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Combustion systems incorporate multiple factors that impact the combustion process. These include:
In all combustion systems, these factors must work together to ensure that the combustion process, whether conventional or advanced, achieves the required performance and emissions goals.
This paper discusses some aspects related to combustion chamber geometry, in-cylinder flow and compression ratio.
Diesel combustion is known to be very lean with A/F ratios of 25:1 at peak torque, 30:1 at rated speed/maximum power conditions, and over 150:1 at idle for turbocharged engines. Yet, this extra air does not enter into the combustion process. It is rather heated during combustion and exhausted—causing diesel exhaust to be lean. Even though the average air-fuel ratio is lean, if proper care is not taken in the design process, regions of the combustion chamber can be fuel rich and lead to excessive smoke emissions. A key objective in designing the combustion bowl then is that good mixing of fuel and air is achieved and that these fuel rich regions are avoided as much as possible. Turbulence in the air motion within the combustion bowl is found to be beneficial to the mixing process that can be used to achieve this goal. Swirl induced by the intake port can be enhanced or squish can be generated by the piston as it approaches the cylinder head to create more turbulence during the compression stroke through proper design of the bowl in the piston crown.
Designs for early 1990s combustion bowls were generally described as “re-entrant”. Figure 1 shows examples of a re-entrant bowl (solid line) with a “straight sided” or “Mexican hat” combustion bowl design shown in the dashed line. Elements for the optimization of this design include consideration of different ratios of radii of the combustion bowl to the cylinder bore, different chamfer angles of the re-entrant lip, and different aspect (bowl diameter/bowl depth) ratios .
Figure 1. Comparison between Straight-Sided and Re-Entrant Bowl