How to Calculate Vehicle Speed

Car transmissions can look complicated, and the actual working can seem to be even more complicated. A conventional constant mesh gear box consists of an input shaft from the engine, a counter shaft and a main shaft which delivers power to the differential via propeller shaft. To know more in detail about the working of a constant mesh gearbox, please visit the page on the following link: 

• How manual transmission works

As you can see in the diagram, a 4-speed constant mesh gearbox consists of a set of 11 gears (including the gears between the input shaft and the counter shaft). Now let’s calculate the gear ratio between the gears and how a gear ratio can affect the final drive given to the wheels? How to calculate the speed of the vehicle?

1^st gear:

Let’s say the gear A (driving) has 10 teeth and gear B (driven) has 35 teeth. Gear ratio is the ratio of number of teeth in the driven gear to the number of teeth in the driving gear.

Gear ratio = number of teeth in the driven gear / number of teeth in the driving gear

Therefore, 1^st gear ratio can be calculated as

G1 = T_B / T_A

G1 = 35/10 = 3.5 : 1

T_B = Number of teeth in gear B

T_A = Number of teeth in gear A

The differential has its own gear ratio which is known as the Differential gear ratio (G_D). In this case, let’s assume that G_D = 3.5. Now the G_D is fixed and cannot be altered.

Now to calculate the speed at which the wheels are rotating, we need to bring into picture the final gear ratio. Final gear ratio decides at what speed the wheels are driven. It is a product of both transmission gear ratio and the differential gear ratio together.

Final Gear Ratio (G_F) = G1 X G_D

G_F = 3.5 X 3.5 = 12.25

Yes, I know it is very complicated and you are lost somewhere in understanding the whole concept. To explain you in simple words, the final value of G_F = 12.25 indicates that for 12.25 revolutions of the engine crankshaft, the wheels will revolute only once.

Consider your engine running at 2000 rpm, then wheels will rotate at (2000/12.25) rpm.

To calculate the speed of the vehicle:

Let’s consider the tire is 0.35 m in diameter, therefore the circumference of the tire is 

C = πD

C = π(0.35)

C = 1.1 m (approx.)

Hence, for every 12.25 revolutions of the crankshaft, the wheels will cover 1.1 m.

Now let’s consider the engine speed in revolution per hour (rph) = 2000 x 60 = 1,20,000 rph.      

Now the vehicle speed can be calculated using the above values. The vehicle speed at 1^st gear at an engine speed of 1,20,000 rph is

Vehicle speed = (Engine speed in rph / final gear ratio) X circumference of the tire

Vehicle Speed = (1,20,000 rph / 12.25) X 1.1 m

= 10,775 meters per hour (approx.)

= 10.775 km/h

The vehicle speed for the other gear ratios can be calculated by following the same procedure as above. Let’s calculate:

2^nd Gear:

Let’s assume the 2^nd gear ratio, G2 = 2.5 : 1.

Differential gear ratio (G_D) = 3.5 : 1

Final gear ratio = 2.5 X 3.5 = 8.75 : 1

Engine speed = 200000 rph          

Vehicle speed at 2^nd gear = (200000/8.75) X 1.1

 = 25142.85 m/h 

= 25 km/h (approx.)

3^rd gear:

Let’s assume the 3^rd gear ratio, G2 = 1.8 : 1.

Differential gear ratio (G_D) = 3.5 : 1

Final gear ratio = 1.8 X 3.5 = 6.3 : 1

Engine speed = 200000 rph          

Vehicle speed at 3^rd gear = (200000/6.3) X 1.1 

= 34920.63 m/h 

= 35 km/h (approx.)

4th Gear:

Let’s assume the 4^th gear ratio, G2 = 1 : 1.

Differential gear ratio (G_D) = 3.5 : 1

Final gear ratio = 1 X 3.5 = 3.5 : 1

Engine speed = 200000 rph          

Vehicle speed at 4^th gear = (200000/3.5) X 1.1 

= 62857.14 m/h 

= 63 km/h (approx.)

