How is engine power measured?
Engine power is measured with a device called a dynamometer.
Dynamometers
are so named because the earliest versions were literally a
dynamo (i.e., electrical
generator) connected to the engine crankshaft. Engine power
output was measured by
converting it to electricity and measuring the electrical
energy. Most modern
dynamometers use a fluid brake to load the engine and measure
its power. Dynamometers
are sometimes referred to by the nickname “dyno.”
There are two major types of dynamometers used to measure
automotive engine power:
-
Chassis dynamometers
-
Engine dynamometers
The difference between a chassis dynamometer and an engine
dynamometer is that the
chassis dynamometer loads the engine at the drive wheels of the
vehicle (through
the drivetrain) and the engine dynamometer loads the engine at
its flywheel.
The problem with measuring engine power from the drive wheels
is that all the
drivetrain components (transmission, differential, etc.)
consume some of the engine’s
power. The power you measure at the wheels is considreably less
than the power
produced by the engine. To determine the engine’s actual power
output with a chassis
dynamometer is very difficult because you have to calculate (or
estimate) the
drivetrain’s parasitic power consumption, and add that to the
measured power at the
wheels. Drivetrain losses are usually estimated as a percentage
of the engine’s net
power output.
Drivetrain power losses can vary significantly from one test
run to the next. The
type, quality, and condition of the drivetrain components, the
temperature of the oil
in the transmission and differential, and even tire temperature
and pressure can all
affect drivetrain power loss. If you’re doing serious engine
tuning or R&D, you
need to know exactly what happens to engine power output when
you make small changes,
and adding the many drivetrain variables makes that kind of
precision almost
impossible. With an engine dynamometer, you don’t have to
factor in drivetrain
losses.
One advantage of a chassis dynamometer is somewhat more “real
world” performance
measurement, although estimating the inconsistent drivetrain
power loss on a
not-brand-new vehicle compromises the quality of the
information. The biggest
advantage of a chassis dyno is that you can get some
performance measurement on a
running vehicle, without having to remove the engine.
What is brake horsepower?
The term “brake horsepower” simply refers to power measured by a
dynamometer. All
dynamometers work by applying a brake against the engine to
load it, and measuring
how much brake force it takes to overcome the engine’s power.
Some people mistakenly
think this term refers to power measured at the wheels of a
vehicle, but brake
horsepower can be measured with either an engine dynamometer or
a chassis dynamometer.
What is the difference between gross
power and net power?
The “gross” power measurement is derived from an engine
dynamometer, with the engine
under ideal laboratory conditions. This includes using a
specialized dynamometer intake
and exhaust system, no engine-driven accessories (such as water
pump, alternator, power
steering pump, etc.), no emission control devices such as AIR
or EGR, special fuel
formulations, and fuel delivery and ignition tuning that could
never be used on a street
vehicle. These were the type of engine power ratings that auto
manufacturers advertised
before the 1972 model year.
There were two main disadvantages of using gross power ratings:
-
They were not representative of the power produced by an
engine installed
in a vehicle.
-
The testing method was not standardized nor openly
documented.
With gross power ratings, you had no way to compare ratings
from different engines,
even if they were made by the same company. Of course, Ford
would try to get the
best numbers they could from each engine, but you can bet they
put more effort into
getting good numbers from a Boss 351C 4V than from a 97ci,
4-cylinder Pinto engine.
Starting in MY 1972, the US Federal Trade Commission (FTC)
mandated that automobile
manufacturers use “SAE net” power ratings when they advertised
claims of engine power.
The SAE net power ratings were based on testing standards
developed by the Society of
Automotive Engineers (SAE). Ultimately, this was a benefit to
the entire automotive
industry, as the manufacturers all played by the same rules
when rating engine power
output.
The net power measurement is still derived from an engine
dynamometer, but with the
engine using a production induction system and air filter,
production exhaust
manifolds and exhaust system (including mufflers and catalytic
converters), production
emission controls including AIR and EGR, production fuel
delivery and ignition tuning
specifications, and all the engine-driven accessories required
for a particular
application.
The advantage of SAE net power ratings was that advertised
power more closely
represented the performance of an engine in a real vehicle, and
the ratings were
comparable between different engines because a standardized
testing method was used.
The 1971 and 1972 M-block 400 makes a good example to compare
gross (260hp) and net
(172hp) power ratings. There was no significant change in the
400’s specification
(compression ratio, camshaft profile and timing, etc.) from
1971 to 1972, yet the
advertised power rating dropped by about 33%. That lower rating
was entirely a result
of switching from gross power to net power ratings, and that
drop of about one-third
is typical of most engines.
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