Transmission Systems

Purpose

The clutch connects and disconnects the engine from the transmission. It lets the driver (1) start the engine without load, (2) engage the vehicle smoothly from rest, (3) change gears, and (4) protect the drive line from shock.

Single plate dry clutch

 Engine flywheel   Clutch plate     Pressure plate  Cover
 |==============|  |====lining===|  |=======|=======|
 |  (friction   |<-| disc, hub   |<-| springs push  |
 |   face)      |  | on splined  |  |  plate        |
                    | gearbox in- |  [release lever] <- release
                    | put shaft   |                    bearing

Main parts

• Flywheel: drives the clutch cover and gives one friction surface.
• Clutch (friction) plate: steel disc with friction linings riveted on both sides, with a hub sliding on the splined input shaft of the gearbox. Torsional (damper) springs absorb shocks.
• Pressure plate: a heavy cast-iron ring pressed against the clutch plate by springs.
• Springs, release levers, release (throwout) bearing, clutch pedal linkage or hydraulic cylinder.

Working

1. Engaged (pedal released): springs clamp the clutch plate between the flywheel and pressure plate; torque is transmitted by friction to the gearbox shaft.
2. Disengaged (pedal pressed): the release bearing pushes the levers, which pull the pressure plate back; the plate is free and no torque is transmitted.

The torque transmitted: $T = \mu W R_m n$, where $\mu$ is friction coefficient, $W$ the axial force, $R_m$ the mean radius and $n$ the number of friction surfaces (2).

Diaphragm spring vs coil spring clutch

(a) Manual gearbox

A gearbox gives different gear ratios so that torque and speed can be matched with road load, and provides neutral and reverse.

Sliding mesh: gears on the main shaft slide along splines to mesh with the counter shaft gears. Straight spur gears are used and mesh only when the gears are stationary, so the driver must "double-declutch"; there is noise and wear.

Constant mesh: all gears on the main shaft are always meshed with the countershaft gears and spin freely on the main shaft. A dog clutch slides on the splined main shaft to lock the chosen gear. Helical gears are used, so it is quiet and strong, but the dog clutch speeds must still be matched.

Synchromesh: a constant mesh box with a synchroniser (blocking ring with a cone) before each dog clutch.

 Input -> [Constant gear] --> counter shaft
              |                |  gears 1,2,3 (always meshed)
 main shaft:  gear 3   gear 2   gear 1
              [sync ring + sleeve] [sync ring + sleeve]
 Shift fork moves the sleeve

When the lever is moved, the sleeve pushes the cone of the blocking ring against the gear cone; friction equalises the speeds, and only then does the sleeve slide on the dogs of the gear, so shifting is smooth and silent.

(b) Transfer case

A transfer case is a gearbox fitted on a four-wheel-drive (4WD) vehicle after the main gearbox. It takes power from the transmission and splits it between the front and rear propeller shafts through gears or a chain. Many have a high range (1:1) for road and a low range (about 2.5:1) for off-road, extra torque multiplication, and some have a centre differential or clutch to avoid wheel windup on hard ground.

Torque converter

A fluid coupling that also multiplies torque; it replaces the clutch in an automatic transmission.

 Engine side      Turbine       Stator
 Impeller (pump)  (to gearbox)  (one-way clutch)
  )) vanes ((  ->  )) vanes ((  -> )) vanes ((
  <------------------ oil circulates ------------>

Three elements

1. Impeller (pump): welded to the housing and driven by the engine; the vanes throw the oil outward.
2. Turbine: connected to the gearbox input; the oil leaving the impeller strikes its vanes and drives it.
3. Stator: fixed to the housing through a one-way clutch, located between turbine and impeller.

Torque multiplication: at low turbine speed (car starting) the oil leaving the turbine moves against the impeller direction. The stator vanes redirect this oil to flow in the direction of impeller rotation, so the oil adds energy to the impeller and the turbine torque becomes higher than the engine torque (about 2 to 2.5 times at stall). As the turbine speed approaches the impeller speed (the coupling point), oil leaves the turbine in the correct direction, the one-way clutch frees the stator, and the unit acts as a plain fluid coupling (multiplication ratio of 1).