Dual Clutch Transmission

Dual clutch transmission (DCT) uses a set of two clutches to operate the odd and even gear sets separately. One clutch is used to operate even set of gears and the other clutch will operate the odd set of gears. It is also known as semi-automatic transmission.

Working Principle:

When we look at a conventional manual clutch transmission, a clutch pedal is used to first cut off the power supply from the engine to the gearbox and then gear stick shift is used to change the gear and then later on releasing the clutch pedal, the clutch is engaged with the flywheel and the power supply from the engine to the gearbox resumes. Therefore, there is no continuous flow of power from engine to wheels. This results in torque shifts and can result in the passengers thrown forward or backward as gears are changed by an unskilled driver.

In the case of DCT, there is no clutch pedal. The clutches are controlled by suitable electronics and hydraulics mechanism. One clutch will control the odd gear ratios (1, 3 and 5) and the other clutch will control the even gear ratios (2, 4). The trick behind this is to achieve lightning fast gear changes without interrupting the power supply from the engine.

Design of Dual Clutch Transmission:

A dual clutch transmission uses 2 multiplate clutch assemblies. Even single plate clutches can be used instead of multiplate clutches. There are 2 transmission shafts that carry the power from engine to the gearbox. The outer transmission shaft is connected to the outer clutch assembly.

The outer transmission shaft is hollow, which allows us to insert the inner transmission shaft and connect it to the inner clutch assembly. The outer transmission shaft is supplies power to the even gears (2 and 4) and the inner transmission shaft supplies power to the odd gears (1, 3 and 5).

Both the clutches are controlled hydraulically. It consists of a piston assembly placed against a stack of clutch plates and friction discs. When clutch is engaged, hydraulic pressure from the piston forces the clutch plates and friction discs to move against the pressure plate. The entire clutch assembly is locked and rotates together as a single unit. When the clutch is disengaged, the return springs help to pull the clutch plates and friction discs so that the transmission shaft rotates freely.

Working:

To understand the working in a better way, I suggest you to learn how a manual transmission works by clicking on the following link:

How manual transmission works?

The gear shifting is similar to a conventional manual transmission. Dog clutch and synchronizers are used to engage an individual gear to the output shaft.

1^st Gear:

The inner clutch assembly is engaged and the dog clutch engages itself with the 1^st gear on the output shaft. Make a note that the outer clutch will be disengaged during this process. Once we reach a high enough speed in the 1^st gear, the selector rod will automatically engage the dog clutch to the 2^nd gear on the output shaft. As soon as the outer clutch is engaged, the 2^nd gear is achieved in no time.

When the outer clutch is engaged, the inner clutch will be disengaged, hence cutting off the supply through 1^st gear.

2^nd Gear:

The outer clutch assembly is engaged now and since the dog clutch is already connected with the 2^nd gear, instantaneous acceleration is achieved. The inner clutch will be disengaged during this process.

3^rd Gear:

When the speed in the 2^nd gear is high enough, the dog clutch engages with the 3^rd gear in order to be ready to supply the power during gear shift. As soon as the inner clutch is engaged and outer clutch is disengaged again, the 3^rd gear is achieved. The inner and outer clutch engagement cycle continues for every odd and even gear shifting.

4^th and 5^th Gear:

The outer clutch is engaged to achieve 4^th gear and inner clutch is engaged to achieve 5^th gear.

Reverse Gear:

Reverse gear can be controlled by any one of the clutches. In this case, the outer clutch is used to achieve reverse gear. An idle gear ‘I’ is inserted to mesh between the gears ‘H’ and ‘R’ to reverse the rotation of the wheels.

Advantages of DCT:

·         Even with automatic engagement and disengagement of clutch, drivers can tell computers when to take action with the help of paddles or gearshift.

·         It provides smooth acceleration by avoiding torque shifts or gear shift shocks that are usually experienced in a car with manual transmission.

·         Fuel economy can be improved dramatically by up to 10%.

·         Can handle high torque demands of high performance cars.

The only main disadvantage of DCTs would be the manufacturing cost of it since they involve two clutches and two transmission shafts.

