Environmental Engineering Reference
In-Depth Information
3.2 Launch and boosting
Virtually the complete impression of vehicle performance is gleaned during the
first couple of seconds of a brisk take-off and acceleration. How smoothly
the vehicle accelerates, whether or not shift events are noticeable and if any dri-
veline shudder or vibration is present contribute to the overall impression. Beyond
the initial launch, and particularly when the automatic transmission torque con-
verter is transitioning out of torque multiplication, the benefit of hybrid boosting
becomes noticeable.
In ISG type of direct drive systems having power levels less than 10 kW, the
boost impact is not noticeable above 3,000 engine rpm. But up to that speed,
boosting by the electric M/G is noticeable and does benefit vehicle acceleration
because engine output torque is augmented. We saw in the introduction to this
chapter how adding engine torque to the driveline dramatically improved the total
vehicle capability, provided the correct matching is employed. If the transmission
and final drive gear ratios are too 'tall', the acceleration will not be as brisk, even
with torque augmentation.
3.2.1 First two seconds
The most noticeable launch feel occurs during the first two seconds when the
automatic transmission torque converter is delivering double the engine torque to
the driveline. Manual transmissions generally require 18% higher transmission gear
ratios to 'make up' for the torque boost of the hydraulic torque converter.
The initial performance feel creates an image of the vehicle in the driver's
mind, an image that defines the particular vehicle, what some would call brand
DNA.
3.2.2 Lane change
Another measure of vehicle performance is lane change during passing and
the attendant need for acceleration varies from either 30 to 60mph or from 50 to
70mph depending on geographical location and driving habits. To illustrate the
vehicle's performance during passing manoeuvres, we take the same Focus five door
and compare its WOT performance in terms of time to accelerate for both of the
speed intervals noted. Table 3.4 summarizes the findings of running the simulation
for three cases: Case (1) 0% grade and single occupant, Case (2) 7% grade and single
occupant and Case (3) again 0% grade but with four occupants.
Table 3.4 Vehicle lane change and passing manoeuvres
Manoeuvre
30-60 mph time (s)
50-70 mph time (s)
Case 1: 0% grade, 1 occupant
6.1
5.9
Case 2: 3% grade, 1 occupant
11.6
21.0
Case 3: 0% grade, 4 occupants
7.0
7.2
