Radiation and convection

One might expect the term "radiator" to apply to devices that transfer heat primarily by thermal radiation while a device which relied primarily on natural or forced convection would be called a "convector". In practice, the term "radiator" refers to any of a number of devices in which a liquid circulates through exposed pipes (often with fins or other means of increasing surface area), notwithstanding that such devices tend to transfer heat mainly by convection and might logically be called convectors. The term "convector" refers to a class of devices in which the source of heat is not directly exposed.

Turbocharging versus supercharging

In contrast to turbochargers, superchargers are mechanically driven by the engine. Belts, chains, shafts, and gears are common methods of powering a supercharger, placing a mechanical load on the engine. For example, on the single-stage single-speed supercharged rollls-royce merlin engine, the supercharger uses about 150  horsepower (110 kw). Yet the benefits outweigh the costs; for the 150 hp (110 kW) to drive the supercharger the engine generates an additional 400 horsepower, a net gain of 250 hp (190 kW). This is where the principal disadvantage of a supercharger becomes apparent; the engine must withstand the net power output of the engine plus the power to drive the supercharger.

Another disadvantage of some superchargers is lower adiabatic efficiency as compared to turbochargers. Adiabatic efficiency is a measure of a compressor's ability to compress air without adding excess heat to that air. The compression process always produces heat as a byproduct of that process; however, more efficient compressors produce less excess heat. Roots superchargers impart significantly more heat to the air than turbochargers. Thus, for a given volume and pressure of air, the turbocharged air is cooler, and as a result denser, containing more oxygen molecules, and therefore more potential power than the supercharged air. In practical application the disparity between the two can be dramatic, with turbochargers often producing 15% to 30% more power based solely on the differences in adiabatic efficiency.

By comparison, a turbocharger does not place a direct mechanical load on the engine (however, turbochargers place exhaust back pressure on engines, increasing pumping losses). This is more efficient because it uses the otherwise wasted energy of the exhaust gas to drive the compressor. In contrast to supercharging, the primary disadvantage of turbocharging is what is referred to as "lag" or "spool time". This is the time between the demand for an increase in power (the throttle being opened) and the turbocharger(s) providing increased intake pressure, and hence increased power.

Throttle lag occurs because turbochargers rely on the build up of exhaust gas pressure to drive the turbine. In variable output systems such as automobile engines, exhaust gas pressure at idle, low engine speeds, or low throttle is usually insufficient to drive the turbine. Only when the engine reaches sufficient speed does the turbine section start to spool up, or spin fast enough to produce intake pressure above atmospheric pressure.

A combination of an exhaust-driven turbocharger and an engine-driven supercharger can mitigate the weaknesses of both. This technique is called twincharging.

In the case of electro motive diesel two-stroke engines, the mechanically-assisted turbocharger is not specifically a twincharger, as the engine uses the mechanical assistance to charge air only during starting. Once started, the engine uses true turbocharging. This differs from a turbocharger that uses the compressor section of the turbo-compressor only during starting, as a two-stroke engines cannot naturally aspirate, and, according to SAE definitions, a two-stroke engine with a mechanically-assisted compressor during starting is considered naturally aspirated.

Stopping Sight Distance

Stopping sight distance is defined as the distance needed for drivers to see an object on the roadway ahead and bring their vehicles to safe stop before colliding with the object. The distances are derived for various design speeds based on assumptions for driver reaction time, the braking ability of most vehicles under wet pavement conditions, and the friction provided by most pavement surfaces, assuming good tires. A roadway designed to criteria employs a horizontal and vertical alignment and a cross section that provides at least the minimum stopping sight distance through the entire facility.
Stopping sight distance is influenced by both vertical and horizontal alignment. For vertical stopping sight distance, this includes sight distance at crest vertical curves , headlight sight distance at sag vertical curves , and sight distance at under crossings .
For crest vertical curves, the alignment of the roadway limits stopping sight distance .  Sag vertical curves provide greater stopping sight distance during daylight conditions, but very short sag vertical curves will limit the effective distance of the vehicle’s headlights at night.  If lighting is provided at sag vertical curves, a design to the driver comfort criteria may be adequate.  The length of sag vertical curves to satisfy the comfort criteria over the typical design speed range results in minimum curve lengths of about half those based on headlight criteria.
For horizontal curves, physical obstructions can limit stopping sight distance .  Examples include bridge piers, barrier, walls, back slopes, and vegetation.