 Related topics:

• Continuously Variable Transmission (CVT)

• How manual transmission works

• Sliding Mesh Gearbox

• Multi Plate Clutch (Spring Type)

• Single Plate Clutch

• Front Wheel Drive (FWD)

• Things not to do in a manual transmission

Things not to do in a manual transmission

In this article, I will talk about 5 bad habits that one may follow while driving a manual transmission vehicle. Not following these don’ts might affect the transmission and engine performance of the vehicle.

1.       Don’t rest your hand on the gearshift:

This habit is commonly seen among the experienced drivers who prefer to rest their hand on the gearshift and steer the vehicle with the other hand. We all know that during gear shift, selector fork engages the dog clutch to the respective gears.

If we rest our hand on the gearshift, we might apply unwanted pressure on the selector fork against the dog clutch. This can lead to wearing of the selector fork and the dog clutch. The scenario looks like forcing gear change without pressing on the clutch pedal which can damage the components.

The best way to deal with this situation is to use both the hands for steering. Gearshift should be touched only while changing the gear. Whenever you need a gearshift, press the clutch pedal, use your hand to shift the gear and then put your hand back on the steering wheel.

2.       Put the transmission on neutral at the stop light:

Drivers should have the practice of leaving the vehicle at neutral at stop lights instead of pressing on the clutch pedal and shifting to 1^st gear. The clutch assembly has a release bearing which presses against the diaphragm spring when the clutch pedal is pressed. This applies unnecessary pressure on both the release bearing and the diaphragm spring which leads to wearing.

The best thing to do is to bring the gearshift to neutral and release the clutch pedal to avoid any unnecessary wearing within the clutch.

3.       Never use clutch to hold the vehicle on a slope:

When you have halted your vehicle on a steep hill, there is always a tendency for the vehicle to roll back. In this scenario, we should never use the clutch to hold ourselves from rolling back, as the clutch plate and pressure plate would be rotating at different speeds and this would lead to the wearing of the friction material on the clutch plate.

Hand brakes come in handy under this situation. Apply hand brakes to prevent the vehicle from rolling back. When you want to move forward, press on the clutch pedal and shift the gear to 1^st gear. Then slowly start releasing the clutch pedal. As you feel a forward movement, quickly release the handbrake and then press on the accelerator to get the forward movement. With regular practice, one should be able to control it perfectly.

4.       Downshift the gear at lower vehicle speed:

On some occasions people get lazy to downshift the gear at a lower vehicle speed. In other words, we shouldn’t apply a lot of throttle when we are at a lower engine rpm. For example, if you are at 5^th gear and the vehicle is running at 40 kmph, it is not a good idea to press on the throttle because the engine will be at a huge gearing disadvantage. You are simply going to ask the engine to put more effort to accelerate the vehicle. This will affect the fuel economy.

So if you are running the car at a lower speed, it would be in your best interest to downshift to a lower gear for better and smoother acceleration.

5.       Don’t rest your foot on the clutch pedal:

Resting your foot on the clutch pedal can affect the power transmitted to the transmission because the clutch may not be fully engaged. In this situation, you might also be wearing the clutch disc due to slipping.

6.       Don’t start your vehicle at 2^nd gear:

This is again commonly seen among professional drivers who seem to have better control over clutch pedal. Moving the vehicle by up-shifting directly from neutral to 2^nd gear will lead to engine lugging. It simply means you are asking the engine to put more effort in accelerating your car which can be done quite easily at 1^st gear. We all know the fact that the transmission transmits maximum torque at 1^st gear; hence it is always desirable to launch the vehicle at 1^st gear.

       


Other Topics:

• Top 12 features of U.S President's car

• How manual transmission works

• Sliding Mesh Gearbox

• Multi Plate Clutch (Spring Type)

• Single Plate Clutch

Front Wheel Drive (FWD)

Front wheel drives (FWD) or (front engine, front wheel drive) vehicles have the engine, transmission and driven wheels at the front of a vehicle. This means that the rear wheel axle is not equipped with a differential. The front wheels axle instead has a differential to split the torque.

In a FWD, the engine can either be mounted longitudinally or transversely. The disadvantage with longitudinal placement is that the bonnet becomes too long. Hence, transverse mounting is preferred over longitudinal mounting.

