Combustion chambers and Types

Combustion chambers

There are basically four types of combustion chambers they are,

a.   Bathtub and wedge.

b.   Blow in piston.

c.   Four valve pent proof.

d.   Hemispherical.

Types of Combustion chambers

The major combustion chamber design objectives which relate to engine performance and emissions are: (1) a fast combustion process, with low cycle-by cycle variability, over the full engine operating range; (2) a high volumetric efficiency at wide-open throttle; (3) minimum heat loss to the combustion chamber walls; (4) a low fuel octane requirement.

Many methods for producing a "fast bum" have been proposed. These include ways of making the combustion chamber shape more compact, moving the spark plug to a more central location within the chamber, using two plugs, and increasing in-cylinder gas motion by creating swirl during the induction process or during the latter stages of compression. A faster combustion process relative to more moderate bum rate engines does result in a direct engine efficiency gain, other factors being equal. The magnitude of this direct gain is relatively modest. Experimental studies of the effect ofan increase in burn rate from moderate to fast at constant engine load, speed, and mixture composition show that this effect is a few percent at most.23 Computer simulations of the engine operating cycle confirm these experimental observations: while a decrease in total burn duration from 100 to 60' (slow to moderate burn) does result in a 4 percent decrease in bsfc, a decrease in bum duration from 60 to 20" gives only a further 1.5 percent bsfcdecrea~e.~Of greater importance is the fact that the faster bum process is more robustand results in the engine being able to operate satisfactorily with much more EGR, or much leaner, without a large deterioration in combustion quality. Faster burning chamber designs exhibit much less cycle-by-cycle variability. This ability to operate with greater dilution at part load while maintaining a short burn duration and low cycle-by-cycle variability, permits much greater control of NO, within the engine with 20 or more percent EGR without any substantial increase in HC emissions (see Fig. 11-29), or permits very lean operation. In both cases the efficiency gain relative to moderate burn rate engines, which must operate with less dilution, is sizeable.24High volumetric efficiency is required to obtain the highest possible power density. The shape of the cylinder head affects the size of valves that can be incorporated into the design. Effective valve open area, which depends on valve diameter and lift, directly affects volumetric efficiency. Swirl is used in many modern chamber designs to speed up the burning process and achieve greater combustion stability. Induction-generated swirl appears to be a particularly stable in-cylinder flow. Swirl results in higher turbulence inside the chamber during combustion, thus increasing the rate of flame development and propagation. Generating swirl during the intake process decreases volumetric efficiency.

Heat transfer to the combustion chamber walls has a significant impact on engine efficiency. It is affected by cylinder head and piston crown surface area, by the magnitude of in-cylinder gas velocities during combustion and expansion, by the gas temperatures and the wall temperatures. The heat-transfer implications of a combustion chamber should be included in the design process. Knock effectively limits the maximum compression ratio that can be used in any combustion chamber; it therefore has a direct impact on efficiency. Knock is affected by all the factors discussed above. It is the hardest of all the constraints to incorporate into the design process because of its obvious complexity. Knowledge of the fundamentals of spark-ignition engine combustion, in cylinder gas motion, and heat transfer has developed to the point where a rational procedure for evaluating these factors for optimum combustion chamber development and design can be defined. The next two sections develop such a procedure burning chamber designs exhibit much less cycle-by-cycle variability. This ability to operate with greater dilution at part load while maintaining a short burn duration and low cycle-by-cycle variability, permits much greater control of NO, within the engine with 20 or more percent EGR without any substantial increasein HC emissions (see Fig. 11-29), or permits very lean operation. In both cases the efficiency gain relative to moderate burn rate engines, which must operate with less dilution, is sizeable. High volumetric efficiency is required to obtain the highest possible power density. The shape of the cylinder head affects the size of valves that can be incorporated into the design. Effective valve open area, which depends on valve diameter and lift, directly affects volumetric efficiency. Swirl is used in many modern chamber designs to speed up the burning process and achieve greater combustion stability. Induction-generated swirl appears to be a particularly stable in-cylinder flow. Swirl results in higher turbulence inside the chamber during combustion, thus increasing the rate of flame development and propagation. Generating swirl during the intake process decreases volumetric efficiency. Heat transfer to the combustion chamber walls has a significant impact on engine efficiency. It is affected by cylinder head and piston crown surface area, by the magnitude of in-cylinder gas velocities during combustion and expansion, by the gas temperatures and the wall temperatures. The heat-transfer implications of a combustion chamber should be included in the design process. Knock effectively limits the maximum compression ratio that can be used in any combustion chamber; it therefore has a direct impact on efficiency. Knock is affected by all the factors discussed above. It is the hardest of all the constraints to incorporate into the design process because of its obvious complexity.

