SLIPPER CLUTCH

While riding our bikes, at some stages, we do experience jerk in case of sudden downshifts. These jerks are also not good for the transmission system of the bike as these occur due to engine braking caused by a sudden downshift an release of the clutch wire. Bikes which are more powerful can even lock up their rear wheels and it can become dangerous for the rider as well as others on the road.

Slipper clutch, also known as back torque limiter(as it limits the amount of torque on the rear wheel) is used to reduce the above discussed effects and provide safety to the riders. The actual difference between the slipper clutch and normal clutch is the clutch hub. There are different types of slipper clutches available, however, the most common type is the ramp-type slipper clutch, which is the variety used in most of the motorcycles we use on the street. A slipper clutch assembly has ramps built into the inner basket as well as the pressure plate. These ramps have the ability to allow for a sliding or slipping motion in the direction of the rotation of the wheel.

ADVANTAGES:

• The biggest advantage is, of course, the safety it provides.
• It also enhances the performance
• Bike becomes easier to ride as it forgives few of your mistakes.
• Decreases wear and tear of transmission.

DISADVANTAGES:

• They are expensive to own and maintain
• Engine braking, which can help you in case of brake failure, isn't much useful with slipper clutch.
• Not every mechanic an repair the faults/ failures in slipper clucth assembly.

Throttle By Wire

An Internal Combustion Engine (ICE) needs air and fuel for initiating combustion inside the cylinders. Fuel is supplied by injectors or a carburettor, Air is taken from the air intake and the flow of fuel into the engine's combustion chamber is controlled/regulated by the throttle, also known as butterfly valve.The throttle valve which opens and closes to adjust the air-flow into the engine is controlled by a cable connecting the accelerator pedal.

           

                                       valve                                                        cable

The throttle cable has almost become extinct on today’s motor vehicle. The newer, drive by wire or throttle by wire technique was first used back in the year 1988 by BMW in its 7 series.BMW has named it EML which is abbreviation of a German term meaning Electronic Throttle Control.

Actually, what this technology does is it replaces the  mechanical linkage between the accelerator pedal and the throttle butterfly, which used to be via a cable or via rods and linkages by some sophisticated electronic control modules, sensors and actuators. This system is also referred to as Fly-by-Wire.Earlier the electronic throttle actuation or EML was based upon the option of becoming a production line ‘add on’ system. It utilised its own Electronic Control Module (ECM), without the requisition of additional hard wear (and programming) to the vehicle’s original ECM. This was achieved by inputting minimal data into the vehicle’s ECM via a serial link from the electronic throttle actuator’s separate control unit.

Today’s systems have a specific ECM that incorporates the necessary programming to facilitate the input signals from the throttle pedal potentiometers and signal outputs to the electronic throttle body.

Drive-By-Wire or Throttle-By-Wire eliminates the use of a cable for controlling the throttle valve. The accelerator pedal is connected to a potentiometer. The various positions of the pedal act as the potentiometer inputs. These inputs are received by the ECU.Based on the inputs, the ECU is programmed to run an actuator.The actuator, in turn, opens the throttle valve or butterfly valve to a specified amount. The actuation process is carried out by a servomotor. Throttle Position Sensor (TPS)  is integrated into the throttle by wire . The TPS sends the position signals of the throttle valve back to the ECU. The signals from the Throttle Position Sensor and the input from the potentiometer connected to the accelerator pedal, helps the ECU to perform the ‘throttling' with greater accuracy.

ADVANTAGES:

• No wear and tear of throttle cable.
• Eliminates significant amount of weight by eliminating the use of mechanical linkages.
• Safety features like ESC, Hill hold assist, and Adaptive Cruise Control system  take inputs from the throttle.
• Maintains control even if the driver floors the accelerator pedal.

DISADVANTAGES:

• Increases the cost.
• Costlier to replace/maintenance cost is higher.
• Throttle response can't be adjusted manually, which some people love to do.
• Leads to more energy consumption
• Delay in response from the engine. 

AWD vs 4WD: What's the difference

4WD:

Also referred as four-by-four or 4x4, this system is typically used to handle rough and tough situations such as off road or mud/slush.  Basically, it is a system which sends power to all four wheels equally without vectoring(controlling of power between the wheels or axels), which means that all four wheels will spin at same rate respective to the power supplied from the engine.It consists of a device called transfer case which divides power between the front and rear axles. One of the major drawbacks of this system is that it becomes difficult to turn at high speeds as the wheels on the inner side(according to the turn) has less distance to cover than the wheel on the outer side. This leads to the vehicle sliding outwards of the curve. 