The power from the engine is transmitted to the transaxle via a clutch. Clutch allows the gearbox to be completely disengaged with the engine in order to help in smoother gear transition. Transaxle is a drive axle that combines both the transmission and differential functions together.

The transaxle then supplies the power to the front wheels via constant velocity (CV) joints.

Advantages of a FWD:

·         It is compact in structure and hence saves a lot of space. With a transversely mounted engine, the bonnet length can be kept a lot shorter.

·         The ground clearance can be kept low since a tunnel won’t be required in the chassis to accommodate the propeller shaft.

·         More traction on the driven front wheels since the front axle will be carrying more load than the rear axle.

·         Thrust will be in the intended direction which is not the case in rear wheel drives.

Disadvantages of a FWD:

·         Vehicles with high powered engines will tend to move from left to right due to torque steer. The power generated by the engine causes torque steer.

·         Front wheels will undergo more wear due to the heavy load, braking and steering. Rear wheels won’t feel that much heat and thus uneven wearing of the tires take place.

·         Under acceleration, the weight is transferred from the front side of the vehicle to the rear. This reduces the traction between front wheels and the surface.

Single Plate Clutch

A vast majority of cars rely on single plate friction clutches which will be explained in this article. One of its components is the flywheel which is connected to the engine crankshaft. The flywheel is coated with a friction surface on one side.

A clutch disc equipped with 2 frictional surfaces is brought in contact with the flywheel. In general, if the friction surface of the flywheel comes in contact with the friction surface of the clutch disc, the clutch disc will start rotating along with the flywheel.

Clutch Disc:

The clutch disc is made of several parts such as:

·         A hub which is fixed to the clutch disc. The hub has internal teeth which are engaged with the transmission shaft or driven shaft.

·         Friction material on both the sides. Friction materials today are usually made of ceramics rather than asbestos to increase the co-efficient of friction.

·         A cover plate is provided to cover the hub and the disc.

·         A set of 4 springs are provided to dampen the vibrations and for smoother operation when the clutch is engaged with the flywheel.

The wheels of a vehicle will only rotate if the clutch disc is in contact with the flywheel.

Pressure Plate:

A pressure plate is imperative for engaging and disengaging the clutch disc with the flywheel. The clutch disc is sandwiched between the flywheel and the pressure plate. A diaphragm spring is provided in the pressure plate to allow for a flexible movement of the plate. A clutch release bearing is provided in front of the spring to facilitate the compression and release of the spring. The clutch pedal is connected to the release bearing.

Clutch Cover:

A clutch cover is used to cover the entire clutch assembly. It is riveted to the flywheel. Therefore, as the flywheel rotates, the clutch cover also rotates at the same speed irrespective of the engagement and disengagement of the clutch disc.

Working of Single Plate Clutch:

When the clutch pedal is pressed, the release bearing moves in a direction towards the flywheel. The inner portion of the diaphragm spring moves in towards the clutch disc, whereas the outer portion of the spring moves in the opposite direction. As a result, the pressure plate which is connected to the outer portion of the diaphragm spring moves away from the clutch disc. Therefore, the clutch disc is disengaged from the flywheel. The power from the engine now cannot be transferred to the gearbox. The driver utilizes this time to shift gears from bottom to top gear or vice-versa.

When the clutch pedal is released, the force on the diaphragm spring is also released and the pressure plate now pressed against the clutch disc. Consequently, the clutch disc is engaged with the flywheel.

          

Related Topics:

• Multiplate Clutch

Multi Plate Clutch (Spring Type)

You would be aware of the fact that multi plate clutches are extensively used in bikes rather than cars. Instead, single plate clutches are deployed in cars. The reason for the multi plate clutches not being deployed in cars is:

Since the space is constricted in bikes, the value of R is small and due to the flanges and bolts, the value of r also cannot be changed beyond a limit. The smaller radius transmits lesser torque. Single plate clutch has a bigger value for R and hence more amount of torque is transmitted.

Why don’t cars have multi plate clutch?

