Mode switches among SI, SACI, and HCCI combustion and their influence on drive cycle fuel economy

A vehicle and driver model from (Nuesch et al., 2013) is here integrated with a finite state model describing mode switches among spark-ignited (SI) combustion and the advanced combustion modes spark assisted compression ignition (SACI) and homogeneous charge compression ignition (HCCI). The model is used to quantify the influence of mode switch fuel penalties on drive cycle fuel economy considering the federal test procedure (FTP-75) and highway fuel economy test (HWFET). The simulation under the assumed fuel penalties and dynamics shows a very small fuel loss due to harmful switches. Harmful switches are mode switches that lead to short stays in the advanced combustion, where the penalty in switching is greater than the benefit achieved after the switch. The simulations also highlight the benefits of integrating the SACI mode. Apart from extending the fuel efficiency improvements beyond the HCCI range, it is postulated that SACI combustion is easier to reach (lower fuel penalty and faster response than the HCCI-SI switch) from both SI and HCCI, creating a bridge and an economic destination in the overall speed-load space. The combined operation of SACI and HCCI leads to substantially higher improvements, especially for the FTP-75 drive cycle, than using either mode individually.