The role of fuel in determining the high load limit of controlled auto-ignition engines

Abstract

Controlled Auto-Ignition (CAI) engines have the potential to increase fuel economy while lowering nitrogen oxide and soot emissions. One hurdle that is currently being faced is the engine's inability to operate at high loads due to a large Maximum Pressure Rise Rate (MPRR). To address this issue, this research has been focused on analytically determining the optimum fuel that can be used in a CAI engine to reduce the MPRR and extend the high load limit. The strategy is to use the fuel ignition characteristics to maximize the impact of stratification on reducing the MPRR with sequential ignitions. To quantify the impact of the selected fuel on the high load limit, the fuel's ignition delay curve under constant volume conditions, as a function of the initial concentration and temperature was used. A parametric model of the fuel chemistry was created so that different functional dependences of the ignition delay curve could be produced through adjustment of the model parameters. Then, the ignition delay curve was parameterized, and various artificial fuels were created. The artificial fuels were then tested in an engine simulation under different operating conditions and temperature distributions. The results from the engine simulations provide insight into the characteristics of the optimum fuel ignition delay time versus temperature relationship. As expected, the conclusions depend on the initial boundary conditions applied; particularly the initial cylinder temperature distribution. With a constant temperature applied to the entire charge, the MPRR is reduced when ignition occurs in the Negative Temperature Coefficient (NTC) region. When ignition occurs in the NTC region, the radical concentration in the later stages of the combustion process is reduced, which reduces the MPRR. When an initial quadratic temperature distribution is applied, different regions of the combustion chamber can ignite at different periods, hence reducing the MPRR. However, ignition in the NTC region negates this positive effect, and should therefore be avoided. The functional dependency of the ignition delay time versus initial temperature for the optimum fuel must therefore be created based on the expected initial charge temperature distribution.