1.4.4. Compression Ratio

The compression ratio, which is defined as the ratio of the cylinder volume at the bottom center to the volume at the top center, is a critical factor in combustion efficiency and pollutant formation. A high compression ratio enhances the combustion efficiency, but the higher temperatures cause more NOx to form, thus increasing the emissions.

Automotive engines in the 1970s had much higher compression ratios since the emissions standards were much lower. With stricter environmental regulations on emissions, the compression ratios were reduced to meet the new standards. In the 1990s, technological improvements in catalytic converters and improvements in combustion efficiency allowed higher compression ratios and improved fuel economy. An additional consideration, especially for diesel engines, is that the materials used for the piston and combustion chamber must be able to withstand the peak temperatures and pressures encountered with high compression ratios and high boost levels.

Figure 1.3:  Different Piston Shapes Used to Achieve Desired Compression Ratio for an Engine

Different Piston Shapes Used to Achieve Desired Compression Ratio for an Engine

Thus the geometric design of intake/exhaust ports, cylinder heads, valves and pistons involves interplay and trade-offs between the volumetric and combustion efficiency, pollutant formation, packaging considerations, materials choices and manufacturing tolerances. The ability to accurately analyze the engine fluid dynamics plays a key role in optimizing the engine to efficiently deliver the power needed, while meeting emissions standards and respect packaging and manufacturing constraints.

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