A Parametric Study of Knock Control Strategies for a Bi-Fuel Engine

Until a proper fueling infrastructure is established, vehicles powered by natural gas must have bi-fuel capability in order to avoid a limited vehicle range. Although bi-fuel conversions of existing gasoline engines have existed for a number of years, these engines do not fully exploit the combustion and knock properties of both fuels. Much of the power loss resulting from operation of an existing gasoline engine on compressed natural gas (CNG) can be recovered by increasing the compression ratio, thereby exploiting the high knock resistance of natural gas. However, gasoline operation at elevated compression ratios results in severe engine knock. The use of variable intake valve timing in conjunction with ignition timing modulation and electronically controlled exhaust gas recirculation (EGR) was investigated as a means of controlling knock when operating a bi-fuel engine on gasoline at elevated compression ratios. Dynamometer testing was performed on a Saturn 1.9 liter 4-cylinder dual-overhead-cam engine equipped with 12.7:1 high compression pistons. Four intake cam configurations were tested including the stock cam profile and phasing, 18° and 38° retarded phasings of the stock cam and a specially designed 292° long duration cam. Knock occurrence and severity were determined from in-cylinder pressure data and power, torque and emissions performance were measured. A thermodynamic engine model was also developed to predict the occurrence of knock given various engine configurations and operating conditions. The model employed a zero-dimensional, steady state thermodynamic cycle simulation and an empirical Arrhenius autoignition correlation to predict the onset of knock. The accuracy of the model was adjusted to fit experimental data taken from knocking cycles. The model was then employed to investigate additional intake valve timing, and ignition timing strategies through interpolation of measured results recorded at compression ratios of 9.5:1 and 12.7:1. Model results showed that a compression ratio of 11.5:1 with intake cam durations of 274° for gasoline operation and 244° for CNG operation would result in minimum wide-open throttle performance differences between gasoline and CNG operation.