Overhead Camshaft SOHC,DOHC & more...

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Overhead Camshaft (OHC)

Overhead camshaft (OHC) valvetrain configurations place the camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters directly instead of using pushrods. When compared directly with pushrod (or OHV) systems with the same number of valves, the reciprocating components of the OHC system are fewer and in total will have less mass. Though the structures that support the system may become more complex, most engine manufacturers easily accept the added complexity in trade for better engine performance and greater design flexibility. The OHC system can be driven using the same methods as an OHV system, these methods may include using a timing belt, chain, or in less common cases, gears.

Many OHC engines today employ Variable Valve Timing and multiple valves to improve efficiency and power. OHC also inherently allows for greater engine speeds over comparable cam-in-block designs.

There are two overhead camshaft layouts:
• Single overhead camshaft (SOHC)
• Double overhead camshafts (DOHC)

Single overhead camshaft

Single overhead camshaft is a design in which one camshaft is placed within the cylinder head. In an inline engine this means there is one camshaft in the head, while in a V engine there are two camshafts: one per cylinder bank.

The SOHC design is inherently mechanically more efficient than a comparable pushrod design. This allows for higher engine speeds, which in turn will by definition increase power output for a given torque. The cams operate the valves directly or by a short rocker as opposed to overhead valve pushrod engines, which have tappets and long pushrods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.

SOHC designs offer reduced complexity compared to pushrod designs when used for multivalve heads, in which each cylinder has more than two valves.

Double overhead camshafts

A double overhead camshaft (also called double overhead cam, dual overhead cam or twincam) valvetrain layout is characterized by two camshafts being located within the cylinder head, where there are separate camshafts for inlet and exhaust valves. In engines with more than one cylinder bank (V engines) this designation means two camshafts per bank, for a total of four.

Double overhead camshafts are not required in order to have multiple inlet or exhaust valves, but are necessary for more than 2 valves that are directly actuated (though still usually via tappets). Not all DOHC engines are multivalve engines — DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared, however today DOHC is synonymous with multivalve heads, since almost all DOHC engines have between three and five valves per cylinder.

History

The first DOHC engines were two valve per cylinder designs from companies like Fiat (1912), Peugeot (1913), Alfa Romeo (6C- 1925, 512 - 1940), Maserati (Tipo 26, 1926), and Bugatti (Type 51, 1931). Most Ferraris used two valve per cylinder DOHC engines as well.

When DOHC technology was introduced in mainstream vehicles, it was common for the technology to be heavily advertised. While the technology was used at first in limited production and sports cars, the Fiat group is historically credited as the first car company to use a belt driven DOHC engine across their complete product line, comprised of coupes, sedans, convertables and station wagons, in the mid-1960s.

http://www.samarins.com/glossary/dohc.html OHV, SOHC, DOHC engine animated diagrams

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Valvetrain

A traditional reciprocating internal combustion engine uses valves to to control air and fuel flow into and out of the cylinders, facilitating combustion. Valvetrain is an all-encompassing term used to described the mechanisms and parts which control the operation of the valves.

Layout

Valvetrains are built in several configurations, each of which varies slightly in layout but still performs the task of opening and closes the valves at the time necessary for proper operation of the engine. These layouts are differentiated by the location of the camshaft within the engine:

• Overhead camshaft
The camshaft (or camshafts, depending on the design employed) is located above the valves within the cylinder head, and operates either indirectly or directly on the valves.

• Cam-in-block
The camshaft is located within the engine block, and operates directly on the valves, or indirectly via pushrods and rocker arms. Because they often require pushrods they are often called pushrod engines.

• Camless
This layout uses no camshafts at all. Technologies such as solenoids are used to individually actuate the valves.

Parts
As stated above, the valvetrain is the mechanical system responsible for operation of the valves. Valves are usually of the poppet type, although many others have been developed such as sleeve, slide and rotary valves.

Poppet valves typically require small coil springs, appropriately named valve springs, to keep them closed when not actuated by the camshaft. They are attached to the valve stem ends, seating within spring retainers. Other mechanisms can be used in place of valve springs to keep the valves closed: Formula 1 engines employ pneumatic cylinder heads in which fluid pressure closes the valves, while motorcycle manufacturer Ducati uses desmodromic mechanisms to manually close the valves.

