2006-01-1087 Analysis of Load and Speed Transitions in an HCCI Engine Using 1-D Cycle Simulation and Thermal Networks Kyoungjoon Chang, Aristotelis Babajimopoulos, George A. Lavoie, Zoran S. Filipi and Dennis N. Assanis University of Michigan Copyright © 2006 SAE International ABSTRACT Exhaust gas rebreathing is considered to be a practical enabler that could be used in HCCI production engines. Recent experimental work at the University of Michigan demonstrates that the combustion characteristics of an HCCI engine using large amounts of hot residual gas by rebreathing are very sensitive to engine thermal conditions. This computational study addresses HCCI engine operation with rebreathing, with emphasis on the effects of engine thermal conditions during transient periods. A 1-D cycle simulation with thermal networks is carried out under load and speed transitions. A knock integral auto- ignition model, a modified Woschni heat transfer model for HCCI engines and empirical correlations to define burn rate and combustion efficiency are incorporated into the engine cycle simulation model. The simulation results show very different engine behavior during the thermal transient periods compared with steady state. Hot walls advance the ignition timing, while cold walls may result in misfire. Realizable operating regions during the thermal transitions are very dependent on the wall temperatures and are quite different from the steady state. This implies that thermal inertia must be considered in order to fully optimize HCCI engine operation. INTRODUCTION The Homogeneous Charge Compression Ignition (HCCI) engine has been vigorously studied in the last decade because of its high thermal efficiency (potentially 15- 20% higher than conventional gasoline engines) and ultra low NOX and PM emissions compared with SI (gasoline Spark Ignition) and CI (diesel Compression Ignition) engines [1, 2]. The basic idea is to employ a premixed air-fuel mixture that is sufficiently lean or dilute to keep flame temperatures below about 1900K to help keep NO x and particulate production low. Consequently, the HCCI engine with lean burn characteristics is a very good candidate for future clean and economical passenger vehicle applications. In spite of these great benefits, it has been very difficult to apply HCCI technology to real production engines. There are major challenges that must be overcome to make the HCCI engine practical. First, the ignition timing and combustion phasing in the HCCI engine cannot be directly controlled because there is no direct trigger, such as spark timing in SI engines or injection timing in CI engines; second, it has low power density because of its lean combustion nature [3]; and finally, the HCCI engine has limited operating range due to knock-like rapid combustion under some conditions and misfire under others. A number of studies on how to expand the HCCI operating region have been conducted [4, 5], but running the HCCI engine at all operating conditions that a passenger vehicle requires cannot yet be achieved. Recently, strategies on how to enlarge the operating range of the HCCI engine have been studied because its limited operating zone impedes the realization of practical application of HCCI. Enlarging the HCCI operating range at steady state by controlling the coolant temperature was experimentally investigated on a single cylinder research engine with different valve events [5]. It was shown that the upper limit can be extended by reducing the coolant temperature and the lower limit can be extended by increasing the coolant temperature. Even though the operating region of HCCI may be expanded to a wider zone, there will be regions which HCCI cannot cover during the actual driving. The dual combustion mode of SI and HCCI is a very good candidate to compensate for the HCCI engine deficit of narrow operating window. Research on how to make smooth and robust combustion mode transition from SI to HCCI and HCCI to SI is currently being performed by several groups [6, 7, 8]. Understanding the engine behavior in combustion mode transition between SI and HCCI is very critical in order to have a full advantage of this strategy [8].