still widespread, from Harley-Davidson on VW Beetle up to the current Corvette . As one of the first OHV engines of the pipeline engine is from 1905. [1]
OHC

The concept OHC (overhead camshaft, also SOHC – Single Overhead Camshaft), the camshaft is located in the cylinder head and is either through a series of gears, a vertical shaft , a chain or a belt . driven Clément-Bayard in 1908, the first OHC valve control on his motor racing before. [1] The valves via rocker arms or cam actuated. The rotating drive between the crankshaft and cylinder head reduces the inertia and higher speed is possible. This design is the most widespread, the variations are endless. CIH ( In Head Camshaft ) is an OHC-variant, in which the camshaft is located laterally in the cylinder head.
DOHC

In DOHC (double overhead camshaft Sheet), two camshafts are located in the cylinder head, one of which the intake valves and the other operates the exhaust valves. The first DOHC engine was racing the engine of Peugeot from the year 1912. [2] The moving masses are even lower, and so even for higher machine speeds, but also less wear at low shaped combustion chamber, and the placement of spark plugs is easy. When DOHC engine, the valves are now using tappets operated. Modern racing engines and motorcycle engines are exclusively equipped with DOHC valve train. DOHC does not automatically mean four valves per combustion chamber, basically with the DOHC valve control can also be operated two valves per combustion chamber (see Kawasaki Z1 ).
valve assembly

The arrangement of the valves to each other is made between parallel, V-shaped and radially:
Parallel valves

With an overhead camshaft parallel (in series) hanging valves are operated directly by bucket tappets. At a camshaft via tappets, pushrods and rocker arms simple
V-shaped arrangement

In this arrangement, the valve at an angle to each other in the cylinder head hanging valves a V. This results, compared to a cylinder head with overhead valves in parallel a more favorable combustion chamber shape . For cylinder heads with this valve assembly usually are the inlet and outlet opposite . This arrangement can be used for both two-valve as well as for multi-valve engines, such a cylinder head can be constructed as a four-valve engine with water cooling owing to the two adjacent outlet channels, since locally a considerable amount of heat must be dissipated. In an OHC cylinder head the valves are actuated by rocker; the DOHC cylinder head most directly by bucket tappets, rarely short-cam.
Radial arrangement

In this valve assembly for a four-valve engine, the valves of the inlet and outlet do not hang in parallel next to each other, but the valves of the same function are arranged at an angle to each other, so that the valve plate not form a roof-shaped combustion chamber (like the V-shaped overhead valves ), but a roughly hemispherical combustion chamber. With the ability to order in this angle between the exhaust valves an additional coolant channel, the cylinder head in this design is thermally resistant. In addition, the valve openings in said valve assembly to be greater than that of a cylinder head with overhead valves in parallel. Each inclined to one another hanging valves are actuated by a camshaft with cam conically milled or also obliquely arranged cam followers, and the direct actuation of each require a camshaft for intake and exhaust valves (DOHC). When valve actuation via rocker arms, a camshaft is sufficient (eg Honda Radial Four Valve Combustion Chamber). Examples: Rolls-Royce Condor , Honda XR 500 R , Honda XL 600 , current engines of MV Agusta

A special form of this valve assembly of the radial four-valve engine of Diametral Ludwig Apfelbeck .
valve actuation
Valve drive with cup tappets:
First Cams of the camshaft
Second Tappets
Third Valve spring
4th Valve stem
5th Gas duct to the inlet (fresh gas) or outlet (exhaust)
6th Valve plate, which seals the combustion chamber (7) against the valve seat ring
7th Combustion chamber

On the type of valve control is distinguished:

