Power transmission

Clutches

  • Main clutch (in drive train)
    • Between engine and manual transmission and they are needed for:
      • Starting phase to bridge the speed difference from zero to the lowest engine speed (=idle speed)
      • Interrupting the load, while changing the gear (shifting) in mechanical transmission
    • All-wheel drive for connection of a drive axle
  • Shifting clutch (in transmission): Shifting in gear train (manual transmission, gear step in automatic transmissions).

Caloric energy equals to integral of variation of mechanical power which is equal to torque times velocity variation


Variations

Motivation:
Combustion engine can't transmit torque without any velocity, therefore we need a starting clutch. If we did not have any transmission, there would not have any efficiency loss and we would always be at the highest gear —> direct connection between motor and shaft. Transmission has at least 4 gears, but there is always a speed gap.



Also useful for electric motors since permanent magnet electric motors should be disconnected from the DT to increase overall efficiency. In order to shift the maximum efficiency driving point to another velocity.

Construction
When someone presses the pedal, it will move the ejector onwards, which actuates the disk spring. The disk spring is connected to the pressure plate, making it release the friction lining. This will cut the transmission of power from the motor to the power train, making it possible to 1- start the motor since there will not have any load, 2- change gears


  • High energy consumption in clutch during coupling (sliding), equal to the energy needed to accelerate the inertial mass at start.
  • Temperature difference between friction side and back side of pressure plate and flywheel → conical deformation → Effective friction surface is reduced
  • THERE IS NO CONVERSION OF TORQUE


  • Load capacity
  • Start on level plane: 36 Nm/cm2
  • Start on drive up-hill: 300 Nm/cm2


  • Diameter
  • Small diameter
    • (+) build size
    • (+) costs
    • (+) inertia of clutch plate
    • (+) weight
  • Large diameter
    • (+) torsion damping curve
    • (+) fading stability
    • (+) coating durability
    • (+) pedal force.

Friction clutch
Common main clutch. Force F presses the clutch plates agains a friction disk. Therefore the Torque the clutch makes is equal to the force times the friction coefficient times the mean friction radius times the number of friction pairs.


Clutch pressure:

  • Dry clutch passenger vehicle: p < 20 [N/cm2]
  • Dry clutch heavy vehicle: p < 8
  • Wet clutch heavy vehicle: p < 5

Dimensioning
ƒ(transferable torque, thermal strain)
Design of clutch to have Mc max ~ 1.5 x Me max


Types:

  • Wet clutch - friction coefficient decreases, therefore we need more pressure and more friction pairing
  • Dry clutch - higher friction coefficient, therefore less presure

Dynamics

  • Idle operation
    • oscillation of motor pistons excites the springs installed within the clutch
    • model it with a spring and a friction damper
    • motor torque accelerates the inertia times acceleration of delta + damping times the deformation (turning angle) + stiffness times the difference of the turning angle
  • assuming a oscillation of the deformation angle or the torque takes place, just multiply the initial by e^jwt. Derive it twice and then j^2 = 1 (complex numbers)


  • Driving elasticity
    switches the operation zone to the resonance line (around 40-70Hz) —> it has a tradeoff of resonance and transmitting high frequency excitations
    • Principle of operation for driving elasticity:
      • allows for a rotational angle between transmission input shaft and clutch plate
      • allows the generation of friction energy from oscillation angle
      • friction dampens the vibrations from the drive train (rumbling, rattling)

Using dual mass flywheel
more comfortable, increase of dampimp

Flow clutch (hydraulic rotation converter)

  • Input shaft connected to pump which rotates and, due to the centrifugal forces, throw the oil in it outwards. This oil will make the turbine rotate, transmitting the torque to the output shaft. The pump spins faster than the turbine.


