Comparison of Engine Operational Modes with Respect to Compression Ignition Engine Knock 2018-01-0219
Diesel knock and ringing combustion in compression ignition (CI) engines are largely an unavoidable phenomenon and are partially related to the overall effectiveness of the fuel injection process. Modern electronic fuel injection systems have been effective at reducing the intensity of knock in CI engines, largely through optimization of fuel injection timing, as well as higher operating pressures that promote enhanced fuel and air mixing. In this effort, a single-cylinder CI engine was tested under a number of different combustion strategies, including a comparison of mechanical and electronic injection systems, increasing fuel injection pressures for biodiesel fuels, and the usage of dual-fuel combustion with compressed natural gas (CNG). Using in-cylinder pressure traces and engine operational data, the difference in injection mechanisms, fuel preparation, and their effects on knock intensity is clearly illustrated. This allows a means of comparison across multiple engine combustion modes, including mechanical vs. electronic injection, injection of various fuels through a common injection system, and standard operation against dual-fuel operation. In particular, the effect of higher injection pressures and fuel flow rates on reducing knock in some cases is highlighted. In addition, cyclic variability in the severity of ringing combustion appears to be a function of the ability of the fuel system to re-pressurize between engine cycles. This offers a potential means to provide for diagnosis of engine fuel pressurization system deficiencies through variability in ringing combustion, even if these deficiencies would not be discernible from normal measured operational characteristics.