Valvetrain

A control valve or a valve drive is a mechanism that controls the valves, thereby controlling the charge cycle by opening and closing the air intake and exhaust gas outlet channels in a reciprocating engine.

It is used in almost all four-stroke engines - exceptions like the damper motor are rare. In two-stroke engines, however, no valves are mainly used except for large two-stroke diesel engines have an exhaust valve.

In general, a valve of a camshaft (English Camshaft ) via a push rod, rocker arm and rocker arm is opened. The few exceptions are pneumatic, hydraulic or solenoid valves. Valves are closed by coil springs, rarely by torsion bars, gas springs or a closing cam. The camshaft is driven by the crankshaft, in four -stroke motors with a reduction ratio of 2:1.

In operation valves during each second more than once - in marine diesels just twice, but about a hundred times in racing engines - opened and closed, so accelerated and again brought to a standstill. The movements of the valves and the piston in the cylinder must be precisely synchronized.

  • 3.1 Tasks, stresses and construction
  • 3.2 valve actuation 3.2.1 Direct actuation
  • 3.2.2 Operation via lever

Types

While the valves are constructed predominantly uniformly, there are significant differences in design of the valve arrangement, the valve actuation assembly and the camshaft of the camshaft drive common embodiments.

Basic types

Is generally distinguished according to the arrangement of the valves and the camshaft to the cylinder.

Poppet

Wherein the first motor only the outlet valve is controlled, the intake valve is "automatically" is opened by the negative pressure of the aspirating plunger. The blow valve closes the end of the intake stroke by a spring, the exhaust valve is actuated by a plunger of a cam. Motors with sniffer valves can achieve designed only to 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.

Inlet over outlet, controlled exchange ( IOE)

The IOE ( Inlet Exhaust Over ) " depends on ", the inlet valve in the cylinder head and is actuated by a camshaft in the cylinder on the bumper and the rocker arm. Including the stationary exhaust valve is arranged directly driven by the camshaft. IOE is also called the " change " or " unchecked ". The valve control IOE has mostly exposed rocker arms, so that the lubrication could be a problem. With IOE and the poppet valve will be referred to Anglican language area, which is located above the exhaust valve.

Related valves ( SV)

A very simple, now common only in case of power generators and lawn mowers construction is called SV ( Side Valves, " lateral valves ", also Standing Valves, "standing valves " ): The camshaft is located in the crankshaft and sliding across ram the "standing next to the cylinder " valves in the cylinder head inside. The design of the cylinder head is called " Flathead " and has been known since 1904. The cylinder head is the side valve just a simple cover. The cylinder block is complex, however, because it also contains the gas ducts and the valve guides in addition to the cylinder bore. The combustion chamber has an unfavorably large surface area. Accordingly, this type of valve control system is only suitable for low compression ratio and low engine speeds. Advantages are the low-profile design of the engine and the simple casting of the cylinder head.

A special form of the side-valve engine of the T- head engine, is at the inlet and outlet valves mounted on the opposite side. This design with two down camshafts was at the beginning of the 20th century for high-performance engines (eg Mercedes, Laurin & Klement, Stutz, American LaFrance ) common, but no longer has any meaning.

Overhead valves ( OHV )

In OHV (Overhead Valve ) "hang" all the valves in the cylinder head, the camshaft is in the crankcase, in V engines over the crankshaft. This is also called " lower camshaft ". The valves are actuated via tappets, pushrods and rocker arms. The combustion chamber can be formed favorable. This design was common at the beginning of mass motorization and is still spreading, Harley- Davidson VW Beetle up to the current Corvette. As one of the first overhead valve engines of the pipe-motor applies from 1905.

Overhead camshaft ( OHC )

With the concept OHC ( overhead camshaft, also SOHC - Single Overhead Camshaft) engine is the camshaft in the cylinder head and is driven either by a series of gears, a vertical shaft, push rods, a chain or a timing belt. Clément- Bayard presented in 1908 before an early OHC valve timing on his racing engine. The valves are operated by bucket tappets, rocker or drag lever. The rotating drive between the crankshaft and cylinder head reduces the inertia and higher speeds are possible. This construction method is the most widely used, the variations are numerous. CIH ( Camshaft in Head ) is an OHC variant in which the camshaft is located in the cylinder head side; Rocker arms are required.

