VACUUM CLUTCH

The vacuum clutch is operated by the vacuum existing in the engine manifold. Fig shows the mechanism of a vacuum clutch. It consists of a vacuum cylinder with piston, solenoid operated valve, reservoir and a non-return valve. The reservoir is connected to the engine manifold through a non return valve. Vacuum cylinder is connected to the reservoir through solenoid operated valve. The solenoid is operated from the battery and the circuit incorporates a switch which is placed in the gear lever. The switch is operated when the driver holds the lever to change gears.

When the throttle is wide opened, the pressure in the inlet manifold decreases due to which the non-return valve closes, isolating the reservoir from the manifold. Thus a vacuum exists in the reservoir all the time.In the normal operation, the switch in the gear lever remains off, the solenoid operated valve remains in its bottom position. In this positions the atmospheric pressure acts on both the side of the vacuum cylinder, because the vacuum cylinder is open, so also atmosphere though a vent. When the driver holds the lever to change the gear, the switch is closed; energizing the solenoid which pulls the valve up. This connects one side of vacuum cylinder to the reservoir. Due to the difference of pressure on the vacuum cylinder piston, it moves. This movement of the piston is transmitted by a linkage to the clutch, causing it to disengage. When the driver is not operating the gear lever, the switch is open and the clutch remains engaged due to the force of springs.

ELECTROMAGNETIC CLUTCH

In this system the clutch is controlled by means electric current supplied to the field windings in the flywheel. The fly wheel is attached with the field winding, which is given electric current by means of battery, dynamo or alternator. The construction feature of main components is almost similar to the single plate clutch. When electric current is supplied to the windings the flywheel will attract the pressure plate and clutch plate is forced between pressure plate and flywheel resulting in engagement. When the supply to the winding is cut off the clutch is disengaged by releasing the pressure plate due to the force exerted by the helical springs or tension springs. Electromagnetic clutch consists of a clutch release switch. When then driver holds the gear lever to change the gear, the switch is operated cutting off the current to the winding which causes the clutch disengaged.When the vehicle is stalling, the engine speed is lower & the dynamo output is low, the clutch is not firmly engaged. Therefore, three springs are also provided on the pressure plate which helps the clutch engaged firmly at low speed also.

The forces of the electromagnet can be regulated by means of an electrical resistance provided with acceleration system and controlled by the accelerator pedal. When the speed is increased, the accelerator pedal is pressed and the resistance is gradually cut off and thus in this way, force of electromagnet is increased and clutch transmission becomes more rigid

CONE CLUTCH

Cone clutch are wedge clutch provides a positive drive when the external face of the male cone member engages with the internet face of recessed conical member. The facing is usually fitted to the female or recessed member in order to improve heat dissipation and durability. Normally cone clutch are used with epicyclic gear trains for a higher torque transmission. The energy which a cone clutch can absorb during on engagement is less compared to the energy absorbed by a multiple clutch. But it is compact, cheaper and requires low clamping load due to the wedging action. The cone clutches are loaded by spring or hydraulic cylinders. Wedge angle and accurate axial alignment are the two important factors for good cone clutch performance. If the wedge angle is very less, it results in excessive wedge action and fierce engagement. This in turn results in difficult operation for disengagement. If the wedge angle is too large it reduces torque transmission capacity of the clutch and make the clutches to skid. Semi-cone angle of 12-16 are commonly used for effective torque transmission.

The torque transmitted by a cone clutch is given by  

T = μW (r₁ + r₂) / 2 sin α

Where, r₁ and r₂ are the radius of large and small cone (friction) in meters. α is the semi – cone angle.

During the engagement of clutch the driven member is forced towards the driving cone by the spring force. Hence the power is transmitted from the engine to the driving cone, driving cone to driven cone and driven to the gear box. When the clutch is to be disengaged the driven cone is to be pulled off by means of actuates cenkages and contact surfaces are separated hence no power is transmitted to the clutch shaft.

      OVERDRIVE

Overdrive is a device to step up the gear ratio in the car. It is fitted in between transmission and the propeller shaft. It enables a high cruising speed to be attained with a comparatively low engine speed (upto 20 – 25%) on long journeys. This results in less wear of the engine and decreases vibration and noise. As the friction lows at lower speeds is less, there is a saving of fuel also with the overdrive. Overdrive is generally fitted on top gear only. But in some sport cars, over drives are also fitted on gears other than the top gear which increases the torque ratios available. For examples, when overdrive is fated on top, third and second gear, seven forward speeds or torque ratios are available. The overdrive may be operated either manually or automatically at a predetermined speed.