Tappet – here the cam directly actuates a hollow cylinder, the “cup” that pushes directly through a shim (“shim”) on the valve stem. The distance between the cam and valve is lowest, also does not force deflection held by towing or rocker, in this design lightweight, very stiff, making it suitable for high speeds. The shim is usually open at the top of the cover of the plunger. This actuation also provides the lowest overall height of the valve control.
Cam follower / rocker – between the cam and valve is a lever, which is mounted in its own axis. Advantage: valve is easy to adjust, even V-shaped overhead valves with an OHC engine can be controlled with a single camshaft.
Rocker – work like a rocker. The rocker has two arms. The tilting axis is located between the cam shaft (or plunger) on the one hand and the valve on the other side. CIH engines may be equipped with an adjustable tappet between the cam and rocker arm furnished to the valve lash adjuster (rare). Normally actuates the camshaft a rigid plunger, and the valve clearance is adjusted by a screw on the rocker arm. With an overhead camshaft, the valve clearance on the storage of the rocker arm is adjusted by the pivot point, ie, the tilt axis, moved, or a cam is rotated. Except the valve clearance is also changing the length of the lever arm, whereby the valve lift is slightly changed ( lever principle ).
Desmodromic
When Desmodromic, for example, the Ducati is used motorcycles, there is a so-called Priority. There is typical valve spring is not in the desmodromic. Instead, the valves are opened and closed via drag lever via an additional locking cams of the camshaft and a locking lever. This design is extremely smooth because of the camshaft no additional spring force is overcome. The system is thus also suitable for high speeds. The valve clearance must be adjusted separately for the opening and closing. Newer designs have a dual hydraulic valve clearance compensation. A weak additional valve spring to close allows the engine to start and without them on a cold engine would be no compression possible. With interlocking desmodromic valve actuation serves the same cam (double contour cam) for opening and closing.

Selection Criteria

The selection of the valve train on the part of the designer depends on several criteria:

Number of operable valves: If you want to equip a four-valve engine with a single camshaft, so not all valves are operated directly by bucket tappets, so often two camshafts are used. Logically, the same goes for five-valve dual camshaft.
Position of the injectors and spark plugs in the cylinder head
Maximum engine speed: about 65-150/min in marine diesels (low-speed), about 3500 rpm for truck diesels, about 6000-8000/min in passenger car gasoline engines until 17.000/min in motorcycle engines , up to 20,000 rpm with formula -1 – and four-stroke motorcycle racing engines.

camshaft drive

For transferring the rotational movement of the crankshaft to the (upper) camshaft there are various possibilities. Are widespread:

Timing chain or
Timing , other options are
Spur gears or (especially on older bikes), the
Bevel and very rarely drive with a pair of
Pushrods . In this type of drive is provided that the cam shaft is arranged parallel to the crankshaft, which is the predominant method of construction.

Due to the principle of a four stroke engine, the camshaft rotates constantly at half crankshaft speed – apart from four-stroke radial engines , details see there.

To optimize fuel consumption, the car manufacturers are increasingly turning to variable valve timing . Here are dependent on speed of the camshaft by adjusting the control times of the intake valves changed, partly also the exhaust valves. This enables the filling of the cylinder and thus the efficiency of the engine to improve in a wider speed range. Further optimization is possible by the adapted change of valve lift, so if the throttle is superfluous, so the Valvetronic BMW or FIAT MultiAir.

In radial engines because of the different form the control waves are called “drum cam”, “cam ring” or “cam”. They usually run coaxially with the crankshaft, so that suitable as drive gears only considered.

Typically, the cams on a camshaft arranged such that the adjustment movement radially to the rotating shaft is, taking the cam contours extend generally parallel to the shaft, the plunger is perpendicular to it. Of Ludwig Apfelbeck also engines have been designed with chamfered cam, in which the ram can take different angles to the camshaft. In some sports motorcycle engines in turn are conical ground cams used.

Rarely are constructions in which the valve control movement is axially in the direction of the camshaft, so some of the British Chater-Lea motorcycles and in constructions of Richard kitchens . The cams actuate this pivoted lever.