The bigger the diameter of the clutch, the more the torque can be transferred through it. This can be proved based on the torque equation:

T = N * µ * W * (R+r)/2

T- Torque

N- Number of contacting surfaces

µ- co-efficient of friction

W- Normal force acting on clutch

R- Outer radius of clutch

r- Inner radius of clutch

Design of Multi plate clutch (spring type):

Multi plate clutch is made of several friction discs, unlike the single plate clutch that has only one friction disc with 2 friction surfaces are used to transmit power from the engine to the transmission.

Multi plate clutches consist of a series of friction discs (3 to 6) that are splined to a clutch shaft. Each friction disc has a friction surface on either side to increase the co-efficient of friction of the clutch. The clutch shaft is connected to the engine flywheel.

Steel discs are placed between the friction discs. The steel discs have external teeth that sit and lock within the clutch housing. The clutch housing transmits the power to the gearbox.

The entire friction discs and steel discs assembly is known as a clutch pack. A pressure plate is placed at the end of the clutch pack. The pressure plate engages the entire clutch pack together due to the force of the coil springs. The coil springs apply spring force on the pressure plate, which in turn applies pressure on the clutch pack.

Working of Multi plate clutch (spring type):

The clutch pack is pressed together with the help of spring force. When the clutch pedal is released, the rotation of friction discs will result in the rotation of steel discs and vice-versa.

When the clutch pedal is pressed, the friction and steel discs are no longer engaged and they start rotating freely without transmitting any power to the transmission.  

Most of the bikes use wet type multi plate clutch in order to avoid overheating of the clutch.

        

Sliding Mesh Gearbox

Sliding mesh gearbox is the simplest type of gearbox. It looks similar to a constant mesh gear box, except that the main shaft gears are not always in contact with the counter shaft gears.

The individual gear ratio is obtained by sliding the selected gear wheel axially. The gear wheels are splined in the main shaft and can be slid to obtain different gear ratios.

One major problem with sliding mesh is the absence of synchronizer units as found in the constant mesh. While changing the gear ratio, the speeds have to be matched before engagement of the gears. Due to the unavailability of synchronizer teeth, the gears might collide with each other and generate a lot of noise. This can even damage the gears.

The gears are provided with spur straight teeth, in order to avoid any side thrust while engagement. This type of gearbox is used in very few vehicles where compact assembly is required. The shaft and gears are made of low alloy nickel-chromium-molybdenum steel.  

Design:

The input shaft from the engine drives the counter shaft. Both input and counter shaft gears are in constant mesh with each other. All the other gears on the counter shaft are rigidly fixed and rotate with the shaft. The main shaft or the output shaft is held in the same axis as that of the input shaft.

The main shaft gears are not in constant mesh with the counter shaft gears. The gear wheels on the main shaft can be slid axially to achieve different gear ratios. The different gear ratios are obtained as following:

Neutral Gear:

In this case, none of the main shaft gears are meshed with the counter shaft gears. Hence, the drive from the input shaft is not transferred to the main shaft. The wheels remain stationary.

1^st Gear:

In 1^st gear, the gears D and F are meshed to form a lower gear ratio. Since the gear F is bigger (30 teeth) and has more number of teeth than the gear D (10 teeth), the main shaft rotates at a lower speed. The torque is highest and speed is lowest at 1^st gear. This is to facilitate easy movement of the car from standstill.

If the counter shaft is rotating at 1000 rpm (N1), then the main shaft speed (N2) can be calculated as

N1/N2 = T_F/T_D

N2 = N1 X (T_D / T_F)

N2 = 1000 X (10/30)

N2 = 333.33 rpm

Where,  N1 = speed of counter shaft

              N2= speed of main shaft

              T_D = number of teeth in gear D

              T_F = number of teeth in gear F

2^nd Gear:

In 2^nd gear, the gear E is slid and meshed with gear C. The number of teeth in gear E (T_E) is reduced, or example T_E = 22. The number of teeth in gear C is increased (T_C= 15). This increases the speed of the car and the torque is reduced.