Trunk pistons

Trunk pistons are long, relative to their diameter. They act as both piston and also as a cylindrical crosshead. As the connecting rod is angled for part of its rotation, there is also a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this.

Trunk pistons have been a common design of piston since the early days of the reciprocating internal combustion engine. They were used for both petrol and diesel engines, although high speed engines have now adopted the lighter weight slipper piston.

A characteristic of most trunk pistons, particularly for diesel engines, is that they have a groove for an oil ring below the gudgeon pin, not just the rings between the gudgeon pin and crown.

The name 'trunk piston' derives from the trunk engine, an early design of marine steam engine. To make these more compact, they avoided the steam engine's usual piston rod and separate crosshead and were instead the first engine design to place the gudgeon pin directly within the piston. Otherwise these trunk engine pistons bore little resemblance to the trunk piston: they were of extremely large diameter and were double-acting. Their 'trunk' was a narrow cylinder placed mounted in the centre of this piston. Media related to trunk pistons at Wikimedia Commons

The regimes of lubrication

As the load increases on the contacting surfaces three distinct situations can be observed with respect to the mode of lubrication, which are called regimes of lubrication:

• Fluid film lubrication is the lubrication regime in which through viscous forces the load is fully supported by the lubricant within the space or gap between the parts in motion relative to one another (the lubricated conjunction) and solid–solid contact is avoided.
  • Hydrostatic lubrication is when an external pressure is applied to the lubricant in the bearing, to maintain the fluid lubricant film where it would otherwise be squeezed out.
  • Hydrodynamic lubrication is where the motion of the contacting surfaces, and the exact design of the bearing is used to pump lubricant around the bearing to maintain the lubricating film. This design of bearing may wear when started, stopped or reversed, as the lubricant film breaks down.

• Elastohydrodynamic lubrication: Mostly for nonconforming surfaces or higher load conditions, the bodies suffer elastic strains at the contact. Such strain creates a load-bearing area, which provides an almost parallel gap for the fluid to flow through. Much as in hydrodynamic lubrication, the motion of the contacting bodies generates a flow induced pressure, which acts as the bearing force over the contact area. In such high pressure regimes, the viscosity of the fluid may rise considerably. At full elastohydrodynamic lubrication the generated lubricant film completely separates the surfaces. Contact between raised solid features, or asperities, can occur, leading to a mixed-lubrication or boundary lubrication regime.

• Boundary lubrication (also called boundary film lubrication): The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and pressure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant tenacious layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided. Boundary lubrication is also defined as that regime in which the load is carried by the surface asperities rather than by the lubricant.

Besides supporting the load the lubricant may have to perform other functions as well, for instance it may cool the contact areas and remove wear products. While carrying out these functions the lubricant is constantly replaced from the contact areas either by the relative movement (hydrodynamics) or by externally induced forces.

Lubrication is required for correct operation of mechanical systems pistons, pumps, bearings, turbines, cutting tools, etc. where without lubrication the pressure between the surfaces in close proximity would generate enough heat for rapid surface damage which in a coarsened condition may literally weld the surfaces together, causing seizure.

In some applications, such as piston engines, the film between the piston and the cylinder wall also seals the combustion chamber, preventing combustion gases from escaping into the crankcase.

Diesel Oxidation Catalyst (DOC)

For compression-ignition, the most commonly used catalytic converter is the Diesel Oxidation Catalyst (DOC). This catalyst uses O₂ (oxygen) in the exhaust gas stream to convert CO (carbon monoxide) to CO₂ (carbon dioxide) and HC (hydrocarbons) to H₂O (water) and CO₂. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping to reduce visible particulates. These catalysts are not active for NO_x reduction because any reductant present would react first with the high concentration of O₂ in diesel exhaust gas.

