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Homogeneous charge compression ignition (HCCI) is one of the most promising piston-engine concepts for the future, providing significantly improved efficiency and emissions characteristics relative to current technologies. This paper presents a framework for controlling an HCCI engine with exhaust recompression and direct injection of fuel into the cylinder. A physical model is used to describe the HCCI process, with the model states being closely linked to the thermodynamic state of the cylinder constituents. Separability between the effects of the control inputs on the desired outputs provides an opportunity to develop a simple linear control scheme, where the fuel is used to control the work output and the valve timings are used to control the phasing of combustion. The controller is tested on both a single and multi-cylinder HCCI engine, demonstrating the value of a physical model-based control approach that allows an easy porting of the control structure from one engine to another. Experimental results show good tracking of both the work output and combustion phasing over a wide operating region on both engines. In addition, the controller is able to balance out differences between cylinders on the multi-cylinder engine testbed, and reduce the cycle-to-cycle variability of combustion.

Homogeneous charge compression ignition (HCCI) is a promising low-temperature combustion strategy for reducing NOx emissions and increasing efficiency in internal combustion engines. However, HCCI has no direct combustion initiator and, when achieved by reinducting or trapping residual exhaust gas with a variable valve actuation (VVA) system, becomes a dynamic process as the temperature of the residual gas couples one cycle to the next. These characteristics of residual-affected HCCI present a challenge for control engineers and a barrier to implementing HCCI in a production engine. In order to address these challenges, this paper outlines physics-based control strategies for both the VVA system and the HCCI combustion process. The results show that VVA system control can provide arbitrary valve timings on a cycle-to-cycle basis, enabling tight control of HCCI. By abstracting these valve timings further into an inducted gas composition and an effective compression ratio, model-based controllers can be developed to control simultaneously load and combustion timing in an HCCI engine.

Combustion dynamics leading to thermoacoustic instability in a rearward-facing step stabilized premixed flame is experimentally examined with the objective of investigating the fluid dynamic mechanism that drives heat release rate fluctuations, and how it couples with the acoustic field. The field is probed visually, using linear photodiode arrays that capture the spatiotemporal distribution of CH* and OH*; an equivalence ratio monitor; and a number of pressure sensors. Results show resonance between the acoustic quarter wave mode of the combustion tunnel and a fluid dynamic mode of the wake. Under unstable conditions, the flame is convoluted around a large vortex that extends several step heights downstream. During a typical cycle, while the velocity is decreasing, the vortex grows, and the flame extends downstream around its outer edge. As the velocity reaches its minimum, becoming mostly negative, the vortex reaches its maximum size, and the flame collides with the upper wall; its leading edge folds, trapping reactants pockets, and its trailing edge propagates far upstream of the step. In the next phase, while the velocity is increasing, the heat release grows rapidly as trapped reactant’ pockets are consumed by flames converging towards their centers, and the upstream flame is dislodged back downstream. The heat release rate reaches its maximum halfway into the velocity rise period, leading the maximum velocity by about 90°. In this quarter-wave mode, the pressure leads the velocity by 90° as well, that is, it is in phase with the heat release rate. Numerical modeling results support this mechanism. Equivalence ratio contribution to the instability mechanism is shown to be minor, i.e., heat release dynamics are governed by the cyclical formation of the wake vortex and its interaction with the flame.

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H 2 pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO x emission benefits from the air jet, while, with H 2 addition, NO x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO x . The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO x reduction.

The control of a class of combustion systems, susceptible to damage from self-excited combustion oscillations, is considered. The controller injects some fuel unsteadily into the burning region, thereby altering the heat release, in response to an input signal detecting the oscillation. An adaptive control design, called self-tuning regulator (STR), has recently been developed, which attempts to meet the apparently contradictory requirements of relying as little as possible on a particular combustion model while providing some guarantee that the controller will cause no harm. This paper focuses on an extension of the STR design, when, as a result of stringent emission requirements and to the danger of flame extension, the amount of fuel used for control is limited in amplitude. A Lyapunov stability analysis is used to prove the stability of the modified STR when the saturation constraint is imposed. Simulation and experimental results show that in the presence of a saturation constraint the self-excited oscillations are damped more rapidly with the modified STR than with the original STR.

Active control of pressure oscillations has been successfully applied to a lean premixed prevaporized (LPP) combustion rig operating at atmospheric conditions. The design of the rig is based on the primary stage of the Rolls-Royce RB211-DLE industrial gas turbine. Control was achieved by modulating the fuel flow rate in response to a measured pressure signal. The feedback control is an adaptive, model-based self-tuning regulator (STR), which only requires the total time delay between actuation and response to achieve control. The STR algorithm achieves a reduction of up to 30 dB on the primary instability frequency. This performance was an improvement of 5–15 dB over an empirical control strategy (simple time-delay controller) specifically tuned to the same operating point. Initial robustness studies have shown that the STR retains control for a 20% change in frequency and a 23% change in air mass flow rate.