In order to tackle this problem, modern 4wd systems are "part-time" i.e., they operate in 2wd mode on normal roads and can be swithced to 4wd when needed. This also increases the fuel efficiency and also decreases the load on the transmission system, which in overall terms, reduces the maintenance and running cost of the vehicle.

Some 4wd vehicles have a transfer case with different settings like 4low or 4 high modes. Where, 4low is used for low speed maneuverability and 4 high can be used for attaining high speeds on slippery surfaces.

ALL WHEEL DRIVE(AWD)

All-wheel-drive is a much more modern and more complex system than 4wd. It is all about varying the amount of power sent to the wheels either mechanically or electronically. It is kind of similar to the part time four wheel drive system.It's also completely automated. It doesn't need any kind of intervention from the driver and hence is much easier to use.They actually detect the spinning of wheels by constantly monitoring the difference in speed between different wheels and reduces power if detects any loss of traction.

The problem here is that due to more complex system, rather than simple mechanical one, these systems are less robust and can't be repaired everywhere. This limits their use to a drastic extent.It also isn't much capable in extreme conditions. AWD system having more parts, increases the overall weight of the vehicle and hence contributes in reducing the fuel efficiency of the vehicle.

BLIND SPOT DETECTION

While changing lanes on highways, we see the ORVMs of the required side. But, it isn't always enough. There is a spot behind your vehicle in which, if any vehicle is present, is not visible in the rear view mirror and this blind spot leads to many accidents and the driver of either vehicle can't be blamed for that. It's just a spot where ,if anything is present, is not visible to us, and interesting fact is that not many people are aware of that. Still, If side view mirrors on a car are adjusted in a particular way, there is no blind spot on the sides. But, most of the drivers aren't aware of this adjustment, infact, most of us don't even adjust our mirrors which clearly isn't a good practice.

Working:

Blind spot detection (BSD) was developed by Volvo a decade ago. BSD tracks traffic just behind you as well as what’s coming alongside. The alert stays active until the car in the adjacent lane is in front of you, or at least directly alongside and you’d have to be blind not to see it. It doesn’t care if you are in your lane or have drifted a bit into the next and are at risk of sideswiping another car. BSD uses ultrasonic or radar sensors on the side and rear of the car.

The visual alert is a yellow (usually) indicator in the side mirror glass, inside edge of the mirror housing, or on the A pillar inside the car. It lights when it senses a car in the blind spot and flashes if the turn signal is flashing. You’ll also get an audible alert (beeping) or an induced vibration or light shake of the steering wheel if the turn signal is flashing.

Is it worth?

Blind-spot monitoring is one of the most useful tools for keeping safe. If you pay attention to the audible or visual warnings, they can minimize your changes of merging into another vehicle. Systems with collision prevention functionality add further value to blind-spot monitoring because they actively steer or brake, adding another layer of security during your drive.

AIR SUSPENSION

All of us are aware with suspension systems. Normally, vehicles come with coil and leaf spring suspension system.This type is more abundant because of its simple mechanical nature and comparatively low chances of failures. But, the limitation of coil and leaf springs are also too many. They can't be adjusted on the go and their adjustments are done at the time of fitting itself. This makes it a tough choice for customers in the market. If you buy a car with softer suspension, it will be good for bad roads, but won't be stable enough on the highways. Similarly, a car with stiffer suspension can give you back pain if driven on bad roads for long time, but is much more stable at high speeds on highways. In order to overcome this dilemma of choosing between soft and stiff, a more dynamic air suspension system is also available in the market, called air suspension. Hydropneumatic suspension systems are also of dynamic type.

Air suspension is a type of vehicle suspension powered by a compressor which is driven by the engine of the vehicle or some electric system. Air suspension system mainly uses Bellows, which is a balloon like structure made up of rubber. The compressor is used to inflate and deflate the bellows, which in turn increases or decreases the ride height.The purpose of air suspension is to provide a smooth, constant ride quality, but in some cases is used for sports suspension.