Basically to the valve control system of the rotating camshaft is necessary – the timed actuation of plungers is with camshafts, which rotate by less than one full turn back and forth, or moved in translation “cams” are possible. Such a design was used but not yet in the series. Valves can also be continued by means of hydraulic, pneumatic or electro-magnetically operated, so for high-speed racing engines, such as in Formula 1 .

twin overhead camshafts driven by a timing chain

twin overhead camshafts driven by a toothed belt

Camshaft drive by helical gears

Camshaft drive by vertical shaft

Camshaft drive by pushrods

valve
Exhaust valve, the valve head right, left the ring groove to wedge the spring plate

To control the charge cycle in a combustion engine with four-stroke engines and partly almost exclusively in two-stroke engines are used Tellerhubventile. The valve consists of the valve plate, the inlet or outlet of the channel in the closed state seal against the valve seat suitable ground or machined in the cylinder head. The valve disc is parabolically over into the valve stem, to the passing gases to provide the lowest possible resistance. The valve stem is a long, thin cylinder, which is fixedly connected to the plate.
tasks, and construction loads

Valves are thermally and mechanically highly stressed components, which are additionally exposed to corrosive influences. The mechanical stresses arise due to deflection of the valve disc under ignition pressure, knock down by closing (shock) and by inertia forces due to high accelerations. By appropriate thickness and shape of the plate, these voltages are influenced.

From the combustion chamber, the valves take her with a large surface area to heat. The outlet valve is heated during the opening and by the hot flue gases flowing on the top. In the valve, the heat flows to the valve seat, especially, a smaller part on the shaft to the valve guide. Intake valves reach temperatures of 300 ° C to 500 ° C, exhaust valves 600 ° C to 800 ° C [3] . If the seal on the valve seat during the combustion phase is not proper, caused localized overheating and partial melting, which can lead to failure of the valve.

To improve the heat conduction through the shaft for the latter is particularly high demands made hollow and filled with sodium. The movements of the liquid at temperatures above 97.5 ° C sodium enhance the heat transfer. Then the valve temperatures can be lowered by up to 100 ° C. To reduce wear, the seat by welding of Stellite – a special carbide alloy – be armored.

The material of the valve must have high temperature strength and oxidation resistance. For this special steels were developed. Apart from steel with high-speed motors are lighter partly titanium alloys used.

Due to wear in the cylinder heads often valve seats installed. As part of the technical development of this growing requirements regarding the type of material, precision and power stability are provided.

Among others, the valve seat inserts in the inlet and outlet for large diesel engines (power units for ships and stationary power generators) in the centrifugal casting process produced. The incorporated materials (nickel-based alloys, heat-resistant ferritic steels, as well as application-specific materials) not only provide optimum thermal load up to 400 ° C, but also a high wear and corrosion resistance, the use of different fuels such as heavy oil, diesel, sulfur-free fuels and gas.

For aluminum cylinder heads in any case, a seat will be provided (alloy centrifugal casting, in special cases, cast iron with a coefficient of thermal expansion in the amount of light metal). In highly stressed engines, especially for the exhaust valves seat rings made of alloy centrifugal casting are also used in cast iron cylinder heads. The valve seats are pressed or shrunk. Through the combination of suitable materials, contact welding between valve face and valve seat ring is avoided and thus prevent scuffing. First valve seat inserts were only in gas engines and engines used with aluminum cylinder head, as in gasoline engines through to the 1980s exclusively used leaded gasoline , a protective layer between the valve plate and cylinder head made ​​and so prevented the contact welding. In diesel engines takes the soot that task. Burning of valve plates in their seats is prevented by Verdrehvorrichtungen.