Reduction in NO_x emissions from compression-ignition engines has previously been addressed by the addition of exhaust gas to incoming air charge, known as EGR. In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles to meet new federal emissions requirements. There are two techniques that have been developed for the catalytic reduction of NO_x emissions under lean exhaust conditions - (SCR) and the lean NO_x trap or NOx. Instead of precious metal-containing NOx adsorbers, most manufacturers selected base-metal SCR systems that use a reagent such an ammonia to reduce the NO_x into nitrogen. Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. One trademark product of urea solution, also referred to as Diesel Exhaust Fluid (DEF), is adblue

 diesel exhaust contains relatively high levels of particulate matter (soot), consisting in large part of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction, so particulates are cleaned up by a soot trap or (DPF). Historically, a DPF consists of a Cordierite or Silicon Carbide substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. Contemporary DPFs can be manufactured from a variety of rare metals that provide superior performance (at a greater expense). As the amount of soot trapped on the DPF increases, so does the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are required to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration may also be limited to prevent extreme exotherms from damaging the trap during regeneration. In the U.S., all on-road light, medium and heavy-duty vehicles powered by diesel and built after 1 January 2007, must meet diesel particulate emission limits that means they effectively have to be equipped with a 2-Way catalytic converter and a diesel particulate filter. Note that this applies only to the diesel engine used in the vehicle. As long as the engine was manufactured before 1 January 2007, the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007.

Angular contact of ball bearing

An angular contact ball bearing uses axially asymmetric races. An axial load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to want to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact bearings better support "combined loads" (loading in both the radial and axial directions) and the contact angle of the bearing should be matched to the relative proportions of each. The larger the contact angle (typically in the range 10 to 45 degrees), the higher the axial load supported, but the lower the radial load. In high speed applications, such as turbines, jet engines, and dentistry equipment, the centrifugal forces generated by the balls changes the contact angle at the inner and outer race. Ceramics such as silicon nitride are now regularly used in such applications due to their low density (40% of steel). These materials significantly reduce centrifugal force and function well in high temperature environments. They also tend to wear in a similar way to bearing steel—rather than cracking or shattering like glass or porcelain.

Most bicycles use angular-contact bearings in the headsets because the forces on these bearings are in both the radial and axial dire

HAPPY CHRISTMAS

Faith makes all things possible,
Hope makes all things work,
Love makes all things beautiful,
May you have all the three for this Christmas.
MERRY CHRISTMAS!

Field regulation

Automotive alternators require a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the (ECU). The field current is much smaller than the output current of the alternator; for example, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.

The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches a certain speed.

Older automobiles with minimal lighting may have had an alternator capable of producing only 30 A. Typical passenger car and light truck alternators are rated around 50-70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems. Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water-cooled or oil-cooled.

In recent years, alternator regulators are linked to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator.

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed. This is similar to very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which achieve an efficiency around 60%. Larger permanent magnet alternators can achieve higher efficiencies. Large AC generators used in power stations run at carefully controlled speeds and have no constraints on size or weight. They have much higher efficiencies, as high as 98%.

Automobile air conditioning

Automobile air conditioning systems cool the occupants of a vehicle in hot weather, and have come into wide use from the late twentieth century. air conditioners use significant power; on the other hand the drag of a car with closed windows is less than if the windows are open tocool the occupants. There has been much debate on the effect of air conditioning on the fuel efficiency of a vehicle. Factors such as wind resistance, aerodynamics and engine power and weight have to be factored into finding the true variance between using the air conditioning system and not using it when estimating the actual fuel mileage. Other factors on the impact on the engine and an overall engine heat increase can have an impact on the cooling system of the vehicle.

History

A company in New York City in the United States, first offered installation of air conditioning for cars in 1933. Most of their customers operated limousines and luxury cars.

The packard motor car company was the first automobile manufacturer to offer an air conditioning unit into its cars, beginning in 1939. These air conditioners were manufactured by Bishop and Babcock Co, of Cleveland Ohio. The "Bishop and Babcock Weather Conditioner" also incorporated a heater. Cars ordered with the new "Weather Conditioner" were shipped from Packard's East Grand Boulevard facility to the B&B factory where the conversion was performed. Once complete, the car was shipped to a local dealer where the customer would take delivery.