Knowledge of the fundamentals of spark-ignition engine combustion, in cylinder gas motion, and heat transfer has developed to the point where a rational procedure for evaluating these factors for optimum combustion chamber development and design can be defined. The next two sections develop such a procedure burning chamber designs exhibit much less cycle-by-cycle variability. This ability to operate with greater dilution at part load while maintaining a short burn duration and low cycle-by-cycle variability, permits much greater control of NO, Within the engine with 20 or more percent EGR without any substantial increase in HC emissions, or permits very lean operation. In both cases the efficiency gain relative to moderate burn rate engines, which must operate with less dilution, is sizeable. High volumetric efficiency is required to obtain the highest possible power density. The shape of the cylinder head affects the size of valves that can be incorporated into the design. Effective valve open area, which depends on valve diameter and lift, directly affects volumetric efficiency. Swirl is used in many modern chamber designs to speed up the burning process and achieve greater combustion stability. Induction-generated swirl appears to be a particularly stable in-cylinder flow. Swirl results in higher turbulence inside the chamber during combustion, thus increasing the rate of flame development and propagation. Generating swirl during the intake process decreases volumetric efficiency.

Heat transfer to the combustion chamber walls has a significant impact on engine efficiency. It is affected by cylinder head and piston crown surface area, by the magnitude of in-cylinder gas velocities during combustion and expansion, by the gas temperatures and the wall temperatures. The heat-transfer implications of a combustion chamber should be included in the design process.

Knock effectively limits the maximum compression ratio that can be used in any combustion chamber; it therefore has a direct impact on efficiency. Knock is affected by all the factors discussed above. It is the hardest of all the constraints to incorporate into the design process because of its obvious complexity. Knowledge of the fundamentals of spark-ignition engine combustion, in cylinder gas motion, and heat transfer has developed to the point where a rational procedure for evaluating these factors for optimum combustion chamber development and design can be defined. The next two sections develop such a procedure burning chamber designs exhibit much less cycle-by-cycle variability. This ability to operate with greater dilution at part load while maintaining a short burn duration and low cycle-by-cycle variability, permits much greater control of NO, within the engine with 20 or more percent

EGR without any substantial increase in HC emissions (see Fig. 11-29), or permits very lean operation. In both cases the efficiency gain relative to moderate burn rate engines, which must operate with less dilution, is sizeable. High volumetric efficiency is required to obtain the highest possible power density. The shape of the cylinder head affects the size of valves that can be incorporated into the design. Effective valve open area, which depends on valve diameter and lift, directly affects volumetric efficiency. Swirl is used in many modern chamber designs to speed up the burning process and achieve greater combustion stability. Induction-generated swirl appears to be a particularly stable in-cylinder flow. Swirl results in higher turbulence inside the chamber during combustion, thus increasing the rate of flame development and propagation. Generating swirl during the intake process decreases volumetric efficiency. Heat transfer to the combustion chamber walls has a significant impact on engine efficiency. It is affected by cylinder head and piston crown surface area, by the magnitude of in-cylinder gas velocities during combustion and expansion, by the gas temperatures and the wall temperatures. The heat-transfer implications of a combustion chamber should be included in the design process. Knock effectively limits the maximum compression ratio that can be used in any combustion chamber; it therefore has a direct impact on efficiency. Knock is affected by all the factors discussed above. It is the hardest of all the constraints to incorporate into the design process because of its obvious complexity. Knowledge of the fundamentals of spark-ignition engine combustion, in cylinder gas motion, and heat transfer has developed to the point where a rational procedure for evaluating these factors for optimum combustion chamber development and design can be defined. The next two sections develop such a procedure.