2008-01-0984 Active Combustion Control of Diesel HCCI Engine: Combustion Timing M. Hillion, J. Chauvin and O. Grondin IFP, France. N. Petit Ecole des Mines de Paris, France Copyright c  2008 Society of Automotive Engineers, Inc. ABSTRACT We propose a model based control strategy to adapt the injection settings according to the air path dynamics on a Diesel HCCI engine. This approach complements existing airpath and fu- elpath controllers, and aims at accurately controlling the start of combustion. For that purpose, start of injection is adjusted based on a Knock Integral Model and intake manifold condi- tions. Experimental results are presented, which stress the rele- vance of the approach. INTRODUCTION In the context of environmental restrictions and sustainable de- velopment, pollution standards have become more and more stringent over the last 15 years. Engine pollutant emission re- duction has then become a topic of major interest for engine development. Lately, two main strategies have been explored: namely after-treatment and improved combustion modes. For Diesel engines, cost of after treatment devices are usually high. In turn, this has spurred a major interest in developments of cleaner combustion modes such as the Highly Premixed Com- bustion modes (HPC), including Homogeneous Charge Com- pression Ignition (HCCI). HCCI combustion requires the use of high Exhaust Gas Recirculation (EGR) rates. The air charge ad- mitted in the cylinder is significantly diluted, which reduces ni- trogen oxides (NO x ) emissions by lowering the maximum tem- perature during combustion (see [1]). In this paper, we focus on the HCCI mode which has a great potential in terms of NO x reduction and is also the most chal- lenging from a control perspective (see [2] and [3]). The several phases of the combustion can be described accord- ing to the timeline detailed in Figure 1. There are two main phases corresponding to the airpath subsystem (which involves the intake manifold, the intake throttle, the turbocharger, and the EGR valve) and the fuelpath subsystem (which consists of the injectors). The HCCI combustion mode is highly sensitive to the thermodynamic conditions at the intake. Accurate airpath control and adaption of the fuelpath are thus required to manage the HCCI combustion. Airpath controllers have long been proposed (see [4], [5] and their references). They result in efficient tracking of the in- take manifold variables (reference total mass, burned gases rate (BGR), and temperature of the intake charge) even during tran- sients. Usually, three main actuators are employed (EGR valve, intake throttle and turbocharger). Classic fuelpath controllers can be described as follows. During the cylinder compression phase, fuel is injected and mixed to the compressed air and burned gas mixture. The fuel vaporizes and auto-ignites after the so-called ignition delay (see Figures 1 and 2). Standard fuelpath control strategies focus on controlling injected fuel mass. Eventually, a smoke limiter can be added on, providing a fuel mass limiter based on a fuel/air ratio limitation to avoid smoke emissions during transients. This is usually suf- ficient to produce the torque requested by the driver. As we will now discuss it, these controllers are often not suf- ficient to maintain a stable Diesel HCCI combustion. In fact, and by contrast to conventional Diesel combustion mode, slight offsets of cylinder initial conditions (e.g. pressure, tempera- ture, or composition) easily cause problems. In practice, if the fuelpath controller is not coordinated to the airpath controller, combustion stability is jeopardized during transients. This is clearly visible in Figure 3e where combustion timings are re- ported. Large overshoots or undershoots reflect the fact that the combustion is not stabilized during transients. Actually, this can easily lead to stall. This can be seen in Figure 3 where experi- mentally observed misfire is reported (at time 30s). In details, one can see in Figure 3a that the produced torque drastically drops during transient resulting in stall. In the setup used to ob- tain these results, a classic airpath and fuelpath control strategy 1