PRIMARY BRAKE SHOE--The brake shoe in-
POWER STROKE--The piston stroke from top dead
stalled facing the front of the vehicle; usually a
center to bottom dead center during which the
self-energizing shoe.
fuel-air mixture burns and forces the piston down
so the engine produces power.
PRIMARY CIRCUIT--The low-voltage part of an
automotive ignition system.
POWER TAKEOFF--An attachment for connecting
an engine to power-driven auxiliary machinery
PRIMARY WINDING--The winding of a trans-
when its use is required.
former connected to the electrical power source;
for example, the low-voltage winding in an ignition
POWER--The rate of doing work or the rate of
coil.
expanding energy.
PRIME MOVER--The source of mechanical power
POWERPLANT--The engine or power-producing
used to drive the generator, pump or compressor.
mechanism on a vehicle or equipment.
PRIMER--An auxiliary fuel pump operated by hand
POWER UNIT--A combination of pump, pump
to feed additional fuel into the engine to produce a
drive, reservoir, controls, and conditioning com-
richer mixture for starting or to purge air from the
ponents which may be required for its application.
system.
PREIGNITION--Premature ignition of the fuel-air
PROPELLER SHAFT--See DRIVE SHAFT.
mixture being compressed in the cylinder on the
compression stroke.
PROTON--A basic particle of matter having a
positive electrical charge, normally associated with
PRESSURE--The amount of force distributed over
the nucleus of the atom.
each unit of area, usually expressed in pounds per
square inch.
PSI (POUNDS PER SQUARE INCH)--A measure
of force per unit area.
PRESSURE, ABSOLUTE--The sum of atmospheric
and gauge pressures.
PTO (POWER TAKEOFF)--A location on a trans-
mission or transfer case from which an operating
PRESSURE, ATMOSPHERIC--Pressure exerted by
shaft from another unit can be driven.
the atmosphere at any specific location.
PUMP--A device that converts mechanical force and
PRESSURE, BACK--The pressure encountered on
motion into hydraulic fluid power.
the return side of a system.
PUMP, AXIAL PISTON--A pump having multiple
PRESSURE, DIFFERENTIAL--The difference in
pistons disposed with their axes parallel.
pressure between any two points of a system or a
component.
PUMP, CENTRIFUGAL--A pump that produces
fluid velocity and converts it to pressure head.
PRESSURE, HEAD--The pressure due to the height
of a column or body of fluid. It is usually expressed
PUMP, FIXED-DISPLACEMENT--A pump in
in feet.
which the displacement per cycle cannot be varied.
PRESSURE, OPERATING--The pressure at which a
PUMP, RADIAL PISTON--A pump having multiple
system operates.
pistons disposed radially actuated by an eccentric
element.
PRESSURE, PRECHARGE--The pressure of com-
pressed gas in an accumulator prior to the admis-
PUMP, VARIABLE-DISPLACEMENT--A pump
sion of a liquid.
in which the volume of fluid per cycle can be
varied.
PRESSURE, PROOF--The nondestructive test pres-
sure in excess of the maximum rated operating
PUSH ROD--On overhead valve engines, it is the
pressure.
device that transmits motion from the camshaft and
valve lifter to the rocker arm to open the valve.
PRESSURE, STATIC--The pressure in a fluid at rest.
RACE--The inner or outer ring that provides a contact
PRESSURE SWITCH--An electrical switch oper-
surface for balls or rollers to ride on.
ated by the increase or decrease of fluid pressure.
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