Two overhead camshafts ( DOHC)

For DOHC ( double overhead camshaft engl. ) there are two camshafts in the cylinder head, one of which the intake valves and the other operates the exhaust valves. The first DOHC engine was the racing engine of Peugeot from the year 1912., The moving masses are even lower, so even higher speed possible, but also less wear at low shaped combustion chamber, and the placement of spark plugs is easy. When DOHC engine, the valves are operated by bucket tappets today. Modern racing engines and motorcycle engines are exclusively equipped with DOHC valve train. DOHC does not automatically mean four valves per combustion chamber; can in principle with the DOHC valve control two valves are operated each combustion chamber ( see Kawasaki Z1).

Valve assembly

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

Parallel valves

With an overhead camshaft, the parallel overhead ( in series) valves are operated directly by bucket tappets, with CIH engines via tappets and rocker arms, at a camshaft in the cylinder via tappets, pushrods and rocker arms.

V -shaped configuration

In this valve arrangement, at an angle to one another in the cylinder head form a hanging valves V. This is in comparison with a cylinder head with overhead valves in parallel with a more favorable combustion chamber shape. For cylinder heads with this valve assembly are usually in the inlet and outlet facing each other. This arrangement can be used for both two-valve, as well as multi -valve engines; such a cylinder head can be built as a four-valve engine with liquid cooling because of the two adjacent outlet channels, as there is a considerable amount of heat to be dissipated locally. OHC in a cylinder head, the valves are operated by rocker arms; the DOHC cylinder head most directly by bucket tappets, rare short drag lever.

Radial arrangement

In this valve arrangement for a four-valve engine, the valves of the intake and exhaust do not depend in each case parallel to each other, but the valves of the same function are mutually arranged at an angle to each other, so that the valve heads do not form a roof-shaped combustion chamber (as in V- shaped overhead valves ), but an approximately hemispherical combustion chamber. Due to the possibility of arranging an additional coolant channel in the angle between the exhaust valves, the cylinder head in this design is thermally resistant. In addition, the valve openings may be larger than that of a cylinder head with overhead valves in parallel with this valve arrangement. The obliquely to each other hanging valves are actuated by a camshaft with cam taper ground or also diagonally arranged rocker arms; the direct operation each requires a camshaft for intake and exhaust valves (DOHC ). When the valve actuation via rocker arms enough a camshaft (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 is the radial diametral four-valve engine by Ludwig Beck apple.

Valve actuation

By type of valve control is distinguished:

  • Tappets - here actuated cam having a cup-shaped hollow cylinder, which - apart from lash adjusters - presses directly on the valve stem. The distance between the cam and the valve is at its lowest in this design, also does not force deflection by towing or rocker arms instead, thus this type of construction is light, very stiff and is therefore suitable for high speeds. This actuating offers the lowest overall height of the valve control -. Among the options the valve clearance adjustment and automatic play compensation see under bucket tappets.
  • Cam followers / rocker arm - located between the cam and valve, a lever, which is mounted on a separate axis. Advantage: Valve clearance is easy to adjust; and V - shaped overhead valves in an OHC engine can be controlled by a single camshaft.
  • Rocker - work similar to a rocker arm. The rocker arm 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 motors can be equipped with an adjustable plunger between the cam and rocker arms for valve clearance compensation ( rare). Normally, the camshaft actuates 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 bearing of the rocker arm is adjusted by the pivot point, that is, the tilting axis is moved or rotated with an eccentric. Besides the valve clearance is also changing the length of the lever arm, whereby the valve is changed slightly ( lever rule ).
  • Desmodromik The Desmodromic, which is used for example in Ducati motorcycles, it is a so-called positive control. The typical valve spring does not exist in the Desmodromik. Instead, the valves are opened and closed by rocker arms via an additional locking cams of the camshaft, and a lock lever. This design is especially smooth because of the camshaft no additional spring force is overcome. The system is therefore also suitable for high speeds. However, the valve clearance must be for the opening and closing set separately. Newer designs have a dual hydraulic valve clearance compensation. Weak additional valve spring for closing prevents starting of the engine; without them no compression would be possible with the engine cold. With form-fitting desmodromic valve actuation same cam is used (double contour cams) to open and close.

Selection criteria

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

  • Number of valves to be actuated: If you want to equip a four-valve engine with a single camshaft, so can not be directly operated by bucket tappets for all valves, therefore, often two camshafts are used. Logically, this is also true for five -valve engine with two camshafts.
  • Position of the spark plugs and fuel injectors in the cylinder head
  • Maximum speed of motor: about 65-150/min in marine diesels ( slow-speed), about 3500 rpm for truck diesel engines, such 6000-8000/min in passenger car gasoline engines until 17.000/min motorcycle engines, to 20,000 rpm for formula -1 - and four- stroke motorcycle racing engines.