  • Starting clutch for heavy vehicle

  • For automatic transmission
  • Used as a retarder - like a hydrodynamic brake used for trucks to brake just using the shear stress on the fluid because it will heat up. Needs to cool the oil and put it again on the clutch

Equation Mc = ke' x n^2 x d^5


Clutch factor K'e
Geometries:

  • Routing torus (highest)
  • Bulging disk
  • Storage room (lowest)


PROS

  • Low space requirement
  • Diamater adjusted to aplication
  • Good damping from ICE speed oscilations


CONS:

  • low efficiency
  • high torque at stand still —> need to cool the oil
  • continuous operation —> slip of 3%

Haldex Clutch

  • Only for AWD placed behind the transmission and the rear
  • Further development of the viscous clutch
  • A bunch of lamellas connected via toothing. The end of one of the shafts is tilted, so that when the shafts are spinning in different velocities, it presses a hydraulic piston that presses all the plates and connects the shafts and transmit the torque to the other axle.

During curves, is not good to have a haldex clutch becase one axle will be turning with different velocities on purpose, that's why there is a valve to regulate it

Gearbox

Mechanical stepped GB

  • FD - driving force at max power for gear i
  • Gap need to be bridged by the friction clutch. If too big, then too much heat and losses
  • trasmission ratio i = i[gx] x i[d] (typical value for id is 4 and for ig of 1st gear is also 4)

Normal driving gear: tangent point of Fd and gear before 0%
Top speed gear: tangent point where it intersects with 0%
Resting gear or eco gear: tangent point after. 0%


Truck: up to 16 gears
passangers: up to 9 gears -> automatic
Input/output/counter shaft with cog wheels



Selecting gears

  • Geometric gear factor: dividing each consecutive gear ratio you always get the same value= gear step factor (see formula)
  • Progressive gear factor: i increases from last to 1st gear (see formula)

Planetary transmission

  • Planet gears
  • Sun gear
  • Sun gear shaft
  • Planet carrier
  • Ring gear

Kutzbach plan

  • lines from the center and edges of the planet gears, assume velocity and than connect the points. 2 speed given (input and fixed) and one resulting. conect the h of the points to the origin and extend these lines until the radius. this will give the sizes for determining i

PROS:

  • Compact
  • Teeth are in continuous interaction (but this brings friction and splashing, reducing efficiency)
  • Shifting under load with the main clutch but this requires a brake for each gear
  • Shifting without torque disruption (good for automatic transmission)

CONS:

  • expensive and lower efficiency

Flow trasmission

Speed manual transmission with fixed countershaft

  • has dog clutches
  • needs shifting clutches
  • drive shaft moves
    cog wheels

Speed manual transmission without fixed countershaft

  • needs shifting clutches
  • differential on the right side
  • escadinha
  • FWD
  • Throughput from the countershaft

With fixed countershaft for coaxial throughput

  • escadinha reversa
  • RWD with engine in the front

Unsynchronized transmission

  • needs a synchronizing element
  • needs sliding shift sleeves
  • selector fork moves the sleeves back and forth
  • blocker ring guarantees everything is rotating with the same velocity and teeth are in position for coupling.

Phase 1 - bring synchronize ring to position
Phase 2 - start sync with some friction until its locked
Phase 3 - bring back the sync ring

Hydrodynamic torque converter

  • Pump
  • Turbine
  • Idler brings back the fluid to the pump. Idler is fixed to housing

Mo = Mi + ML
i = input
o = output
L - idler


converter factor k


Mi = ki x ni^2 x di^5


SLIP OF AROUND 10%, design point around 30%


Graphs - Efficiency depends on the ratio of po/pi=(kono/kini). As Ko decreases with lower slips, efficiency decreases.

Trilok transmission

  • Idler no longer fixed, but connected to transmission housing over a free-wheel F
  • with friction clutch

PROS:

  • At low engine rotations, the transferred torque is very small, so stalling the engine is not possible.
  • Raising the engine rotations for startup brings at start rotations nI highest torque MO atnO =0!
  • No wear, no sliding friction.