Depending on the design used, the valves are actuated directly by a rocker arm, finger or bucket tappet. Overhead camshaft engines use fingers or bucket tappets, upon which the cam lobes contact, while cam-in-block engines use rocker arms. Rocker arms are actuated by a pushrod, and pivot on a shaft or individual ball studs in order to actuate the valves.

Pushrods are long, slender metal rods seated within the engine block. At the bottom ends the pushrods are fitted with lifters, either solid or hydraulic, upon which the camshaft, located within the cylinder block, makes contact. The camshaft pushes on the lifter, which pushes on the pushrod, which pushes on the rocker arm, which rotates and pushes down on the valve.

Camshafts must actuate the valves at the appropriate time in the combustion cycle. In order to accomplish this the camshaft is linked to and kept in synchronisation with the crankshaft (the main shaft upon which the pistons act) through the use of a metal chain, rubber belt or geartrain. Because these mechanisms are essential to the proper timing of valve actuation they are named timing chains, timing belts and timing gears, respectively.

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Camshaft

The camshaft is an apparatus used in piston engines to operate poppet valves. It consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes or cams protruding from it, one for each valve. The cams force the valves open by pressing on the valve, or on some intermediate mechanism, as they rotate.

The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control the flow of fuel intake and exhaust, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain called a timing belt or timing chain. In some designs the camshaft also drives the distributor and the oil and fuel pumps. Also on early fuel injection systems, cams on the camshaft would operate the fuel injectors.

In a two-stroke engine that uses a camshaft, each valve is opened once for each rotation of the crankshaft; in these engines, the camshaft rotates at the same rate as the crankshaft. In a four-stroke engine, the valves are opened only half as often; thus, two full rotations of the crankshaft occur for each rotation of the camshaft.

Depending on the location of the camshaft, the cams operate the valves either directly or through a linkage of pushrods and rockers. Direct operation involves a simpler mechanism and leads to fewer failures, but requires the camshaft to be positioned at the top of the cylinders. In the past when engines were not as reliable as today this was seen as too much bother, but in modern gasoline engines the overhead cam system, where the camshaft is on top of the cylinder head, is quite common. Some engines use one camshaft each for the intake and exhaust valves; such an arrangement is known as a double or dual overhead cam (DOHC), thus, a V engine may have 4 camshafts.

The rockers or cam followers sometimes incorporate a mechanism to adjust and set the valve play through manual adjustment, but most modern auto engines have hydraulic lifters, eliminating the need to adjust the valve lash at regular intervals as the valvetrain wears.

Sliding friction between the surface of the cam and the cam follower which rides upon it is considerable. In order to reduce wear at this point, the cam and follower are both surface hardened, and modern lubricant motor oils contain additives specifically to reduce sliding friction. The lobes of the camshaft are usually slightly tapered, causing the cam followers or valve lifters to rotate slightly with each depression, and helping to distribute wear on the parts. The surfaces of the cam and follower are designed to "wear in" together, and therefore when either is replaced, the other should be as well to prevent excessive rapid wear. In some engines, the flat contact surfaces are replaced with rollers, which eliminate the sliding friction and wear but add mass to the valvetrain.

In addition to mechanical friction, considerable force is required to overcome the valve springs used to close the engine's valves. This can amount to an estimated 25% of an engine's total output at idle, reducing overall efficiency. Two approaches have been tried to reclaim this "wasted" energy but have proven difficult to implement:

• Springless valves, like the desmodromic system employed today by Ducati
• Camless valvetrains using solenoids or magnetic systems have long been investigated by BMW, and are currently being prototyped by Valeo and Ricardo

Camshaft computer animation

Camshaft 4 cyl. engine

Components of a typical, four stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.

Double overhead cams control the opening and closing of a cylinder's valves

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Crankshaft

The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation. It typically connects to a flywheel, to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsional or vibrational damper at the opposite end, to reduce the torsion vibrations often caused along the length of the crankshaft by the cylinders farthest from the output end acting on the torsional elasticity of the metal. The crankshaft was invented by the Turkish inventor Al-Jazari in the 12th century.

