Camshaft:~

Camshaft by Rishabh Sharma on Scribd

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.

History:~

An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Turkey (Diyarbakır) by Al-Jazari in 1206. He employed it as part of his automata, water-raising machines, and water clocks such as the castle clock. The cam and camshaft later appeared in European mechanisms from at least the 14th century, or possibly earlier.
Among 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.

Uses:~

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

Automotive:~

Materials

Camshafts can be made out of several types of material. These include:

Chilled iron castings: Commonly used in high volume production, chilled iron camshafts have a good wear resistance since the chilling process hardens them. Other elements are added to the iron before casting to make the material more suitable for its application.

Billet Steel: When a high quality camshaft or low volume production is required, engine builders and camshaft manufacturers choose steel billet. This is a much more time consuming process, and is generally more expensive than other methods. However the finished product is far superior. CNC lathes, CNC milling machines and CNC camshaft grinders will be used during production. Different types of steel bar can be used, one example being EN40b. When manufacturing a camshaft from EN40b, the camshaft will also be heat treated via gas nitriding, which changes the micro-structure of the material. It gives a surface hardness of 55-60 HRC. These types of camshafts can be used in high-performance engines.

Timing

A steel billet racing camshaft with noticeably broad lobes (very long duration)

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 the air/fuel mixture intake and exhaust gases, 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. Direct drive using gears is unusual because of the cost. The frequently reversing torque caused by the slope of the cams tends to cause gear rattle which for an all-metal gear train requires further expense of a cam damper. Rolls-Royce V8 (1954) used gear drive as unlike chain it could be made silent and to last the life of the engine. Where gears are used in cheaper cars, they tend to be made from resilient fibre rather than metal, except in racing engines that have a high maintenance routine. Fibre gears have a short life span and must be replaced regularly, much like a timing belt. In some designs the camshaft also drives the distributor and the oil and fuel pumps. Some vehicles may have the power steering pump driven by the camshaft. With some early fuel injection systems, cams on the camshaft would operate the fuel injectors. Honda redesigned the VF750 from chain drive to gear drive VFR750 due to insurmountable problems with the VF750 Hi-Vo inverted chain drive.

An alternative used in the early days of OHC engines was to drive the camshaft(s) via a vertical shaft with bevel gears at each end. This system was, for example, used on the pre-WW1 Peugeot and Mercedes Grand Prix cars. Another option was to use a triple eccentric with connecting rods; these were used on certain W.O. Bentley-designed engines and also on the Leyland Eight.

In 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; thus, two full rotations of the crankshaft occur for each rotation of the camshaft.

The timing of the camshaft can be advanced to produce better low RPM torque, or retarded for better high RPM power. Either of these moves the overall power produced by the engine down or up the RPM scale respectively. The amount of change is very little (usually < 5 deg), and affects valve to piston clearances.

Duration

Duration is the number of crankshaft degrees of engine rotation during which the valve is off the seat. As a generality, greater duration results in more horsepower. The RPM at which peak horsepower occurs is typically increased as duration increases at the expense of lower rpm efficiency (torque).

Duration can often be confusing because manufacturers may select any lift point to advertise a camshaft's duration and sometimes will manipulate these numbers. The power and idle characteristics of a camshaft rated at .006" will be much different than one rated the same at .002".

Many performance engine builders gauge a race profile's aggressiveness by looking at the duration at .020", .050" and .200". The .020" number determines how responsive the motor will be and how much low end torque the motor will make. The .050" number is used to estimate where peak power will occur, and the .200" number gives an estimate of the power potential.

A secondary effect of increased duration is increasing overlap, which is the number of crankshaft degrees during which both intake and exhaust valves are off their seats. It is overlap which most affects idle quality, inasmuch as the "blow-through" of the intake charge which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation. In reality, increasing a camshaft's duration typically increases the overlap event, unless one spreads lobe centers between intake and exhaust valve lobe profiles.

Lift

The camshaft "lift" is the resultant net rise of the valve from its seat. The further the valve rises from its seat the more airflow can be released, which is generally more beneficial. Greater lift has some limitations. Firstly, the lift is limited by the increased proximity of the valve head to the piston crown and secondly greater effort is required to move the valve's springs to higher state of compression. Increased lift can also be limited by lobe clearance in the cylinder head construction, so higher lobes may not necessarily clear the framework of the cylinder head casing. Higher valve lift can have the same effect as increased duration where valve overlap is less desirable.

Higher lift allows accurate timing of airflow; although even by allowing a larger volume of air to pass in the relatively larger opening, the brevity of the typical duration with a higher lift cam results in less airflow than with a cam with lower lift but more duration, all else being equal. On forced induction motors this higher lift could yield better results than longer duration, particularly on the intake side. Notably though, higher lift has more potential problems than increased duration, in particular as valve train rpm rises which can result in more inefficient running or loss of torque.

Cams that have too high a resultant valve lift, and at high rpm, can result in what is called "valve bounce", where the valve spring tension is insufficient to keep the valve following the cam at its apex. This could also be as a result of a very steep rise of the lobe and short duration, where the valve is effectively shot off the end of the cam rather than have the valve follow the cams’ profile. This is typically what happens on a motor over rev. This is an occasion where the engine rpm exceeds the engine maximum design speed. The valve train is typically the limiting factor in determining the maximum rpm the engine can maintain either for a prolonged period or temporarily. Sometimes an over rev can cause engine failure where the valve stems become bent as a result of colliding with the piston crowns.

Position

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.

Number of camshafts:~

While today some cheaper engines rely on a single camshaft per cylinder bank, which is known as a single overhead camshaft (SOHC), most modern engine designs (the overhead-valve or OHV engine being largely obsolete on passenger vehicles), are driven by a two camshafts per cylinder bank arrangement (one camshaft for the intake valves and another for the exhaust valves); such camshaft arrangement is known as a double or dual overhead cam (DOHC), thus, a V engine, which has two separate cylinder banks, may have four camshafts (colloquially known as a quad-cam engine).

More unusual is the modern W engine (also known as a 'VV' engine to distinguish itself from the pre-war W engines) that has four cylinder banks arranged in a "W" pattern with two pairs narrowly arranged with a 15-degree separation. Even when there are four cylinder banks (that would normally require a total of eight individual camshafts), the narrow-angle design allows the use of just four camshafts in total. For the Bugatti Veyron, which has a 16-cylinder W engine configuration, all the four camshafts are driving a total of 64 valves.

