COMBUSTION CHAMBER DESIGN

The fuel 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.

Open Combustion Chamber

The open 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.

Precombustion Chamber
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 process:

• During 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.
• As 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 velocity.
• The 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.

Turbulence 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.

Spherical (Hypercycle) Chamber
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 follows:

1. As the air enters the combustion chamber, the shape of the intake port (fig. 5-5) introduces a swirling effect to it.
2. During the compression stroke, the swirling motion of the air continues as the temperature in the chamber increases (fig. 5-5).
3. As 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).
4. As 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.