a. During the compression stroke of the engine, air
Open Chamber (Fig. 5-2). The open chamber
is forced into the precompression chamber and, because
is the simplest form of chamber. It is suitable only for
the air compressed, it is hot. At the beginning of
slow-speed, four-stroke cycle engines, but is used widely
injection, the precombustion chamber contains a definite
in two-stroke cycle diesel engines. In the open chamber,
volume of air.
the fuel is injected directly into the space at the top of the
cylinder. The combustion space, formed by the top of
b. As the injection begins, combustion begins in the
the piston and the cylinder head, usually is shaped to
The burning of the fuel
provide a swirling action of the air as the piston comes
combined with the restricted passage to the main
up on the compression stroke. There are no special
combustion chamber creates a tremendous amount of
pockets, cells, or passages to aid the mixing of the fuel
pressure in the precombustion chamber. The pressure
and air. This type of chamber requires a higher injection
and the initial combustion cause a superheated fuel
pressure and a greater degree of fuel atomization than is
charge to enter the main combustion chamber at a
required by other combustion chambers to obtain an
acceptable level of fuel mixing. This chamber design is
very susceptible to ignition lag.
c. The entering mixture hits the hollowed-out piston
top, creating turbulence in the chamber to ensure
5-10. Precombustion Chamber (Fig. 5-3).
complete mixing of the fuel charge with the air. This
precombustion chamber is an auxiliary chamber at the
mixing ensures even and complete combustion. This
top of the cylinder.
It is connected to the main
chamber design will provide satisfactory performance
combustion chamber by a restricted throat or passage.
with low fuel injector pressures and coarse spray
The precombustion chamber conditions the fuel for final
patterns because a large amount of vaporization takes
combustion in the cylinder. A hollowed-out portion of the
place in the combustion chamber. This chamber also is
piston top causes turbulence in the main combustion
not very susceptible to ignition lag, making it more
chamber as the fuel enters from the precombustion
suitable for high-speed applications.
chamber to aid in mixing with air. The following steps
occur during the combustion process:
turbulence chamber is similar in appearance to the
precombustion chamber, but its function is different.
There is very little clearance between the top of the
piston and the head, so that a high percentage of the air
between the piston and the cylinder head is forced into
the turbulence chamber during the compression stroke.
The chamber usually is spherical, and the opening
through which the air must pass becomes smaller as the
piston reaches the top of the stroke, thereby increasing
the velocity of the air in the chamber. This turbulence
speed is approximately 50 times crankshaft speed. The
fuel injection is timed to occur when the turbulence in the
chamber is the greatest. This ensures a thorough mixing
of the fuel and the air, with the result that the greater part
of combustion takes place in the turbulence chamber
itself. The pressure created by the expansion of the
burning gases is the force that drives the piston
downward on the power stroke.
5-12. Spherical Combustion Chamber (Fig. 5-5).
The spherical combustion chamber is
Figure 5-2. Open Combustion Chamber.