John
| Thanks for the info on that.
| How do you measure the HP on an engine and what is NM.
To measure the horsepower, which is the rate of doing work, you put some
sort of instrumented load on it - usually a "dynamometer". This is
usually a (big) dynamo or alternator from which the electricity
generated gives a measure of power. Incidentally 1 horsepower is 33,000
foot pounds per minute, in other words a horse can raise 33,000 pounds
(about 15 tons) by one foot in a minute.
NM stands for Newton Metres. A Newton is the metric measurement of
forces (1 N =3D 1 kg.m/s2), and is roughly the force exerted downwards by
a 100 gram weight (at earth gravity). So a Newton Metre is the torque
(force x distance =3D torque) you would get from roughly a 100g weight on
the end of a metre long bar.
It's a bit illogical mixing Horsepower (imperial unit) with NM (metric
unit), but these are the units that people tend to be familiar with in
the context of engines. The metric unit of power is a Watt, usually
quoted in kiloWatts (kW) for engines. 1 HP is about 0.76 kW.
So why measure both torque and power? They are different: torque is a
measure of the force that the cylinders can exert, power is the rate at
which they are delivering this force.
I find the easiest way to visualise this difference is to think of a
steam locomotive that is on the verge of starting: the torque available
from the steam in the cylinders is huge, but the power is zero because
no actual work is being done until the wheels start to turn.
At the other end of the scale turbines have a superb power to weight
ratio because while their typical torque is low, they spin so fast that
they deliver a large amount of power.
Confused? I hope not, but don't worry if you are. I doubt one person in
ten could explain the difference between mass and weight, and one in a
hundred the difference between torque and power.
Christopher Bell
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