Numerical Prediction of Knock in a Bi-Fuel Engine

Dedicated natural gas engines suffer the disadvantages of limited vehicle range and relatively few refueling stations. A vehicle capable of operating on either gasoline or natural gas allows alternative fuel usage without sacrificing vehicle range and mobility. However, the bi-fuel engine must be made to provide equal performance on both fuels. Although bi-fuel conversions have existed for a number of years, historically natural gas performance is degraded relative to gasoline due to reduced volumetric efficiency and lower power density of CNG. Much of the performance losses associated with CNG can be overcome by increasing the compression ratio. However, in a bi-fuel application, high compression ratios can result in severe engine knock during gasoline operation. Variable intake valve timing, increased exhaust gas recirculation and retarded ignition timing were explored as a means of controlling knock during gasoline operation of a bi-fuel engine.

To avoid the prohibitive cost of grinding and testing multiple intake cams, a numerical model was developed to predict the occurrence and intensity of knock given various intake valve timing profiles, and geometric compression ratios. The autoignition model employed a zero-dimensional, steady-state thermodynamic cycle simulation to calculate the pressure and temperature of the end-gas region. Heat release due to combustion was modeled according to empirical mass burn relations derived from an extensive database of in-cylinder pressure measurements recorded from dynamometer testing of a Saturn 1.9 liter spark ignition engine. The onset of autoignition was predicted according to and empirical Arrhenius ignition delay correlation calibrated using experimental data taken during knocking and non-knocking operation of the Saturn engine. The model was used in conjunction with experimental testing with selected intake valve timings and compression ratios to determine a combination, which would produce minimum drivability difference between CNG and gasoline. Results of this parametric study showed that a compression ratio of 11.5:1 with an intake duration of 274° for gasoline operation and an intake duration of 244° degrees for CNG operation resulted in balanced wide-open throttle performance between the two fuels without the risk of knock during gasoline operation.