Camshaft drive

For the transmission of rotary motion from the crankshaft to the ( overhead ) camshaft there are various possibilities. Widely used are:

  • Timing chain or
  • Timing belt; other possibilities are
  • Spur gears or ( especially in older motorcycles ) the
  • Bevel and very rarely with the drive with a pair of
  • Push rods. In this type of drive is provided that the cam shaft is disposed parallel to the crankshaft, which is the predominant method of construction.

Due to the principle turns the camshaft of a four-stroke engine always at half crankshaft speed - apart from four-stroke radial engines, for further details see there.

To optimize fuel consumption automobile manufacturers are increasingly relying on camshaft adjustment. Here, depending on the speed by moving the camshaft, the control times of the intake valves changed, partly also the outlet valves. Thus, the filling of the cylinder and thus the efficiency of the engine can be improved in a wider rpm range. Further optimization is possible by the adapted change of valve lift, thereby possibly also the throttle is superfluous, as in the Valvetronic BMW or MultiAir by Fiat.

With star engines, the control shaft will be called a " cam ring ", " cam ring " or " cam " because of the different shape. They usually run coaxial with the crankshaft, so come as a drive only gears into consideration.

Typically, the cams are arranged on a camshaft so that the plunger is moved radially relative to the rotary shaft; while the cam contours usually run parallel to the shaft, the plunger is perpendicular to it. Ludwig Apfelbeck However, motors are constructed with a sloped cam in which the follower can assume other angles to the camshaft. In some sports motorcycle engines turn conically ground cams are used.

Rarely are structures with axial control valve movement toward the camshaft, so in some of the British Chater Lea motorcycles and in designs by Richard kitchens. The cams operating any rotatably mounted lever.

In principle, no rotating camshaft for valve control necessary - the timed actuation of plungers is also camshafts rotate by less than a full rotation back and forth, or moved in translation " cam tracks " possible. Such a construction has not yet been used in the series. Valves can be operated by hydraulic, pneumatic or electromagnetic continues, with high-revving race engines, as in Formula 1

Two overhead camshafts driven by a toothed belt

Camshaft drive by helical gears

Camshaft drive by vertical shaft

Camshaft drive by pushrods

Valve

To control the charge cycle in a combustion engine almost exclusively and partially in two-stroke engines Tellerhubventile be used for four-stroke engines. The valve consists of the valve plate, which seals off the inlet or outlet channel in the closed state against the ground or turned to fit the valve seat in the cylinder head. The valve disk is a parabola on the valve stem to provide the least possible resistance to the gas flowing past. The valve stem is a long thin cylinder, which is fixedly connected to the plate.

Tasks, stresses and construction

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

From here take the combustion chamber valves with large surface area to heat. The outlet valve is heated during the opening and by the hot flue gases flowing on the upper side. In the valve, heat flows mainly to the valve seat, a smaller portion on the stem to valve guide. Intake valves reach temperatures of 300 ° C to 500 ° C, outlet valves 600 ° C to 800 ° C. If the seal on the valve seat during the combustion phase is not properly incurred local overheating and partial melting, leading to the failure of the valve.

To improve the heat conduction through the shaft it is made ​​hollow for very high demands and filled with sodium. The movements of the liquid at temperatures above 97.5 ° C sodium enhance the heat transfer. Thus, the valve can be lowered to temperatures up to 100 ° C. To reduce wear, the seat by welding Stellite - a special hard metal alloy - be armored. The fluid inside the valve also leads to a vibration damping.

The material of the valve must have high temperature strength and oxidation resistance. It requires special RSH steels have been developed. A typical name is X15NiCr25.20 (material Nr.1.4841 ) (or 1.4541 stainless steel ( X6CrNiTi18 -10) ). In addition to steel some lighter titanium alloys are used in high-revving engines. Mario Illien sat in the Ilmor engines, which he developed for Formula 1 vehicles from Mercedes, the first time the material beryllium as an alloying addition.

Reasons of wear, valve seat rings are installed in the cylinder heads often. In the course of technical progress steadily growing requirements in terms of the type of material, precision and power stability are placed on them.

Among other things, the valve seat rings in the inlet and outlet for large diesel engines ( propulsion units for ships and stationary power generators ) are produced by centrifugal casting. The materials ( nickel -based alloys, heat-resistant ferritic steels as well as application-specific materials) used for this purpose not only provide good temperature resistance up to 400 ° C, but also a high wear and corrosion resistance with the use of different fuels such as heavy oil, diesel, sulfur-free fuels and gas.

