Motores
Richard Prystupa
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Motores (Modulo 160401a)
Es un dispositivo mecánico en el cual la energía
química de la oxidación del combustible es
convertida en energía calorífica, la cual a su vez es
convertida en energía mecánica.
La relación es normalmente desde 7:1 hasta 15:1 por
peso (aire/combustible).
?QUE ES TORQUE?
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Es la fuerza aplicada en una palanca que hace rotar
alguna cosa, provocando un momento torsional.
Expresado en lb*pie, lb*plgs, o Newton-metros.
Tq = F x distancia del radio en pie
Lectura del torque
Tornillo o tuerca conectada
empieza a torcerse
Fuerza
QUE ES POTENCIA?
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Es cuán rápido podemos realizar el trabajo.
1 HP = 746 Watts
1 HP = 550 lb. pie./sec.
HP = T x RPM
5252
¿Cuál es la
unidad de
potencia?
Si
RANGO DE CABALLOS DE FUERZA DE UN
MOTOR (p. 8)
CABALLOS DE FUERZA INDICADO
 Son los caballos de fuerza calculados teóricamente.
 No se encuentran previstas las perdidas por fricción
o bombeo, o la energía necesaria para mover otros
accesorios.
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CABALLOS DE FUERZA POR FRICCION
(CFFr)
Es la energía perdida por fricción y bombeo.
(Ej. Transmisión, rodamientos, poleas, bombas)
FHP = IHP – BHP (CFFr= CFI – CFF)
CABALLOS DE FUERZA DE FRENO
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Es la energía existente
medida en el extremo
del cigüeñal (Volante).
Este es el desarrollo de
los caballos de fuerza
existente en el motor en
operación.
BHP = IHP – FHP.
(CFF= CFI-CFFr)
PRESION
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Es una fuerza por unidad de área.
Ej. lbs.*Plgs2 o kPa (1 lb*Plgs2 = 6.9 kPa)
VACIO (p. 9)
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Es la presión por debajo de la presión atmosférica.
(14.7 lbs*Plgs2 o 0 psig @ sea level).
PRESION ATMOSFERICA
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Es debido al peso de la atmósfera sobre la superficie de la
tierra que a nivel del mar existe 14.7 lbs*Plgs2 atmosférica.
Presión ejercida sobre unidad
unidades de área. 16 400 pies sobre el nivel del mar.
Pr = 7.7 Lbsa*Plgs2
A nivel del mar
Pr = 14.7 Lbsa*plgs2
Aceleración
de la gravedad
RELACION CILINDRO CARRERA (p. 10)
CILINDRO
 El diámetro interior del cilindro
 Medido a una precisión de .001”
CARRERA
 Es el desplazamiento del pistón
desde el PMS al PMI o viceversa.
Øint
PMS
Carrera
PMI
DESPLAZAMIENTO DEL MOTOR
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Ej.
Cilindro = 4.001”
Carrera = 3.480”
PMS
Motor de 8 cilindros
¿Cual es el
desplazamiento cúbico del CARRERA
Motor?
PMI
V = ∏r² x H
V = 3.14 (2) ² x 3.480” x #
cil.
V = 349.865 Plgs3 o
350 Plgs3.
CILINDRO
MOTOR CUADRADO (p. 10)
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Cuando el diámetro interior del cilindro es
igual a la carrera.
4”
4”
MOTOR SOBRECUADRADO
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Cuando el Øint. del cilindro es mayor que la carrera, por
consiguiente, el pistón tiene menos recorrido.
Son encontrados típicamente en motores automotriz y altas
aceleraciones.
Típicamente son de motores pequeños – 327, 350, 400 GM
4”
3”
MOTOR SUBCUADRADO
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Cuando el Øint. del cilindro es menor quela carrera.
Grandes torques de salida a bajas rpm.
Son hallados sobre grandes, pequeños motores en movimiento.
Motores de bloques grandes – 396, 427, 454 son determinados
por su peso y dimensiones externas, no el desplazamiento en
Plgs3.
3”
4”
VOLUMEN DE LA HOLGURA
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Es el volumen remanente sobre el pistón,
cuando este esta en el PMS.
Pistón @ PMS
Carrera
Holgura
COMPRESSION RATIO
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Is how much air/fuel mixture is compressed by
volume.
It is the ratio of the total volume of the cylinder and
combustion chamber clearance at BDC compared
to the clearance volume at TDC.
What would the
compression ratio
be in this example?
COMPRESSION RATIO
What would the compression ratio be in this example?
15:1
VOLUMETRIC EFFICIENCY (p. 11)
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The ratio expressed as a percentage of the
volume of atmospheric air drawn into the
cylinder on the intake stroke (4 stroke natural
aspiration) compared to the displacement.
VE = Actual Output x 100
Theoretical Output
SCAVENGE EFFICIENCY
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It is the ratio expressed as a percentage of the fresh
air contained in the cylinder to the total volume of
air and exhaust gases in the cylinder at the time the
port closes.
Associated with two-stroke engines.
THERMAL EFFICIENCY
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States how well the engine changes fuel energy
into mechanical energy.
