The How’s and Why’s are critical in writing specification and
averting failure. In the long-game of 8-12 years of living with a truck, it
really does pay to get it right on the front end. Retroactive adjustments in
specification (through refit and/or reprogramming) can cost many times more
than if it was spec’ed properly on the front-end, or worse yet, compromise the
longevity and usefulness of the truck in the first place.
Today’s topic: Power
Engine manufacturers will talk about 2-basic engine ratings,
torque and horsepower (throw in “torque rise” if you are talking about big
trucks). There are lots of misunderstandings about how these two interrelate. This
often this causes misuse of the terms and they aren’t interchangeable. An
executive summary of the terms and what they mean? Torque is a physical
measurement of how much effort an engine can produce, whereas horsepower is a
calculation of how much work that effort can produce over a period of time.
When engines are being developed or optimized, they are installed
on an instrument called a dynamometer. In the crudest sense a dynamometer is a
device that can take something that makes effort while in radial motion (IE: an
engine’s rotation) and measure its output in terms of force. This is done by
placing a fluid coupling between the engine’s output flange and a scale. On these
devices the amount of “coupling” between the engine and the load can be varied,
allowing the engine to run at maximum capability across a variety of rotational
speeds. The effort produced is then measured though a system of levers and transducers
to determine the force the engine makes, usually measured in lbs./ft in
non-metric speaking countries.
So, let’s say you are running an engine at full capability, but
stalling it with the coupling so it is only capable of turning 2000 RPM. Your dynamometer
has an effective 3-foot lever which it measures force from, and at the end of
that lever you are measuring 85lbs of effort. In this scenario our test engine
makes 255 lbs./ft torque (potential twist) at that rotational speed (85lbs X 3
ft).
Regarding horsepower: the nature of any internal combustion engine
is that it will make peak torque at a speed that is less (sometimes
significantly) than it is capable of turning. After an engine reaches peak
torque, the effort it can produce will drop off quickly as engine speed
increases, but as the engine is turning faster it makes more “power” anyway
(remember, work & time). But if this same test engine is still generating
160 lbs./ft of torque at 4500 RPM, we can do some math a calculate that it is
generating 137HP {(160X4500)/5252} at that speed. Horsepower then is a relative
measurement of the amount of work an engine can get done by applying effort
over time. These are hypothetical figures produced by a hypothetical engine,
but to put it into perspective this is somewhat typical of what your mom’s full-size
station wagon would have produced in the mid 1970’s, so this is kind of a “just
get it done” engine in the passenger car world.
Segueing to a big-truck engine we will find that the engine speeds
drop dramatically, and at the same time torque ratings rise a lot. These are
large engines and they turn slowly. There are a variety of technical reasons
for this, but for expediency we will say it is easier to control the dynamic
forces associated with their big, heavy internal parts and combustion processes
at lower speeds, and the lower speeds reduce fuel consumption.
Torque-rise is the difference (in percentage) between the maximum
torque an engine will produce, and the amount of torque it is producing at its
maximum rated horsepower.
Let’s say our 12.9 liter diesel in the shiny new Kenworth we just
bought is producing 410HP at 1850 RPM (kind of a typical mid-upper range fleet
rating). I won’t bore you to tears with the math, but that means the engine is
producing 1164 lbs./ft of torque at 1850 RPM. Now let’s say this same engine
will produce its peak torque at 1300 rpm, and that figure is 1450 lbs./ft. This
gives you a 25% torque-rise (25% more effort available at the torque peak than
at HP peak).
Why is this important? Remember, the slower an engine turns, he
less fuel it consumes (Fewer rotations mean fewer firing sequences, fewer times
filling the cylinders, fewer times injecting diesel to cause combustion), so
there is incentive to want to turn them as slowly as practical while
maintaining sufficient power. So, while it is possible to spin this engine at
1850 rpm all day, it isn’t cost effective. You need access to all that power to
accelerate the load, but not to cruise or maintain speed.
If however we choose to gear the truck so the engine is turning
1500 rpm at its optimal road speed, we are above the speed at which the engine
makes maximum effort (1300 RPM), but below where it makes maximum power (1850
RPM). That allows us to optimize gear selection so that as our driver starts
into a hill at cruise, the truck will slow (and so will the engine) but likely
only to the point that the torque has increased enough to counter the
additional load (due to torque rise). You’ve used the available torque-rise as
a buffer to counter the truck’s tendency to slow-down on hills.
Hopefully this has given you a better understanding of
interpreting the technical-ese associated with gearing and engines. Do it right
and both your owners and drivers are happy. You burn less fuel so your costs
are less and the driver isn’t having to downshift at every small hill. Next time
you shake his hand, you may not feel calluses from all the shifting he’s had to
do!
© 2018 D.W. Williams