The overhead camshaft design adds more valvetrain components that ultimately incur in more complexity and higher manufacturing costs, but this is easily offset by many advantages over the older OHV design: multi-valve design, higher RPM limit and design freedom to better place valves, ignition (Spark-ignition engine) and intake/exhaust ports.

Maintenance:~

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, and in particular the valves and valve seats in the combustion chamber.

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 adds mass to the valvetrain.

Camshaft bearings are similar to crankshaft main bearings, being pressure-fed with oil. 

However, OHC camshaft bearings do not always have replaceable bearing shells, meaning that a new cylinder head is required if the bearings suffer wear due to insufficient or dirty oil.

Alternatives

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. Some approaches to reclaiming this "wasted" energy include:

• Springless valves, like the desmodromic system employed today by Ducati
• Camless valvetrains using solenoids or magnetic systems have long been investigated by BMW and Fiat, and are currently being prototyped by Valeo and Ricardo
• The Wankel engine, a rotary engine which uses neither pistons nor valves, best known for being used by Mazda in the RX-7 and RX-8 sports cars.
• Koenigsegg has developed an electric valve actuator as a more fuel efficient and space saving alternative to the traditional camshaft.

Ignition systems

In mechanically timed ignition systems, a separate camshaft is geared to the engine and operates a breaker that triggers a spark at the correct points in the combustion cycle.

Electrical:~

Before the advent of solid state electronics, camshaft controllers were used to control the speed of electric motors. A camshaft, driven by an electric motor or a pneumatic motor, was used to operate switches in sequence. By this means, resistors or tap changers were switched in or out of the circuit to vary the speed of the main motor. This system was mainly used in electric multiple units.

Supercharger

Roots type supercharger on AMC V8 engine for dragstrip racing

A supercharger is an air compressor that increases the pressure or density of air supplied to an internal combustion engine. This gives each intake cycle of the engine more oxygen, letting it burn more fuel and do more work, thus increasing power.
Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or chain connected to the engine's crankshaft.
When power is provided by a turbine powered by exhaust gas, a supercharger is known as a turbosupercharger – typically referred to simply as a turbocharger or just turbo. Common usage restricts the term supercharger to mechanically driven units.

History

In 1848 or 1849 G. Jones of Birmingham, England brought out a Roots-style compressor.
In 1860, brothers Philander and Francis Marion Roots, founders of Roots Blower Company of Connersville, Indiana, patented the design for an air mover for use in blast furnaces and other industrial applications.

The world's first functional, actually tested engine supercharger was made by Dugald Clerk, who used it for the first two-stroke engine in 1878. Gottlieb Daimler received a German patent for supercharging an internal combustion engine in 1885. Louis Renault patented a centrifugal supercharger in France in 1902. An early supercharged race car was built by Lee Chadwick of Pottstown, Pennsylvania in 1908 which reportedly reached a speed of 100 mph (160 km/h).
The world's first series-produced cars with superchargers were Mercedes 6/25/40 hp and Mercedes 10/40/65 hp. Both models were introduced in 1921 and had Roots superchargers. They were distinguished as "Kompressor" models, the origin of the Mercedes-Benz badging which continues today.

On March 24, 1878 Heinrich Krigar of Germany obtained patent #4121, patenting the first ever screw-type compressor. Later that same year on August 16 he obtained patent #7116 after modifying and improving his original designs. His designs show a two-lobe rotor assembly with each rotor having the same shape as the other. Although the design resembled the roots style compressor, the "screws" were clearly shown with 180 degrees of twist along their length. Unfortunately, the technology of the time was not sufficient to produce such a unit, and Heinrich made no further progress with the screw compressor. Nearly half a century later, in 1935, Alf Lysholm, who was working for Ljungstroms Angturbin AB (later known as Svenska Rotor Maskiner AB or SRM in 1951), patented a design with five female and four male rotors. He also patented the method for machining the compressor rotors.

Types of supercharger

There are two main types of superchargers defined according to the method of gas transfer: positive displacement and dynamic compressors. Positive displacement blowers and compressors deliver an almost constant level of pressure increase at all engine speeds (RPM). Dynamic compressors do not deliver pressure at low speeds; above a threshold speed, pressure increases with engine speed.

Positive displacement

An Eaton MP62 Roots-type supercharger is visible at the front of this Ecotec LSJ engine in a 2006 Saturn Ion Red Line.

Lysholm screw rotors with complex shape of each rotor, which must run at high speed and with close tolerances. This makes this type of supercharger expensive. (This unit has been blued to show close contact areas.)

Positive-displacement pumps deliver a nearly fixed volume of air per revolution at all speeds (minus leakage, which is almost constant at all speeds for a given pressure, thus its importance decreases at higher speeds).

Major types of positive-displacement pumps include:

• Roots
• Lysholm twin-screw
• Sliding vane
• Scroll-type supercharger, also known as the G-Lader

Compression type

Positive-displacement pumps are further divided into internal and external compression types.
Roots superchargers are external compression only (although high-helix roots blowers attempt to emulate the internal compression of the Lysholm screw).

• External compression refers to pumps that transfer air at ambient pressure into the engine. If the engine is running under boost conditions, the pressure in the intake manifold is higher than that coming from the supercharger. That causes a backflow from the engine into the supercharger until the two reach equilibrium. It is the backflow that actually compresses the incoming gas. This is an inefficient process and the main factor in the lack of efficiency of Roots superchargers when used at high boost levels. The lower the boost level the smaller is this loss, and Roots blowers are very efficient at moving air at low pressure differentials, which is what they were invented for (hence the original term "blower").

All the other types have some degree of internal compression.

• Internal compression refers to the compression of air within the supercharger itself, which, already at or close to boost level, can be delivered smoothly to the engine with little or no back flow. This is more effective than back flow compression and allows higher efficiency to be achieved. Internal compression devices usually use a fixed internal compression ratio. When the boost pressure is equal to the compression pressure of the supercharger, the back flow is zero. If the boost pressure exceeds that compression pressure, back flow can still occur as in a roots blower. Internal compression blowers must be matched to the expected boost pressure in order to achieve the higher efficiency they are capable of, otherwise they will suffer the same problems and low efficiency of the roots blowers.

Capacity rating

Positive-displacement superchargers are usually rated by their capacity per revolution. In the case of the Roots blower, the GMC rating pattern is typical. The GMC types are rated according to how many two-stroke cylinders, and the size of those cylinders, it is designed to scavenge. GMC has made 2–71, 3–71, 4–71, and the famed 6–71 blowers. For example, a 6–71 blower is designed to scavenge six cylinders of 71 cubic inches (1,163 cc) each and would be used on a two-stroke diesel of 426 cubic inches (6,981 cc), which is designated a 6–71; the blower takes this same designation. 