Design

Large engines are usually multicylinder to reduce pulsations from individual firing strokes, with more than one piston attached to a more complex crankshaft; but many small engines, such as those found in mopeds or garden machinery, are single cylinder and use only a single piston, simplifying crankshaft design. The crankshaft has a linear axis about which it rotates, typically with several bearing journals riding on replaceable bearings held in the engine block, the main bearings. As the crankshaft undergoes a great deal of sideways load from each cylinder in a multicylinder engine, it must be supported by several such bearings, not just one at each end; this was also a factor in the rise of V8 engines with their shorter crankshafts, in preference to straight-8 engines. High performance engines will often have more main bearings than their lower performance cousins, for this reason. In addition, to convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crank pins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach. The distance of the axis of the crank throws from the axis of the crankshaft determines the piston stroke measurement, and thus engine displacement; a common way to increase the power of an engine is to increase the stroke. This also increases the reciprocating vibration, however, limiting the high RPM capability of the engine; in compensation, it improves the low speed operation of the engine, as the longer intake stroke through smaller valve(s) results in greater turbulence and mixing of the intake charge. For this reason, even such high speed production engines as current Honda engines are classified as long-stroke, in that the stroke is larger than the diameter of the cylinder bore. In production V or flat engines, neighboring connecting rods attach side by side to the same crank throw, simplifying crank design.

The configuration and number of pistons in relation to each other and the crank leads to straight, V or flat engines. The same basic engine block can be used with different crankshafts, however, to alter the firing order; for instance, the 90 degree V6 engine configuration, usually derived by using six cylinders of a V8 engine with what is basically a shortened version of the V8 crankshaft, produces an engine with an inherent pulsation in the power flow due to the "missing" two cylinders, often reduced by use of balance shafts. The same engine, however, can be made to provide evenly spaced power pulses by using a crankshaft with an individual crank throw for each cylinder, spaced so that the pistons are actually phased 60 degrees apart, as in the GM 3800 engine. Similarly, while production V8 engines use 4 crank throws spaced 90 degrees apart, racing engines often use a "flat" crankshaft with throws spaced 180 degrees apart, accounting for the higher pitched, smoother sound of IRL engines compared to NASCAR engines, for example. In engines other than the flat configuration, it is necessary to provide counterweights for the reciprocating mass of each piston and connecting rod; these are typically cast as part of the crankshaft, but occasionally are bolt-on pieces. This adds considerably to the weight of the crankshaft; crankshafts from Volkswagen, Porsche, and Corvair flat engines, lacking counterweights, are easily carried around by hand, compared to crankshafts for inline or V engines, which need to be handled and transported as heavy chunks of metal.

Many early aircraft engines (and a few in other applications) had the crankshaft fixed to the airframe and instead the cylinders rotated, known as a rotary engine design.

In the Wankel engine, the rotors drive the eccentric shaft, which can be considered the equivalent of the crankshaft in a piston engine.

Crankshaft (red), pistons (gray) in their cylinders (blue), and flywheel (black)

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Piston

In general, a piston is a sliding plug that fits closely inside the bore of a cylinder.

Its purpose is either to change the volume enclosed by the cylinder, or to exert a force on a fluid inside the cylinder.

Internal combustion engine

Most pistons fitted in a cylinder have piston rings. Usually there are two spring-compression rings that act as a seal between the piston and the cylinder wall, and one or more oil control rings below the compression rings. The head of the piston can be flat, bulged or otherwise shaped. Pistons can be forged or cast. The shape of the piston is normally rounded (but can be different, see NR500 ). A special type of cast piston is the hypereutectic piston. The piston is an important component of a piston engine and of hydraulic pneumatic systems.

In an Otto or Diesel engine, the head of the piston forms one wall of an expansion chamber inside the cylinder. The opposite wall, called the cylinder head, contains inlet and exhaust valves for gases.

As the piston moves inside the cylinder, it transforms the energy from the expansion of a burning gas (usually a mixture of petrol or diesel and air) into mechanical power (in the form of a reciprocating linear motion). From there the power is conveyed through a connecting rod to a crankshaft, which transforms it into a rotary motion, which usually drives a gearbox through a clutch.