COMPONENTS:

• Vulcanized, heavy-duty rubber air springs(bellows) at each of the vehicle's   wheels
• An air compressor in the vehicle's trunk or under the hood of the vehicle
• A storage tank for compressed air, which allows you to store air at around an average of 150 PSI
• Valve blocks which direct air to the four springs from the storage reservoir through a set of solenoids, valves, and o-rings.
• ECAS computer that communicates between the vehicle's main computer to calculate where to direct air pressure.
• Air pipes connecting from the storage tank to the air springs that channel the flow of air throughout the suspension system.
• A desiccant-filled drier canister to keep the internal recesses of the system dry

BENEFITS/ADVANTAGES:

• Increased driver as well as passenger comfort
• Reduced noise and vibrations from the road
• Lesser wear and tear
• Adjustable ride height
• Adjustable stiffness

DRAWBACKS:

• Much high initial cost, hence used mostly in luxury cars
• Higher maintenance cost
• Fuel efficiency may decrease due to increase in weight(since its components are heavier)
• Not easily repairable(most mechanics can't repair such complex systems. Hence failure in remote areas may cause trouble.

HEADS UP DISPLAY

Head-up display (HUD) is a form of augmented reality that presents data on a transparent display in a way that users don’t have to look away from their usual viewpoints. It was initially developed for military aviation as far back as the 1950s, displaying altitude, speed and even targeting systems on the front of the cockpit. This allowed pilots to receive information at eye-level by looking forward with their head positioned “up”, instead of having to look away at another piece of equipment which can lead to distraction.

Heads-up display (HUD) systems can project information on the windshield, tell you where to turn, notify you of lane markings, identify how close the car in front of you is and even give you restaurant information all right in front of your eyes -- and while you're actually able to see those objects or places.

The technology used  for HUD varies on the system. Some cars use transparent phosphors on the windshield that reacts when a laser shines on it. When the laser is off, you don't see anything, but when the light is on the information is projected on the glass. Others use a similar system but incorporate mirrors to project the images on the windshield. 

Heads up display has various safety benefits as well. It can be used with advanced features such as front camera to project the road ahead on the windshield itself in case of bad weather conditions when the screen fogs up or the driver is unable to see the road ahead clearly.This can also be used to improve night driving and hence reducing the number of accidents which occur due to some form of hindrance in the vision.Some adaptive cruise control systems already make safety calculations to determine the best route for a car around a certain situation, but the next step is to actually project that information, in real-time, on the car's windshield.

Not only cars but now-a-days motorcycle helmets are also coming with heads up display to display information to the rider.

HUD has become more accessible over time and the cheapest car in India currently having HUD is Kia Seltos.

Hopefully, it will become more accessible as the technology itself isn't much expensive and more and more cars should be equipped with it, which eventually, will lead to a safer future for all of us.

Read More

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Cruise Control System

Introduction

                                                        

Cruise control is a system used in automobiles to control the speed of the vehicle.It actually takes over the throttle of the vehicle. It was used for the first time in the year 1900. It was used to maintain the speed of the car whether it went uphill or downhill. It is quite helpful for all-day long drives as it helps you to keep your feet free on long, empty highways. Not only cars but cruiser /touring bikes also offer cruise control.Now-a-days more advanced cruise control systems called as adaptive cruise control systems are used in some expensive vehicles. The adaptive cruise control system can be of:

• Automatic Braking Type: the automatic braking type use either a single or combination of sensors (radar, lidar, and camera) to allow the vehicle to keep pace with the car it is following, slow when closing in on the vehicle in front and accelerating again to the preset speed when traffic allows.
• Dynamic Set Speed Type: The dynamic set speed uses the GPS position of speed limit signs, from a database. Some are modifiable by the driver.
• Dynamic radar cruise control: uses a camera and millimeter-wave radar to maintain a set point distance from vehicles in front of the car; the system will automatically slow down or speed up based on the vehicles in front.

How does it work?

Cruise control  engages the throttle valve by a cable connected to an actuator, rather than by pressing a pedal. The throttle valve controls the power and speed of the engine by limiting how much air it takes in (since it’s an internal combustion engine).

The driver can set the cruise control with the cruise switches, which usually consist of ON, OFF, RESUME/Accelerate, SET/Decelerate and CANCEL. These are commonly located on the steering wheel or on the windshield wiper or turn signal stalk. The RES/ACCEL knob sets the speed of the car. One tap will accelerate it by 1 kmph, two by 2 kmph and so on.You can also keep it pressed to continuously increase the speed. The same is with SET/Decelerate Tapping the knob in the opposite direction will decelerate the vehicle. As a safety feature, the cruise control system will disengage as soon as you hit the brake pedal.

As you can see in the video, the cruise control can be enabled after 40kmph and as soon as the brake pedal is pressed/tapped, the cruise control gets cancelled.

