Of all the factors given in this formula, only
cylinder pressure (P) and engine rpm (N) can be
changed during the normal operation of the
engine. The remaining factors are constant.
BRAKE HORSEPOWER is the actual amount
of power that an engine can deliver at a certain
speed with a wide-open throttle. The term brake
horsepower is derived from the braking device
(usually a dynamometer) that is applied to
measure the horsepower an engine develops. The
dynamometer consists of a resistance-creating
device, such as an electric armature revolving in
a magnetized field. A paddle wheel revolving in
a fluid may also be used to absorb the energy.
An ENGINE DYNAMOMETER maybe used
to test an engine that has been removed from the
vehicle it drives. If the engine does not develop
the manufacturers recommended horsepower and
torque at specific rpms, the engine must be tuned
up or repaired.
The CHASSIS DYNAMOMETER can give a
quick report on engine conditions by measuring
output at various speeds and loads. It is useful
in shop testing and adjusting automatic
On the chassis dynamometer (fig. 3-1), the
driving wheels of the vehicle are placed on rollers.
The engine drives the wheels, and the wheels drive
the rollers. By loading the rollers varying amounts
and by running the engine at different rpms,
nearly all normal driving conditions can be
simulated. The tests and checks can be made
without the interference of body noises, as
happens when the vehicle is driven on the road.
FRICTION HORSEPOWER is the difference
between indicated horsepower and brake horse-
power. Actually, it is the power required to
overcome friction within the engine, such as
friction between engine parts, resistance in driving
accessories, and, among other things, loss due to
pumping action of the pistons. The latter maybe
compared to the effort required to raise the handle
of a hand-operated tire pump. It may be difficult
to define friction horsepower properly, but with
proper maintenance, it can be reduced to improve
the mechanical efficiency of the engine.
Figure 3-1.Chassis dynamometer.