However, because 6–71 is actually the engine's designation, the actual displacement is less than the simple multiplication would suggest. A 6–71 actually pumps 339 cubic inches (5,555 cc) per revolution (but as it spins faster than the engine, it can easily put out the same displacement as the engine per engine rev).

Aftermarket derivatives continue the trend with 8–71 to current 16–71 blowers used in different motor sports. From this, one can see that a 6–71 is roughly twice the size of a 3–71. GMC also made 53 cu in (869 cc) series in 2–, 3–, 4–, 6–, and 8–53 sizes, as well as a "V71" series for use on engines using a V configuration.

Dynamic

Dynamic compressors rely on accelerating the air to high speed and then exchanging that velocity for pressure by diffusing or slowing it down.
Major types of dynamic compressor are:

• Centrifugal
• Multi-stage axial-flow
• Pressure wave supercharger

Supercharger drive types

Superchargers are further defined according to their method of drive.

• Belt (V-belt, Synchronous belt, Flat belt)
• Direct drive
• Gear drive
• Chain drive

Temperature effects and intercoolers

Supercharger CDT vs. Ambient Temperature. 

One disadvantage of supercharging is that compressing the air increases its temperature. When a supercharger is used on an internal combustion engine, the temperature of the fuel/air charge becomes a major limiting factor in engine performance. Extreme temperatures will cause detonation of the fuel-air mixture (spark ignition engines) and damage to the engine. In cars, this can cause a problem when it is a hot day outside, or when an excessive level of boost is reached.

It is possible to estimate the temperature rise across a supercharger by modeling it as an isentropic process.

${\displaystyle {\frac {T_{2}}{T_{1}}}}$

${\displaystyle =\,\!}$

${\displaystyle \left({\frac {p_{2}}{p_{1}}}\right)^{\frac {\gamma -1}{\gamma }}}$

Where:

${\displaystyle T_{1}\,\!}$ = ambient air temperature

${\displaystyle T_{2}\,\!}$ = temperature after the compressor

${\displaystyle p_{1}\,\!}$ = ambient atmospheric pressure (absolute)

${\displaystyle p_{2}\,\!}$ = pressure after the compressor (absolute)

${\displaystyle \gamma \,\!}$ = Ratio of specific heat capacities = ${\displaystyle C_{p}/C_{v}\,\!}$ = 1.4 for air

${\displaystyle C_{p}\,\!}$ = Specific heat at constant pressure

${\displaystyle C_{v}\,\!}$ = Specific heat at constant volume

For example, if a supercharged engine is pushing 10 psi (0.69 bar) of boost at sea level (ambient pressure of 14.7 psi (1.01 bar), ambient temperature of 75 °F (24 °C)), the temperature of the air after the supercharger will be 160.5 °F (71.4 °C). This temperature is known as the compressor discharge temperature (CDT) and highlights why a method for cooling the air after the compressor is so important.

While it is true that higher intake temperatures for internal combustion engines will ingest air of lower density, this only holds correct for a static, unchanging air pressure. i.e. on a hot day an engine will intake less oxygen per engine cycle than it would on a cold day. However, the heating of the air, while in the supercharger compressor, does not reduce the density of the air due to its rise in temperature. The rise in temperature is due to its rise in pressure. Energy is being added to the air and this is seen in both its energy, internal to the molecules (temperature) and of the air in static pressure, as well as the velocity of the gas.

Inter-cooling makes no change in the density of the air after it has been compressed. It is only removing the thermal energy of the air from the compression process. i.e. the inter-cooler only removes the energy put in by the compression process and does not alter the density of air, so that the air/fuel mixture is not so hot that it causes it to ignite before the spark ignites it, otherwise known as pre-ignition.

Two-stroke engines

For two-stroke engines, scavenging is required to purge exhaust gasses. In small engines this is commonly achieved by using the crankcase as a blower, the descending piston during the power stroke compresses air in the crankcase used to purge the cylinder. Scavenging blowing should not be confused with supercharging, no charge compression takes place. As the volume change produced by the lower side of the piston is the same as the upper face, this is limited to scavenging and cannot provide any supercharging.

Larger engines usually use a separate blower for scavenging and it was for this type of operation that the Roots blower was developed. Historically many designs of blower have been used, from separate pumping cylinders, 'top hat' pistons combining two pistons of different diameter the larger one being used for scavenging, various rotary blowers and centrifugal turbocompressors, including turbochargers. Turbocharging two-stroke engines is difficult, but not impossible, as an exhaust-driven turbocharger does not provide any boost until it has had time to spin up to speed. Purely turbocharged two stroke engines may thus have difficulty when starting, with poor combustion and dirty exhausts, possibly even four-stroking. Some two-stroke turbochargers have a mechanical drive through a clutch, used for starting.

Simple two-stroke engines with ported inlet and exhaust cannot be supercharged since the inlet port always closes first. For this reason, two-stroke Diesel engines usually have mechanical exhaust valves with separate timing to allow supercharging. Regardless of this, two-stroke engines require scavenging at all engine speeds and so turbocharged two-stroke engines must still employ a blower, usually Roots type. This blower may be mechanically or electrically driven, in either case the blower may be disengaged once the turbocharger starts to deliver air.

Automobiles

1929 "Blower" Bentley. The large "blower" (supercharger), located in front of the radiator, gave the car its name.

In 1900, Gottlieb Daimler, of Daimler-Benz (Daimler AG), was the first to patent a forced-induction system for internal combustion engines, superchargers based on the twin-rotor air-pump design, first patented by the American Francis Roots in 1860, the basic design for the modern Roots type supercharger.

The first supercharged cars were introduced at the 1921 Berlin Motor Show: the 6/20 hp and 10/35 hp Mercedes. These cars went into production in 1923 as the 6/25/40 hp (regarded as the first supercharged road car) and 10/40/65 hp. These were normal road cars as other supercharged cars at same time were almost all racing cars, including the 1923 Fiat 805-405, 1923 Miller 122 1924 Alfa Romeo P2, 1924 Sunbeam, 1925 Delage, and the 1926 Bugatti Type 35C. At the end of the 1920s, Bentley made a supercharged version of the Bentley 4½ Litre road car. Since then, superchargers (and turbochargers) have been widely applied to racing and production cars, although the supercharger's technological complexity and cost have largely limited it to expensive, high-performance cars.

