COMBUSTION CHAMBER DESIGN
injected into the combustion chamber must be mixed thoroughly with the compressed air and
distributed as evenly as possible throughout the chamber if the engine is to function at
maximum efficiency and exhibit maximum drivabilty. A well-designed engine uses a
combustion chamber that is designed for the intended usage of the engine. The injects used
should compliment the combustion chamber. The combustion chambers described on the
following pages are the most common and cover virtually all of the designs that are
currently in use. These are the open chamber, precombustion chamber, turbulence chamber,
and spherical (hypercycle) chamber.
combustion chamber (fig. 5-2)
is the simplest form of chamber. It is suitable for only slow-speed, four-stroke cycle
engines, but is widely used in two-stroke cycle diesel engines. In the open chamber, the
fuel is injected directly into the space on top of the cylinder. The combustion space,
formed by the top of the piston and the cylinder head, usually is shaped to provide s
swirling action of the air, as the piston comes up on the compression stroke. There are no
special pockets, cells, or passages to aid the mixing of the fuel and air. This type of
chamber requires a higher injection pressure and a greater degree of fuel atomization than
is required by other combustion chambers to obtain an acceptable level of fuel mixing. To
equalize combustion in the combustion chamber, use a multiple orifice-type injector tip
for effective penetration. This chamber design is very susceptible to ignition lag.
The precombustion chamber (fig. 5-3)
is an auxiliary chamber at the top of the cylinder. It is connected to the main combustion
chamber by a restricted throat or passage. The precombustion chamber conditions the fuel
for final combustion in the cylinder. A hollowed-out portion of the piston top.causes
turbulence in the main combustion chamber, as the fuel enters from the precombustion
chamber to aid in mixing with air. The following steps occur during the precombustion
the compression stroke of the engine, air is forced into the precombustion chamber and,
because the air is compressed, it is hot. At the beginning of injection, the precombustion
chamber contains a definite volume of air.
the injection begins, combustion begins in the precombustion chamber. The burning of the
fuel, combined with the restricted passage to the main combustion chamber, creates a
tremendous amount of pressure in the combustion chamber. The pressure and the initial
combustion cause a super-heated fuel charge to enter the main combustion chamber at a high
entering mixture hits the hollowed-out piston top, creating turbulence in the chamber to
ensure complete mixing of the fuel charge with the air. This mixing ensures even and
complete combustion. This chamber design provides satisfactory performance with low fuel
injection pressures and coarse spray patterns because a large amount of vaporization
occurs in the precombustion chamber. This chamber also is not very susceptible to ignition
lag, making it suitable for high-speed operations.
Chamber The turbulence chamber (fig. 5-4)
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 a high
percentage of the air between the piston and cylinder head is forced into the turbulence
chamber during the compression stroke. The chamber is usually spherical, and the small
opening through which the air must pass causes an increase in air velocity, as it enters
the chamber. This turbulence speed is about 50 times crankshaft speed. The fuel injection
is timed to occur when the turbulence in the chamber is greatest. This ensures a thorough
mixing of the fuel and air, causing the greater part of combustion to take place in the
turbulence chamber. The pressure, created by the expansion of the burning gases, is the
force that drives the piston downward on the power stroke.
The spherical (hypercycle) combustion chamber (fig. 5-5)
is designed principally for use in the multifuel diesel engine. The chamber consists of a
basic open type chamber with a spherical shaped relief in the top of the piston head. The
chamber works in conjunction with a strategically positioned injector and an intake port
that produces a swirling effect, as it enters the chamber. Operation of the chamber is as
the air enters the combustion chamber, the shape of the intake port (fig. 5-5) introduces
a swirling effect to it.
the compression stroke, the swirling motion of the air continues as the temperature in the
chamber increases (fig. 5-5).
the fuel is injected, approximately 95 percent of it is deposited on the head of the
piston and the remainder mixes with the air in the spherical combustion chamber (fig. 5-5).
combustion begins, the main portion of the fuel is swept off the piston head by the
high-velocity swirl that was created by the intake and the compression strokes. As the
fuel is swept off of the head, it burns through the power stroke, maintaining even
combustion and eliminating detonation (fig. 5-5).
Figure 5-2.Open combustion chamber.
Figure 5-3.Precombustion chamber.
Figure 5-4.Turbulence chamber.
Figure 5-5.Spherical chamber.