Motores de combustión

Saab Variable Compression
“Estudio termodinámico del
nuevo motor SVC”
• Objetivos del estudio.
• Referencias.
• Fuente de Información.
SVC - a unique new engine concept
Saab is now launching an entirely new engine concept named SVC,
which stands for Saab Variable Compression. Owing to the SVC
engine's unique design, it offers performance on a par with units twice
its size but with the fuel consumption of a small engine. The SVC
engine is a five-cylinder 1.6 litre unit producing 225 bhp, and it
delivers no less than 305 Nm of torque.
What is unique about the SVC engine is that it features variable
compression. In other words, the ratio between the piston's
displacement volume and the volume of the combustion chamber is not
constant, as it is in a conventional engine. Instead, the SVC registers
current needs and decides how much the mixture of fuel and air is to be
compressed in the cylinder prior to ignition. The upper part, the monohead, can be inclined up to four degrees to achieve optimum
compression, which means that the engine always works at its most
efficient level.
It is this variable compression in combination with considerable
overboost and a scaled-down cylinder displacement that makes the SVC
design so strong and at the same time so fuel-efficient. Generous
overboost means it is possible to supply more fuel to the engine as and
when needed. This in turn promotes both greater torque and higher
power output. A smaller cylinder displacement also means the engine is
lighter and operates with lower friction, so it uses fuel more efficiently
compared to a conventional engine. Fuel consumption can be reduced by
up to 30 percent - while retaining existing performance levels.
25 years ago, Saab was the world's first manufacturer to develop a turbo
engine for everyday use. Ever since, we have remained at the forefront of
development in the field of engine efficiency. The new engine represents a
major leap forward in petrol engine technology.
Destacable
• Compresión variable
• Motor Otto de 1,6 Litros le extraen 225 [Hp] y
305 [ N·m]
• Consume un 30% menos de combustible, gracias
también al overboost (sobrealimentador).
• 8 < rC < 14
Motor
Funcionamiento
Tabla Comparativa
A u to m ó v il
P oten cia H p
S a a b 9 -3
2 L
185
Subarú Im preza T urbo 2 L
218
P eugeot
3 L V6 DOHC
210
A u d i T T 1 ,8 T
Q u a ttro
225
M e rc e d e s B .
2 ,1 L D O H C
115
O pel O m ega
2 ,5 L
130
V o lv o V 7 0
2 0 V T 5 2 ,3 L
240
¿Por qué?
¿Qué pasa dentro del cilindro?
Varían:
• rc
• p1
• V
Ciclo Otto sobrealimentado
p
3
2
+
7
6
+
4
1
5
V
Indice de Compresión
rC 
V D  VC
VC
Aquí lo que varía es VC , por lo tanto cuando
VC   rC 
En motores Otto normales 6,5 < rC <11
Algunas relaciones termodinámicas
imep
p3

1
k
 1   rc
k




1

1

k 1

rc
rc
 

T1
1
rc  1 


 CV 
k 1
*
Q
r
c

imep =: indicated mean effective pressure
Efectividad con la cual el volumen desplazado por el pistón
que es usada para producir trabajo con respecto
a la máxima presión
rC   ( imep/p3 ) 
* Gas ideal con CV y Cp constantes






 f ,i  1 
xr 
1
1
rC
 pe



p
i 


Si rC   f,i 
k 1
1
k
rC 
Q*
1 
k 1

C

T

r
V
1
C







1
k
Si rC   xr
Eficiencia termodinámica v/s rC
f,i
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
rv
Efecto de la sobrealimentación
Pot 
 A
 f   V  V D  Q HV   a , i  F
2
Con sobrealimentación aumenta la potencia
a,i 
Conclusiones
• A bajas revoluciones me conviene que el
índice de compresión sea alto ya que así
aprovecho bien la mezcla
• A altas revoluciones bajo relación de
compresión o si no se produce
autodetonación
• A altas presiones aumentan los NOX