There were many reasons why this early air conditioner unit was unsuccessful: 1) The main evaporator and blower system took up half of the trunk space. (This problem would go away as trunks became larger in the post-war period.) 2) The system was less efficient than those that would follow in the post-war years. 3) It had no temperature thermostat or shut-off mechanism other than switching the blower off. (Cold air would still enter the car with any movement as the drive belt was continuously connected to the compressor--later systems would use electrically operated clutches to remedy this problem.) 4) The several feet of plumbing going back and forth between the engine compartment and trunk proved unreliable in service. 5) Finally, the biggest reason this early system failed was that it cost US $274.00 (equivalent to $4,544 today) an enormous amount of money in post-depression/pre-war America.

Packard fully warranted and supported this conversion, and marketed it well. However, given the limitations above, it was unsuccessful. Subsequently, the option was discontinued after 1941.

Chrysler Airtemp

The 1953 chrysler imperial was the first production car in twelve years to actually have automobile air conditioning, following tentative experiments by packard in 1940 and cadillac in 1941. walter p. chrysler had seen to the invention of  airtemp air conditioning back in the 1930s for the chrysler building, and had ostensibly offered it on cars in 1941-42, and again in 1951-52, but none are known to have been sold in the latter form until the 1953 model year. In actually installing optional Airtemp air conditioning units to its Imperials in 1953, chrysler beat Cadillac, buick and old mobile which added air conditioning as an option in the 1953 model year.

Airtemp was more sophisticated and efficient than the complicated rival air conditioners of 1953. It recirculated, rather than merely cooled, the air inside the vehicle, and it was also the highest capacity unit available on an automobile. It was also simple to operate, with a single switch on the dashboard marked with low, medium, and high positions, which the driver selected as desired. The system was capable of cooling a Chrysler from 120 degrees to 85 degrees in about two minutes, and of completely eliminating humidity, dust, pollen and tobacco smoke at the same time. Since it relied on fresh air, and drew in sixty percent more of it than any contemporary system, Airtemp avoided the staleness associated with automotive air conditioning at the time. It was silent and unobtrusive. Instead of plastic tubes mounted on the package shelf as on GM and on other cars, small ducts directed cool air toward the ceiling of the car where it filtered down around the passengers instead of blowing directly on them, a feature that modern cars have lost.

Constant-velocity joint

Constant-velocity joints (aka homokinetic or CV joints) allow a drive shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. They are mainly used in front wheel drive and many modern rear wheel drive carswith independent rear suspension typically use CV joints at the ends of the rear axle halfshafts, and increasingly use them on the propshafts.

Constant-velocity joints are protected by a rubber boot, a CV gaiter. Cracks and splits in the boot will allow contaminants in, which would cause the joint to wear quickly.

Double Cardan Shaft

A configuration known as a double Cardan joint drive shaft partially overcomes the problem of jerky rotation. This configuration uses two U-joints joined by an intermediate shaft, with the second U-joint phased in relation to the first U-joint to cancel the changing angular velocity. In this configuration, the angular velocity of the driven shaft will match that of the driving shaft, provided that both the driving shaft and the driven shaft are at equal angles with respect to the intermediate shaft (but not necessarily in the same plane) and that the two universal joints are 90 degrees out of phase. This assembly is commonly employed in rear wheel drive vehicles, where it is known as a drive shaft or propeller (prop) shaft.

Even when the driving and driven shafts are at equal angles with respect to the intermediate shaft, if these angles are greater than zero, oscillating moments are applied to the three shafts as they rotate. These tend to bend them in a direction perpendicular to the common plane of the shafts. This applies forces to the support bearings and can cause "launch shudder" in rear wheel drive vehicles. The intermediate shaft will also have a sinusoidal component to its angular velocity, which contributes to vibration and stresses.

