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9)
Speed-Torque Curves – practical implications
Having said all that, let's examine the real life situation of
two different Engines having the same ‘peak torque’ but occurring
at different Rpm levels. One is designed for a Utility Vehicle and the
other for a pleasure to drive Passenger Car.
A close look at the above pair of Torque/Bhp/Rpm curves would reveal that:
i) The Utility Vehicle’s ‘peak-torque’
is designed to occur at 2500 Rpm and consequently, its ‘peak-power’
@ 48 KW occurs at 5000 Rpm.
ii)
Like wise, the Car’s peak torque is designed to occur at 5000
Rpm and therefore, peak Power @ 65 KW at 6000 Rpm.
iii)
Both Engines develop the same torque of ~ 110 NM @ 3750 Rpm and therefore,
the same Power of ~ 42 KW at this Rpm.
iv)
To illustrate how the two different set of Curves affect the ‘feel’
of the particular Vehicle, let’s assume that both are ‘cruising’
at 3750 rpm coz at this point, both produce the same amount of Torque.
v)
Now let’s assume that both vehicles under such a condition hit
an up gradient and consequently, the engine rpm ‘tends’
to drop, say, to 3000 rpm. Consequently, in case of the Car, the Torque
drops too and therefore, to keep it going at the same linear speed,
it’d need a down-shift to keep the engine from stalling eventually
– resulting in a drop in its linear speed over the gradient.
vi)
On the other hand, in case of the Utility Vehicle, the ‘impending’
drop in its engine rpm results in an increase of the torque available
and therefore, by increased throttle opening it’d respond positively
and be able to keep going up the gradient at the same linear speed in
the same/cruise gear.
10)
Conclusion
Since in
case of the Car, coz its peak torque occurs at a later/higher Rpm, it’d
accelerate much better through the gears compared to the utility vehicle.
Further, since the Car’s max power also occurs at a higher engine
Rpm and is higher than the Utility Vehicle’s, it’s ‘top’
speeds will also be higher. This of course assumes that the other factors
affecting ‘speed’ such as their respective wind resistances
and ‘gross’ weights are identical too.
On the other hand, for ‘lugging’ around City b2b traffic,
the Utility Vehicle is going to demand a lot fewer Gear Changes than the
Car – which would be a pleasure to push around on the Highways!
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