In light metal cylinder heads, a seat ring must in any case be provided ( alloy centrifugal castings, in special cases, cast iron with a thermal expansion coefficient in the amount of light metal ). In highly stressed engines seat rings made ​​of alloy centrifugal casting are also used in cast-iron cylinder heads in particular for the exhaust valves. The valve seat rings are pressed or shrunk. The combination of suitable materials contact welding between valve face and valve seat ring is avoided and thus prevent scuffing. First, valve seat rings have been used in gas engines and engines with aluminum cylinder head, as in gasoline engines by the exclusively used until the 1980s, leaded gasoline, a protective layer between the valve plate and cylinder head formed and prevented the contact welding. For diesel engines, the carbon black takes on this task. Sticking of valve discs in their seats is prevented by the twisting.

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

Valve actuation

Direct operating

With direct operation, the valve is opened directly by the overhead camshaft, closed it is of the valve spring. The force is transferred via the spring plate, the valve is held by two wedges which engage in an annular groove on the upper valve stem. Between the cam and valve stem as the transfer member is a tappet arranged on the slides of the cams during operation. Move the plunger, the resulting lateral forces away 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 tappets. Since the cam and plunger must slide against each other under high surface pressure, the material combination is important. Hardened steel and gray cast iron are well suited. In order to avoid uneven wear is allowed to frequently rotate the plunger about its axis. To it is laterally offset from the center cam from 1 to 3 mm. In addition to rigid plungers, in which the valve clearance can be adjusted by the contribution of platelets of different thickness ( 1/10-mm-Abstufung ) or changes to valve or valve seat, there are also ram with automatic clearance adjustment, called lash adjusters.

Actuation of levers

Rocker arms are a relic of the engines with camshaft in, here they were driven by bumpers as the transmission link. But also for an overhead camshaft are used as the valves in the combustion chamber are arranged in a V-shape. At the fulcrum of the rocker arm, a high tracking force, so that the storage has to be designed particularly stiff. For the Kipphebelverhältnis i = L2/L1 values ​​between 1 and 1.3 are generally pursued as a compromise between low surface pressure on the plunger, low moving mass and high stiffness. The force of the rocker arm to be transmitted as possible axially to the valve stem in order to avoid a lateral force on the valve stem. Rocker arms are usually cast or forged. Cheap versions are made from molded sheet, but these are less rigid. It is advantageous to adjust the valve clearances on the stationary levers storage, but the adjustment is normally seated against the case of forged levers, making the moving mass of the valve train increases. Drag lever (or swing arm ) are much less stressed than rocker arms. Changes in the support point have less influence on the kinematics. When you drag lever the installation of a hydraulic lash adjuster in the lever pivot is a very clever engineering design that takes up little space, the overall elasticity changes only slightly and is therefore often preferred over other solutions.

Valve clearance

In operation, heated metal expands, so that the valve becomes longer. Therefore it can happen that it no longer closes completely. Even if the valve seat is worn more quickly than the cam, the valve is unable to close completely.

Then threatens the burning of the valve seat and the valve disc, as the hot combustion gases flow past the closed valve, and not quite hardly can dissipate heat to the cylinder head. Due to the gases flowing through, power loss also occurs, since the part of the combustion gases, escapes past the valve, its energy is not write to the piston.

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

Therefore, the manufacturers give a range for the valve clearance to, in the order of a few hundredths to tenths of a millimeter, usually based on the cold engine. The valve clearance should at regular maintenance intervals ( according to mileage, partly operating hours) are checked and if necessary adjusted, if the engine has no automatic valve clearance compensation.

The game is usually measured with feeler gauges and then set differently depending on the type: If rocker arms are actuated by push rods, located in the levers often countered screw on which the adjustment can be made. In other constructions, the fulcrum of the lever is adjusted. Tappets, however, have hardened spacer plate ( "shims " ), which are placed between the tappet and the cam and tappet and valve stem. These are available in fine gradations of thickness and replaced if necessary correction.

Many newer engines have hydraulic tappets, where no more valve clearance is necessary. Then Regular maintenance is required, but the hydraulic tappets wear out and cause even after extended time, you may rattle and loss of power. Hydraulic valve lash settings can also be constructed within or levers for lifting the lever mounting point ( "Hydro Lifter ").

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