The plate of the inlet valve usually has a larger diameter than that of the exhaust valve, since in this way the power output for a given combustion chamber diameter is at its highest. The shank is in turn the exhaust thicker to promote the heat transport. The heat is further transferred to the valve guide, and from there to the cylinder head, which is usually cooled by cooling water, rarely by air.
valve actuation
Direct push

In the direct operation, the valve is open directly through the overhead camshaft, it is closed by the valve spring. Which force is transmitted via the spring plate, the valve is held by two wedges which engage in an annular groove at the upper valve stem. Between the cam and valve stem as the transmission member is a valve lifter is disposed on the cam rolls sliding during actuation. He must keep the resulting lateral forces from the valve stem. The plunger diameter is determined by the maximum ram speed, the cam width is determined by the surface pressure between the camshaft and the tappet. Since cam and tappet slide under high surface pressure on each other to the material combination is important. Hardened steel and cast iron are particularly well suited. In order to avoid uneven wear of the ram is allowed to rotate about its axis is often. Purpose, it is laterally offset from the middle of the cam by 1 to 3 mm. In addition to the rigid tappets in which the valve clearance is adjusted by the contribution of platelets of different thicknesses (1/10-mm-Abstufung), there are also automatic tappet clearance adjustment, known hydraulic tappets.
Operation via lever
Rocker arm of a motor vehicle engine – the thread of the adjusting screw to the right

Rocker arms are a relic of the camshaft in engines, here they were driven out of bumpers as a transfer member. But even with an overhead camshaft, they are used, particularly if the valves are arranged in the combustion chamber of V-shaped. At the pivot point of the rocker arm, a high contact force, so that the storage must be designed especially stiff. For i = l Kipphebelverhältnis 2 / l 1, in general be aimed at values ​​from 1 to 1.3, as a compromise between low surface pressure on the plunger, less moving mass, and high stiffness. The force of the rocker arm to be transmitted to the valve stem axially as possible in order to avoid a lateral force on the valve stem. Rocker arms are usually cast or forged. Cheap variants consist of deep-drawn sheet metal, but they are less stiff. It is advantageous to adjust the valve clearance on the arm rest storage, but the adjuster sits in forged levers compared to normally, making the moving mass of the valve train increases. Cam (or rocker) are much smaller forces than rocker. Changes in the support point have less influence on the kinematics. When cam to install a hydraulic valve clearance compensation in the lever pivot is a very clever design solution that requires minimal space, the overall elasticity changes only slightly and is therefore preferred over other solutions often.
lash

In operation, heated metal expands, so that the valve becomes longer. Thus it can happen that it will not close completely. Even if the valve seat is worn more quickly than the cam, the valve can no longer close completely.

Then threatens the burning of the valve seat or the valve plate, as the hot combustion gases flow past the valve and it is not fully closed can hardly dissipate heat to the cylinder head. Because of the gases flowing through it also occurs in a power loss. (The background is that the valve is closed, the combustion gases must do work and their thermal energy partially converted into mechanical energy. If the valve leaks, the thermal energy is still in the gas, which leads to higher temperatures and the damage to the valve.)

To ensure a safe closing of the valve under all operating conditions, a valve clearance is provided – a small distance in the valve train somewhere. This distance is reduced with operating temperature, but must not be zero. Excessive play in turn causes loss of power, excessive noise and excessive wear of the valve train.

The manufacturers therefore specify a range for the valve clearance, in the order of a few hundredths to tenths of a millimeter, generally related to the cold engine. The valve clearance should at regular maintenance intervals (according to mileage, sometimes even hours) are checked and adjusted if necessary, if the engine has no automatic valve clearance compensation.

The game is usually with feeler gauges is measured and then set differently depending on the type: When rocker arms, which are actuated by push rods, is often located in the levers countered one screw, to which the setting can be made. In other constructions, the fulcrum of the lever is adjusted. Tappets have hardened against insertion plate (“shims”), which are placed between the plunger and the cam and follower and valve stem. These are available in subtle gradations of thickness and replaced if necessary correction.

Many newer engines have hydraulic lifters in which no valve clearance is needed. Regular maintenance is then eliminated, but the lash themselves and cause wear to high maturity may rattle and loss of power. Hydraulic valve lash settings can be constructed within or levers to lift the lever mounting point (“hydraulic lifters”).

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