Most engines are about 25 to 35% efficient. (Most
goes out the exhaust).
BASIC ENGINE OPERATION (p. 12)
INTERNAL COMBUSTION ENGINES
 Fuel is burnt inside the engine in the cylinders.
INTERNAL COMBUSTION ENGINES
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The 3 main requirements to allow fuel to burn in an
engine are: Fuel, Air and Ignition.
INTERNAL COMBUSTION ENGINES
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The compression process generates heat, in some
cases it is enough to ignite the mixture without a
spark. (Diesel engine).
INTERNAL COMBUSTION ENGINES
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Upon the power stroke, the piston transfers the
energy to the connecting rod which then is
transferred to the crankshaft into rotary motion.
Four Stroke (Cycle) Engines (p. 13 – Fig. 8)
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On a 4 cycle engine, it takes 720 degrees of the
crank to rotate to complete 1 cycle.
Four Stroke (Cycle) Engines
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On the intake stroke, the intake valve opens before
the piston reaches TDC (valve overlap) and begins
to move downwards pulling air/fuel mixture into the
cylinder.
Four Stroke (Cycle) Engines
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Slightly past BDC, the intake valve closes and the
piston moves upward compressing the air/fuel
mixture.
Four Stroke (Cycle) Engines
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The air/fuel mixture is ignited at TDC (both valves
are still closed – compression stroke).
Four Stroke (Cycle) Engines
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The piston moves past TDC as complete burning of
the air/fuel mixture begins to take place.
The expanding gases push the piston downward in
the cylinder producing power (power stroke).
Four Stroke (Cycle) Engines
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Slightly before the piston reaches BDC, the exhaust
valve opens and the piston pushes the burned gases
out of the cylinder as it moves up (exhaust stroke).
Four Stroke (Cycle) Engines
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As the piston nears TDC, the exhaust valve starts
closing and the intake valve starts opening and the
cycle begins again. This is known as valve overlap
and occurs at the end of the exhaust stroke.
Four Stroke (Cycle) Engines
Four Stroke (Cycle) Engines
TWO STROKE (CYCLE) ENGINES (p.15-fig. 9)
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Two stroke engines complete one cycle in 360
degrees.
2 stroke engines may use valves or ports.
TWO STROKE (CYCLE) ENGINES
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When the piston is moving downward, spent
gases begin leaving the piston cylinder.
TWO STROKE (CYCLE) ENGINES
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The air/fuel mixture begins entering the
cylinder close to the bottom of the stroke.
TWO STROKE (CYCLE) ENGINES
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As the piston starts moving upward, air/fuel is still
entering the piston chamber but is stopped early in
the stroke.
The remainder of the stroke is used to compress the
air/fuel mixture in the piston chamber.
TWO STROKE (CYCLE) ENGINES
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Near TDC, the mixture is ignited and the expanding
gases begin to push the piston downwards.
TWO STROKE (CYCLE) ENGINES
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Near the end of the cycle, the exhaust valve (or
port) opens and the spent gases begin exhausting.
2 stroke engines need to be artificially aspirated to
force out the exhaust gases and to push in the fresh
air/fuel mix.
TWO STROKE (CYCLE) ENGINES
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The intake valve (or port) opens while the piston is
still moving downward and the cycle begins again.
TWO STROKE (CYCLE) ENGINES
TWO STROKE vs. FOUR STROKE
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simpler and lighter
do not have valves
fire once every revolution,
(~75% more hp than 4
stroke)
can work in any
orientation
not as efficient as 4 stroke
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fire once every two
revolutions
More efficient than 2 stroke
Better fuel consumption
No mixing oil/fuel
CRANKSHAFT ROTATION
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Is determined as if viewing the engine from the main
power takeoff or flywheel end. (Rear)
If the engine turns to the right its rotation clockwise
(CW).
If engine rotates to the left its rotation is
counterclockwise (CCW).
NUMBERING OF CYLINDERS
FIRING ORDER (p. 17)
FIRING ORDER
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The position of the crankshaft throws and the lobes
on the camshaft determine the firing order of an
engine.
The order is designed to give an even number of
pulses throughout the complete rotation of the
crankshaft.
This is the sequence of order for the cylinders to
receive ignition.
RUNNING MATES (Fig. 11)
This applies to four stroke engines only because
every cylinder fires in one complete revolution
(360 degrees) on 2 stroke engines.
 Running mates refer to pistons which reach TDC
simultaneously, but only one fires. (720˚ cycle)
 Assists in balancing of the crank and pistons.
Running mate arrangements:
 4 cyl. Inline engine: 1-4, 2-3.
 8 cyl. Inline engine: 1-8, 2-7, 3-6, 4-5.
 V 6 engine: 1-6, 2-5, 3-4.
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RUNNING MATES (Crankshaft Throws)
ENGINE CLASSIFICATION (p. 18)
Engines are classified by:
 cylinder and crankshaft arrangements.
 valve arrangement.
 position of camshaft.
 cooling methods.
 induction methods.
 engine speeds.
 operating (stroke) cycle.
 ignition methods and type of fuel consumed.