Mathematically, this can be shown as follows: If  and  are the angles for the input and output of the universal joint connecting the drive and the intermediate shafts respectively, and  and  are the angles for the input and output of the universal joint connecting the intermediate and the output shafts respectively, and each pair are at angle  with respect to each other, then:

If the second universal joint is rotated 90 degrees with respect to the first, then . Using the fact that  yields:

and it is seen that the output drive is just 90 degrees out of phase with the input shaft, yielding a constant-velocity drive.

Supersession of carburetors

In the 1970s and 1980s in the US, the federal government imposed increasingly strict exhaust emission regulations. During that time period, the vast majority of gasoline-fueled automobile and light truck engines did not use fuel injection. To comply with the new regulations, automobile manufacturers often made extensive and complex modifications to the engine carburetor(s). While a simple carburetor system is cheaper to manufacture than a fuel injection system, the more complex carburetor systems installed on many engines in the 1970s were much more costly than the earlier simple carburetors. To more easily comply with emissions regulations, automobile manufacturers began installing fuel injection systems in more gasoline engines during the late 1970s.

The open loop fuel injection systems had already improved cylinder-to-cylinder fuel distribution and engine operation over a wide temperature range, but did not offer further scope to sufficient control fuel/air mixtures, in order to further reduce exhaust emissions. Later closed loop fuel injection systems improved the air/fuel mixture control with an exhaust gas oxygen sensor and began incorporating a catalytic converter to further reduce exhaust emissions.

Fuel injection was phased in through the latter '70s and '80s at an accelerating rate, with the US, French and German markets leading and the UK and Commonwealth markets lagging somewhat. Since the early 1990s, almost all gasoline passenger cars sold in fist world markets are equipped with electronic fuel injection (EFI). The carburetor remains in use in developing countries where vehicle emissions are unregulated and diagnostic and repair infrastructure is sparse. Fuel injection is gradually replacing carburetors in these nations too as they adopt emission regulations conceptually similar to those in force in Europe, Japan, Australia and North America.

Many motorcycles still utilize carburetored engines, though all current high-performance designs have switched to EFI.

 nascar finally replaced carburetors with fuel-injection, starting at the beginning of the 2012 nascar sprint cup series season.

Operation of spark plug

The plug is connected to the high voltage generated by an ignition coil or magneto. As the electrons flow from the coil, a voltage difference develops between the central 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. The ionized gas becomes a conductor and allows electrons to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to 'fire' properly, although it can go up to 45,000 volts. They supply higher current during the discharge process resulting in a hotter and longer-duration spark.

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' heard when observing a spark, similar to lightning and thunder.

The heat and pressure force the gases 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 as though the timing was advanced.

Operation of ABS

Typically ABS includes a central electronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects a wheel rotating significantly slower than the others, a condition indicative of impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. Conversely, if the ECU detects a wheel turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel. This process is repeated continuously and can be detected by the driver via brake pedal pulsation. Some anti-lock systems can apply or release braking pressure 15 times per second. Because of this, the wheels of cars equipped with ABS are practically impossible to lock even during panic braking in extreme conditions.

The ECU is programmed to disregard differences in wheel rotative speed below a critical threshold, because when the car is turning, the two wheels towards the center of the curve turn slower than the outer two. For this same reason, a differentia is used in virtually all roadgoing vehicles.

If a fault develops in any part of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault is rectified.

Modern ABS applies individual brake pressure to all four wheels through a control system of hub-mounted sensors and a dedicated micro controller. ABS is offered or comes standard on most road vehicles produced today and is the foundation for electronic stability control systems, which are rapidly increasing in popularity due to the vast reduction in price of vehicle electronics over the years.

Modern electronic stability control systems are an evolution of the ABS concept. Here, a minimum of two additional sensors are added to help the system work: these are a steering wheel angle sensor, and a gyroscopic sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car does not coincide with what the steering wheel sensor reports, the ESC software will brake the necessary individual wheel(s) (up to three with the most sophisticated systems), so that the vehicle goes the way the driver intends. The steering wheel sensor also helps in the operation of cornering brake control (CBC), since this will tell the ABS that wheels on the inside of the curve should brake more than wheels on the outside, and by how much.

