Date of Graduation

1997

Document Type

Thesis

Abstract

A numerical thermodynamic model was developed and employed to investigate the use of elevated compression ratios, variable valve timing, and ignition timing modulation as a means of controlling knock and improving the performance of a bi-fuel engine. The proposed strategy employed an increased compression ratio to boost power and torque when operating on natural gas. An increased intake duration and retarded ignition timing were then employed to control knock during gasoline operation. Dynamometer testing was performed on a 1.9 liter Saturn{dollar}\\sp\\circler{dollar} engine to provide empirical data for model calibration and to evaluate the effects of proposed changes on engine emissions. Tests were conducted with a compression ratio of 12.7:1 and four intake configurations, including the stock Saturn{dollar}\\sp\\circler{dollar} cam, the stock cam retarded by 19{dollar}\\sp\\circ{dollar} and by 38{dollar}\\sp\\circ{dollar}, and a specially designed extended duration cam. Experimental results showed that at low to moderate power levels, knock could be controlled through increased exhaust gas recirculation (EGR) and moderate retardation of the ignition timing. Retarding or extending the intake valve open duration was shown to further reduce knock intensity. However, at wide-open throttle, knock could not be reduced to acceptable levels without a severe loss in performance. The analytical model was then employed to evaluate additional compression ratio and intake cam configurations through interpolation of experimental results. Analytical results showed that a compression ratio of 11.5:1 with an intake duration of 274{dollar}\\sp\\circ{dollar} would result in good steady-state, wide-open throttle performance and minimum driveability differences between the two fuels. In this configuration, predicted power output differed by less that 5 percent between gasoline and natural gas operation.

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