Ways of making power

There are two ways that a piston engine can make power. These are the two-stroke cycle and the four-stroke cycle. A two stroke engine produces power every stroke, while a four stroke engine produces power every other stroke. Older designs of small two-stroke engines produced more pollution than four stroke engines, however modern two-stroke designs, like the Vespa ET2 Injection utilise fuel-injection and are as clean as four-strokes. Large diesel two-stroke engines, as used in ships and locomotives, have always used fuel injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-storey houses, has pistons nearly 1 metre in diameter and is one of the most efficient mobile engines in existence. In theory, a four stroke engine has to be larger than a two stroke engine to produce an equivalent amount of power. Two stroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two stroke engines needed more maintenance, even though they have less moving parts and tended to wear out faster than four stroke engines, however fuel-injected two-strokes achieve better engine lubrication and cooling and reliability should improve considerably.

piston + connecting rod

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Connecting rod

In a reciprocating piston engine, the connecting rod or con rod connects the piston to the crank or crankshaft.

Internal combustion engines

In modern automotive internal combustion engines, the connecting rods are most usually made of steel for production engines, but can be made of aluminium (for lightness and the ability to absorb high impact at the expense of durability) or titanium (for a combination of strength and lightness at the expense of affordability) for high performance engines, or of cast iron for applications such as motor scooters. They are not rigidly fixed at either end, so that the angle between the con rod and the piston can change as the rod moves up and down and rotates around the crankshaft.

The small end attaches to the piston pin, gudgeon pin (the usual British term) or wrist pin, which is currently most often press fit into the con rod but can swivel in the piston, a "floating wrist pin" design. The big end connects to the bearing journal on the crank throw, running on replaceable bearing shells accessible via the con rod bolts which hold the bearing "cap" onto the big end; typically there is a pinhole bored through the bearing and the big end of the con rod so that pressurized lubricating motor oil squirts out onto the thrust side of the cylinder wall to lubricate the travel of the pistons and piston rings.

The con rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and relaxing with every rotation, and the load increases rapidly with increasing engine speed. Failure of a connecting rod is one of the most common causes of catastrophic engine failure in cars, frequently putting the broken rod through the side of the crankcase and thereby rendering the engine irreparable; it can result from overheating, fatigue near a physical defect in the rod, lubrication failure in a bearing due to faulty maintenance, or from failure of the rod bolts from a defect, improper tightening, or re-use of already used (stressed) bolts where not recommended. Despite their frequent occurrence on televised competitive automobile events, such failures are quite rare on production cars during normal daily driving. This is because production auto parts have a much larger factor of safety, and often more systematic quality control.

When building a high performance engine, great attention is paid to the con rods, eliminating stress risers by such techniques as grinding the edges of the rod to a smooth radius, shotpeening to relieve internal stress, balancing all con rod/piston assemblies to the same weight and Magnafluxing to reveal otherwise invisible small cracks which would cause the rod to fail under stress. In addition, great care is taken to torque the con rod bolts to the exact value specified; often these bolts must be replaced rather than reused. The big end of the rod is fabricated as a unit and cut or cracked in two to establish precision fit around the big end bearing shell. Therefore, the big end "caps" are not interchangeable between con rods, and when rebuilding an engine, care must be taken to ensure that the caps of the different con rods are not mixed up. Both the con rod and its bearing cap are usually embossed with the corresponding position number in the engine block.

Recent engines such as the Ford 4.6 liter engine and the Chrysler 2.0 liter engine, have connecting rods made using powder metallurgy, which allows more precise control of size and weight with less machining and less excess mass to be machined off for balancing. The cap is then separated from the rod by a fracturing process, which results in an uneven mating surface due to the grain of the powdered metal. This ensures that upon reassembly, the cap will be perfectly positioned with respect to the rod, compared to the minor misalignments which can occur if the mating surfaces are both flat.

A major source of engine wear is the sideways force exerted on the piston through the con rod by the crankshaft, which typically wears the cylinder into an oval cross-section rather than circular, making it impossible for piston rings to correctly seal against the cylinder walls. Geometrically, it can be seen that longer con rods will reduce the amount of this sideways force, and therefore lead to longer engine life. However, for a given engine block, the sum of the length of the con rod plus the piston stroke is a fixed number, determined by the fixed distance between the crankshaft axis and the top of the cylinder block where the cylinder head fastens; thus, for a given cylinder block longer stroke, giving greater engine displacement and power, requires a shorter connecting rod (or a piston with smaller compression height), resulting in accelerated cylinder wear.

In certain types of engine, master/slave rods are used rather than the simple type shown in the picture above. The master rod carries one or more ring pins to which are bolted the much smaller big ends of slave rods on other cylinders. Radial engines typically have a master rod for one cylinder and slave rods for all the other cylinders in the same bank. Certain designs of V engines use a master/slave rod for each pair of opposite cylinders. On the other hand, some V engines use simple rods side by side on a single crankpin, or separate crankpins for each cylinder.