ABS equipment may also be used to implement a  (TCS) on acceleration of the vehicle. If, when accelerating, the tire loses traction, the ABS controller can detect the situation and take suitable action so that traction is regained. More sophisticated versions of this can also control throttle levels and brakes simultaneously.

Upon the introduction of the subaru lagacy in 1989, Subaru networked the four channel anti-lock brake function with the all wheel drive system so that if the car detected any wheel beginning to lock up, the variable assists the all wheel drive system installed on vehicles with the automatic transmission would engage to ensure all wheels were actively gripping while the anti-lock system was attempting to stop the car.

legendary willys jeep

Differential (mechanical device)

A differential is a device, usually, but not necessarily, employing gears, which is connected to the outside world by three shafts, chains, or similar, through which it transmits torque and rotation. The gears or other components make the three shafts rotate in such a way that , where , , and  are the angular velocities of the three shafts, and  and  are constants. Often, but not always,  and  are equal, so  is proportional to the sum (or average) of  and . Except in some special-purpose differentials, there are no other limitations on the rotational speeds of the shafts, apart from the usual mechanical/engineering limits. Any of the shafts can be used to input rotation, and the other(s) to output it. 

In automobiles and other wheeled vehicles, a differential is the usual way to allow the driving road wheels to rotate at different speeds. This is necessary when the vehicle turns, making the wheel that is travelling around the outside of the turning curve roll farther and faster than the other. The engine is connected to the shaft rotating at angular velocity . The driving wheels are connected to the other two shafts, and  and  are equal. If the engine is running at a constant speed, the rotational speed of each driving wheel can vary, but the sum (or average) of the two wheels' speeds can not change. An increase in the speed of one wheel must be balanced by an equal decrease in the speed of the other. (If one wheel is rotating backward, which is possible in very tight turns, its speed should be counted as negative.)

It may seem illogical that the speed of one input shaft can determine the speeds of two output shafts, which are allowed to vary. Logically, the number of inputs should be at least as great as the number of outputs. However, the system has another constraint. Under normal conditions (i.e. only small tyre slip), the ratio of the speeds of the two driving wheels equals the ratio of the radii of the paths around which the two wheels are rolling, which is determined by the track-width of the vehicle (the distance between the driving wheels) and the radius of the turn. Thus the system does not have one input and two independent outputs. It has two inputs and two outputs.

A different automotive application of differentials is in epicyclic gearing. A gearbox is constructed out of several differentials. In each differential, one shaft is connected to the engine (through a clutch or functionally similar device), another to the driving wheels (through another differential as described above), and the third shaft can be braked so its angular velocity is zero. (The braked component may not be a shaft, but something that plays an equivalent role.) When one shaft is braked, the gear ratio between the engine and wheels is determined by the value(s) of  and/or  for that differential, which reflect the numbers of teeth on its gears. Several differentials, with different gear ratios, are permanently connected in parallel with each other, but only one of them has one shaft braked so it can not rotate, so only that differential transmits power from the engine to the wheels. (If the transmission is in "neutral" or "park", none of the shafts is braked.) Shifting gears simply involves releasing the braked shaft of one differential and braking the appropriate shaft on another. This is a much simpler operation to do automatically than engaging and disengaging gears in a conventional gearbox. Epicyclic gearing is almost always used in automatic transmission, and is nowadays also used in some hybrid and electric vehicles.

Non-automotive uses of differentials include performing analog arithmetic. Two of the differential's three shafts are made to rotate through angles that represent (are proportional to) two numbers, and the angle of the third shaft's rotation represents the sum or difference of the two input numbers. An equation clock that used a differential for addition, made in 1720, is the earliest device definitely known to have used a differential for any purpose. In the 20th Century, large assemblies of many differentials were used as analog computers, calculating, for example, the direction in which a gun should be aimed. However, the development of electronic digital computers has made these uses of differentials obsolete. Practically all the differentials that are now made are used in automobiles and similar vehicles. This article therefore emphasizes automotive uses of differentials.