To understand the working of an overdrive, consider the above figure. It consists of an epicyclic gear train in which the sun gear is free to rotate on the input shaft, while the carrier can move on splines, on the input shaft. A freewheel clutch is also fitted on the input shaft splines. The output shaft is connected to the ring. When the sun gear is locked with the casing i.e., it becomes stationary, of the output shaft is increased i.e., overdrive is engaged.

When the sun gear is locked to the carrier or to the ring, solid drive through n is obtained. Thus depending on the locking of the sun gear with ring gear or with carrier the overdrive or the normal direct drive is obtained. There is another possible control of the mechanism i.e., when the sun wheel is kept free to rotate on the input shaft. In this case there is direct drive through the freewheel clutch when the engine develops power. However when the accelerator is brought to zero position and the engine is simply idling, the output shaft tends to override the input shaft. The roller of the freewbeel clutch in this case no longer remains wedged and the car free wheels. Thus for gear changing one simply has to lift his foot off the accelerator pedal, the clutch need not be operated.

TORQUE CONVERTER

The torque converter is modified form of fluid flywheel. Fluid flywheel is used for the transmission of power, whereas torque converters are used to transmit the power with varied torque as per the requirement. In addition the driving member (impeller / pump) and driven member (turbine), it consists of a reaction member also (stator). In its simplest form it consists of an impeller connected to the crankshaft, turbine connected to output shaft and stator mounted on overrunning clutch on stationary component impeller is normally an integral part of the converter housing. (It is generally welded to the cover half during the manufacturing). 

Turbine and stator are enclosed within the welded housing. The stator incorporates a one-way clutch and mounts on a stationary support shaft that is grounded to the transmission case directly or indirectly through the transmission pump assembly. The impeller and turbine blades are designed with special features. The curved shape of the impeller blades in a backward direction gives added acceleration and energy to the oil as it leaves the impeller. The curved shape of the turbine vanes is designed to absorb as much energy as possible from the moving oil as it passes through the turbine. The vane curvature has two functions that give the turbine excellent torque absorbing capacity. It reduces shock losses due to the sudden change in oil direction between the impeller and turbine. It also takes advantage of the hydraulic principle that the more the direction of a moving fluid is diverted, the greater the fore that fluid exerts on the diverting surface. The fluid impact is absorbed along the full length of the vane surface as the fluid reverses itself. The stator is the third bladed member of the converter. During the torque phase, its function is to redirect the fluid flow as it leaves the turbine and reenters the impeller. This assists the impeller rotation and gives a thrust boost to the fluid discharge.

Converter starts operating when the impeller starts rotating, with the engine providing the required input. The impeller creates a centrifugal pumping head or vortex flow. At the same time, the fluid must follow the rotational inertia or the effort of the impeller. These two fluid forces combine to produce a resultant force in the form of an accelerated jet stream against the turbine vanes. The impeller and turbine attempts to act as an effective fluid coupling by featuring curved impeller and turbine vanes rather than a straight radial design. The turbine vanes reverse the fluid direction. The curved turbine vanes provide efficient energy transfer, but the reentry of the remaining fluid thrust back to the impeller, works against the impeller and crankshaft direction. Hence, it is necessary to introduce the stator element to make the converter work. The stator is employed between the turbine, outflow and impeller inflow to reverse the direction of the fluid and make it flow in the same direction as that of the impeller. Instead of the fluid opposing the impeller, the fluid energy now assists the impeller and crankshaft rotation. This results in boosting the rpm of’ the impeller. This allows the impeller to accelerate more and recycle the fluid with a greater thrust against the turbine vanes. The purpose of using the remaining fluid energy to drive the impeller is referred as regeneration gain. The stator is mounted on a one-way clutch. During the torque phase, the stator remains locked and at coupling speed it overruns.

