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A vehicle and driver model from [1] 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.

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A methodology is introduced to analyze the drive cycle fuel economy of a vehicle equipped with a multimode combustion engine, utilizing commercial cam phasers and two-step cam profile switching. The analysis is based on a longitudinal vehicle model with manual transmission. The engine model employs a finite state machine describing mode switches between two distinct combustion modes, namely, spark-ignited and homogeneous charge compression ignition. Preliminary combustion mode switch experiments were used to parameterize the model. The influence of mode switch fuel penalties on drive cycle fuel economy was quantified through application of the model to the federal test procedure (FTP-75), the highway fuel economy test (HWFET) and the US06 supplemental federal test procedure. The mode switches were analyzed individually in terms of their benefit on fuel economy and a distinction was made between harmful and beneficial mode switches. Mode switches were defined as harmful whether their fuel penalty is greater than the benefits originating from spending time in the subsequent homogeneous charge compression ignition mode. A parametric study was conducted to investigate the impact of harmful mode switches on fuel economy as a function of the fuel penalties during the switch. In the case of high fuel penalties, supervisory control becomes an important tool for minimizing the number of harmful mode switches. One possible supervisory strategy discussed is a smoothing strategy, in which a mode switch is delayed by introducing a dwell time.

Highly diluted, low temperature homogeneous charge compression ignition (HCCI) com-bustion leads to ultralow levels of engine-out NOx emissions. A standard drive cycle, however, would require switches between HCCI and spark-ignited (SI) combustion modes. In this paper we quantify the efficiency benefits of such a multimode combustion engine, when emission constraints are to be met with a three-way catalytic converter (TWC). The TWC needs unoccupied oxygen storage sites in order to achieve acceptable performance. The lean exhaust gas during HCCI operation, however, fills the oxygen storage and leads to a drop in NOx conversion efficiency. If levels of tailpipe NOx become unacceptable, a mode switch to a fuel rich combustion mode is necessary in order to deplete the oxygen storage and restore TWC efficiency. The resulting lean-rich cycling leads to a penalty in fuel economy. Another form of penalty originates from the lower combustion efficiency during a combustion mode switch itself. In order to evaluate the impact on fuel economy of those penalties, a finite state model for combustion mode switches is combined with a longitudinal vehicle model and a phenomenological TWC model, focused on oxygen storage. The aftertreatment model is calibrated using combus-tion mode switch experiments from lean HCCI to rich spark-assisted HCCI (SA-HCCI) and back. Fuel and emission maps acquired in steady-state experiments are used. Differ-ent depletion strategies are compared in terms of their influence on drive cycle fuel econ-omy and NOx emissions. It is shown that even an aggressive lean-rich cycling strategy will marginally satisfy the cumulated tailpipe NOx emission standards under warmed-up conditions. More notably, the cycling leads to substantial fuel penalties that negate most of HCCI’s efficiency benefits. [DOI: 10.1115/1.4028885]