An optimal controller using the system identification (SI) method was developed for a swirl-stabilized spray combustor operating between 30 and 114 kW. The efficacy of the controller was tested with two different nozzle configurations. The first consisted of a dual-feed nozzle whose primary fuel stream was utilized to sustain combustion, while the secondarystreamwasused for active control. The second configuration used a single-feed nozzle with two different swirling air streams. An LQG-LTR (linear quadratic Gaussian-loop transfer recovery) controller was designed using the SI-based model to determine the active control input, which was in turn used to modulate the secondary fuel stream. Using this controller, the thermoacoustic oscillations, which occurred under lean operating conditions, were reduced to the background noise level. A time-delay controller was also implemented for comparison purposes. The results showed that the LQG-LTR controller yielded an additional pressure reduction of 14 dB compared...

We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) and one-step kinetics to describe the unsteady combustion process. The analysis incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations and is cast in the form of a first order filter. We are able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations, as function of the operating conditions, for example, the mean equivalence ratio and mass flow rate. The model predicts a 180° sudden shift in phase between imposed flow oscillations and resulting heat release between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability as the equivalence ratio is lowered or the average mass flow rate is increased. A number of experimental investigations, in which the inlet nozzles were choked to minimize equivalence ratio oscillations, are used to corroborate this conclusion. Next, the heat release-mass flow relationship is coupled with the acoustic field and is applied to predict instability conditions in high swirl combustion. The predictions agree qualitatively with the corresponding experimental observations.

It is well-known that an HCCI-based engine has a high operating efficiency and low emissions. However, the HCCI process is sensitive to disturbances that can cause the engine to misfire, especially at low loads. In this paper, we propose an efficient, physics-based model for studying engine state evolution in the presence of misfires. The model relies on a simple recursive mass balance model, simplified thermodynamic mechanisms, and use of the “integrated Arrhenius rate” with a modified threshold to model combustion phasing, occurrence of misfire, and partial burn during misfire. Simulation results for our reduced-order model, compared to a high-resolution HCCI simulation proposed in a previous work, show good match.

In HCCI mode with negative valve overlap, the understanding of the engine behavior in case of misfire and delayed combustion is important to provide a complete control strategy. A hybrid continuous zero dimensional model for gasoline HCCI, based on simplified chemical kinetics and a separate airflow model is introduced. CHEMKIN is used to simulate the chemical kinetics, whereas the airflow and the injection is simulated using MatLab. The model is compared to experimental data. The introduced model is used to analyze the effect of misfire and late combustions on the dynamics of the system. A state transition map is proposed to distinguish between misfire with and without recovery. Control strategies to improve the misfire recovery are suggested.

In this paper we propose a model of a two stroke homogeneous charge compression ignition (HCCI) engine which captures the important characteristics of gas exchange processes taking place in a two stroke mode. The model allows a direct flow of gases between intake and exhaust volumes and hence offers a more detailed gas modeling than single volume models widely accepted in the literature. We compare simulation results obtained from the model with experimental data collected with a single cylinder HCCI engine. We also propose a simple control strategy allowing effective tracking of reference signals in indicated mean effective pressure (IMEP) and combustion phasing.

Thermoacoustic instability in premixed combustors occurs occasionally at multiple frequencies, especially in configurations where flames are stabilized on separating shear layers that form downstream of sudden expansions or bluff bodies. While some of these frequencies are related to the acoustic field, others appear to be related to shear flow instability phenomena. It is shown in this paper that shear flows can support self-sustained instabilities if they possess absolutely unstable modes. The associated frequencies are predicted using mean velocity profiles that resemble those observed in separating flows and for profiles obtained from numerical simulations, and are shown to match those derived from experimental and numerical investigations. It is also shown that the presence of density profiles compatible with premixed combustion can affect this frequency and can change the absolute instability mode into a convectively unstable mode thereby reducing the possibility of the generation of self-sustained oscillations. A qualitative prediction of the pressure amplitudes resulting from these shear layer modes is shown to be consistent with experimental measurements. The results from the stability analysis are combined with those using the Proper Orthogonal Decomposition (POD) method to yield a reduced-order model.

Combustion instability is one of the distinct characteristics in continuous combustion processes such as gas turbines, ramjet engines and afterburners. This combustion instability often occurs near blow limit and high thermal out conditions and generates large amplitudes of heat release and pressure oscillations. Active control has been used to suppress combustion instability, but due to complex dynamics of the combustion processes, modeling of the combustion system has often been limited the application of the active control. Recently, we developed a combustion model which is applicable to kinetically controlled combustion instability. In this paper, we first examine the sensitivity of the model characteristics, which show that the model parameters change by 100% for a 10% change in the operating conditions. Next, we propose a self-tuning controller, and show that it effectively suppresses the pressure oscillations in the presence of changing operating conditions.

We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) model and one-step kinetics to describe the unsteady combustion process. The model incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations, and is cast in the form of a first order filter. The model is able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations, as function of the operating conditions, e.g., the mean equivalence ratio and mean mass flow rate. The model predicts a sudden shift in phase in the region between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability. We use this novel model to predict combustion instability conditions in high swirl combustion, and demonstrate that these predictions agree qualitatively with experimental studies.