Lock-up clutch: at cruising speed a friction clutch locks the turbine to the impeller housing. It removes slip (about 2 to 5 %), improving fuel economy and reducing heat.

Gear changes

• A planetary gearset (sun, planet carrier, ring gear) gives different ratios when one member is held by a brake band or a multi-plate clutch and another is driven or locked. By changing which members are held, several forward ratios and reverse are obtained.
• A hydraulic control unit (valve body) uses oil pressure from the pump. A governor sends road-speed pressure and the throttle (or TV) valve sends engine load pressure. The shift valves compare them and apply or release the clutches and bands, so up-shifts and down-shifts are automatic.
• Modern units use the ECU and solenoids instead of the governor.

Advantages are smooth driving and easy operation; disadvantages are lower efficiency than manual, high cost, and complex service.

(a) Continuously variable transmission (CVT)

A CVT provides an infinite number of ratios between two limits, without fixed gear steps.

 Engine -> [Primary pulley]==steel belt==[Secondary pulley] -> wheels
            (moves apart/close)           (moves close/apart)
 Low speed: primary small radius, secondary large radius
 High speed: primary large radius, secondary small radius

• Two conical (V) pulleys, each with one movable half, connected by a steel push-belt or chain.
• Hydraulic pressure (controlled by the ECU) changes the width of the pulley grooves. When the primary pulley closes, the belt rides at a larger radius on the primary and a smaller radius on the secondary, so the ratio decreases (higher speed).
• The engine can then stay near its best efficiency speed.
• Advantage: smooth, no shift shock, better economy.
• Limitation: belt slip and wear limit the torque capacity; the "rubber-band" drone feel; costly to repair.

(b) Dual clutch transmission (DCT)

A DCT has two clutches, one for odd gears (1, 3, 5) and one for even gears (2, 4, 6, R), each with its own input shaft (a hollow shaft over a solid shaft).

 Engine -> [Clutch A: odd gears 1,3,5 ]
        -> [Clutch B: even gears 2,4,6 ]
 One gear engaged, next gear pre-selected

• While one gear carries power, the next gear is already selected on the other shaft. At the shift, clutch A opens as clutch B closes, so there is almost no power interruption (shift time under 100 ms).
• Actuated electro-hydraulically by the transmission control unit (TCU); wet clutch for high torque, dry clutch for light cars.
• Advantage: fast shift and good efficiency, like a manual with automatic convenience.
• Limitation: low-speed jerk and clutch overheating in traffic, complex and costly.

Given: $m = 1300$ kg, $T_e = 150$ N m, $G_1 = 3.8$, $G_4 = 0.8$, $G_f = 4.1$, $\eta = 0.9$, $r = 0.30$ m, $f = 0.015$.

(i) Wheel torque in first gear

$$T_w = T_e\, G_1 G_f\, \eta = 150 \times 3.8 \times 4.1 \times 0.9 = 2103.3\ \text{N m}$$

(ii) Tractive force

$$F_t = \frac{T_w}{r} = \frac{2103.3}{0.30} = 7011\ \text{N}$$

(iii) Initial acceleration

Rolling resistance:

$$F_{rr} = f\, m g = 0.015 \times 1300 \times 9.81 = 191.3\ \text{N}$$ $$a = \frac{F_t - F_{rr}}{m} = \frac{7011 - 191.3}{1300} = 5.25\ \text{m/s}^2$$

This is the engine-limited value; the real value is smaller because of drag, rotating inertia and the limit of tyre grip (about $\mu g \approx 7$ m/s² on dry road for a driven axle carrying only part of the weight).

(iv) Vehicle speed at 3000 rpm

$$v = \frac{2\pi N r}{60\, G\, G_f}$$

First gear:

$$v_1 = \frac{2\pi \times 3000 \times 0.30}{60 \times 3.8 \times 4.1} = 6.05\ \text{m/s} = 21.8\ \text{km/h}$$

Top gear:

$$v_4 = \frac{2\pi \times 3000 \times 0.30}{60 \times 0.8 \times 4.1} = 28.7\ \text{m/s} = 103.4\ \text{km/h}$$

Answer: $T_w = 2103$ N m; $F_t = 7011$ N; $a = 5.25$ m/s²; $v = 21.8$ km/h (first) and $103.4$ km/h (top) at 3000 rpm.