Cruise controlled cars are also said to have 1st level of Automation. However, with increase in traffic over highways, the use of cruise control somewhere gets limited. In order to tackle this, the demand of adaptive cruise control system has increased and hopefully, in near future, more and more cars/bikes will be equipped with it.

READ MORE

 

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Dashboard Lights: What do they convey?

The instrument cluster contains the various gauges and indicators that drivers depend on to learn important information on the current status of the automobile. In a vehicle, potentially hazardous problems could occur at any point of time. Dashboard lights are integrated into the instrument cluster of the dashboard in order to provide full information about various components of the car and it also warns the driver if anything goes wrong.Gauges provide the information for speed, distance, heat and fuel. Indicator lights provide warnings and updates like the check engine light and the low fuel light. Different vehicles have different warnings available. 

Instrument cluster

Basically, Instrument clusters provide drivers with a centralized and easily viewable location for displaying all critical system information.

There are some of the various type of symbols displayed on the instrument panel located on the dashboard:

• Warning Symbols:

These include symbols such as 1.Engine Temperature Warning Light, 2.Battery Charge Warning Light,3.Oil Pressure Warning Light and 4.Brake Warning Light.

               

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• Safety Symbols:

These lights indicate an issue with your car that needs either an immediate action or checking.

A few most commonly used symbols are:

1.Reduced Power Warning, 2. Seat Belt Indicator, 3.Parking Brake Light, 4.Check Engine or Malfunction Indicator Light (MIL)

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There are also some more indicators which are commonly used to display the current lighting status of the vehicle such as Parking light indicator, High Beam, Low Beam and Fog light indicator.

• Diesel-Powered Vehicle Symbols:

Diesel powered vehicles have some more specific warning/indication lights:

1. Glow plug Indicator:

This light means that the engine’s glow plugs are warming up and the engine should not be started until the light goes out.So you need to wait for a few seconds after turning on ignition and only if the light goes off, you should fire the engine. If it flashes, an issue has been detected, such as a worn out glow plug.

2. Fuel filter warning:

                                                              

This light means that the diesel fuel filter is full, and needs to be emptied to avoid engine damage.

These were some of the commonly used warning lights. You must have information about these lights and you must keep an eye over it while driving the vehicle so that you don't get into big trouble.

THE CLUTCH TRICK:MAKE YOUR CLUTCH LAST A LIFETIME!

Did You know that the clutch plate of your vehicle can run for a lifetime(not literally though!)? Lets Find Out:

What is a clutch?

Clutch is basically a mechanical device which is used to transfer the power from the engine's shaft to the wheel. It is a very important part of the transmission system of a vehicle. Not only manual transmission systems have a clutch but even automatic transmission systems have a clutch(it's just not physically visible to us though).But, we clutch's overall life isn't in our hands in an automatic transmission system unlike a manual one where the life of our clutch plate depends mostly on our driving style and a few other factors.

Working of a clutch:

Video Credit: Simon Raisbeck

The above video explains the working of a clutch

In order to­ put a car to complete stop without killing the engine, the wheels need to be disconnected from the engine somehow. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the slippage between them.

  This design allows you to disengage the wheels from the engine in order to change gear, allowing drivers to have a great amount of control over the speed of their car.

How to know your clutch has a problem?

Here are a few symptoms which can help you diagnose the problem and save your clutch before it gets completely worn out:

• Poor Performance: The vehicle no longer responds to your inputs as it used to do.
• Noise: The clutch plate or gear may make some growling noise when in neutral or while changing gears
• Gear shifting not smooth: Gear gets stuck while changing
• Unable to change gears: This is the last stage and after this you may need a replacement.

Now that we have discussed about the problems, let's discuss how not to get these problems, or, atleast, delay them for as long as possible.

Making your clutch last long

Did you know that your clutch can last for over 2,50,000 kms! It all depends on the way you use it. A few changes to your driving style can save you a lot of money. Wearing of clutch can also damage other related parts and the repair bills may come out to be very very expensive.

This applies to every vehicle having a manual transmission.

If you observe keenly, you will find out that most of the riders on motorcycles keep their clutch half-engaged most of the time, which is not a good practice. you must not keep your clutch lever/pedal half pressed, it drastically reduces the lifespan of the clutch. 

If you ride/drive like this, your clutch may need repairs/replacement within 20-30,000 kms only. Don’t go round corners or slow down for traffic lights with the clutch semi-depressed.