Supercharging versus turbocharging

A G-Lader scroll-type supercharger on a Volkswagen Golf Mk1.

Keeping the air that enters the engine cool is an important part of the design of both superchargers and turbochargers. Compressing air increases its temperature, so it is common to use a small radiator called an intercooler between the pump and the engine to reduce the temperature of the air.
There are three main categories of superchargers for automotive use:

• Centrifugal turbochargers – driven from exhaust gases.
• Centrifugal superchargers – driven directly by the engine via a belt-drive.
• Positive displacement pumps – such as the Roots, Twin Screw (Lysholm), and TVS (Eaton) blowers.

Roots blowers tend to be only 40–50% efficient at high boost levels; by contrast centrifugal (dynamic) superchargers are 70–85% efficient at high boost. Lysholm-style blowers can be nearly as efficient as their centrifugal counterparts over a narrow range of load/speed/boost, for which the system must be specifically designed.

Mechanically driven superchargers may absorb as much as a third of the total crankshaft power of the engine and are less efficient than turbochargers. However, in applications for which engine response and power are more important than other considerations, such as top-fuel dragsters and vehicles used in tractor pulling competitions, mechanically driven superchargers are very common.
The thermal efficiency, or fraction of the fuel/air energy that is converted to output power, is less with a mechanically driven supercharger than with a turbocharger, because turbochargers use energy from the exhaust gas that would normally be wasted. For this reason, both economy and the power of a turbocharged engine are usually better than with superchargers.

Turbochargers suffer (to a greater or lesser extent) from so-called turbo-spool (turbo lag; more correctly, boost lag), in which initial acceleration from low RPM is limited by the lack of sufficient exhaust gas mass flow (pressure). Once engine RPM is sufficient to raise the turbine RPM into its designed operating range, there is a rapid increase in power, as higher turbo boost causes more exhaust gas production, which spins the turbo yet faster, leading to a belated "surge" of acceleration. This makes the maintenance of smoothly increasing RPM far harder with turbochargers than with engine-driven superchargers, which apply boost in direct proportion to the engine RPM. The main advantage of an engine with a mechanically driven supercharger is better throttle response, as well as the ability to reach full-boost pressure instantaneously. With the latest turbocharging technology and direct gasoline injection, throttle response on turbocharged cars is nearly as good as with mechanically powered superchargers, but the existing lag time is still considered a major drawback, especially considering that the vast majority of mechanically driven superchargers are now driven off clutched pulleys, much like an air compressor.

Turbocharging has been more popular than superchargers among auto manufacturers owing to better power and efficiency. For instance Mercedes-Benz and Mercedes-AMG previously had supercharged "Kompressor" offerings in the early 2000s such as the C230K, C32 AMG, and S55 AMG, but they have abandoned that technology in favor of turbocharged engines released around 2010 such as the C250 and S65 AMG biturbo. However, Audi did introduce its 3.0 TFSI supercharged V6 in 2009 for its A6, S4, and Q7, while Jaguar has its supercharged V8 engine available as a performance option in the XJ, XF, XKR, and F-Type, and, via joint ownership by Tata motors, in the Range Rover also.

Twincharging

In the 1985 and 1986 World Rally Championships, Lancia ran the Delta S4, which incorporated both a belt-driven supercharger and exhaust-driven turbocharger. The design used a complex series of bypass valves in the induction and exhaust systems as well as an electromagnetic clutch so that, at low engine speeds, boost was derived from the supercharger. In the middle of the rev range, boost was derived from both systems, while at the highest revs the system disconnected drive from the supercharger and isolated the associated ducting. This was done in an attempt to exploit the advantages of each of the charging systems while removing the disadvantages. In turn, this approach brought greater complexity and impacted on the car's reliability in WRC events, as well as increasing the weight of engine ancillaries in the finished design.

The Volkswagen TSI engine (or Twincharger) is a 1.4-litre direct-injection motor that also uses both a supercharger and turbocharger.

Aircraft

Altitude effects

The Rolls-Royce Merlin, a supercharged aircraft engine from World War II. The supercharger is at the rear of the engine at right

A Centrifugal supercharger of a Bristol Centaurus radial aircraft engine.

Superchargers are a natural addition to aircraft piston engines that are intended for operation at high altitudes. As an aircraft climbs to higher altitude, air pressure and air density decreases. The output of a piston engine drops because of the reduction in the mass of air that can be drawn into the engine. For example, the air density at 30,000 ft (9,100 m) is ¹⁄₃ of that at sea level, thus only ¹⁄₃ of the amount of air can be drawn into the cylinder, with enough oxygen to provide efficient combustion for only a third as much fuel. So, at 30,000 ft (9,100 m), only ¹⁄₃ of the fuel burnt at sea level can be burnt. (An advantage of the decreased air density is that the airframe experiences only about 1/3 of the aerodynamic drag. Plus, there is decreased back pressure on the exhaust gases. On the other hand, more energy is consumed holding an airplane up with less air in which to generate lift.)

A supercharger can be thought of either as artificially increasing the density of the air by compressing it or as forcing more air than normal into the cylinder every time the piston moves down.
A supercharger compresses the air back to sea-level-equivalent pressures, or even much higher, in order to make the engine produce just as much power at cruise altitude as it does at sea level. With the reduced aerodynamic drag at high altitude and the engine still producing rated power, a supercharged airplane can fly much faster at altitude than a naturally aspirated one. The pilot controls the output of the supercharger with the throttle and indirectly via the propeller governor control. Since the size of the supercharger is chosen to produce a given amount of pressure at high altitude, the supercharger is oversized for low altitude. The pilot must be careful with the throttle and watch the manifold pressure gauge to avoid overboosting at low altitude. As the aircraft climbs and the air density drops, the pilot must continuously open the throttle in small increments to maintain full power. The altitude at which the throttle reaches full open and the engine is still producing full rated power is known as the critical altitude. Above the critical altitude, engine power output will start to drop as the aircraft continues to climb.

Effects of temperature

Supercharger CDT vs. Altitude.

As discussed above, supercharging can cause a spike in temperature, and extreme temperatures will cause detonation of the fuel-air mixture and damage to the engine. In the case of aircraft, this causes a problem at low altitudes, where the air is both denser and warmer than at high altitudes. With high ambient air temperatures, detonation could start to occur with the manifold pressure gauge reading far below the red line.