The torque multiplication occurs when the turbine is turning at a slower speed than the impeller and the stator is stationary / reactionary. This is called torque phase, slip phase or stall phase. This sequence generates a boost in output torque. Recycling of the fluid permits more of the impeller input to be used in increasing the jet stream velocity and turning effort of the turbine. By helping the impeller to accelerate the fluid thrust against the turbine, the stator provides the basis for torque multiplication The curved turbine blades absorb the energy from the impeller discharge until the force of fluid is great enough to overcome the turbine resistance to motion. The converter torque is equal to the product of effective fluid force and working radius of the turbine (torque= force x lever arm radius). It is similar to the torque multiplication by gear reduction. The maximum throttle occurs with the engine at wide open throttle (WOT) and zero turbine speed. This is commonly referred as torque rating or stall torque of the converter. For best efficiency, engineering design of the three-element converter keeps the maximum torque ratings within a range of 2:1 to 2 5:1.

During the torque phase, vortex flow is the predominant force in the fluid. Therefore the fluid cycles like a continuous chain from the impeller to the turbine and back to the impeller through the locked stator. This action is continuous until the turbine speed is at nine-tenths of the impeller speed, at which the converter has achieved a speed ratio more than 90%. After a moment at stall the turbine and vehicle starts moving. Once the turbine starts, it becomes easier and easier for the fluid force to drive the turbine and vehicle. The turbine rpm actually starts to gain and approach impeller speed. As the turbine gains the speed, the turbine lever arm absorbs less and less of the fluid force and converter torque output gradually drops. 

The fluid thrust under vortex influence is trying to hit a moving target that is moving away from it faster and faster. Finally when the converter has reached a speed ratio of more than 90%, the converter enters the coupling phase of the operation. The stator is no longer needed and must freewheel with the rotary flow. The vortex effect on the fluid has dropped significantly and the rotary flow is now the main force. The rotating inertia of the fluid mass, impeller and the turbine form a hydraulic lock or bond. The converter is now in coupling phase and the torque ratio is 1:1. When the speed difference between the impeller and turbine is at its minimum, it is referred as coupling phase. It occurs when the torque converter is operating at its greatest hydraulic efficiency. At this point the stator freewheels and there is no torque multiplication.

FLUID COUPLING

Fluid coupling is the simplest form of the hydrodynamic drive consisting of two similar members with straight radial vanes referred as impeller (pump) and turbine. It is used to transmit the power from the engine to the remaining parts of the transmission. Since the fluid coupling is always a major part of the engine flywheel assembly, it is also called fluid flywheel.

The working principle of a fluid coupling can be understood easily with the help of two fans facing each other. When one fan is turned on and the air stream causes the second fan to turn even though it is not switched on. The first fan is the driving member or the impeller and the second fan is the driven member or the runner. This is the simple fluid coupling with air serving the function of fluid.  The figureshows the simple construction of a fluid flywheel. It consists of a two half dough nut shaped shells equipped with interior fins that radiates from the hubs. One shell is mounted on the crankshaft and is called impeller or driving member. The other shell is mounted on the driven shaft and is called runner or driven member. The two shells are very close with their ends facing each other and enclosed in housing, so that they can be turned without touching each other. The housing is filled with liquid / fluid. When the engine drives the impeller it sets up the fluid mass into motion, creating a fluid force. The path of the fluid force strikes on a solid object, the turbine. 

The impact of the fluid jet stream against the turbine blades sets the turbine in motion. With this energy cycle has been completed: mechanical to fluid and back to mechanical. When the impeller spins up, two separate forces are generated in the fluid. One is rotary flow, which is the rotational effort or the inertia of the impeller rotation. The other is vortex flow which circulates the fluid members (it is at right angles to the rotary flow) and is caused by the centrifugal pumping action of the rotating impeller. The lag of the runner behind the impeller is known as slip, and depends upon the engine speed and load. The slip is maximum with the vehicle at rest (turbine stationary) and the throttle open to cause the impeller to start circulating oil. As explained earlier the oil is having both rotary and vertex flow at this time. The oil flies out against the curved interior due to the centrifugal force. The rotary flow starts the movement of the runner. As the turbine begins to rotate and catch up impeller speed, flow gradually decreases because of the counter pumping action of the turbine. This permits the rotary action to become greater influence on the fluid and the resultant thrust becomes more effective in propelling the turbine. Finally, at greater speed ratios over 90% the rotary inertia or momentum of the fluid and coupling members forms a hydraulic lock or bond, and the coupling members turns as a single unit. This is referred as coupling point. In an ideal liquid coupling, the runner would attain the same speed as the impeller, so as to receive all the power imparted by the engine. In commercial designs the runner speed becomes almost equal to that of the impeller only under the best operating conditions, when the efficiency of the coupling is highest.