RV FEATURE
The terms ‘power’ and ‘torque’ are interrelated
but often misunderstood. In the tow vehicle
context, power is the ability for the engine to
perform a certain amount of work over a given
length of time. If you want to drive fast, you need
a lot of power to do so. Until recently that ability
was measured in horsepower.
But if you need to climb a steep hill, your vehicle
needs a lot of torque – you only need a lot of
power if you want to climb that hill quickly.
The concept of measuring power stems (in
1702) from a Thomas Savery. He felt it feasible
to compare the work done over time by horses’
ability to lift weight. Eighty years later, engineer
James Watt used that concept to help market his
new steam engine (a working replica of which is
in Sydney’s Power House museum).
James Watt determined that a typical brewery
horse could lift 33,000 pounds (14,960 kg) 30 cm
per minute. It was later found that this was too
low and also that the horse’s peak power for a
short time is many times higher.
By comparison, humans can produce 1.2 horse
power (hp) briefly and about 0.1 hp indefinitely.
Usain Bolt needed to exert 3.5 hp for his
9.58-second 100-metre world record in 2009.
Horse power can now also be expressed in watts:
in much of the world, 1 hp is about 745 watts.
Americans and the French regard it as marginally
less (presumably having slightly inferior horses).
Torque, however, is a force that rotates things
and its origin goes back to Archimedes! A car
engine’s pistons create torque that spins the
crankshaft, i.e., a force is applied at a distance,
so measuring torque involves a distance and an
applied force.
TORQUE FOR TOWING
In the context of tow vehicles, power is a
combination of torque and engine speed. A heavy
truck’s engine typically produces a great deal of
torque, but because it does so only at low engine
speed (as compared with cars), it does not have
or need (for example) the power of a Ferrari that
revs up to 8000 rpm.
To enable that truck to use the available power
to propel it on a flat road at 100 km/h – or climb
a steep hill at far less speed – gearing is used
to increase the effective torque. This is much like
using a lever to lift a heavy rock – you move the
top of that lever a long way, and the bottom of the
lever only moves a centimetre or so. The power
you exert, however, remains much the same, but
the torque is much greater.
The supreme example of the difference between
power and torque is the BMW i3’s electric engine.
Its power is only 114 kW (152 hp) but it produces
“POWER IS A COMBINATION OF
TORQUE AND ENGINE SPEED.”
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