A supercharger optimized for high altitudes causes the opposite problem on the intake side of the system. With the throttle retarded to avoid overboosting, air temperature in the carburetor can drop low enough to cause ice to form at the throttle plate. In this manner, enough ice could accumulate to cause engine failure, even with the engine operating at full rated power. For this reason, many supercharged aircraft featured a carburetor air temperature gauge or warning light to alert the pilot of possible icing conditions.

Several solutions to these problems were developed: intercoolers and aftercoolers, anti-detonant injection, two-speed superchargers, and two-stage superchargers.

Two-speed and two-stage superchargers

In the 1930s, two-speed drives were developed for superchargers. These provided more flexibility for the operation of the aircraft, although they also entailed more complexity of manufacturing and maintenance. The gears connected the supercharger to the engine using a system of hydraulic clutches, which were initially manually engaged or disengaged by the pilot with a control in the cockpit. At low altitudes, the low-speed gear would be used in order to keep the manifold temperatures low. At around 12,000 feet (3,700 m), when the throttle was full forward and the manifold pressure started to drop off, the pilot would retard the throttle and switch to the higher gear, then readjust the throttle to the desired manifold pressure. Later installations automated the gear change according to atmospheric pressure.

Another enhancement was the use of two compressors (also known as stages) in series, such two-stage superchargers were also always two-speed. After the air was compressed in the low-pressure stage, the air flowed through an intercooler radiator where it was cooled before being compressed again by the high-pressure stage and then possibly also aftercooled in another heat exchanger. Two-stage compressors provided much improved high altitude performance, as typified by the Rolls-Royce Merlin powered Supermarine Spitfire Mk IX and the North American Mustang. In some two-stage systems, damper doors would be opened or closed by the pilot in order to bypass one stage as needed. Some systems had a cockpit control for opening or closing a damper to the intercooler/aftercooler, providing another way to control temperature. Rolls-Royce Merlin engines had fully automated boost control and all the pilot had to do was advance the throttle, the control system would limit boost as necessary until maximum altitude was reached.

Turbocharging

A mechanically driven supercharger has to take its drive power from the engine. Taking a single-stage single-speed supercharged engine, such as the Rolls-Royce Merlin, for instance, the supercharger uses up about 150 hp (110 kW). Without a supercharger, the engine could produce about 750 horsepower (560 kilowatts), but with a supercharger, it produces about 1,000 hp (750 kW)—an increase of about 400 hp (750 - 150 + 400 = 1000 hp), or a net gain of 250 hp (190 kW). This is where the principal disadvantage of a supercharger becomes apparent. The engine has to burn extra fuel to provide power to drive the supercharger. The increased air density during the input cycle increases the specific power of the engine and its power-to-weight ratio, but at the cost of an increase in the specific fuel consumption of the engine. In addition to increasing the cost of running the airplane this has the potential to reduce its overall range. On the other hand, with more engine power the airplane can carry more fuel. In military types, this has often been done using external drop tanks, for example in the American P-38 Lightning, P-47 Thunderbolt, P-51 Mustang, and F6F Hellcat fighter planes.

With their external fuel tanks and supercharged or turbocharged engines, the P-38 and the P-51 could fly from England to Berlin and back, the P-47 could fly from England to the Ruhr and back, and the F6F had the longest range of any fighter based on aircraft carriers of the war. Also, the P-51 could fly even further - from Iwo Jima to Tokyo and back. These ranges were much longer than those of any Nazi German, British, Japanese, Canadian, or Soviet fighter planes of World War II. These American fighters also had excellent fighting performance at high altitudes.

As opposed to a supercharger driven by the engine itself, a turbocharger is driven using the exhaust gases from the engines. The amount of power in the gas is proportional to the difference between the exhaust pressure and air pressure, and this difference increases with altitude, helping a turbocharged engine to compensate for changing altitude.

The majority of high-altitude aircraft engines used during World War II used mechanically driven superchargers, because these had three significant manufacturing advantages over turbochargers. Turbochargers - used by large American aircraft engines such as the Allison V-1710 (used in the P-38) and the Pratt & Whitney R-2800, required additional ducting expensive high-temperature metal alloys in the gas turbine and preturbine section of the exhaust system, but they were very useful in high-altitude bombers and some fighter planes. The size of the ducting alone was a serious problem. For example, both the F4U Corsair and the P-47 Thunderbolt used the same multicylinder radial engine, but the large barrel-shaped fuselage of the P-47 was needed because of the amount of ducting to and from the turbocharger in the rear fuselage. The F4U used a two-stage supercharger with compact intercooler layout.

Turbocharged piston engines are also subject to many of the same operating restrictions as those of gas turbine engines. Turbocharged engines also require frequent inspections of their turbochargers and exhaust systems to search for possible damage caused by the extreme heat and pressure of the turbochargers. Such damage was a prominent problem in the early models of the American B-29 Superfortress high-altitude bombers used in the Pacific Theater of Operations during 1944–45.
Turbocharged piston engines continued to be used in a large number of postwar airplanes, such as the B-50 Superfortress, the KC-97 Stratofreighter, the Boeing Stratoliner, the Lockheed Constellation, and the C-124 Globemaster II.

In more recent times most aircraft engines for general aviation (light airplanes) are naturally aspirated, but the smaller number of modern aviation piston engines designed to run at high altitudes use turbocharger or turbo-normalizer systems, instead of a supercharger driven from the crank shafts. The change in thinking is largely due to economics. Aviation gasoline was once plentiful and cheap, favoring the simple but fuel-hungry supercharger. As the cost of fuel has increased, the ordinary supercharger has fallen out of favor. Also, depending on what monetary inflation factor one uses, fuel costs have not decreased as fast as production and maintenance costs have.

Effects of fuel octane rating

Until the late 1920s all automobile and aviation fuel was generally rated at 87 octane or less. This is the rating that was achieved by the simple distillation of "light crude" oil. Engines from around the world were designed to work with this grade of fuel, which set a limit to the amount of boosting that could be provided by the supercharger, while maintaining a reasonable compression ratio.
Octane rating boosting through additives was a line of research being explored at the time. Using these techniques, less valuable crude could still supply large amounts of useful gasoline, which made it a valuable economic process. However, the additives were not limited to making poor-quality oil into 87-octane gasoline; the same additives could also be used to boost the gasoline to much higher octane ratings.

