Automobile transmission system

Topics: Torque, Gear, Transmission Pages: 22 (2121 words) Published: April 21, 2014
Vehicle Design

Transmissions Part 1
Dr. Alasdair Cairns

Content



Transmission fundamentals
– Principles of gears (revision)
– Road load losses



Gear ratio optimisation
– Performance
– Fuel economy

Transmissions – Overview

Why do we need a gearbox in a road car?

Transmissions – Practical Use
Seven practical reasons why a gearbox is
required:
1. To increase tractive effort when the vehicle is moved from rest 2. To improve hill climbing or descending ability
3. To allow the engine to be operated near to peak torque during vehicle acceleration
4. To allow the engine to be operated near to peak power at the required maximum vehicle speed (“Vmax”)
5.
5 To allow the engine to be operated at the most efficient point for a given vehicle speed (within the gear ratios available)
6. To avoid engine stall at low vehicle speeds
7. To allow the vehicle to be easily driven in either forward or reverse direction

4

© MAHLE

Transmissions – Mechanical Leverage (1)


P T 

where:
P is power (W)
T is torque (Nm)
 is angular velocity (rad/s)




Engine speed ranges are typically limited to 850-7000rpm



Required wheel speeds are 0-1800rpm (road car) or 0-2500rpm (F1)



When the speed is too low or the load too g
p
great the engine stalls
g



The primary function of the gearbox is therefore to maintain the optimum torque and engine speed for a given vehicle condition, within the engine speed range: 1.
1 For a given engine power by gearing down the wheel speed we can increase the available power,
torque at the road wheel
2. By gearing up the wheel speed we can effectively widen the speed range available….

4

© MAHLE

Transmissions – Mechanical Advantage (2)

P1  P2
Driver
(G1)






4

Driven
(G2)



T1 1  T2  2

Mechanical Advantage refers to an increase in torque or force that a mechanism achieves g
q
through power transmission
Power is the product of force and velocity
Example showing a 2.7:1 gear ratio
If we assume negligible transmission losses:
• By reducing the speed by a factor of 2.7 the torque available is increased by 2.7 • Akin to a leverage effect
© MAHLE

Gear Trains
1 z 1

2 z 2

3

z3
4 z 4

Gears 2 and 3 are called idler gears (they are both driving and driven) They do not affect the overall gear ratio but change the direction of rotation of the final gear…..

Transmissions – Cascade Diagram






Data for a current production C-class vehicle fitted with a 5-speed gearbox and 1.6 litre SI engine Such tractive effort curves are commonly used in industry to categorise the performance of the powertrain. The peaks in the curves usually correspond to peak engine torque The traction limit h

Th t ti li it shows th grip li it of th t
the i limit f the tyres (b
(based upon an assumed constant t
d
d
t t tyre f i ti
friction
coefficient of 0.95, which is well above the real world value) Most manufacturers will design to exceed the red line to give higher tractive force throughout the gears......

Vehicle Road Load

1
2
Fd   v Cd A
2


where:
Fd = drag force
 = fluid density
v = fl id velocity
fluid l it
Cd = coefficient of drag
(0.25-0.45)
A = projected frontal area

The vehicle road load is the sum of the effects of:
1. Air resistance: vehicle drag effects, the forces of which increase in proportion to
t v2 and b
d become d i
dominant at hi h speeds. It f ll
t t higher
d
follows th t th power l
that the
loss i
is
3
proportional to v
2. Rolling resistance: due to friction between the tyres and the road (see U-link )
notes)





Ideal values of both can be calculated
In industry, the influence of road load is often carefully measured by producing coast-down curves under tightly controlled conditions ( f
(performed with the
transmission in neutral, with strict rules for weight, tyre condition, weather etc.) Gradient...
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