Listing things you should do in order to get the most out of your clutch plate in points:

1. Don’t ride the clutch
2. Shift to neutral at traffic lights
3. Use handbrake when parking
4. Be quick with gear shifts
5. Change gears as less as possible

That's it, and you are done! Happy Riding/Driving.

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Turbochargers

Future Power source for EVs

Differential Locking

TURBOCHARGERS: WHY SO COMMON

Turbochargers

With each passing day, we are seeing the increase in use of turbocharged engines. In context to Indian automobile market, we can see many smaller cars are now being fitted with turbochargers.Automotive Turbocharger Market size exceeded USD 16 billion in 2019 and is estimated to grow at a CAGR(Compounded annual growth rate) of over 7% between 2020 and 2026.

This substantial growth is expected because of its increasing adoption in passenger, commercial, and heavy-duty vehicles to provide enhanced fuel efficiency.

Why so popular?

A turbo can significantly boost an engine's horsepower without significantly increasing its weight, which is the huge benefit that makes turbos so popular! It is also one of the key reasons behind increase in use of turbos in smaller capacity vehicles.  The following chart shows the significant effect of turbochargers:

Strict emission norms all over the world also result in increase in demand of turbochargers.For instance, in August 2019, the Supreme Court of India announced that Bharat Stage VI (BS-VI) emission norms will be applicable in India from 1^st April 2020, completely banning the sale of vehicles equipped with BS-IV engines.The implementation of such regulations by government authorities has encouraged automobile manufacturers to integrate automotive turbochargers in passenger cars and heavy-duty vehicles, fueling the market revenue.

What does it do?

Turbocharger improves power output of engines using exhaust gas to increases overall air intake in the combustion chamber. Turbocharger allows more compressed air inside the engine’s intake manifold, resulting in more efficient fuel combustion and hence superior power output, improved fuel efficiency and lower exhaust emission.

It basically draws in ambient air and compresses it before it enters into the intake manifold at increased pressure.This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the kinetic energy of the engine's exhaust gases. It may also be used to increase fuel efficiency without increasing power.

Popularity in petrol powered cars

Diesel vehicles predominantly use turbochargers
Initially, turbocharger penetration was primarily driven by diesel vehicle with high diesel share geographies like Europe and India having high turbocharger penetration.

Its gaining prominence in petrol vehicles for improved fuel efficiency and reduced emission, and higher power output. In India, small cars (< 4,000 mm) enjoy favourable tax duties as compared to larger cars(>4000 mm). However, these small cars also have engine size (petrol < 1,200 cc, diesel <1,500 cc) restriction which ultimately limits power output. Recently, in order to address these concerns, some manufacturers have launched turbo charged petrol engines in < 4,000 mm PVs(passenger vehicles) which provides superior power output as well as improved fuel efficiency.

Turbocharger vs Supercharger

A turbocharger is powered by a turbine driven by the engine's exhaust gas, whereas, a supercharger is mechanically driven by the engine using a belt drive, chain drive, etc.Compared with a mechanically driven supercharger, turbochargers tend to be more efficient, but less responsive. By comparison, a turbocharger does not place a direct mechanical load on the engine, although turbochargers place exhaust back pressure on engines, increasing pumping losses. Turbo Lag is a phenomena in which a hesitation or slowed throttle response is observed when accelerating as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost which can also be referred to as spooling.

More Topics:

Sodium-Ion Batteries

Differential Locking

Electronic Stability Control

IoT in cars

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Sodium Ion Batteries: The future powerplant for EVs?

Introduction

We all are already much familiar with Lithium-ion batteries as we use them on a daily basis. It is present in our mobile phones, laptops and tablets, digital cameras, camcorders, handheld game consoles & torches (flashlights) and much more. It is also used to power Power Tools such as cordless drills and a variety of other equipments.

Current-day electric vehicles are also being powered by this technology. But, Li-ion batteries have their own limitations(at least in context of EVs). 

• Over their lifespan batteries degrade gradually leading to reduced capacity. 
• Lithium-ion batteries can be a safety hazard since they contain a flammable electrolyte.
• They don't provide enough range.
• They take lots of time to be charged fully.
• If a lithium-ion battery is damaged or crushed(in case of accidents) then problems may arise. External short circuit can trigger the battery explosion.
• It's quite expensive(hence EVs are quite expensive when compared to their IC Engine counterparts).