Higher-octane fuel resists auto ignition and detonation better than does low-octane fuel. As a result, the amount of boost supplied by the superchargers could be increased, resulting in an increase in engine output. The development of 100-octane aviation fuel, pioneered in the USA before the war, enabled the use of higher boost pressures to be used on high-performance aviation engines, and was used to develop extremely high-power outputs – for short periods – in several of the pre-war speed record airplanes. Operational use of the new fuel during World War II began in early 1940 when 100-octane fuel was delivered to the British Royal Air Force from refineries in America and the East Indies. The German Luftwaffe also had supplies of a similar fuel.

Increasing the knocking limits of existing aviation fuels became a major focus of aero engine development during World War II. By the end of the war, fuel was being delivered at a nominal 150-octane rating, on which late-war aero engines like the Rolls-Royce Merlin 66 or the Daimler-Benz DB 605DC developed as much as 2,000 hp (1,500 kW).

ENGINE STROKE DEFINATION

                        ENGINE STROKE DEFINATION

In a 4 stroke engine, piston strokes (movement from Bottom Dead Center to Top Dead Center or the opposite) are necessary to complete a working cycle.Intake stroke (from TDC to BDC): fresh mixture in SI (Spark Ignition) engine or fresh air in diesel engine is intaken into the cylinder through the intake valves, which can open with a light advance before the TDC and can close with a certain delay after the BDC, to maximize the mass inducted.

Compression stroke :~ (from BDC to TDC): fresh mixture in SI engine or fresh air in diesel engine is compressed with all valves closed. Towards the end of the compression stroke, the combustion is initiated via spark ignition (Spark Ignition engine) or fuel injection (diesel engine).

Power stroke :~  (from TDC to BDC): the hot burned gases expand, pushing down the piston and applying on it a work which is five times (or more) greater than the work applied by the piston during the compression stroke. Towards the end of power stroke, the exhaust valves can start to open and part of the burned gases are expelled from the cylinder thanks to the pressure differential.

Exhaust stroke :~ (from BDC to TDC): the piston expels the remaining burned gases. Towards the end of the exhaust stroke, the intake valves can open while shortly after the TDC the exhaust valve can close, this is called an overlap. After that, a new cycle can begin.
Although the cycle is completed in 4 strokes in 2 crank revolutions, there are 6 operating phases that can be highlighted, since different phases may occur during a single stroke:

• Intake
• Compression
• Combustion
• Expansion
• Exhaust (Blowdown)
• Exhaust (Displacement)

It should be noticed that 2 operating phases are requested to replace the burned gases with fresh mixture.

2 Stroke Principle

In 2 strokes engine, the complete operating cycle just requires two piston strokes (i.e. 1 crankshaft revolution).

                   2 STROKE ENGINE

In order to obtain higher power output, the two strokes used for the gas exchanges are suppressed, and substituted by scavenging process. The scavenging process is defined by the displacement of the burned gases when the piston is approaching the end of the power stroke by means of a fresh charge which has been pressurized.

In the simplest design, the fresh charge has been pressurized thanks to the crankcase itself, the volume of which varies in opposition with the cylinder volume, so that the minimum crankcase volume (and then the maximum pressure) is reached when piston is at the BDC in the main cylinder.

A more compact design in comparison with 4 stroke engine is possible because the intake and exhaust valves can be replaced by ports (holes) in the cylinder liner, the opening and closing of which can be directly controlled by the piston motion.

The two strokes are the following:

Compression stroke :~ after closing the inlet and exhaust ports, the piston compresses the cylinder charge (in the meantime, the volume in the crankcase increases, drawing fresh charge into the crankcase by depression). Towards the end of the compression stroke, the combustion is initiated by spark ignition (SI engine) or fuel injection (diesel engine).

Power stroke :~ the hot burned gases expand, pushing down the piston. Towards the end of this stroke, the exhaust port opens and part of the burnt gases are expelled from the cylinder thanks to the pressure differences. Afterwards, the scavenging ports are opened, and the pressurized fresh charge moves out the burned gases, so that a new cycle can start again after the piston has reached the BDC.
Again, as for the 4 stroke engine, 6 different phases take place during the 2 strokes:

• Scavenging
• Intake
• Compression
• Combustion
• Expansion
• Blowdown

However, to achieve such a cycle, a pressure controlled valve on the scavenging port is needed. If simple ports in the cylinder walls are used, the intake port edge must be in a lower position than the exhaust port, to allow the blowdown phase. This would cause a short circuit of part of the induced fresh charged at the beginning of the compression stroke, as the exhaust port remains open for a while after the intake port closure.

scavenging

The scavenging process represents the Achille’s heel of the 2 stroke engine, since in its simplest layout with simple ports in the cylinder walls, part of the fresh charge will flow directly to the exhaust port, causing high fuel consumption and HC emissions in SI engine.


For those reasons, the use of 2 strokes SI engines has been limited to small power utility engines (such as lawn mowers, saw chains, outboard engines for boat propulsion…), where the cons were thought to be acceptable due to the high simplicity, low cost and high power density of these engines.

2 strokes engines are also used for large diesels for marine and stationary applications (about 1 meter bore) where they are usually preferred to 4 strokes due to the excessively high thermo-mechanical stresses that valves should withstand (stress increases with valve diameter, which is proportional to cylinder bore).

How Torque Converters Work

Torque Converter by Rishabh Sharma on Scribd

If you've read about manual transmissions, you know that an engine is connected to a transmission by way of a clutch. Without this connection, a car would not be able to come to a complete stop without killing the engine. But cars with an aut­omatic transmission have no clutch that disconnects the transmission from the engine. Instead, they use an amazing device called a torque converter. It may not look like much, but there are some very interesting things going on inside.

­ In this article, we'll learn why automatic transmission cars need a torque converter, how a torque converter works and what some of its benefits and shortcomings are.

 The Basics

Just like manual transmission cars, cars with automatic transmissions need a way to let the engine turn while the wheels and gears in the transmission come to a stop. Manual transmission cars use a clutch, which completely disconnects the engine from the transmission. Automatic transmission cars use a torque converter.
A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still.

Inside a Torque Converter

The parts of a torque converter (left to right): turbine, stator, pump

As shown in the figure above, there are four components inside the very strong housing of the torque converter:

• Pump
• Turbine
• Stator
• Transmission fluid

The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the engine is running at. The fins that make up the pump of the torque converter are attached to the housing, so they also turn at the same speed as the engine. The cutaway below shows how everything is connected inside the torque converter.

The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside, much as the spin cycle of a washing machine flings water and clothes to the outside of the wash tub. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center.