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Spark plug

A spark plug (also very rarely nowadays in British English, a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder. Early patents for spark plugs included those by Nikola Tesla (in U.S. Patent 609,250 for an ignition timing system, 1898), Richard Simms (GB 24859/1898, 1898) and Robert Bosch (GB 26907/1898). Karl Benz is also credited with the invention. But only the invention of the first commercially viable high-voltage spark plug as part of a magneto-based ignition system by Robert Bosch's engineer Gottlob Honold in 1902 made possible the development of the internal combustion engine.

Internal combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the air and then inject diesel fuel into the heated compressed air mixture where it autoignites. Compression-ignition engines may use glow plugs to improve cold start characteristics.

Spark plugs may also be used in other applications such as furnaces where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.

Operation

The spark plug is connected to thousands of volts generated by the ignition coil. As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. An ionized gas becomes a conductor and an ionized gas can pass electrons.

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" you hear when watching a spark, similar to lightning and thunder.

The heat and pressure force the gasses to react with each other and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded and a large one like the timing was advanced for that individual cycle.

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Four-stroke cycle

The four-stroke cycle of an internal combustion engine is the cycle most commonly used for automotive and industrial purposes today (cars and trucks, generators, etc). The Thermodynamics cycles used in internal combustion reciprocating engines are the Otto Cycle (the ideal cycle for spark-ignition engines) and the Diesel Cycle (the ideal cycle for compression-ignition engines). The Otto Cycle consists of Adiabatic compression, heat addition at constant volume, Adiabatic expansion and rejection of heat at constant volume. It was conceptualized by the French engineer, Alphonse Beau de Rochas in 1862, and independently, by the German engineer Nicolaus Otto in 1876. The four-stroke cycle is more fuel efficient and clean burning than the two-stroke cycle, but requires considerably more moving parts and manufacturing expertise. Moreover, it is more easily manufactured in multi-cylinder configurations than the two-stroke, making it especially useful in high-output applications such as cars. The later-invented Wankel engine has four similar phases but is a rotary combustion engine rather than the much more usual, reciprocating engine of the four-stroke cycle.

The Otto cycle is characterized by four strokes, or straight movements alternately, back and forth, of a piston inside a cylinder:

1) intake (induction) stroke
2) compression stroke
3) power (combustion) stroke
4) exhaust stroke

The cycle begins at top dead center, when the piston is at its uppermost point. On the first downward stroke (intake) of the piston, a mixture of fuel and air is drawn into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s), and the following upward stroke (compression) compresses the fuel-air mixture.

Four-stroke cycle (or Otto cycle)The air-fuel mixture is then ignited, usually by a spark plug for a gasoline or Otto cycle engine, or by the heat and pressure of compression for a Diesel cycle of compression ignition engine, at approximately the top of the compression stroke. The resulting expansion of burning gases then forces the piston downward for the third stroke (power), and the fourth and final upward stroke (exhaust) evacuates the spent exhaust gases from the cylinder past the then-open exhaust valve or valves, through the exhaust port



Valve timing
In its original configuration, the four-stroke engine relies entirely on the piston's motion to draw in fuel and air (Naturally Aspirated Engine), and to force out the exhaust gasses. As the piston descends on the intake (inlet) stroke, the increasing volume within the cylinder causes a partial vacuum which draws in the air/fuel mixture. This relies on atmospheric pressure. The intake valve then closes, the piston ascends, and the mixture is compressed and ignited, causing the piston to descend again. As the exhaust valve opens, the piston ascends once more and forces the exhaust gases out. This was the technique used in early four-stroke engines. It was soon discovered, however, that at rotational speeds approaching 100 revolutions per minute (RPM) or greater, the exhaust gasses could not change direction quickly enough to exit past the exhaust valve by the piston's motion alone.

At high rotational speeds, consistent flow through the intake and exhaust ports is maintained by allowing the intake and exhaust valves to be open simultaneously at top dead center (known as valve overlap). The momentum of the exhausting gas maintains the outward flow and creates a suction effect on the cylinder known as scavenging, helping to draw the intake charge into the cylinder. In order to retain efficiency, however, the exhaust valve must be closed soon enough so that too much fuel/air mixture from the intake port is not drawn into the engine's exhaust, wasting fuel. In a high-power situation such as racing, where high engine speeds and forced induction are common, this wasted fuel charge can serve to cool the exhaust valve and prevent detonation.

