56 Auto Fundamentals
Combustion Chamber Design
The size and shape of the combustion cham-
ber is very important. A well designed chamber
will allow high compression ratios without the
occurrence of detonation or preignition. Com-
pression ratios of 10.5 to 1 and higher were once
widely used. Modern engines have compression
ratios of around 8, 8.5, or 9 to 1, which provides
both good performance and emission control.
Ricardo Combustion Chamber
Many years ago a man named Ricardo devel-
oped a combustion chamber for L-head engines
that has been modified for use in many I-head
engines. He found that by causing a violent tur-
bulence in the compressed air-fuel charge and
by locating the spark plug near the center of the
turbulence, fl ame travel through the charge was
smooth and compression could be raised consid-
erably without detonation.
Modern designs locate the combustion
chamber under the valves and allow the piston
to come very close to the head. As the piston
travels up on compression, it forces the fuel into
the area below the valves. As the piston comes
close to the head, the remaining mixture is rap-
idly squeezed, causing it to shoot into the com-
bustion chamber, setting up a rapid turbulence.
The area between the piston top and the head is
referred to as the squish area, Figure 3-18.
Hemispherical Combustion Chamber
The hemispherical combustion chamber is
compact and allows high compression with very
little detonation. By placing the valves in two
planes, it is possible to use larger valves, thereby
improving air-fuel intake and exhaust scavenging.
The hemispherical combustion chamber was
fi rst used many years ago on high-performance
engines. Today, it is used on a number of modern
engines. Figure 3-19 illustrates the hemispherical
combustion chamber and valve confi guration.
Wedge Shape Combustion Chamber
Another popular design is the wedge
combustion chamber. When the wedge shape is
incorporated in the head, a fl at top piston will pro-
duce a desirable squish effect. When the top of the
engine block is machined at an angle of around
10°, it is possible to build the cylinder head fl at.
Piston
Port
Flame starts
at plug
Piston
Flame starts
at plug
B
A
Figure 3-17.
Detonation. A—Proper fl ame travel. B—Two fl ame fronts
increase burning rate and cause detonation.
Goodheart-Willcox Publisher
Piston
Port
Turbulence
Squish area
Figure 3-18.
When the top of the piston nears the head, the trapped
air-fuel mixture in the “squish” area shoots into the
combustion chamber, causing turbulence. This is known
as the Ricardo principle.
Goodheart-Willcox Publisher