An engine dyno test is run on a machine called a dynamometer.
Essentially what is done is that the engine is taken out of a
vehicle and installed on this machine which has a method of
causing the engine to have to work against an adjustable load.
One method has a generator as the load. It is called a Schenk
Eddy Current dyno. The engine is run up to a certain rpm, and
then the excitation to the generator field is increased. This
increases the load on the engine. The engineers watch their
instruments to be sure that the engine is able to carry this added
load without losing rpm. Exhaust gas temperatures, coolant temperatures, oil pressure, specific fuel consumption, and engine
emissions are all monitored. But the most important factor monitored
is the torque output of the engine. The torque, or twisting force,
produced by the engine is recorded at all rates of engine revolution,
and, using a formula, horsepower is calculated for each point on the
chart. Engineers are able to see where the engine is having problems
by the shape of the torque curve on the chart. I remember how
the press described the torque curve of one engine. They said that
the engine had "a pleasant, two-stage power delivery," with one
stage between 2000 and 4000 rpm and the second stage between
6000 and 9000 rpm. Only a few years later, these same reporters
were complaining about the horrible "flat spot" between 4000 and
6000 rpm. Another type of dynamometer uses a rotating drum filled
with water. This drum has adjustable vanes to restrict the passage
of water from one chamber to another. This is typical of the roller
dyno that you will find at TuneUp Masters. But there is a 10 to
15% loss in getting the power of your engine to the dyno, so this
type is not used for precision tests. The tiniest dyno I ever saw was
being used by an engineer to measure the stall torque of a little
electric motor being used as a servo in an aerospace application.
The engineer had wrapped a string around a pully attached to the
motor. The other end of the string was attached to a tiny spring
scale. The engineer was measuring inch-ounces of torque. In an
engine dynamometer test, the engineers are measuring hundreds of
foot-pounds of torque. Torque is the reaction of the engine to a
load. One foot-pound of torque is the force produced by a one-pound
load on a one-foot lever arm. Usually a spring scale is not used to
measure the load. A piezo-electic load cell is used instead. It has
a crystal that produces an electric signal proportionate to the
distortion of the crystal. |