After ignition of the fuel/air charge, as the piston approaches bottom dead center, combustion slows. Just before the charge is finished burning, the exhaust valve is opened at approximately twenty degrees of crankshaft rotation before bottom dead center. This allows the still-expanding gasses inside the cylinder to push out through the exhaust port, starting exhaust flow and giving the exhaust flow momentum. Though a small amount of force is lost through the exhaust port that could be driving the piston, the force that the piston must exert on the gasses to exhaust them from the cylinder is reduced, resulting in increased efficiency.

Exhaust systems in many situations are a compromise between cost of production, optimum flow, low emissions, and low noise levels. Also, exhaust gas must be kept away from the air that the engine's driver or pilot or operator breathes. Restrictions in an exhaust system, including emissions equipment, mufflers, and simple exhaust tubing can restrict proper exhaust flow. In multi-cylinder applications, in which many cylinders share a common exhaust pipe, pressure waves created by cylinders exhausting gas can impede flow of exhaust from other cylinders. Since this prevents exhaust gas from exiting the cylinder, the overlap of the intake valve can result in reversion, when exhaust gas enters the intake port. The internal pressure problems due to a multi-cylinder engine sharing a common intake plenum can be overcome by using a carburetor or injector for each cylinder.

Accomplishing maximum volumetric efficiency for a given engine is not a formulaic process. Variables such as flow rates , overlap, valve lift, porting specifications and the location of valve events create a large set of variables. Different intake and exhaust equipment is tested at different speeds and loads, and the end result is usually a compromise between power, emissions, and cost, except in situations where maximum power is desired regardless of cost or emissions (such as racing.) The new volumetric efficiency and valve run are in animations

Valve train
The valves are typically operated by a camshaft, which is a rod with a series of projecting cams (lobes), each with a carefully calculated profile designed to push the valve open by the required degree at the right moment and to hold it open as required as the camshaft rotates. Between the valve stem and the cam is a tappet, a cam follower, which accommodates variations in the line of contact of the cam. The location of the camshaft varies, as does the quantities. Some engines have overhead cams, or even dual overhead cams, as in the illustration above, in which the camshaft(s) directly actuate(s) the valves through a tappet. This design is typically capable of higher engine speeds due to fewer moving parts in the valve train. In other engine designs, the cam shaft is placed in the crankcase and its motion transmitted by a push rod, rocker arms, and valve stems.

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Cylinder head

In an internal combustion engine, the cylinder head sits atop the cylinders and consists of a platform containing most part of the combustion chamber and the location of the valves and spark plugs. In a flathead engine, the mechanical parts of the valve train are all contained within the block, and the head is essentially a flat plate of metal bolted to the top of the cylinder bank; this simplicity leads to ease of manufacture and repair, and accounts for the flathead engine's early success in production automobiles and continued success in small engines, such as lawnmowers. This design, however, requires the incoming air to flow through a convoluted path, which limits the ability of the engine to perform at higher rpm, leading to the adoption of the overhead valve head design.

In the overhead valve head, the top half of the cylinder head contains the camshaft in an overhead cam engine, or another mechanism (such as rocker arms and pushrods) to transfer rotational mechanics from the crankshaft to linear mechanics to operate the valves (pushrod engines perform this conversion at the camshaft lower in the engine and use a rod to push a rocker arm that acts on the valve). Internally the cylinder head has passages called ports for the fuel/air mixture to travel to the inlet valves from the intake manifold, for exhaust gases to travel from the exhaust valves to the exhaust manifold, and for antifreeze (coolant) to cool the head and engine.

The number of cylinder heads in an engine is a function of the engine configuration. A straight engine has only one cylinder head. A V engine usually has two cylinder heads, one at each end of the V, although Volkswagen, for instance, produces a V6 called the VR6, where the angle between the cylinder banks is so narrow that it utilizes a single head. A boxer engine has two heads.

The cylinder head is key to the performance of the internal combustion engine, as the shape of the combustion chamber, inlet passages and ports (and to a lesser extent the exhaust) determines a major portion of the volumetric efficiency and compression ratio of the engine.