Experimental studies at Stanford University and elsewhere demonstrate that variable valve actuation (VVA) can be used to initiate homogeneous charge compression ignition (HCCI). This is achieved by reinducting combustion products from the previous cycle, thereby increasing the sensible energy of the reactant charge and allowing ignition to occur by compression alone at modest compression ratios. Since the reinducted products act as a thermal sink during combustion, this process lowers the peak combustion temperature, which in turn lowers NOx concentrations. This drop in NOx is one of the major benefits of HCCI.
Other methods besides VVA exist to initiate HCCI, such as heating or pre-compressing the intake air or varying the compression ratio. Regardless of the method chosen, however, HCCI combustion exhibits some fundamental control challenges concerning combustion phasing and load, two critical parameters for any IC engine combustion methodology. Unlike spark ignition (SI) or diesel engines, where the combustion is initiated via spark and fuel injection, respectively, HCCI has no specific event that initiates combustion. Therefore, ensuring that combustion occurs with acceptable timing, or at all, is more complicated than in the case of either SI or diesel combustion. Combustion phasing in HCCI is dominated by chemical kinetics, which depends on both in-cylinder species concentrations and temperature. Load is dependent on both the combustion phasing and the amount of reactant species present in the cylinder. Both load and combustion phasing are therefore coupled and dependent on both in-cylinder species concentrations and temperature.
These parameters are controlled in the system studied solely through the use of the fully flexible VVA system. The control problem then becomes how to vary the valve timings
to achieve the
desired combustion phasing and load on a cycle-to-cycle basis. As noted previously, this is complicated by the lack of a direct combustion initiator. Furthermore, since products are re-inducted from the previous cycle, the composition and temperature of the exhausted products from the previous cycle have a direct impact on the next cycle, introducing cycle-to-cycle dynamics. Understanding the effect of the previous cycles exhausted products and the implications for control of combustion phasing and load during the next cycle must then be clearly understood before a control methodology for HCCI combustion can be completed.
To synthesize a controller to stabilize HCCI and track desired load trajectories using the VVA system, a model of the system with special attention paid to combustion
phasing and cycle to cycle interactions is necessary. This model should be as simple as possible, as it is often difficult to synthesize controllers from more complex
models. In previous and ongoing work a relatively high order (10-state) model of HCCI combustion has been formulated, showing very good correlation with experimental
combustion timing, in-cylinder pressure evolution, work output, maximum rate of pressure rise and exhaust temperature. As well as this model works, it has been noted that a
simpler and lower order description of
HCCI combustion would probably be needed to synthesis a controller. The formulation of a low-order model and its use to synthesize a load and combustion phasing controller,
have been completed. Implementation in simulation and experiment result in the desired tracking of both combustion timing and work output.
Gregory M. Shaver, Matthew J. Roelle, J. Christian Gerdes, Modeling Cycle-to-Cycle Coupling and Mode Transition in HCCI Engines with
Valve Actuation, To appear in a special issue of the IFAC Journal on Control Engineering Practice (CEP)
Gregory M. Shaver, Matthew Roelle, J. Christian Gerdes, Patrick A. Caton and Christopher F. Edwards, Dynamic Modeling of
HCCI Engines Utilizing Variable Valve Actuation, To appear in the ASME Journal of Dynamic Systems, Measurement and
Gregory M. Shaver, Matthew Roelle, J. Christian Gerdes, Jean-Pierre Hathout, Jasim Ahmed, Aleksandar Kojic, Patrick A. Caton and Christopher F. Edwards, A Physically
Based Approach to Control of HCCI Engines with Variable Valve Actuation, To appear in a special HCCI Issue of The
International Journal of Engine Research
Gregory M. Shaver, Matthew J. Roelle, J. Christian Gerdes, Decoupled Control of Combustion Timing
and Peak Pressure on an HCCI Engine, Submitted to the American Control Conference, 2005, Portland, Oregon
Jean-Pierre Hathout, Jasim Ahmed and Aleksandar Kojic, Reduced Order Modeling and Control of an Electrohydraulic Valve System, In the
Proceedings of the 1st IFAC Symposium on Advances in Automotive Control, 2004, Pages 182-187, Salerno, Italy
Gregory M. Shaver and J. Christian Gerdes, Cycle-to-cycle control of HCCI Engines, In the Proceeding of
the 2003 ASME International Mechanical Engineering Congress and Exposition, IMECE2003-41966, Washington D.C.
Gregory M. Shaver, J. Christian Gerdes, Parag Jain and P.A. Caton and C.F. Edwards, Modeling for Control of HCCI Engines, In the Proceedings of the American Control Conference,
2003, Pages 749-754, Denver