 A recent analysis by Bloomberg New Energy Finance predicts that demand for lithium will explode, increasing 1500-fold by 2030. That could send lithium prices skyrocketing because the metal is mined in only a handful of countries.

To overcome these hurdles, numerous researches are going on all over the world. Sodium ion battery is the result of these researches and developments in this field are noticeable. Researchers have created a solid-state sodium battery with a record capacity to store charge and a flexible electrode that allows recharging hundreds of times.

ADVANTAGES

Following table compares the sodium ion batteries with lead-acid and lithium-ion batteries:

Source:wikipedia

Sodium is inexpensive and can be found in seawater so is virtually limitless. Na-ion batteries are also more stable and more powerful as well.

CHALLENGES

A big problem is the fact that the Sodium batteries lose 0.3 volts if you compare them to Lithium batteries. It means, it will drain out charge faster than the Lithium ion batteries. For every 3 volts of battery consumption for Lithium ion batteries, the Sodium ion batteries will lose 3.9 volts of charge. This unfortunate factor makes them pretty ordinary as charge carriers.

CONCLUSION

Despite those obvious problems, the technology of developing Sodium ion batteries as a substitute of the pricey Lithium ion has been welcomed by the electronics industry in general. The researchers of CEA believe that electrochemistry and solid state chemistry would enable them in creating some tailor made designs to overcome the problems they are facing now. The prototypes are still strictly in research criterion, but it can be hoped that they will pretty soon hit the mass market.

Differential Locking: How does it work?

WHAT IS DIFFERENTIAL?

A differential is nothing but a mechanical device used to drive a pair of wheels while allowing them to rotate at different speeds.Vehicles which don't have differential have same wheel speed on both ends of the axle, driven by a simple chain-drive mechanism.While cornering, the inner wheel travels a shorter distance than the outer wheel, so without a differential either the inner wheel will rotate too quickly or the outer wheel will rotate too slowly, which results in difficult and unpredictable handling, damage to tires and roads, and strain on (or possible failure of) the drivetrain. For example, if the car is making a turn to the right, the main ring gear may make 10 full rotations. During that time, the left wheel will make more rotations because it has farther to travel, and the right wheel will make fewer rotations as it has less distance to travel. The sun gears (which drive the axle half-shafts) will rotate at different speeds relative to the ring gear (one faster, one slower) by, say, 2 full turns each (4 full turns relative to each other), resulting in the left wheel making 12 rotations, and the right wheel making 8 rotations.

Source: www.mathworks.com

ACTIVE DIFFERENTIALS

Active differential is an electronically controlled, relatively new technology . An electronic control unit (ECU) uses inputs from multiple sensors, including yaw rate, steering input angle, and lateral acceleration, and adjusts the distribution of torque to compensate for undesirable handling behaviours such as understeer.Fully integrated active differentials are used on the likes of Ferrari F430, Mitsubishi Lancer Evolution,  and on the rear wheels in the Acura RL.

LOCKING DIFFERENTIALS

One undesirable side effect of an open differential is that it can limit traction under less than ideal conditions. It(open diff.) basically provides same torque to both the wheels in lower gears, and thus at lower speeds, and unless the load is exceptionally high, the drivetrain can supply as much torque as necessary, so the limiting factor becomes the traction under each wheel. If the torque applied to one of the drive wheels is in excess to the threshold of traction, then that wheel will spin.For example, lets imagine a simple rwd vehicle, with one rear wheel on asphalt with good grip, and the other on a patch of  mud/slush. It takes very little torque to spin the side on slippery surface, and because a differential splits torque equally to each side, the torque that is applied to the side that is on asphalt is limited to this amount.

In order to overcome this limitation, the concept of locking differential was introduced. It essentially "locks" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction available to either wheel individually. When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed). 

source:https://www.topgear.com.ph/

Full time four wheel drive vehicles have three differentials:

• In front axle
• Central diff between both axles(transfer case)
• In rear axle

Part-time four-wheel-drive systems don't have a differential between the front and rear wheels; instead, they are locked together so that the front and rear wheels have to turn at the same average speed. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged.

CONCLUSION

So, lets see the pros of locking diffs:

• Distributes power evenly to the wheels
• Enhanced off-road performance
• Improves traction in rough terrain
• Selectable lockers are great for daily drivers
• Some can be activated with the push of a button

Some cons:

• Higher cost for selectable lockers
• Clunky engage and disengage for automatic lockers
• Can put added stress on driveline components if misused
• Selectable lockers can be complicated to fix if they break down