If the counter shaft speed (N1 = 1000 rpm), then the main shaft speed (N2) is

N2 = N1 X (T_c / T_e)

N2 = 1000 X (15/22)

N2 = 681.81 rpm

3^rd gear or Top gear:

In 3^rd gear, the main shaft is slid axially to mesh with the input shaft. In this case, the drive from the input shaft is directly transferred to the main shaft. The input shaft has a gear with internal teeth that mesh with the main shaft gear with external teeth. The vehicle can achieve top speed in top gear and the torque is lowest at this point.

Reverse Gear:

In reverse gear mechanism, an idle gear ‘I’ is used in between the gears G and F. The gears G and F are not in direct contact. The idle gear I is driven by G and the gear I drives the gear F. In this way, the direction of rotation of the main shaft is reversed and hence the vehicle moves backward.

How manual transmission works

Engine crankshaft rotates at a high speed. The high speed power cannot be directly transmitted to the wheels, as it would start rotating at an uncontrollable speed and the driver won’t have any control over the speed on different driving conditions. Therefore, speed reduction is necessary between engine and the wheels. Manual transmission uses a set of gears that help in speed reduction.

The manual gearbox provides a set of gears with different sizes and different number of teeth for different driving conditions. The wheels will be slowest at the 1^st gear and fastest at the top gear. Manual transmission is the most popular form of transmission.

In this article, we will learn about the working of a 4 speed manual gear box with reverse gear.

Why is a transmission necessary?

A vehicle requires moving at different speeds on different conditions. For example, a vehicle moving on a slope requires more torque and hence the vehicle should be operated at lower gear (1^st gear). The higher the torque, the lower is the power from the engine. Whereas, a vehicle moving on a straight surface with less traffic can be operated at higher speeds by switching to higher gears (4^th gear).

Principle of manual transmission:

Manual transmission works on a simple principle of gear ratios. A basic gearbox consists of an input shaft from the engine, an output shaft or main shaft that delivers power to the differential and a counter shaft that transmits the power from input to output shaft.

The power from the input shaft drives the counter shaft and the counter shaft in turn drives he output shaft of main shaft.

Constant Mesh Gearbox:

This is the most common type of manual gearbox used in a vehicle. It consists of an input shaft, a lay shaft or counter shaft, a main shaft or output shaft and a synchromesh device.

The engine drives the input shaft, which in turn drives the counter shaft. The counter shaft gears and the main shaft gears are in constant mesh with each other all the times. That’s why it is called constant mesh gearbox.

The counter shaft gears drive the main shaft gears. But the main shaft gears rotate freely over the bearings and don’t rotate the main shaft unless one of the main shaft gears is locked with the main shaft using the synchromesh device.

The synchromesh device is splined to the main shaft and can slide from left to right or vice-versa. The synchromesh device is commonly known as dog clutch and is operated by means of a selector rod.

Hub and Sleeve arrangement:

There is a hub which is splined to the main shaft and rotates along with the shaft. The hub has external teeth over which a sleeve with internal teeth can slide as per the gear ratio required. Each gear on the main shaft is provided with a synchronizer cone teeth arrangement which rotates freely over a bearing. If the sleeve meshes with the teeth of the synchronizer cone, it is clear that both gear and main shaft will be locked and will start rotating at the same speed.

Synchronizer Ring:

But during the gearbox operation, both the main shaft and main shaft gears will be rotating at different speeds. Hence, meshing the sleeve with the synchronizer cone is a difficult task and can generate a lot of noise. To overcome the problem, a synchronizer ring is provided between the sleeve and the synchronizer gear to match the speed of the gear with the shaft before being meshed with each other.

The synchronizer ring not just rotates along with the hub, but also slides axially.

Engagement of sleeve and synchronizer cone teeth:

When clutch pedal is pressed, the power flow from the engine to transmission is blocked. The sleeve is slid towards the required gear with the help of a selector rod. The sleeve pushes the synchronizer ring against the synchronizer cone.

Due to high frictional force between the cone and the ring, the speed of the gear is matched with the speed of the shaft. When the speeds match, the sleeve is slid further towards the cone and meshes with its teeth. Hence, both gear and the main shaft are locked and both start rotating at the same speed.

