KARTING
DYNOMOMETERS
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PART I - What are they?
By
John Copeland
Dyno-tuned,
Dyno-tested, Dyno-flow, Dyno-carbs, Dyno-cams, Dyno, Dyno, Dyno. Who the
hell is this Dyno guy? It seems like he's stuck his nose into just about
every aspect of karting. A quick spin through the pages of National Kart
News will reveal no less than 100 references to the word "Dyno".
But what does it all mean to the average karter? In the next few months
we're going to take a close look at just what a "dyno" is,
what it does, how it works, and how all that affects you. Hopefully in
the process we'll get a feel for what separates a good dyno from a
worthless one, and how you can tell the difference. We'll also
look at what sort of information you can reasonably expect to get from a
good dyno, and how it may, or may not, apply on the racetrack. It's a
lot to cover, so let's get started.
Webster's
Dictionary defines a dyno (more properly a dynomometer) as "a
device for measuring power (in most cases mechanical power) output from
a source." Now that's pretty clear, huh? A more scientific
definition comes from the American Society of Mechanical Engineering (ASME)
which defines a dynomometer as " Any device for measuring the tangential
forces incident to a rotating "mass." In terms of karting
interest, a dyno is typically a means of accurately measuring the
specific output of an engine, either at the crankshaft or somewhere
further down the driveline, for the purposes of comparison. That
comparison might be between engines, or between different carbs on the
same engine, or between pipes, and on and on. The key element is the
term "accurately measuring." Dynos typically apply some sort
of measurable resistance on the engine. This resistance is called the
"load" and it serves as a benchmark, something to measure the
engine's power against. As we will see, there are a variety of ways to
measure this output, and these are dictated, in most cases, by the basic
mechanism of the dyno. As I mentioned before, the engine output in
question can be measured either at the crankshaft itself, or further
down the driveline, so let's look at that element first.
Chassis
dynos have been around for a long time, in automobile manufacturing as
well as in all forms of motorsports. The idea here is just to move the
vehicle, whether its a go-kart or a bigger car, into position on the
dyno, then to drive the measurement mechanism, either off the drive
shaft, or the axle shaft, or the driving wheels. A crankshaft dyno, on
the other hand, picks up its input directly off the engine, sometimes
directly coupled, sometimes via some speed-reduction mechanism. From the
perspective of ease of use on an existing vehicle, the advantages of the
chassis dyno are obvious. No need to remove the engine from the vehicle,
plus it gives you a more realistic look at the output that is really
going to the ground. A chassis dyno can help unravel what is going on
throughout the drive line; in the engine, the clutch, the transmission,
the drive shaft, the differential, the axles, everything. It gives the
skilled operator a lot of latitude in investigating all the driveline
components from stem to stern. But this very flexibility is a drawback
as well. If the operator wants only to evaluate specific engines or
changes to those engines, all the other components just muddy up the
water, so to speak. And, as you can imagine, it can take a lot larger
facility to accommodate a chassis dyno than something suited to just
engine examination. Nonetheless, chassis dynos have found an important
place in motorsports. Perhaps the most significant jump in chassis dyno
technology occurred in 1965 when Ford Motor Company, having been
thoroughly embarrassed by Ferrari at the 24 Hours of LeMans, built the
first "programmable" automated chassis dyno, capable of
subjecting their GT40 prototypes to all the rigors of a 24 hour race.
Mechanical actuators actually operated the gearshift, the clutch, brake,
and throttle pedals, everything. The racecar sat on the dyno platform
and howled up and down through the gears for 24 hours at a time as the
rear wheels drove rollers in the floor to absorb the output. Although no
official figures were ever released, the project was rumored to have
cost Ford more that $5 Million, but it had the desired results. Ford
finished first and second at LeMans in 1966, and returned to win again
in 1967.
Fortunately
for us, karting doesn't require such extreme measures, nor are chassis
dynos all that desirable in our case. Because most karts in this country
use centrifugal clutches, except gearbox karts of course, and because
these clutches all slip to some degree, using a chassis dyno makes it
somewhat harder to sort out what is really happening in the engine.
