Page 119 - Automotive Engineering

CH AP TER 5 .1 Transmissions and driveline

Engine Engine
speed speed
Engine speed at upshift
2nd
Engine speed
3rd 4th 5th gear
after shift

Constant speed Varying speed
1st gear
gear increments gear increments
(progression could also be linear)
Vehicle speed Vehicle speed
Fig. 5.1-3 Engine speed vs vehicle speed for differing gear ratio progressions.

All of these factors will influence the selection of the gear the lines of constant fuel flow to indicate the max
ratios in practice and possibly cause a compromise torque line.)
between the calculated, ‘ideal’ ratio set for a given car Taking some of the vehicle transmission details as:
and what can be used on an existing vehicle.
Final drive ratio 4.2
Example of the considerations in matching a transmission Fifth gear 0.75
to a vehicle Fourth 1.0
For this example, we will look at some of the factors Third 1.4
which would need to be considered when designing
a gearbox for a road car, in this case a large 4 4. Plotting the engine conditions for 120 km/h (motorway)
Consider the rolling resistance of the 4 4 vehicle in and assuming a loss of 5% in the transmission system
Fig. 5.1-2. Taking a rolling radius of 0.375 m for the give the engine conditions shown on the graph for the
tyre, the torque required at the wheel for any road different gears. The required tractive force at this speed
speed within the range can be calculated. Consider is 1100 N; this equates to a torque of 413 Nm (total)
Fig. 5.1-4 – this is a fuel consumption chart for a large required at the wheels. In theory, this would be a nominal
petrol engine. (A line can be drawn through the top of 103 Nm at each wheel in the case of our 4 4 example.

300
18 20 24
16 22
14
12 30 32
29 31 33 39
250 9 10 37 44
8 41
27 36 38 42
35 47
26 28 34 40 43 45
Further extremes
of ratio choice
200 17 19
Engine torque (Nm) 150 6th gear 1 5th gear
15

100 11 4th gear
7 3rd gear
5 6
4
50
3
2
0
0 1000 2000 3000 4000 5000 6000
Engine speed (r.p.m.)
Fig. 5.1-4 Chart of fuel (mass) flow for a large petrol engine – also shows engine conditions for a range of gear ratios.

112