In a previous Enginology column, we discussed some issues relative to induction and exhaust system design and analysis. In another, the subject of in-cylinder pressure study was also aired. Since then, a few questions have been posed we'd like to address this month, for the purpose of providing some clarification.
On the subject of intake and exhaust system flow, we suggested a relatively simple "rule of thumb" for evaluating (or selecting) passage sizes that could be followed by assuming a value for mean flow velocity. Academically speaking, this approach assumes we are addressing a one-dimensional system and doesn't contemplate such factors as viscous friction, turbulence, or other dynamic disturbances that occur as a function of passage curvature, sudden changes in cross-section areas and related elements. Even though many of the computer-driven simulations that aid the analysis and development of intake and exhaust systems utilize the one-dimensional concept, it's accurate to say there's a practical limitation to the finesse these provide.
Over time, as you read these monthly columns, you'll discover that the treatment we've elected to give topics that have the potential for being infused with academic weight are intentionally delivered in ways to help reduce theory to practice. However, since we've already broached the subject and a couple of more technically-biased questions arose, we'll first dig a bit further into the tuning issue.
For example, flow in inlet and exhaust passages are actually unsteady, interrupted and compressible. Pressure waves, or perturbations, are created that traverse the medium containing them. These move both linearly and non-linearly, based on their respective amplitudes,and add to the complexity in a computational analysis of both intake and exhaust systems.
While we previously suggested intake and exhaust systems could be tuned separately, it's generally believed that an engine's fundamental torque characteristics (essentially the volumetric efficiency curve) is largely governed by its induction system design. And even though similar principles of analysis can be compared to those for an intake system, particularly of a practical nature as previously shared, it's basically during the valve overlap period that an exhaust system might experience initial pressure conditions (atmospheric) as does the induction system. As a result, a comprehensive analysis (or design) of an exhaust system can't be addressed exactly as that for an intake.
However, an assumed mean flow velocity value that's common to both an intake and exhaust system has proven to be a worthwhile approach when simply "ball-parking" a way to examine, compare, or predict the influence of either type of system on an engine's volumetric efficiency or torque curves. Further, believe it or not, it's possible to make a rough determination if an engine has been "over-" or "under-cammed" by comparing actual peak torque rpm to calculated peak torque rpm (using the previously-discussed, one-dimensional approach) and determining if the calculated value (rpm) is above or below the one observed. A condition of "over-camming" is reflected by an observed peak torque rpm that's higher than the one calculated and "under-camming" is indicated by the opposite.
Now, moving over to the inquiries about in-cylinder pressure analysis (engine cycle analysis or ECA), it appears we should have included some thoughts on the effects of combustion cycle-to-cycle variations. In textbook language, this amounts to "cyclic dispersion" over a range of engine speed, time, or both, and relates to variations in peak cylinder pressure as measured from one firing cycle to another, normally sequentially. Why is this important and how can it be addressed?