Topics > Horsepower and Torque
Topic - Horsepower and Torque
one should you care about?
The topics of horsepower and torque can be
confusing. We use these yardsticks to compare engines but what do
they really mean, how are they related and which one is more
important? An old saying is "torque wins races,
horsepower sells cars". It implies the uninformed public is
incorrectly focused on horsepower. Is that so?
fact horsepower and torque are entirely different things and are
measured using different units.
is a measure of twisting force, pure and simple. Torque includes no concept
of motion or time. This is easily understood by anyone who has used
a torque wrench. A torque wrench can measure twisting force on a
bolt that is stationary or turning. In the US torque is normally expressed in foot pounds.
is created any time opposing forces are applied at different points
along a lever arm. Imagine
a weightless one foot long lever arm connected at one end to a
point. If we apply a one pound force at the other end of that lever
arm a one foot pound twisting force will be exerted (one foot pound of torque).
If we apply a two pound
force then a two foot pound twisting force will be exerted.
exert torque at the crankshaft. That torque is coupled through the transmission, driveline
and ultimately to the drive axle. The twisting force at the drive
axle is converted to a linear force where the tire tread meets the road
surface. The linear force is measured in pounds and pushes the car forward.
can easily calculate the linear force that
accelerates the car knowing only the torque at the drive axle and the
diameter of the tires.
force = Axle Torque / Tire Diameter / 2
torque exerted by the engine varies with rpm and is typically
expressed as a curve. The curve is the collection of instantaneous torque measurements at each rpm point.
driveline losses, the axle torque and linear force that pushes the car
proportional to the crankshaft torque for a given gear. That means
that, for a given gear and neglecting air resistance, the car will
always accelerate at the same rate whenever the engine is producing
a given torque. It doesn't matter what RPM the engine is
turning or the speed of the vehicle. A perfectly flat torque
curve would allow the car to accelerate the same at any
the real world torque curves are not flat, they build to peak and
then taper off at high RPM. A car will accelerate hardest in a
given gear when the engine is operating at its torque peak.
that we can figure out how hard the car is accelerating and horsepower doesn't even enter into
is a measure of the rate at which work can be performed. Motion and
time are integral to horsepower which is typically expressed
as foot pounds per minute in the US.
this case work has nothing to do with your boss. Work means moving
an object in opposition to a force (that may remind some of their
boss). Raising one pound a
distance of one foot is one foot pound of work. Raising one
pound a distance of two feet is two foot pounds of work.
combines the concept of work with time. In the 1700s James Watt of
steam engine fame took some measurements and concluded that an average horse could
lift a 550 pound weight one foot in one second. Watt defined
one horsepower as 550 foot pounds per second or 33,000 foot pounds
can show a direct relationship between torque and horsepower.
Remember our one foot lever arm with the pivot at one end and one pound
force at the other? If it rotates about the pivot one full revolution in one
minute it will move that one pound 2*pi*r or 6.28 feet per minute (think about a
point on a tire tread rotating about its axle). If it rotates at 5252
RPM it will move that one pound 6.28 * 5252 or 33,000 feet per
minute, exactly one horsepower. Hence the relationship
between horsepower and torque is defined as follows:
= torque * RPM / 5252
is a linear relationship and says that
horsepower will rise in lockstep with RPM and torque. The horsepower
produced by an engine varies with rpm and is typically
expressed as a curve. The curve is simply the collection of
instantaneous horsepower calculations at each rpm point.
review we know that a) axle torque converted to linear force
accelerates the car b) for a given gear the acceleration will be
greatest at the engine torque peak and has nothing to do with RPM or
speed c) horsepower is a calculated function of torque and
horsepower doesn't matter, right? Wrong! Read on.
multiplication - The
transmission is a torque multiplier. Reduction gears are employed to
make the drive shaft rotate at a slower RPM than the crank shaft
(ignoring overdrive). These reduction gears cause a similar increase
in the drive shaft torque.
an example a hypothetical two-speed transmission might have a first
gear with a 2:1 reduction gears and a second gear with 1:1 gears.
Let's connect that transmission to a an engine that can produce 100
foot pounds of torque at the crank shaft throughout the RPM range.
1:1 second gear rotates the drive shaft at the same speed as the
crankshaft and it will exert exactly the same 100 foot pounds of
torque (neglecting frictional losses). However the 2:1 first
gear rotates the drive shaft at half the speed of the crankshaft but
it will exert 200 foot lbs of torque, twice the torque of the crank
shaft! That means first gear will accelerate the car twice as
hard as second gear.
lies the key to why we care about horsepower. Through reduction
gears maximum drive axle torque for a given vehicle speed is
generated when the engine is operating at its horsepower peak and
not its torque peak (they are rarely the same).
an example recall our car with two speeds, flat torque curve, 100
foot pound engine. Imagine it has a 1:1 final drive ratio that
propels the car at 107 mph (makes the math simple) at 1500 RPM in
second gear using 24 inch diameter tires. We can also drive the car
107 mph at 3000 RPM in first gear. The engine is producing 100
foot pounds of torque in both cases. Which one accelerates the
answer of course is first gear. The following table uses the
formulae presented above and illustrates the value of torque
a result of torque multiplication a car will accelerate fastest at
any given MPH when a gear is selected that puts the engine at its
horsepower peak and not its torque peak.
This is one reason why small high-revving engines can often beat
large low-revving engines. Despite a lower crankshaft torque, the
high-revving engine can deliver greater drive axle torque through
there it is, torque and horsepower. It seems that horsepower is good
for selling cars AND winning races.
Moreland - May 2002
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