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Evaluation of Variable Camshaft Effects on Performance of a High Output, 4-Valve SI Engine
Published January 01, 1990 by Associazione Tecnica Dell'Automobile in Italy
An analytical and experimental evaluation was carried out to determine the effects of variable intake and exhaust cam timing on the performance of a high output, 4-valve SI engine. The engine is free- breathing and has highly tuned intake and exhaust manifolds. The analysis was carried out using an engine analysis code which has the capability to calculate the processes occurring within each individual engine cylinder, and, through gas dynamic analysis, also the flow and pressure dynamics within the intake and exhaust manifolds. The latter capability is essential in simulations of engines which have significant tuning effects which result from the design of their manifolds.
The analytical part of the study first addressed the effects of timing variations of stock intake and exhaust camshafts at wide open throttle. Then a broader study was carried out to determine the effects of variable valve timing coupled with variation of valve-open duration. Brake torque increases of up to 11.5% were observed. At road load operating points, variable timing was found to enable control of exhaust residuals resulting in a favorable impact on fuel economy and NOx generation. At idle, variable timing was found to reduce trapped residuals through a reduction in valve overlap. Non- tuned manifolds were found to produce results similar in magnitude to those seen with the baseline highly tuned manifolds. Experimental data were acquired to address some of the key conclusions of the analytical study, and a good agreement was found between the analysis and the experiment.