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.

        

Common Rail Direct Injection (CRDI)- Diesel

Today’s diesel engines are cleaner, more economical and more fuel efficient than ever before. Common Rail Direct Injection (CRDI) is the most advanced technology in diesel fuel injection. The name implies that the injection takes place directly inside the combustion chamber.

Difference between CRDI and an ordinary diesel injection system:

The difference between an ordinary diesel injection system and CRDI is that in an ordinary system, the fuel has to be pressurized for each injection cycle. In the case of CRDI, a common rail is installed which maintains the pressure of the fuel throughout its entire length. The Electronic Control Unit (ECU) regulates the fuel pressure at different speeds and also modulates the fuel supply required based on the operating conditions. Therefore, with the help of an ECU, fuel compression and injection are made independent of each other. This feature helps in lowering emissions and increasing fuel economy.

History of CRDI:

Robert Huber of Switzerland developed the first prototype of CRDI in 1960. Denso Corporation of Japan developed the common rail system for heavy duty vehicles and it was successfully installed in Hino Rising truck in 1995.

The modern CRDI which is controlled extensively by ECU have electrically controlled injectors replacing the mechanical ones. This system was prototyped in 1990s by Fiat, Magneti Marelli and Elasis. But the German company Bosch GmbH acquired the design for completion and mass production. Bosch ultimately bought the license from Fiat. The first passenger car to have a CRDI was Alfa Romeo 156 2.4 JTD in 1997.

Working of CRDI:

The common rail system is equipped with a high pressure pump which can pressurize the fuel up to 2000 bar. The pressurized fuel is then sent to a common rail which maintains the pressure throughout its entire length. The common rail is fitted with a rail pressure sensor to detect the pressure inside the rail. A pressure limiter valve is also provided in common rail. Controlled via the ECU, the pressure limiter valve limits the pressure in the rail, while at the same time suppressing pressure fluctuations.

In a CRDI, the fuel quantity and timing of injection is governed by the ECU or microprocessor. The common rail supplies fuel to each injector based on the firing order of the engine. The piezoelectric injectors are controlled by the ECU.

To lower the engine noise and vibration, the injector can be controlled to inject a small amount of fuel just before the main injection. This is known as ‘pilot’ injection. The pilot injection is followed by main injection. Some advanced CRDI injectors can achieve multiple injections (5 to 10) during main injection. This helps in lowering noise and emissions. The main injection is usually followed by a post injection.

The start of injection occurs at a pre-determined pressure and also ends at the same pre-determined pressure. Therefore, the pressure is maintained throughout the injection process, thus forming a square injection rate.

        

Related Topics:

• Distributor Fuel injection Pump (Diesel)
• Inline Fuel injection (Diesel)
• Multi point fuel injection (MPFI- Gasoline)

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.

Vehicle Insurance to get costlier

Vehicle insurance is all set to get costlier from April 1, 2016. The Union budget presented in the parliament has prompted the hike in insurance rates of different vehicles. Insurance Regulatory and Development Authority (IRDA) has confirmed a hike of up to 40 % in third party motor insurance premium rates for the year 2016-2017.

The hike for vehicles falling under different category is as follows:

Vehicle Category	Percentage hike
Cars under 1500 cc	40 %
Cars above 1500 cc	25 %
Motorcycles under 75 cc	9.6 %
Motorcycles between 75 cc and 150 cc	15 %
Motorcycles between 150 cc and 350 cc	25 %
Motorcycles beyond 350 cc	-10 %
Auto rickshaws	3.2 %
Public Carriers	15 to 30 %
Goods vehicles or trucks (below 12,000 kgs)	0 %

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