An Evaluation of Knock Determination Techniques for Diesel-Natural Gas Dual Fuel Engines

The recent advent of highly effective drilling and extraction technologies has decreased the price of natural gas and renewed interest in its use for transportation. Of particular interest is the conversion of dedicated diesel engines to operate on dual-fuel with natural gas injected into the intake manifold. Dual-fuel systems with natural gas injected into the intake manifold replace a significant portion of diesel fuel energy with natural gas (generally 50% or more by energy content), and produce lower operating costs than diesel-only operation. Diesel-natural gas engines have a high compression ratio and a homogeneous mixture of natural gas and air in the cylinder end gases. These conditions are very favorable for knock at high loads. In the present study, knock prediction concepts that utilize a single step Arrhenius function for diesel-natural gas dual-fuel engines are evaluated. A heavy duty diesel engine with the capability of running both natural gas and diesel is operated at points where knock occurs and the cylinder pressure traces are recorded. Constants of the Arrhenius function for dual-fuel operation are determined empirically based on the cylinder pressure and temperature histories. Several methods for knock prediction and coefficient fitting are evaluated, and their accuracy is compared. Results indicate that Arrhenius integral methods can determine if a cycle will knock with accuracy up to 72% using coefficients similar to those for methane.