Controlled auto ignition (CAI), also known as homogeneous charge compression ignition (HCCI), is garnering much global R&D focus. Researchers say the low-temperature combustion process offers 20% or greater fuel-efficiency gains when applied to gasoline engines. A related regime, known as PCCI, is being applied to diesel engines.
Numerous OEMs have active CAI development programs in conjunction with major research institutions and powertrain consultancies. In theory, CAI offers attributes of both diesel (high fuel efficiency) and gasoline (low emissions) engines. A nearly homogeneous mixture of fuel, air, and captured exhaust gas is compressed so that the mixture auto-ignites, as in a diesel. Combustion is controlled by the chemical kinetics of the in-cylinder charge, rather than by a conventional flame front.
Thus CAI produces low peak temperatures—and significantly low levels of NOx. In addition, unthrottled operation minimizes pumping losses and improves low-load efficiency. The CAI combustion process can be controlled by a high amount of residual gas and stratification of air and fuel. However, achieving CAI’s potential in the real world, particularly at part-load and sustained high-load operation, is a challenge on which engineers are devoting great attention.
Those involved with developing CAI engines should download SAE technical paper 2009-01-0300, "Operation Strategies for Controlled Auto Ignition Gasoline Engines.” Further insights from the paper were presented by a group of the paper’s authors, including Philip Adomeit, Andreas Sehr, and Henning Kleeberg of FEV, and Georg Stapf of RWTH Aachen University, on April 20 at the SAE World Congress.
The paper investigates both fundamental and application-relevant aspects of CAI control. It presents a reduced-modeling approach to the CAI process which is used to generate data for better understanding of the in-cylinder behavior during load steps, which feeds into the control strategy. The paper notes the key for successful application of CAI combustion in future automotive engines is an extremely accurate control of transient modes and shifts between different operating modes.
Detailed thermodynamic analysis of CAI combustion, optical diagnostics of a transparent test engine, and 3D-CFD analysis with reduced chemical kinetics were used to develop the authors’ necessary fundamental knowledge of the CAI process. In order to deduce measures for stability and operating range extension, the detailed fundamental information was transferred to a 1D model and extended by a multizone approach describing thermodynamic parameters and incorporating reduced reaction kinetics.
Application strategies for CAI were developed on a 500-cm3 single-cylinder research engine with a fully variable valvetrain and direct injection. The authors found that control of the CAI operating range can be achieved by realizing stratification of the in-cylinder charge. Stratification control was possible via valve timing and direct injection.
Based on the thermodynamic requirements, the required variability of the valve train for realization of CAI operation in a multicylinder engine was indentified. The paper noted that FEV is testing multicylinder CAI operating modes on a its 1.8-L SGT engine (DI turbo four cylinder) with fully variable valvetrain. The turbocharging allowed CAI mode extension to higher loads, stated the authors, who indicated the potential for closed-loop control of the combustion process in this application.
The investigations presented in the paper involved different exhaust gas recirculation strategies, exhaust port recirculation (EPR), and combustion chamber recirculation (CCR). For EPR, the exhaust valve open duration is significantly longer, so exhaust gas is drawn back from the exhaust port during the intake stroke. For the CCR strategy the exhaust gas is kept in the combustion chamber due to a highly negative valve overlap.
The fundamental processes in the combustion chamber were investigated for the engine operating point NMEP = 3 bar at 2000 rpm. This point was chosen because it can be operated by both the CCR and EPR strategies. Meshing of the intake ports, combustion chamber including the piston, and valve motion, was done using the es-ice tool. The CFD calculation was performed using the code StarCD.
The authors found that for both EGR strategies similar limitations for CAI operation apply—certain undesirable NVH characteristics (represented by the maximum pressure-rise limit) avoiding insufficient rich mixture, and most importantly the combustion stability limitation. The research established that a lower residual gas fraction (RGF) leads to a lower compression end temperature and, consequently, a retarded combustion event. This was indicated by a longer burn delay and duration, resulting in higher combustion instabilities, characterized by higher standard deviation in NMEP.
The results of the fundamental investigation showed that stratification of the fuel and RGF have a strong effect on auto ignition and the combustion rate. Because of this, the control of the degree of stratification through valve timing and DI strategy enables adjustment of the auto ignition and combustion characteristics in CAI operation.