Small variations in engine output can get "lost" on their way
to the axle shaft. While it is certainly a valid argument that any
changes so small as to not show up at the axle can't very well affect
kart performance, changes are still changes. And the sum total of lots
of little changes may be significant when added up. Those changes can be
good, as in horsepower gains, or bad, but if you can't accurately
measure them, you'll never know. And because we're dealing with such
relatively small power outputs, in comparison to bigger racecars, it's
very important not to lose track of any changes.
That
brings us to the area of shaft dynos. As the name implies, these measure
the output of the engine either directly at the crankshaft, or through
some speed reduction device. Like their chassis dyno cousins, shaft
dynos apply some "load" to the engine output, then use a
variety of means to compare that output to the known load. There are a
variety of ways to apply this load, and each required its own
measurement technique. Probably the simplest and most widely used shaft
dyno system in use in karting is the "Water Brake" dyno. The
basic concept is pretty straightforward. The engine is coupled to a
water pump, generally a 'positive displacement" type where the pump
impeller mechanically forces the water through the pump. A valve is
fitted in the output line of the pump to generate backpressure against
the pump. The pump is mounted so as to allow it to move slightly, as if
it were trying to rotate around it's own input shaft. Once the engine is
running and up in the powerband, the valve is closed slowly to increase
the backpressure on the pump. This causes the engine to work harder to
overcome the backpressure. As the backpressure in the pump increases it
resists the efforts of the engine to drive it. This has two effects; it
makes the engine work harder, slowing it down (or at least preventing it
from gaining RPM, and it tries to rotate the pump housing around it's
own input shaft. This "torquing" of the pump housing can be
measured by something as simple as a spring scale or as sophisticated at
an electronic load cell. A load cell is similar to the sensing device
used in electronic bathroom scales, only much more accurate. The harder
the pump housing wants to twist at a given RPM, the higher the torque
output is. And the higher the torque at a give RPM, the higher the
horsepower. With the throttle wide open, the operator can adjust the
engine RPM by opening and closing the valve on the output side of the
pump. Once the engine has stabilized at the desired RPM, the operator
can then read the spring scale, or the load cell, to determine the
twisting force of the engine at that RPM. Pretty neat, huh? And if all
you're interested in is relative numbers for comparison purposes (is
this carb better, or that one?), that's really all the farther you need
to go. But if you want to convert those output numbers into
"real" torque (and horsepower) figures, you'll have to go a
little farther. Torque is measured in foot/pounds (or inch/pounds, or
some similar unit), and a foot/pound is defined as the amount of energy
required to raise one pound to a height of one foot.
For our purposes, you'll need to convert the output of the measuring
device, be it a spring scale or a load cell, to foot/pounds. The
simplest way of doing that is to locate the measuring device exactly one
foot from the center pivot point of the pump. Then, if the measuring
device indicates a load of 10 pounds, and its located one foot from the
fulcrum point, you have an output of 10 foot/pounds of torque. Can't get
much more direct than that. Of course, if the measuring device is only 6
inches from the fulcrum point, a 10 pound reading will be 10/.5 feet, or
5 foot/pounds. See how it works? But what about horsepower? I thought
you'd be asking. Well, horsepower is a function of Torque and RPM. The
actual formula is (Torque x RPM)/5252.1 = Horsepower. In other words,
horsepower is a derived number, dependent on RPM as well as Torque
(power). I heard it said once that, if you could spin it fast enough,
you could get 100 horsepower out of electric pencil sharpener. But
remember, without a significant amount of torque, you can't drive
anything of any mass. In other words, YOU DRIVE YOUR KART WITH
TORQUE, NOT HORSEPOWER. Later in this series we'll talk about how to
interpret torque output readings from a dyno, and how to take maximum
advantage of what those readings are telling you. I know that everyone
wants to compare horsepower output figures, but the really knowledgeable
folks are looking at the torque numbers. That's where the critical
information is.
As
I said before, there are lots of different ways to configure a dyno, to
provide the load, even to measure the output. For example, rather than
measuring the torque on the pump housing in our example, you could just
as easily measure it as torque on the engine mount plate. That can be a
little trickier, but it works. In the months to come, we'll look at
different kinds of dynos, how they work and the kind of information they
can give you. We'll look at commercially available units and ones you
can build yourself. We'll be contacting manufacturers of dynos suitable
for the karting world and passing on information about their products.
Finally, we'll analyze the kind of information you can get from a good
dyno and how you can apply it to get more out of your karting effort.
Karting
Dynomometers - Part 2
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