The fluid then enters the blades of the turbine, which is connected to the transmission. The turbine causes the transmission to spin, which basically moves your car. You can see in the graphic below that the blades of the turbine are curved. This means that the fluid, which enters the turbine from the outside, has to change direction before it exits the center of the turbine. It is this directional change that causes the turbine to spin.
















In order to change the direction of a moving object, you must apply a force to that object -- it doesn't matter if the object is a car or a drop of fluid. And whatever applies the force that causes the object to turn must also feel that force, but in the opposite direction. So as the turbine causes the fluid to change direction, the fluid causes the turbine to spin.




The fluid exits the turbine at the center, moving in a different
direction than when it entered. If you look at the arrows in the figure above, you can see that the fluid exits the turbine moving opposite the direction that the pump (and engine) are turning. If the fluid were allowed to hit the pump, it would slow the engine down, wasting power. This is why a torque converter has a stator.                  

The Stator

The stator sends the fluid returning from the turbine to the pump. This improves the efficiency of the torque converter. Note the spline, which is connected to a one-way clutch inside the stator. 

The stator resides in the very center of the torque converter. Its job is to redirect the fluid returning from the turbine before it hits the pump again. This dramatically increases the efficiency of the torque converter.

The stator has a very aggressive blade design that almost completely reverses the direction of the fluid. A one-way clutch (inside the stator) connects the stator to a fixed shaft in the transmission (the direction that the clutch allows the stator to spin is noted in the figure above). Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades.             
      
Something a little bit tricky happens when the car gets moving. There is a point, around 40 mph (64 kph), at which both the pump and the turbine are spinning at almost the same speed (the pump always spins slightly faster). At this point, the fluid returns from the turbine, entering the pump already moving in the same direction as the pump, so the stator is not needed.
Even though the turbine changes the direction of the fluid and flings it out the back, the fluid still ends up moving in the direction that the turbine is spinning because the turbine is spinning faster in one direction than the fluid is being pumped in the other direction. If you were standing in the back of a pickup moving at 60 mph, and you threw a ball out the back of that pickup at 40 mph, the ball would still be going forward at 20 mph. This is similar to what happens in the turbine: The fluid is being flung out the back in one direction, but not as fast as it was going to start with in the other direction.

At these speeds, the fluid actually strikes the back sides of the stator blades, causing the stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving through it.

Benefits and Weak Points

In addition to the very important job of allowing your car come to a complete stop without stalling the engine, the torque converter actually gives your car more torque when you accelerate out of a stop. Modern torque converters can multiply the torque of the engine by two to three times. This effect only happens when the engine is turning much faster than the transmission.

At higher speeds, the transmission catches up to the engine, eventually moving at almost the same speed. Ideally, though, the transmission would move at exactly the same speed as the engine, because this difference in speed wastes power. This is part of the reason why cars with automatic transmissions get worse gas mileage than cars with manual transmissions.

To counter this effect, some cars have a torque converter with a lockup clutch. When the two halves of the torque converter get up to speed, this clutch locks them together, eliminating the slippage and improving efficiency

Torque converter

TORQUE CONVERTER by Rishabh Sharma on Scribd

Torque converter

A cut-away model of a torque converter

In modern usage, a torque converter is generally a type of fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. It is usually located between the engine's flexplate and the transmission.
The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. Some of these devices are also equipped with a temporary locking mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.

Hydraulic systems

ZF torque converter cut-away

By far the most common form of torque converter in automobile transmissions is the hydrokinetic device described in this article. There are also hydrostatic systems which are widely used in small machines such as compact excavators.

Mechanical systems

There are also mechanical designs for continuously variable transmissions and these also have the ability to multiply torque. They include the pendulum-based Constantinesco torque converter, the Lambert friction gearing disk drive transmission and the Variomatic with expanding pulleys and a belt drive.

Usage

• Automatic transmissions on automobiles, such as cars, buses, and on/off highway trucks.
• Forwarders and other heavy duty vehicles.
• Marine propulsion systems.
• Industrial power transmission such as conveyor drives, almost all modern forklifts, winches, drilling rigs, construction equipment, and railway locomotives.

Function

Torque converter elements

A fluid coupling is a two element drive that is incapable of multiplying torque, while a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque.

In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating with respect to the prime mover but allows forward rotation.

Modifications to the basic three element design have been periodically incorporated, especially in applications where higher than normal torque multiplication is required. Most commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differing amounts of torque multiplication. For example, the Buick Dynaflow automatic transmission was a non-shifting design and, under normal conditions, relied solely upon the converter to multiply torque. The Dynaflow used a five element converter to produce the wide range of torque multiplication needed to propel a heavy vehicle.

Although not strictly a part of classic torque converter design, many automotive converters include a lock-up clutch to improve cruising power transmission efficiency and reduce heat. The application of the clutch locks the turbine to the impeller, causing all power transmission to be mechanical, thus eliminating losses associated with fluid drive.

Operational phases

A torque converter has three stages of operation:

• Stall. The prime mover is applying power to the impeller but the turbine cannot rotate. For example, in an automobile, this stage of operation would occur when the driver has placed the transmission in gear but is preventing the vehicle from moving by continuing to apply the brakes. At stall, the torque converter can produce maximum torque multiplication if sufficient input power is applied (the resulting multiplication is called the stall ratio). The stall phase actually lasts for a brief period when the load (e.g., vehicle) initially starts to move, as there will be a very large difference between pump and turbine speed.
• Acceleration. The load is accelerating but there still is a relatively large difference between impeller and turbine speed. Under this condition, the converter will produce torque multiplication that is less than what could be achieved under stall conditions. The amount of multiplication will depend upon the actual difference between pump and turbine speed, as well as various other design factors.
• Coupling. The turbine has reached approximately 90 percent of the speed of the impeller. Torque multiplication has essentially ceased and the torque converter is behaving in a manner similar to a simple fluid coupling. In modern automotive applications, it is usually at this stage of operation where the lock-up clutch is applied, a procedure that tends to improve fuel efficiency.

The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the impeller to oppose the direction of impeller rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the impeller by the prime mover. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the one-way stator clutch
.
Unlike the radially straight blades used in a plain fluid coupling, a torque converter's turbine and stator use angled and curved blades. The blade shape of the stator is what alters the path of the fluid, forcing it to coincide with the impeller rotation. The matching curve of the turbine blades helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades is important as minor variations can result in significant changes to the converter's performance.
During the stall and acceleration phases, in which torque multiplication occurs, the stator remains stationary due to the action of its one-way clutch. However, as the torque converter approaches the coupling phase, the energy and volume of the fluid returning from the turbine will gradually decrease, causing pressure on the stator to likewise decrease. Once in the coupling phase, the returning fluid will reverse direction and now rotate in the direction of the impeller and turbine, an effect which will attempt to forward-rotate the stator. At this point, the stator clutch will release and the impeller, turbine and stator will all (more or less) turn as a unit.

Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence, causing the converter to generate waste heat (dissipated in many applications by water cooling). This effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity at low impeller speeds, which allows the turbine to be stalled for long periods with little danger of overheating.

Efficiency and torque multiplication

A torque converter cannot achieve 100 percent coupling efficiency. The classic three element torque converter has an efficiency curve that resembles ∩: zero efficiency at stall, generally increasing efficiency during the acceleration phase and low efficiency in the coupling phase. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid flow interference generated by the stator, and as previously mentioned, is commonly overcome by mounting the stator on a one-way clutch.

Even with the benefit of the one-way stator clutch, a converter cannot achieve the same level of efficiency in the coupling phase as an equivalently sized fluid coupling. Some loss is due to the presence of the stator (even though rotating as part of the assembly), as it always generates some power-absorbing turbulence. Most of the loss, however, is caused by the curved and angled turbine blades, which do not absorb kinetic energy from the fluid mass as well as radially straight blades. Since the turbine blade geometry is a crucial factor in the converter's ability to multiply torque, trade-offs between torque multiplication and coupling efficiency are inevitable. In automotive applications, where steady improvements in fuel economy have been mandated by market forces and government edict, the nearly universal use of a lock-up clutch has helped to eliminate the converter from the efficiency equation during cruising operation.

The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-element designs as used in the Buick Dynaflow and Chevrolet Turboglide could produce more). Specialized converters designed for industrial, rail, or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inefficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication.

The characteristics of the torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. For example, drag racing automatic transmissions often use converters modified to produce high stall speeds to improve off-the-line torque, and to get into the power band of the engine more quickly. Highway vehicles generally use lower stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics.

A design feature once found in some General Motors automatic transmissions was the variable-pitch stator, in which the blades' angle of attack could be varied in response to changes in engine speed and load. The effect of this was to vary the amount of torque multiplication produced by the converter. At the normal angle of attack, the stator caused the converter to produce a moderate amount of multiplication but with a higher level of efficiency. If the driver abruptly opened the throttle, a valve would switch the stator pitch to a different angle of attack, increasing torque multiplication at the expense of efficiency.

Some torque converters use multiple stators and/or multiple turbines to provide a wider range of torque multiplication. Such multiple-element converters are more common in industrial environments than in automotive transmissions, but automotive applications such as Buick's Triple Turbine Dynaflow and Chevrolet's Turboglide also existed. The Buick Dyna flow utilized the torque-multiplying characteristics of its planetary gear set in conjunction with the torque converter for low gear and bypassed the first turbine, using only the second turbine as vehicle speed increased. The unavoidable trade-off with this arrangement was low efficiency and eventually these transmissions were discontinued in favor of the more efficient three speed units with a conventional three element torque converter.

Lock-up torque converters

As described above, impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss.

The first automotive application of the lock-up principle was Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration or as the vehicle slowed down. This feature was also present in some Borg-Warner transmissions produced during the 1950s. It fell out of favor in subsequent years due to its extra complexity and cost. In the late 1970s lock-up clutches started to reappear in response to demands for improved fuel economy, and are now nearly universal in automotive applications.

Capacity and failure modes

As with a basic fluid coupling the theoretical torque capacity of a converter is proportional to ${\displaystyle r\,N^{2}D^{5}}$, where ${\displaystyle r}$ is the mass density of the fluid (kg/m³), ${\displaystyle N}$ is the impeller speed (rpm), and ${\displaystyle D}$ is the diameter(m).In practice, the maximum torque capacity is limited by the mechanical characteristics of the materials used in the converter's components, as well as the ability of the converter to dissipate heat (often through water cooling). As an aid to strength, reliability and economy of production, most automotive converter housings are of welded construction. Industrial units are usually assembled with bolted housings, a design feature that eases the process of inspection and repair, but adds to the cost of producing the converter.

In high performance, racing and heavy duty commercial converters, the pump and turbine may be further strengthened by a process called furnace brazing, in which molten brass is drawn into seams and joints to produce a stronger bond between the blades, hubs and annular ring(s). Because the furnace brazing process creates a small radius at the point where a blade meets with a hub or annular ring, a theoretical decrease in turbulence will occur, resulting in a corresponding increase in efficiency.

Overloading a converter can result in several failure modes, some of them potentially dangerous in nature:

• Overheating: Continuous high levels of slippage may overwhelm the converter's ability to dissipate heat, resulting in damage to the elastomer seals that retain fluid inside the converter. This will cause the unit to leak and eventually stop functioning due to lack of fluid.
• Stator clutch seizure: The inner and outer elements of the one-way stator clutch become permanently locked together, thus preventing the stator from rotating during the coupling phase. Most often, seizure is precipitated by severe loading and subsequent distortion of the clutch components. Eventually, galling of the mating parts occurs, which triggers seizure. A converter with a seized stator clutch will exhibit very poor efficiency during the coupling phase, and in a motor vehicle, fuel consumption will drastically increase. Converter overheating under such conditions will usually occur if continued operation is attempted.
• Stator clutch breakage: A very abrupt application of power can cause shock loading of the stator clutch, resulting in breakage. If this occurs, the stator will freely counter-rotate in the direction opposite to that of the pump and almost no power transmission will take place. In an automobile, the effect is similar to a severe case of transmission slippage and the vehicle is all but incapable of moving under its own power.
• Blade deformation and fragmentation: If subjected to abrupt loading or excessive heating of the converter, pump and/or turbine blades may be deformed, separated from their hubs and/or annular rings, or may break up into fragments. At the least, such a failure will result in a significant loss of efficiency, producing symptoms similar (although less pronounced) to those accompanying stator clutch failure. In extreme cases, catastrophic destruction of the converter will occur.
• Ballooning: Prolonged operation under excessive loading, very abrupt application of load, or operating a torque converter at very high RPM may cause the shape of the converter's housing to be physically distorted due to internal pressure and/or the stress imposed by inertia. Under extreme conditions, ballooning will cause the converter housing to rupture, resulting in the violent dispersal of hot oil and metal fragments over a wide area.