CONS:

  • Price and efficiency.
  • Inferior braking effect compared to rigid engine–transmission connection. - Push-start (without additional technical effort) not possible.

Automated stepped transmission

Fully automatic planetary transmission

  • Clutch - converter + planetary gearbox
    Start: Clutch – converter (no wear, additional torque elevation)

  • Shifting: Purely tractional


    • over clutches and brakes
      (no load detachment, no tractive effort disruption!),
    • electrohydraulic actuation,
    • free wheel for the definition of the direction of rotation or support of torque

  • In comparison to manual transmission, there is


    • higherfriction
    • heavieroilchurning

Shifting programs

  • Best gear is electronically determined (position of selection lever, position and positioning speed of throttle valve) hydraulically put in place by electric valves.
  • Shifting program is adjusted to driving style
  • Possible to hold gear while cornering (shifting is suppressed while cornering depending on bquer and v !). Therefore increase in stability while cornering.
  • Additionally to an automatic mode is also a manual mode (Tiptronic) possible. Here, there is a second shift gate for the selection lever (tipping lever forwards or backwards) or buttons on the steering wheel. Control unit only allows to shift, when rotational speed allows it !

Fully automatic countershaft gearbox

  • Start: Automatic friction clutch
  • Shifting:
    • Load disruption over auto. dry clutch (Start clutch),
    • automatic accelerator release,
    • automatic actuation of shift fork.
  • Efficiencies: Similar to partially automatic transmissions, slightly better, possible through better choice of shifting points.
  • CONS:
    • Load disruption in order to shift,
    • tractive force interruption!

Dual clutch transmission

  • Power train divided in two chunks, even and odd gears.

  • On startup gear one is already in place, friction clutch couple and power is transmitted. When shifting to gear 1 clutch one is released and gear 2 is coupled. Instead of parallel shafts, we can one shaft going inside the other


  • PROS:


    • no traction force interruption
    • better efficiency than with planetary gears
    • less hydraulic than planetary
  • CONS:
    • more hydraulic than conventional contershaft automatic
    • more moving parts

Hybridization

  • P0 - Belt-driven starter
  • P1 - Crankshaft mounted starter
  • P2 - On gearbox input
  • P3 - On gearbox output inside or outside
  • P4 - On axle
  • P5 - On wheel

Stepless mechanical transmission - continuous variable transmission CVT
no need to change gear because the transmission is done via a belt that allows every combination of velocity and torque below the curve

  • variator 2 plate sets
  • pressing/releasing plates to move the belt/chain
  • it can run in 3 modes, compromise, consumption efficiency and performance
  • planetary gear + clutch to lock ring gear for reverse
  • push link belt or tension link chain

PROS

  • infinit choice of gear ratio

CONS

  • limited torque due to belt

Casdan Shafts

  • Between differentials and wheels
  • avoid opperation around critical speed range - 1 at gaussian. frequency equal to sqr stiffness/mass
    • hollow shaft
      actual resonance around 0.6-0.8 theoretical

Differentials

Bevel gear drive axle differential
velocities are different, but torque is equal

  • trasmission sends torque to housing
  • shafts from wheels: suns
  • top and bottom are planets

Joints

  • single universal joint
    • Avoid cardan error: Possible by using two joints
  • double joint
    • attainable, enough for the turning angle of driven wheels.
  • homokinetic joint
    • Torque transfer through balls in symmetry plane.
    • Choose guide surface of balls so, that they are in symmetry plane for every bending angle.
  • homokinetic tripod-type
    • constant velocity
  • special designs (rubber)
  • Rzeppa type
    • constant velocity

Locking differential
deltaF is limited by friction in transmission, not by miu,max !

  • Partial self-locking
    • with increased friction
  • Full self-locking
    • worm gear
    • free wheels

Centre differentials - Power divider
Torques are not equal

  • as a bevel gear transmission
  • one planet for both drive shafts
  • torsen