The same principle is followed to shift to other gears.

Different gear ratios:

Neutral gear:

All the gears on the main shaft are in constant mesh with the gears on the counter shaft. The gears on the main shaft rotate freely and none of the gears are synchromeshed with the main shaft. Hence, no drive is transmitted from the input shaft to the output shaft.

1^st gear:

The smallest gear (lowest number of teeth) in the counter shaft is synchromeshed with the largest gear (highest number of teeth) on the main shaft. Thus we can achieve maximum torque and minimum speed. 1^st gear is ideal for a standing start of the engine.

2^nd gear:

In 2^nd gear, the gear in the middle of the counter shaft is synchromeshed with the 2^nd biggest gear on the main shaft. This increases the speed and reduces the torque to a certain level. 2^nd gear ratio is ideal for cars ascending a hill.

3^rd gear:

The biggest gear on the counter shaft is synchromeshed with the smallest gear on the main shaft to increase the speed further and reduce the torque. This gear ratio is ideal for cruising.

4^th gear:

The input shaft gear is directly synchromeshed with the main shaft to provide a direct drive from the input shaft to the main shaft. The vehicle can reach its top speed at top gear.

Reverse gear:

The reverse gear uses an idle gear to be meshed between an input gear on the counter shaft and an output gear on the main shaft. When the driver selects the reverse gear, the idle gear is slid in between the two gears. This reverses the direction of rotation of the main shaft.

There is no synchronizer cone and ring mechanism for reverse gear. Hence, reverse gear can only be used when the transmission operation is stopped completely and none of the shafts are rotating. The gear ratio is kept low for reverse, since a vehicle requires more torque when it is moving from a standstill.

Related Links:

• Sliding Mesh Gearbox

Continuously Variable Transmission (CVT)

Continuously Variable Transmission (CVT) is a type of automatic transmission that can change through an indefinite number of gear ratios possible between the maximum and minimum values of gear ratio.

A typical mechanical transmission such as a 4 speed gear box has a fixed number of gear ratios. For example,

• 1st gear ratio- 3.4:1
• 2nd gear ratio- 2.5:1
• 3rd gear ratio- 1.7:1
• Final gear ratio- 1:1

In the case of a CVT, it can seamlessly vary gear ratios between the maximum gear ratio (3.4:1) and the minimum gear ratio (1:1). This ensures maximum fuel efficiency.

Design:

CVT transmission doesn't employ the conventional planetary gear system used in automatic transmissions to vary the gear ratio. The most commonly used system for CVT is the pulley system that allows indefinite number of variations in gear ratios between highest and lowest values.

A pulley based CVT uses three main components:

• Electronically controlled and oil pressure supported cones
• The cones form a pair of pulleys- one of them is the 'input driving' pulley and the other is the 'output driven' pulley.
• High strength alloy steel belt running through the pulleys.

The pulleys can vary its diameter when the cones move apart or towards each other. The cones have an angle of 20° and they face each other. Belts form the main link between these pulleys. Rubber belts in the shape of 'V' pass through the gap between the cones. Nowadays, more efficient flexible metal belts are used to handle more torque.

Working:

The engine power is transferred from the input driving pulley to the output driven pulley via the belt. By varying the gap between the cones in the pulleys, an infinite number of gear ratios can be achieved.

The cones can be hydraulically operated or spring operated to change the distance between them. When the cones of a pulley are far apart, the radius of the belt revolving around the pulley becomes smaller. Conversely, if the gap between the cones reduces, the radius of the belt revolving around the pulley increases.

In order to keep the belt tight, if the radius of the belt over one pulley is small, then the radius over the other pulley has to be larger. This relative variation in radii of the belts over the two pulleys helps in varying the gear ratios.

High Gear:

If the radius of the belt over the driven pulley is smaller than its radius over the driving pulley, then the CVT operates at a smaller gear ratio (say 1:1), thereby increasing the vehicle speed.

Low Gear:

Conversely, if the radius over the driven pulley is higher than the driving pulley, the gear ratio is increased (say 3.4:1) and the vehicle speed decreases.