Low-temperature combustion modes, such as homogeneous charge compression ignition, represent a promising means to increase the efficiency and to reduce significantly the emissions of internal combustion engines. Implementation and control are difficult, however, owing to the dependence of the combustion event on the chemical kinetics rather than an external trigger. This work describes a nonlinear control-oriented model developed for a single-cylinder homogeneous charge compression ignition engine, which is physically based on a five-state thermodynamic cycle. This model is aimed at capturing the behavior of an engine which utilizes fully vaporized gasoline-type fuels, exhaust gas recirculation, and intake air heating in order to achieve homogeneous charge compression ignition operation. The onset of combustion, which is vital for control, is modeled using an Arrhenius reaction rate expression which relates the combustion timing to both the charge dilution and the temperature. Despite the fact that homogeneous charge compression ignition combustion is indeed fast, it is not perfectly instantaneous and therefore requires some finite amount of time to occur. To account for this phenomenon within the model, a Δθ term is added which shifts the point of instantaneous combustion from the start of combustion to a point of very-high-energy release based on experimental heat release data. The model is validated against experimental data from a single-cylinder compression ignition engine operating under homogeneous charge compression ignition conditions at two different fueling rates. Parameters relevant to control such as the combustion timing, peak in-cylinder pressure, and pressure rise rates from the simulation agree very well with the experiment in both operating conditions. The extension of the model to other fuels is also investigated via the octane index of several different gasoline-type fuels. Since this nonlinear model is developed from a controls perspective, both the output and the state update equations are formulated such that they are functions of only the control inputs and the state variables, therefore making them directly applicable to state-space methods for control. The result is a discrete-time nonlinear control model which provides a platform for developing and validating various nonlinear control strategies.
This study evaluates the ability of accelerometers to detect combustion phasing of a single-cylinder air-cooled internal combustion engine undergoing homogeneous charge compression ignition. Metrics derived from the measured surface acceleration waveform, surface velocity, and the surface-specific kinetic energy were compared to the 50 per cent energy release location (CA50) on a cyclic basis for three different experimental test cases. The peak surface velocity location showed a robust ability to detect CA50 on a cyclic basis for short combustion durations. Using a simple single-degree-of-freedom vibration model, it is shown the impulsive nature of the combustion load and the natural frequency of the engine structure governs when the peak velocity location will indeed robustly detect CA50 on a on a per-engine cycle basis.
The upper load range of homogeneous charge compression ignition engines is limited by extreme noise levels. Previous studies have investigated the sources of this noise by investigating high-frequency (>4 kHz) acoustic energy typically associated with cylinder cavity resonances excited during high in-cylinder pressure rise rate combustion events. In this work, a broader frequency band (0.5–15 kHz) is investigated experimentally for a single-cylinder homogeneous charge compression ignition engine inside an acoustic sub-enclosure. It has been found that the 0.5–4-kHz frequency band being excited by the rapid in-cylinder pressure rise rate of the homogeneous charge compression ignition combustion event is dominating the acoustic signature of the engine. This indicates that the noise is generated from the structural response of the engine structure to the impulse-like load from the rapid cylinder pressure rise rather than ringing associated with the cavity resonances. A band-level analysis is used to explore the contribution of mechanical and combustion noise on the acoustic signature for various load conditions and combustion phasings. Based on this analysis, a method is proposed where a cylinder pressure rise rate threshold is established for determining the point at which the homogeneous charge compression ignition combustion noise will significantly exceed the mechanical noise level. This threshold, based on the theoretical band-level analysis, agrees with thresholds found from previous literature.
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