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Engine Performance
EXPLAINING
TORQUE
In the engine, torque is the twisting or turning force
developed by the thrust of the pistons and connecting
rods forcing the crankshaft to rotate. To understand how
mush torque is being produced, we must know how much
force is generated from the twisting or turning of the
crankshaft. To determine this we can say that 1 pound (Ib)
of force, measured 1 foot (ft) form the center of the
crankshaft is equal to l lb.ft of torque. Thus the
formula:
Distance *force = torque
Example:
If the lever shown is 2 ft long (distance) and a force of
3 Ib is applied to the lever, torque will be:
Distance * force = torque
2 ft * 3 Ib = 6 Ib.ft of torque
(In the metric system, torque is measured in Newton
meters)
In the automobile engine the maximum torque is given in
pound- feet at a given number of engine rpm. The
volumetric efficiency (VE) of a particular engine
dictates what rpm an engine will develop and how much
torque can be produced. This simply means that as long as
the piston and connecting rods can apply thrust to the
crankshaft, torque will continue to be produced, at low
engine speed, VE and torque are both lower, as the engine
accelerates and reaches the wide- open position, VE and
torque peak together, if the engine rpm continues to
rise, the piston speed increases to such a rate that VE
and torque decrease, causing the engine to lose power
according to the torque horsepower graph in fig a certain
engine that can develop 2800 rpm can reach a maximum
torque of 248 Ib ft, if the engine rpm continues to rise,
the toque begins to drop of, because of the kinetic
energy of the rotating mass, the horsepower continues to
rise to a much a higher peak and then, like torque, it
drops of slightly.
Pound- feet of torque expresses the amount of work done (work,
as distinct from torque, is expressed in units of foot-
pounds (ft. Ib)) whereas horsepower expresses the amount
of work done within a certain time, one of the earliest
uses of steam engines was that of pumping water from
mines, a job previously done by horses. A definite unit
of measurement of the number of foot-pounds of work that
could be done in a given time was established by James
watt in the eighteenth century. He conducted some
experiments and found that "a strong dray horse
" could (with the use of rope and a simple pulley)
draw a 150-Ib weight out of a coal pit through a distance
of 220 ft in 1 min. the work done was 150 times 220, or
33,000 ft. Ib. /min. watt's calculation means that 33,000
ft. Ib of torque in 1 min is equal to 1 hp (horsepower).
In the metric system the power an engine puts out is
often measured in kilowatts (KW): 1.34 hp equals 1 KW,
and hp equal s 0.746 KW. 3 Thermal efficiency.
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Information
author (Dr.Hassan Daadora,Mataria Engineering
Faculty,Cairo)
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